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THESIS

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1293 01801 7453

 

LIBRARY
Michigan State
University

 

 

 

 

This is to certify that the
thesis entitled

An Analysis of Global and Localized Creep Strains and Creep
Properties of Non-Composite and Composite Lead—Free

Solders at Room and Elevated Temperatures
presented by

Jeffrey Lee McDougall

has been accepted towards fulfillment
of the requirements for

Master's Materials Science

degree in

 

 

Major professor

Dan: December 16, 1998

 

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1/98 alClRCJDateDoepGS-p.“

 

 

 

 

 

AN ANALYSIS OF GLOBAL AND LOCALIZED CREEP STRAINS AND CREEP
PROPERTIES OF NON-COMPOSITE AND COMPOSITE LEAD-FREE
SOLDERS AT ROOM AND ELEVATED TEMPERATURES

By

Jeffrey Lee McDougall

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Materials Science and Mechanics

1998

ABSTRACT

AN ANALYSIS OF GLOBAL AND LOCALIZED CREEP STRAINS AND CREEP
PROPERTIES OF NON-COMPOSITE AND COMPOSITE LEAD-FREE
SOLDERS AT ROOM AND ELEVATED TEMPERATURES

By

Jeffrey Lee McDougall

Due to the adoption of “green” manufacturing practices in Europe, as well as the need for
solders with better creep resistance at higher temperatures, there has been new interest in
developing lead-ﬂee solders. Understanding and quantiﬁcation of the effects of creep
deformation is essential for lifetime prediction of electronic systems. Miniature single
shear-lap creep specimens, similar in size to actual solder joints used in electronic
packaging, have been developed to examine the effect of in-situ composite
microstructures on the creep resistance and damage accumulation in lead-free solder
joints. Average and localized strain measurements have been made optically, by
monitoring the displacement of a scratch across the thickness of the solder joint. By
analysis of the local strains, criteria for damage initiation are expressed in terms of the
onset of tertiary creep, which starts sooner in some regions of the microstructure than
others, leading to heterogeneous strain in the joint. The in-situ composite solders result
in a more homogeneous strain evolution, which causes an improved creep resistance at
lower temperatures, and much greater uniform strain at higher strain-rates before a critical

damage level is reached.

This work is dedicated with love and gratitude to mom, Wendy, dad., and of course to
grandma.

Also, I would like to dedicate this to the memory of Gramma and Grampa and my great
grandmother, Gram, my departed and current best friends, Casey and Drexyl, the Glennie
Chapter of the MacDougall Clan for providing me with Scottish blood, Rico for being
Rico, Tommy - who has shown me that one man’s ego can never be too large, Curtis for
introducing me to Atlanta and showing me that space and time are meaningless in the
realm of friendship, Joel - for his Scotch and song, Don - my eternal Glennie Buddy,
Wade - for helping me realize a computer’s true purpose, Cousin Ed and all the boys up
at Alcona Park, and Edna & Jeff Felmlee for making this all possible.

Finally, I dedicate this to Sylvann for mending my broken faith and heart.

iii

ACKNOWLEDGEMENTS

I would like to express sincere gratitude to Dr. Thomas R. Bieler for his guidance,
support, friendship, and belief in my capabilities. Sincere thanks is also directed towards
Dr. Lucas and Dr. Subramanian. I would also like to thank Dr. Case for placing in me the
desire to become a better student. Finally, I want to express sincere appreciation for my
colleagues and friends Sunglak Choi and Alan Gibson for their contributions, both

academic and cultural.

I would also like to acknowledge the Composite Materials and Structure Center for their
ﬁnancial contributions which made this research possible, Dong Yi Seo for the use of his
Macintosh computer, Kurt Neimeyer for his contributions to the miniature creep frame,
and my family and friends for their ﬁnancial support and encouragement throughout my

academic career.

iv

TABLE OF CONTENTS

LIST OF TABLES ........................................................................................................... viii
LIST OF FIGURES ............................................................................................................ ix
CHAPTER 1: INTRODUCTION .................................................................................... 1
CHAPTER 2: REVIEW OF RELATED LITERATURE ............................................. 3
2.1 SOLDER OVERVIEW ............................................................................................ 3
2.1.1 The History of Soldering .................................................................................... 3
2.1.2 Solders and Soldering ........................................................................................ 5

2.2 CURRENT SOLDERING ISSUES .......................................................................... 7
2.3 MICROSTRUCTURE TAILORING ...................................................................... 11
2.4 CREEP OVERVIEW .............................................................................................. 13
2.4.1 History of Creep ............................................................................................... 13
2.4.2 Stress and Strain ............................................................................................... 14
2.4.3 Homologous Temperature ................................................................................ 15
2.4.4 Creep Testing ................................................................................................... 16
2.4.5 The Creep Curve .............................................................................................. 17
2.4.5.1 Secondary Creep-Rate ............................................................................... 20
2.4.5.2 Scattering of Data ..................................................................................... 22

2.4.6 Creep on an Atomic Scale ................................................................................ 22
2.4.7 The Role of Dislocations in Deformation ........................................................ 23
2.4.8 Steady-State Creep-Rate .................................................................................. 24
2.4.9 Secondary Creep-Rate Normalization .............................................................. 26

2.5 CREEP MECHANISMS ........................................................................................ 27
2.5.1 Dislocation Controlled Creep Mechanisms ..................................................... 27
2.5.2 Diffusion-Controlled Creep Mechanisms ........................................................ 28

2.6 CREEP-FATIGUE INTERACTIONS .................................................................... 30
2.6.1 Stress Relaxation .............................................................................................. 32
2.6.2 Fatigue .............................................................................................................. 33
2.6.3 Fatigue of Solder Joints .................................................................................... 35

2.7 CREEP IN SOLDER JOINTS ................................................................................ 40
CHAPTER 3: EXPERIMENTAL PROCEDURE ....................................................... 41
3.1 SAMPLE PREPARATION .................................................................................... 41
3.1.1 Joint Fabrication ............................................................................................... 41
3.1.2 Solder Microstructures ..................................................................................... 43
3.1.3 Polishing .......................................................................................................... 43

3.2 CREEP TESTING .................................................................................................. 47

3.2.1 Miniature Creep Frame .................................................................................... 47

3.2.2 Miniature Electrical Resistance Furnace .......................................................... 47
3.2.3 Deformation of Solder Joints ........................................................................... 49
3.3 DATA EXTRACTION TECHNIQUE ................................................................... 54
3.3.1 Image Enhancement ......................................................................................... 54
3.3.2 Data Extraction ................................................................................................ 55
3.3.3 Reproducibility of Data Extraction .................................................................. 58
3.4 DATA PROCESSING ............................................................................................ 59
3.4.1 Global Creep Strain Measurement ................................................................... 59
3.4.2 Localized Creep Strain Measurement .............................................................. 61
3.4.3 Strain-Rate, Position, and Time ....................................................................... 63
3.5 SEM OBSERVATIONS ......................................................................................... 65
3.5.1 Determination of Stress .................................................................................... 65
3.5.2 Initiation of Fracture ........................................................................................ 65
3.5.3 Location of Fracture ......................................................................................... 66
CHAPTER 4: RESULTS ............................................................................................... 67
4.1 COMPARISON OF EXPERIMENTAL CREEP DATA TO DATA
FOUND IN THE LITERATURE ............................................................................ 67
4.1.1 Room Temperature Composite Data ............................................................... 69
4.1.2 Room Temperature Non-Composite Data ....................................................... 69
4.1.3 85°C Data ......................................................................................................... 70
4.1.4 125°C Data ....................................................................................................... 70
4.2 GLOBAL VS. LOCALIZED CREEP STRAIN ..................................................... 71
4.2.] Room Temperature Composite Creep Data ..................................................... 71
4.2.2 Room Temperature Non-Composite Creep Data ............................................. 73
4.2.3 85°C Composite Creep Data ............................................................................ 75
4.2.4 85°C Non-Composite Creep Data .................................................................... 75
4.2.5 125°C Composite Creep Data .......................................................................... 78
4.2.6 125°C Non-Composite Creep Data .................................................................. 80
4.3 POSITION OF MAXIMUM LOCALIZED CREEP STRAIN ............................... 82
4.4 FRACTURE SURFACES ...................................................................................... 83
4.5 RESULTS OF LOCATION OF F RACTURE EXAMINATION ........................... 86
4.6 RESULTS OF CRACK INITIATION TESTS ....................................................... 87
CHAPTER 5: DISCUSSION ......................................................................................... 90
5.1 CREEP BEHAVIOR ............................................................................................... 90
5.2 HOMOGENEITY OF DEFORMATION ............................................................... 91
CHAPTER 6: CONCLUSIONS .................................................................................... 96
APPENDIX A: CREEP MECHANISMS ......................................................................... 98
APPENDIX B: DATA REDUCTION ............................................................................ 100

vi

APPENDIX C: DISPLACEMENT, STRAIN, AND STRAIN-RATE HISTORIES ..... 105

APPENDIX D: GLOBAL SECONDARY STRAIN-RATE DATA .............................. 125
APPENDIX E: FRACTURE SURFACES AND PROFILES ........................................ 136
APPENDIX F: SOLDER JOINT CREEP DATA .......................................................... 154
REFERENCES ................................................................................................................ 157

vii

LIST OF TABLES

Table 1. Creep Mechanisms .............................................................................................. 99
Table 2. Scratch Displacement Data. .............................................................................. 102
Table 3. Normalized Position and Modiﬁed Scratch Displacement Data ...................... 104
Table 4. Solder Joint Creep Data. ................................................................................... 155

viii

LIST OF FIGURES

Figure l. The Model Shape of a Typical Creep Curve. .................................................... 19
Figure 2. The Process of Stress Relaxation ....................................................................... 36
Figure 3. The Process of Thenno-Mechanical Fatigue ..................................................... 38
Figure 4. Differences Between Normal (a) and Accelerated (b) Thermo-Mechanical
Fatigue Testing Adapted From Darveaux et al. ................................................ 39
Figure 5a. Assembly of the Solder Joint. .......................................................................... 42
Figure 5b. Finished Solder Joint with Magniﬁed View of Joint Area. ............................. 42
Figure 6a. Eutectic 96.SSn-3.5Ag Non-Composite Solder Micrograph. ......................... 44
Figure 6b. Eutectic 96.58n-3.5Ag Composite Solder Micrograph. ................................. 44
Figure 7. Specimen Grip and Polishing Bar. .................................................................... 45
Figure 8. Development of the Miniature Creep Frame. .................................................... 48
Figure 9. Miniature Electrical Resistance Furnace. .......................................................... 50
Figure 10. Creep Frame Set Up for Elevated Temperature Testing .................................. 51

Figure 1]. Displacement of Scratch Due to Creep Shown as:

(a) Raw Captured Image in Negative Format. ................................................. 53

(b) Enhanced Image for use in Datathief ........................................................ 53
Figure 12. Normalized Position Across the Thickness of a Solder Joint ........................... 56
Figure 13. Screen Capture of Data Extraction Using Datathief. ....................................... 56
Figure 14. Scratch Displacement History. ........................................................................ 60
Figure 15. Difference Between Global and Localized Deformation ................................. 60
Figure 16. Cubic Polynomial Curve Fit. ........................................................................... 62

ix

Figure 17. Localized and Global Creep Strain. ................................................................. 62

Figure 18a. Variation of Strain-Rate to Time and Normalized Position for a Composite

Solder Creep Tested at 25°C ........................................................................... 64
Figure 18b. Variation of Strain-Rate to Time and Normalized Position for a Non-

Composite Solder Creep Tested at 85°C. ....................................................... 64
Figure 19. Comparison of Secondary Creep-Rate Data. ................................................... 68

Figure 20. Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 25°C. .......................................................... 72

Figure 21. Displacement (a), Creep Strain (b), and Strain-Rate (c) Histories for Non-
Composite Solder at 25°C. ............................................................................... 74

Figure 22. Displacement (a), Creep Strain (b), and Strain-Rate (c) Histories for
Composite Solder at 85°C. ............................................................................... 76

Figure 23. Displacement (a), Creep Strain (b), and Strain-Rate (c) Histories for Non-
Composite Solder at 85°C. ............................................................................... 77

Figure 24. Displacement (a), Creep Strain (b), and Strain-Rate (c) Histories for
Composite Solder at 85°C. ............................................................................... 79

Figure 25. Displacement (a), Creep Strain (b), and Strain-Rate (c) Histories for
Composite Solder at 85°C. ............................................................................... 81

Figure 26a. Non-Composite Fracture Surface, T = 25°C, I = 21.7 MPa. (Sample 038)...84

Figure 26b. Non-Composite Fracture Surface, T = 25°C, I = 23.5 MPa. (Sample 040). .84

Figure 26c. Composite Fracture Surface, T = 25°C, I = 32.2 MPa. (Sample 052). .......... 85
Figure 26d. Composite Fracture Surface, T = 125°C, 1.’ = 9.1 MPa. (Sample 036) ........... 85
Figure 27. Formation of Circular Areas Under High Stress Conditions:
(a) Formation of Circular Areas. .................................................................... 88
(b) ‘Pop’ Out Upon Further Deformation. ..................................................... 88
(c) Collapse of Circular Area. ........................................................................ 88

Figure 28. Solder Joint Micrographs Before (a) and Aﬁer (b) Further Deformation. ...... 89

Figure 29. Onset of Local Tertiary Creep vs. Global Secondary Creep-Rate. .................. 93

Figure 30.
Figure 31.

Figure 32.

Figure 33.

Figure 34.

Figure 35.

Figure 36.

Figure 37.

Figure 38.

Figure 39.

Figure 40.

Figure 41.

Figure 42.

Figure 43.

Figure 44.

Figure 45.

Strain Homogeneity Ratio vs. Global Secondary Creep-Rate. ........................ 93
Homogeneity of Deformation vs. Average Percent Porosity. .......................... 94

Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)

Histories for Composite Solder at 25°C. ........................................................ 106
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 25°C. ........................................................ 107
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 25°C. ........................................................ 108
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 25°C ................................................. 109
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 25°C ................................................. l 10
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 25°C ................................................. 1 l 1
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 25°C ................................................. 1 12
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 25°C ................................................. l 13
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 85°C. ........................................................ 114
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 85°C. ........................................................ 115
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 85°C ................................................. 1 l6
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 85°C ................................................. l 17
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 85°C ................................................. 118
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 125°C. ...................................................... 119

xi

Figure 46.

Figure 47.

Figure 48.

Figure 49.

Figure 50.

Figure 51.
Figure 52.
Figure 53.
Figure 54.
Figure 55.
Figure 56.
Figure 57.
Figure 58.
Figure 59.
Figure 60.
Figure 61.
Figure 62.
Figure 63.
Figure 64.

Figure 65.

Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 125°C. ...................................................... 120
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 125°C. ...................................................... 121
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 125°C. ...................................................... 122
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 125°C ............................................... 123
Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 125°C. .............................................. 124

Global Secondary Strain Rate for 25°C, 31.1 MPa, Composite Solder ......... 126
Global Secondary Strain Rate for 25°C, 28.9 MPa, Composite Solder ......... 126
Global Secondary Strain Rate for 25°C, 25.8 MPa, Composite Solder ......... 127
Global Secondary Strain Rate for 25°C, 21.9 MPa, Non-Composite Solder 127
Global Secondary Strain Rate for 25°C, 19.9 MPa, Non-Composite Solder 128
Global Secondary Strain Rate for 25°C, 19.8 MPa, Non-Composite Solder 128
Global Secondary Strain Rate for 25°C, 10.5 MPa, Non-Composite Solder 129
Global Secondary Strain Rate for 25°C, 11 MPa, Non—Composite Solder ...129
Global Secondary Strain Rate for 85°C, 10.8 MPa, Composite Solder ......... 130
Global Secondary Strain Rate for 85°C, 10.4 MPa, Composite Solder ......... 130
Global Secondary Strain Rate for 85°C, 12.3 MPa, Non-Composite Solder 131
Global Secondary Strain Rate for 85°C, 17.3 MPa, Non-Composite Solder 131
Global Secondary Strain Rate for 85°C, 16.3 MPa, Non-Composite Solder 132
Global Secondary Strain Rate for 125°C, 9.4 MPa, Composite Solder ......... 132

Global Secondary Strain Rate for 125°C, 9.9 MPa, Composite Solder ......... 133

xii

Figure 66
Figure 67
Figure 68

Figure 69

Figure 70.
Figure 71.
Figure 72.
Figure 73.
Figure 74.
Figure 75.
Figure 76.
Figure 77.
Figure 78.
Figure 79.
Figure 80.
Figure 81.
Figure 82.
Figure 83.
Figure 84.
Figure 85.

Figure 86.

. Global Secondary Strain Rate for 125°C, 11 MPa, Composite Solder ..........

. Global Secondary Strain Rate for 125°C, 8.1 MPa, Composite Solder .........

. Global Secondary Strain Rate for 125°C, 9.7 MPa, Non-Composite Solder
. Global Secondary Strain Rate for 125°C, 31.1 MPa, Composite Solder .......
Fracture Surfaces and Proﬁle for 25°C, 31.1 MPa, Composite Solder ...........
Fracture Surfaces and Proﬁle for 25°C, 28.9 MPa, Composite Solder ...........
Fracture Surfaces and Proﬁle for 25°C, 21.9 MPa, Non-Composite Solder.
Fracture Surfaces and Proﬁle for 25°C, 19.9 MPa, Non-Composite Solder.
Fracture Surfaces and Proﬁle for 25°C, 19.8 MPa, Non-Composite Solder.
Fracture Surfaces and Proﬁle for 25°C, 10.5 MPa, Non-Composite Solder.
Fracture Surfaces and Proﬁle for 85°C, 10.8 MPa, Composite Solder ...........
Fracture Surfaces and Proﬁle for 85°C, 10.4 MPa, Composite Solder ...........
Fracture Surfaces and Proﬁle for 85°C, 12.3 MPa, Non-Composite Solder.
Fracture Surfaces and Proﬁle for 85°C, 17.3 MPa, Non-Composite Solder.
Fracture Surfaces and Proﬁle for 85°C, 16.3 MPa, Non-Composite Solder.
Fracture Surfaces and Proﬁle for 125°C, 9.4 MPa, Composite Solder ...........
Fracture Surfaces and Proﬁle for 125°C, 9.9 MPa, Composite Solder ...........
Fracture Surfaces and Proﬁle for 125°C, 11 MPa, Composite Solder ............
Fracture Surfaces and Proﬁle for 125°C, 8.1 MPa, Composite Solder ...........

Fracture Surfaces and Proﬁle for 125°C, 9.7 MPa, Non-Composite Solder. .

xiii

Fracture Surfaces and Proﬁle for 125°C, 10.9 MPa, Non-Composite Solder.

133

134

134

135

137

138

.139

.140

.141

.142

143

144

.145

.146

.147

148

149

150

151

152

153

CHAPTER 1

INTRODUCTION

As electronic components continue to develop functionally, and less expensive means of
manufacturing are developed, the world becomes continually more dependent upon
electronic technology. However, the use of lead bearing solders is problematic as
countries strive to become more health and environmentally conscious. Outlawing lead
bearing solders would drastically alter the manufacture of electronic components since
eutectic Sn-Pb is the most widely utilized of all solders.

Today’s electronic components are more powerful, smaller, and relatively
inexpensive in respect to their functionality. Although it has been in existence for
thousands of years, soldering technology still represents the vast majority of reliable
electrical connections. Unfortunately, as the functionality and miniaturization of
electronic components continues, the operational temperatures and stresses increase. Use
of soldered components in relatively high temperature environments such as under-the—
hood automotive applications also makes greater demands upon soldering technology.
Issues such as high temperature creep become relevant, considering that room
temperature is over half the melting point (183°C) of eutectic tin-lead solder. As a result,
alternative solders which provide higher performance must be investigated.

In order to combat the detrimental effects of higher operational temperatures, solders
with greater melting temperatures and creep resistance must be developed. Currently,

there has been much interest in the use and development of alternative solders. However,

much of the experimentation is being performed on bulk solder specimens possessing
microstructures unlike the miniature, rapidly solidiﬁed (relatively small grain size) solder
joints encountered in industrial practices (See Section 2.2, page 9). There have been few
investigations utilizing miniature solder joint specimens.

Investigations involving the creep characteristics of solders have dealt with creep in
the classic sense, where the overall or global creep of the specimen was measured. Global
creep data yields results seen in a typical or classic creep curve. Along with classic global
creep, this investigation also considered creep from a localized perspective, i.e., instead of
an measuring the average or global creep strain, creep strains were measured at different
locations across the thickness of the solder joint.

The scope of this research involves examination of creep resistance of miniature
industrial-like solder joints produced from eutectic tin-silver solder with and without
added interrnetallics as reinforcements. Creep characteristics of these solders will be
examined at room and elevated temperatures. New methods of plotting the localized
creep strain-rate were explored. An attempt to determine a criterion for the onset of

tertiary creep deformation is made.

CHAPTER 2

REVIEW OF RELATED LITERATURE

2.1 S OLDER OVER WE W

2.1.1 The History of Soldering

The history of soldering dates back to over 6000 years ago [1]. Artisans from
Mesopotamia ﬁrst used solders in the form of gold-based, hard alloys to join together
copper plates. Egypt was the next region to utilize soldering. Artifacts dating back to
3600 BC were discovered there, as well as in Greece and other Mediterranean areas,
roughly 1000 years later. The most sophisticated example of early soldering comes from
the Bronze Age where the Etruscans produced a gold pendent dating back to 2500 BC, in
which three different types of solders were used within a 5mm proximity. This discovery
was quite surprising, due to the fact that these three different solder alloys had varying
melting temperatures. This is the earliest known example of a soldering process referred
to as ‘step joining’. Evidence of soldering in the western hemisphere dates back to only
around the year 10 AD. Remnants of the La Tolita natives of South America also
indicated the use of gold-based solders.

Currently, it is virtually impossible to determine whether or not these ancient
civilizations were familiar with the use of soft tin-lead solders that are so prevalent today.
Tin decomposes over time when exposed to tannic acid, a substance found in such
organic entities as tree bark. As a result, soldering alloys which may have contained lead

are indistinguishable from lead-free solders. Geographic evidence supports the notion

that the Mediterranean region was not familiar with Sn—Pb solders since tin was not
readily available.

However, rich deposits of tin are found in northern Europe where soft soldering
practices were developed. Historical evidence suggests that soﬁ soldering was developed
by the Celts and Gauls from northern and central Europe around 1900 BC. Their
exceptional use of soft soldering techniques were recorded by the Romans. Cooking
utensils and metals tools could easily be manufactured or repaired over a small wood
fueled ﬁre, due to the low melting temperature of the tin-lead alloys, which were also
high enough to withstand temperatures necessary for the preparation of food or hot water.

Crude examples of soft soldering dating back to 1350 BC were discovered upon the
unearthing of King Tut’s tomb. Evidence also shows that by 350 BC, the Greeks were
using tin-lead solders to seal pumps and other devices produced from bronze. Tin-lead
solders were also readily employed by the Romans to join the lead sheets which lined the
interior of aqueducts.

The application of soldering technology was greatly broadened during the Industrial
revolution [1]. Due to their low cost, availability, and ease of use, tin-lead solders have
prevailed through the twentieth century. With the development of portable heat sources,
the use of soldering in such areas as plumbing, food container sealants, and the
automobile industry vastly increased. It was not until the Second World War, when the
supply of tin suddenly decreased, that alternative solders were given signiﬁcant attention.
Lowering the tin content as well as developing tin free solders was the major concern, but
problems of higher melting temperatures and decreased ﬂuidity were identiﬁed [2].

Today soldering is increasingly relied upon as the primary method of joining electronic

components including resistors, capacitors, and packaged integrated circuits to ceramic

substrates and printed wiring boards [3].

2.1.2 Solders and Soldering

Solders can be classiﬁed as being either soft alloys or hard alloys; 350°C is the melting
temperature that distinguishes the two categories. Solders melting below 350°C are
considered to be soft solders and they typically contain both lead and tin. Other soft
solders can be composed of such elements as bismuth, indium, and cadmium. Hard
solders have melting temperatures which are greater than 350°C and they contain metals
such as gold, aluminum, zinc, and silicon [1].

The process of joining two metals together via a ﬁller metal with a lower melting
temperature than the materials being joined is known as soldering [1]. The soldering
process simply relies on the wetting of the substrate materials by the solder for the
formation of the bond [4]. Soldering is not the only means of joining two metallic
materials. Two other similar methods include brazing and welding. Brazing is a similar
metallurgical joining process which occurs at a temperature greater than 425°C. Brazing
also relies on the wetting of the substrate materials, but some diffusion occurs that
improves the bond strength [4]. When melting of the substrate materials in the region
near the bond occurs, this is known as welding. A ﬁller material can be used, but it is not
always required in this process. Diffusion of the base metals is the means for bond
formation [4]. Welding is a permanent joining technique, whereas soldering and brazing

processes can be reversed with the application of sufﬁcient heat.

Often it is necessary to incorporate a ﬂux into the soldering process. Fluxes serve a
variety of purposes. They are used to clean the solder and substrate surfaces to be joined,
as well as a protective coating before and during the soldering operation [5]. Fluxes were
developed via the metal coatings industry where a clean surface was achieved prior to
coating by immersing the substrate in an acid bath [6]. Fluxes are generally comprised
of organic chemicals and organic salts. A natural resin from pine tress know as rosin can
also be used. Rosin has the added beneﬁt of not reacting until elevated temperatures are
achieved [5].

The nomenclature for solders is based upon the alloy composition. The solder is
described by its composition in weight percent from the most abundant to the least
abundant element. For example, eutectic Sn-Pb solder (628n-38Pb), the standard soft
solder, has a composition of 62 weight % tin with lead as the balance. Solder
composition is quite a diverse matter. There are a wide variety of solders available which
allows for a wide variety of specialized uses. As a result, the soldering process is not a
simple matter of simply surface cleaning along with the addition of heat. There are many
soldering techniques available, and the most prevalent issues concern types of solders,
fluxes, base materials and their ﬁnishes, cleaning agents, and the design of the solder

joint [3].

2.2 CURRENT SOLDERING ISSUES

Today, issues concerning solders and soldering are of paramount importance. Electronic
components are being utilized in an ever increasing number of applications. The
continual miniaturization of more powerful electronic components, makes the soldering
process increasingly difficult. Along with miniaturization, higher temperatures are
generated in certain applications. Also there is the continual concern of legislation or
marketing pressures that could regulate or ban lead, which would dramatically affect the
electronics industry in the United States.

Many lead bearing products have already been outlawed due to noted effects on health
as a result of prolonged exposure. Some of these products include paint and crayons, and
the use of lead bearing solders in plumbing supplies were restricted in 1986. Internal
combustion engines which could operate on unleaded gasoline were produced in the early
1970’s. Means of limiting human exposure to lead are continually being researched [1].
Even though solders comprise about 1% of the lead used in the US, there have been
several attempts made by Congress, including a very recent one, to further reduce the use
of lead, or tax its use severely. Lead bearing solders utilized in electronic components
have yet to come under restriction in the US, although a number of European nations
have already banned the use of lead bearing solders. This is a very relevant issue for
American companies desiring to do business with European nations.

Greater emphasis on the integrity of solder joints has resulted from the heightened
functionality of electronic components [3]. Since the world is relying more and more on

electronic components, solder joint failures must be kept to a minimum and designed in

the most efﬁcient manner. Many of the failures occurring in electronic systems are due to
the failure of solder joints, not the components themselves [1].

In the earlier part of the twentieth century, as soldering became the most reliable
means of electrical connections, the solder joint had only a single purpose which was just
to maintain electrical continuity between components [1]. With today’s silicon chip
technology, solder joints are also expected to provide structural support as well. The
continual decrease in component size along with the need for solder joints to provide both
electrical and structural support results in an increasing demand for stronger and more
creep resistant soldering alloys [1].

It is well known that eutectic Sn-Pb solder coarsens after aging, leading to shortened
life to failure. Thus we need to ﬁnd a better alloy system that not only has no lead
content but also possesses higher thermal fatigue resistance [7]. Current tin-lead solders
lack shear strength, resistance to creep, and resistance to thermo-mechanical fatigue [8].
Eutectic Sn-Ag solder has a melting point of 221°C (compared to 183°C for eutectic Sn-
Pb) so it may be suitable for higher temperature applications such as under-the-hood
electronics in automobiles [9].

In most electronic systems, differential thermal expansion induces stresses resulting in
substantial thermal cycling deformation of solder joint, which leads to eventual failure
[10]. Soft solders are commonly used for electrical interconnects between components in
electronic equipment. The stresses induced in these low melting temperature materials
resulting from thermal expansion mismatch between the joined materials eventually leads
to cracking that can limit component reliability [10]. While fatigue deformation is a

major concern for electronic solders, creep constitutes an important state of deformation

since stress relaxation occurs after a temperature change. In realistic thermal cycles there
is sufﬁcient time for stress relaxation processes to occur and creep induced damage may
result [10]. It has been suggested that solder creep behavior may be directly associated
with the joint fatigue lifetime [11].

Due to difficulties in experimental procedures, much of the data published concerning
solder microstructure and mechanical properties has been based upon bulk solder rather
than actual solder joints. Disagreement has existed regarding the use of bulk solder data
in the design and analysis of actual solder joints. The microstructures of bulk solders
differ from those of solder joints in signiﬁcant ways [9]: Bulk solder does not have Cu
intermetallics that form as a result of interactions between the solder and the Cu substrate.
Solder joints are signiﬁcantly stronger than the bulk tensile specimens [11]. These
intermetallics (whether in the solder or in the solder/base metal interface) reduce the joint
ductility. Secondly, large bulk specimens generally cool more slowly than joints on real
printed wire boards, greatly affecting the creep properties of the solders. The creep
behavior of the solder alloy may in large part determine the joint fatigue lifetime and may
be the best means by which to understand the fatigue results [11]. The Sn-Ag shear-lap
specimens were signiﬁcantly stronger than the bulk tensile specimens [11].
Comparatively, there have been fewer investigations utilizing miniature solder joint
specimens.

There is a need to develop new solders that have similar processing properties to tin-
lead solders, but which are lead-free and have improved mechanical properties and
microstructural stability [8]. Coarsening during aging is more difﬁcult in eutectic Sn-Ag

solder because of the thermally stable intermetallics, therefore its rate of coarsening is

much slower then that in eutectic Sn-Pb [7]. This indicates that known phenomena about

Sn-Pb solders is not indicative of phenomena in other solders.

10

2.3 MICROSTRUCTURE TAILORING

The microstructure of a metal or alloy exposed to temperatures close to half of its melting
point can become dynamic. Due to the processing history of a material, a variety of
microstructures may result just for one particular composition. An investigation
performed by Frost et al. indicated that creep behavior of solder alloys was continually
inﬂuenced by microstructural changes occurring at elevated temperatures [12].
Experimental evidence has shown that Sn-Ag and Sn-Pb solder microstructures can
undergo signiﬁcant coarsening at temperatures similar to operational temperatures
observed in under-the-hood automotive conditions [13, 14].

Eutectic Sn-Ag solder joints are characterized with a relatively ﬁne microstructure
with well dispersed Ag3Sn intermetallics [13] which is excellent for fatigue resistance,
but for creep resistance a coarse grain size is generally desired. The well dispersed Agg, Sn
intermetallics observed in eutectic Sn-Ag solder can show signiﬁcant growth after only
100 hours at temperatures between 60 and 150°C. The naturally forming Cu68n5
intermetallics at the Cu substrate/solder interface have also been shown to coarsen [14].
The presence of these brittle intermetallics at the substrate/solder interface can effect
solder joint reliability. Loss of mechanical strength especially under high loading
conditions is associated with these interfacial intermetallics [1]. In order for Sn-Ag
solders to considered for future use under these high temperature conditions, the
microstructure needs to be stabilized.

By introducing a second phase (dispersoids) into an existing microstructure, grain

boundary mobility can be inhibited which would result in improved microstructural

ll

stability. In addition to inhibiting grain growth, a second phase may also cause a more
homogeneous distribution of strains [13]. Strengthening of the solder alloy against creep
and fatigue deformation can also result through the addition of a second phase [15]. The
composite solder used in this investigation contained a dispersed phase of Cu68n5
intermetallics on the order of 20 volrune percent. Upon initial examination, the
intentionally added intermetallics indicated no improvement in controlling Ag3$n particle
coarsening, but the added Cu6$n5 particles appeared to be quite stable [13]. In an
investigation on Bi-Sn eutectic solder alloy [15] coarsening was suppressed and creep
resistance greatly enhanced throguh the addition of Fe particles

A comparison of creep properties of aged and unaged composite solder as compared
to that of non-composite eutectic Sn-Ag solder was performed by Choi et al. The
composite solder demonstrated dramatic decreases in the strain-rate as compared to the
non-composite solder. Strain-rates decreased by a factor of 100 for aged specimens to
1000 for the unaged specimens [10]. Microstructural observations showed no
displacement of the added CuéSns particles for specimens with strain-rates on the order of
10'7 3". Displacement was apparently limited to the Sn matrix. Under similar loading
conditions, the composite solder showed a decrease in creep strain as compared to the
non~composite solder. As a result, less stress relaxation may occur forcing the failure

process away from the solder and instead to the components it joins [13].

12

2.4 CREEP OVER VIEW

Materials under stress at elevated temperatures can undergo a phenomenon know as
creep. This phenomenon of continuous deformation under a constant load can occur at
all temperatures above absolute zero and is also observed in non-metals [16]. This time-
dependent strain includes elastic, viscous, and plastic deformation, but usually only
plastic deformation is measured [16]. Creep strain is probably the most important time-
dependent damage accumulation factor affecting solder joint reliability [17]. Problems
usually associated with creep occur only at high homologous temperatures [18].

For design purposes, components used under stress at elevated temperatures must be
capable of resistance to distortion over the life span, as well as creep failure should not
occur within the required operating life [18]. Processes of creep deformation at low
temperatures, less than about ﬁfty percent of the melting temperature, are believed to be
related to non-diffusion controlled mechanisms, while creep occurring above this
temperature are generally associated with diffusion controlled mechanisms [16].
However, there still exists a great deal of controversy concerning mechanisms of creep

deformation.

2. 4.1 History of Creep

The phenomenon of time-dependent strain, or creep, was discovered hundreds of years
ago. One of the earliest examples involved churches from the medieval period. Many of
these churches were adorned with steep pitched roofs composed of lead. Since the roofs
were at such a sharp angle, the lead would stretch under its own weight in a manner

similar to liquid running down an inclined plane. This resulted in cracked roofs that

13

needed repair [19]. Another example of creep was discovered in domestic water supplies
consisting of lead pipes. Gradual sagging of the lead pipes would occur which was not
much of a concern, until this caused cracking and eventual bursting of the pipes. The
sagging was more pronounced in pipes which supplied the hot water. Cold water pipes
had fewer problems [19].

In terms of signiﬁcant recorded work on creep, there are several important dates [16].
In 1905 Philips demonstrated that the creep of metals at low stresses was possible.
Andrade also supported this theory in 1910 and again in 1914, however serious
consideration was not given to the creep phenomenon until 1919 when Chevenard
performed experimental work demonstrating that creep was not exclusive to just soft
metals. Other signiﬁcant research was accomplished by Dickenson in 1922. Major work
on the theoretical aspects of creep was done by Becker (1925, 1926) and Orowan in 1934

[16].

2. 4.2 Stress and Strain

Considering the general properties of a typical stress-strain curve, no plastic deformation
will occur until the stress increases beyond the proportional limit of the material, where
the onset of yielding occurs. However, if the material is held under a constant load below
the yield stress at sufﬁciently high temperature conditions, plastic deformation can occur.
This temperature and time-dependent deformation is known as creep and it can occur at
stresses below the macroscopic yield strength by at least one order of magnitude or

greater due to thermal ﬂuctuations which aid the motion of dislocations [l 6].

l4

Shear forces can also induce creep deformation [16]. Shear strain causes the
displacement, dx, of a ﬁnite element in the direction of the shearing force in respect to

the thickness, yo, of the element, where y0 remains constant. The shear strain, y, is given

dc
by, yxy = g. Strain is related to stress through Hooke’s law, which can be stated in

. 0' . . T . .
terms of normal strain, a = E’ and srmple shear strain, y = E. The varrable E rs known

as the elastic or Young’s modulus and G is the shear modulus. Both moduli are
temperature sensitive, i.e. with increasing temperature the moduli decrease. Most creep

tests assume constant load

2. 4.3 Homologous Temperature

Experimental evidence has shown that creep is an extremely temperature sensitive
phenomenon where local thermal agitation provides the additional energy (beyond
mechanical forces) necessary to overcome barriers to creep deformation [16]. The extent
to which a material will deform under constant stress depends greatly on the material’s
melting point. For example, a specimen of nickel (Tmeu =1453°C) under stress at room
temperature may not exhibit any signs of creep, whereas a specimen of eutectic tin-lead
solder (6OSn-40Pb, Tmen =183°C) under similar proportional stress conditions can creep
excessively. An important term when considering the phenomenon of creep is a
material’s homologous temperature which is deﬁned as the ratio of the applied absolute

- - 7:1 to
temperature to the material’s absolute meltrng pomt, THomologous= ”I / 1 .
"16 l

The higher a material’s homologous temperature, the more likely it is to undergo

creep deformation. At room temperature, nickel has a homologous temperature of around

15

0.17, whereas 6OSn-40Pb is at a homologous temperature of 0.65, a value over half of the
absolute melting point. Temperatures where creep becomes important for pure metals are
above 0.4Tmcn. The signiﬁcance of this temperature resides in the fact that atomic
rearrangements of the crystal lattice are possible via diffusion. The homologous
temperature is an important parameter in the proﬁle of creep curves as well as the onset

of creep deformation mechanisms [18].

2.4.4 Creep Testing

In order to characterize creep properties, it is necessary to measure changes in the gage
length with time. No completely satisfactory method of accomplishing this problem has
yet been designed. The most common test method employed is the application of
constant load (tension, but creep tests can also be done in compression) under constant
temperature. Specimens used for a typical creep test are generally cylindrical with
threaded ends which thread directly into the creep machine. The gage lengths typically
range from 10 to 60 mm. It is important to consider that when testing at severe
temperatures where oxidation of the specimen can occur, the specimen needs to be
enclosed in a vacuum or inert atmosphere [20].

Currently the most common method for measuring this change is by using an
extensometer in direct contact with ridges machined onto the gage length of the specimen
or on to enlarged specimen shoulders. The change in gage length between the ridges can
then be monitored via displacement of the extensometer arms. ' Some of the more
common displacement detecting devices which can be used with the extensometer

include dial gages, linear variable differential transformers (LVDT), and differential

16

capacitance transducers. Another method involves monitoring the displacement of scribe
lines on the surface of the specimen via a venrier telescope focused through a viewing
slot in the fumace. This requires manual operation and the scribe lines often disappear or
become indistinct if the specimen surface oxidizes. [20]

Creep tests can take anywhere from several minutes to several years. In general, it is
desirable to avoid accelerated testing and carry out tests in real time. However, in order
to ensure that successive batches of components are being manufactured under the
customer guidelines, or for theoretical means such as determining how added composites
affect the creep characteristics of a given material, accelerated creep tests are commonly
conducted [18]. It is possible to predict the creep rupture life through a relationship
developed by Monkman and Grant. They found that the rupture life is inversely

proportional to the minimum or secondary creep-rate, as. The relationship is given by,
log t, + Clog a, = K, where C and K are constants for a given material, and t, is the

rupture life.

2.4.5 The Creep Curve

Accumulation of strain with respect to time (strain history) is the basic information
resulting from deformation under constant temperature and stress. The total accumulated

creep strain can be described with the equation, 8,0,“, = so + e, where 80 represents the

instantaneous strain upon loading, and e is the time-dependent strain. The creep-rate,

a = ‘18 dt , can be determined by measuring the slope at any point on the creep curve. The

resulting shape of the strain history proﬁle is associated with different stages of creep.

17

51

(I)

The ﬁrst stage in creep, Stage I, is also known as primary creep which is a relatively
short lived phenomenon. This transient period of creep represents a region of decreasing
creep-rate. This stage of creep can be characterized by strain hardening. The second
stage which can be seen from Figure l is known as secondary creep, steady-state creep, or
Stage II. This stage is characterized by a nearly constant creep-rate that results from a
balance between strain hardening and recovery.

The ﬁnal stage of creep, Stage III, also known as tertiary creep occurs from a decrease
in cross-section area of the specimen either because of necking or the formation of
internal voids or cracks which can be associated with metallurgical changes such as
coarsening of precipitate particles, recrystallization, or diffusional changes in the phases

present [16]. Tertiary creep is associated with high stresses due to the decrease in cross-
sectional area. Throughout the creep experiment, the load is held constant. The axial
stress increases as the specimen elongates and decreases in cross-sectional area. The
initial stress is usually the reported stress, not the instantaneous stress [21].

The theoretical shape of a typical creep curve is shown in Figure 1. After an almost
instantaneous strain of the specimen, 8,, due to loading, there is a decrease in the rate of
creep (Stage I) which transforms into a steady state process (Stage II), where there is little
noticeable rate change. This creep-rate then rapidly increases (Stage 111) until the
Specimen fails [21]. Individual creep tests do not always yield all of the three stages. The
Shapes of the creep curves resulting from individual tests can vary greatly according to
temperature, test duration, and the stress value. For example, tests performed at low
Stresses have a tendency to yield a sigmoidal (S-shape) curves as well as curves with short

or long range oscillations [16].

18

 

 

A
l frooture
. t
1 anory Secondary Tertiary
{<1—-——— 1—--—-—-~r> <,_-_ —~—- 11 > <r—III—i>'
l
(o i {ff/ff;j_E[/—/If”!
g‘ 5 ”fr”... £3:— creep rote
Q l -
‘7’ I
|
' ’i
1 so
-‘ l
_.._v___ _ _ _ _ _- _ ___ll -__ _ ___l
Time, 7‘

Adapted from Dieter, page 439.

Figure l. The Model Shape of a Typical Creep Curve.

19

Temperature has a notable effect upon the shape of a creep curve. Under low
homologous temperatures, the total resulting strains are quite low (< 1%) and the creep
curve may only enter stages of primary and secondary creep. At homologous temperatures
in the range of 0.3 or less, the creep strain varies linearly with log(time) when crept in
tension [16]. Failure seldom occurs for tests performed at low homologous temperatures
[18]. However, as the temperature is increased to a homologous value near 0.4, the proﬁle
of the creep curve deviates from this logarithmic form [18]. In contrast, during creep
tests performed at high homologous temperatures, all three stages of creep, including

tertiary, will be experienced. Once tertiary creep is underway, failure is inevitable.

2. 4. 5.1 Secondary Creep-Rate

The minimum creep-rate is the average value of the creep-rate during Stage 11. Generally,
the creep characteristics of a material are described by values obtained from the
‘secondary’ or ‘steady state’ creep-rate, 8'5 , resulting from normal high temperature creep
tests. This eliminates the need for a wider variety of equations representing the creep
properties of materials tested at high homologous temperatures [18]. The creep-rate, é ,
can be determined by measuring the slope of the curve, d8 hit in Stage 11. Even though it
is simple in theory, the measurement of creep resistance is a tedious task which requires
extensive laboratory equipment. Typical tests can run over a wide range of times, from a
few minutes to over ten years [21].

A linear relationship between the secondary creep-rate and temperature can be
obtained by plotting ln(és) vs. (l/T). This indicates that the secondary creep-rate

increases exponentially with temperature [18]. Temperature dependence of this nature

20

can be seen in other processes such as diffusion. Creep can be classiﬁed as an Arrhenius
function [18]. This results in the proportion, as oc exp— (QC / RT) , where QC is the creep

activation energy and R is the universal gas constant. This leads to the general form of

the empirical power law creep equation which is a, = A0" exp[ k?) [22], where a, is

 

the strain-rate or creep-rate, A is a constant, 0' is the stress which has a measurable
exponent given as n, Q; is the creep activation energy, k is Boltzmann’s constant, and T
represents temperature on the absolute scale. When this form of the equation is used to
represent the creep process, values for the activation energies of pure metals in a high
temperature environment, have been measured to be similar to the activation energies for
lattice self diffusion [18]. This indicates that the mechanism for creep at high
temperatures would be controlled by diffusion. This fact also explains the transition
between low and high temperature creep behavior, since the movement of atoms occurs
readily near a temperature of 0.4Tm [18]. Another equation developed by Dorn, which

mixes metallurgical theory and the empirical power law is referred to as a semi-empirical

, DGb " b p ,
equation. This is equation is given by 83 = A—kT—(g) (E) [23], where b rs the

Burger’s vector, D is the diffusivity and is given by the equation D = D0 exp(%—%) , d is

D . . .
the grain size, p is the grain size exponent. The term RT rs referred to as the mobrlrty.

21

 

2.4.5.2 Scattering of Data

Scatter is a quality inherent in creep data. This is due partly to assumptions made during
a creep test. For purposes of convenience, temperature and stress are assumed to be
constant for the duration of a creep test for proper characterization of materials [20].
These substantial assumptions are rarely the actual conditions experienced in practice.
Deviations from these assumptions will result in errors in the determination of creep
strain, strain-rates, and rupture lives [20]. Errors in the axiality (tension or shear) of
loading as well as the skill necessary in setting up the test itself along with typical errors
can arise just as in any experiment from imprecise measurement of specimen dimensions

or a variety of other factors [20].

2. 4.6 Creep on an Atomic Scale

In order to understand creep, it is necessary to consider a material on the atomic level.
Typically, a metal contains many individual crystals which are comprised of a simple
lattice structure of atoms held together via atomic bonding. If a solid is subjected to a
tensile force, the crystal lattice will adjust itself to oppose the applied force and maintain
equilibrium. If a sufﬁcient force were to be imposed upon this polycrystalline material,
resulting in plastic deformation, the deformation is caused mainly from the movement of
dislocations. The deformation which initially occurs is elastic or reversible. Once the
elastic limit has been bypassed, the stress imposed is sufﬁcient enough to cause some of
the dislocations to move across the crystals, resulting in permanent or plastic

deformation. After some deformation occurs, strain will quickly cease, unless the applied

22

load is increased. Deformation is halted because forces are generated to prevent further

deformation. This phenomenon is know as work hardening.

2. 4. 7 The Role of Dislocations in Deformation

As dislocations move through the material as a result of deformation caused by a stress
higher than roughly 10'5 of the shear modulus [24], they multiply rapidly. A consequence
of this rapid multiplication is dislocation crossing and tangling, which impedes the further
movement of other dislocations. Unless the load is increased or extra energy is supplied
in the form of heat, further deformation will not occur. In creep deformation this can be
associated with Stage I as deﬁned previously. The stress necessary to cause continued
deformation in a material is known as the ﬂow stress, 00 [22]. The ﬂow stress can be
deﬁned as a material’s resistance to shape change and is a function of temperature, strain,

and strain-rate [20]. The ﬂow stress, 00 = or + oi , consists of a thermal component,

6]“, and, an athermal component associated with the dislocations inside the material,
6 i [22].

Recovery can be understood in terms of atomic vibration. The higher the temperature,
the greater the degree of vibration and the less time needed for recovery to occur. The
creep-rate reaches a constant value when the rate of strain hardening and rate of recovery
become equal. Since atoms have a greater chance of moving from one atomic position to
another when their degree of vibration is high, the creep-rate increases with increasing
temperature. Diffusion of atoms can occur due to existing vacancies in the lattice. At
elevated temperatures, experimentally calculated activation energies for creep usually

equal the activation energy for self-diffusion [22]. This diffusion tends to make

23

vacancies move into the dislocation cores so that the dislocations shift from one plane of
the crystal to another. This phenomenon is referred to as dislocation climb [19].

During climb of dislocations, the dislocations can either move or annihilate other
dislocations of opposite sign. Since the diffusion of vacancies either towards edge
dislocations (positive climb) or away from edge dislocations (negative climb) is required
for edge dislocations to climb, creep deformation resulting from climb should involve the
activation energy for self diffusion [22]. In this manner, the extensive tangling of
dislocations sorts itself out and many dislocations disappear in a process known as
recovery [19].

A material undergoing creep deformation may not do so in a homogeneous manner.
Changes in the arrangement of dislocations in crystals has been observed using
transmission electron microscopy. Some regions are characterized by being relatively
dislocation free, while other areas possess a high density of dislocation tangles and
networks [20]. As a result of this dislocation density imbalance, the specimen’s state of
internal stress is not uniform since each dislocation has an associated stress ﬁeld that
affects the movement of nearby dislocations [20]. The difference between the applied
stress and these internal stresses is the driving force for the motion of dislocations. As a
result, the inhomogeneous nature of the stress ﬁeld can exhibit itself in some peculiar

ways [20].

2. 4.8 Steady-State Creep-Rate

Confusion exists concerning nomenclature for the creep-rate obtained from Stage II of a

high temperature creep test. The common misconception is to refer to the second stage of

24

creep as ‘steady-state creep’. The term ‘steady-state’ implies an equilibrium process with
respect to dislocations and is reached when the rate of work hardening is balanced by the
rate of recovery. This concept forms the basis for most dislocation creep theories [20].
During steady-state creep, there are as many dislocations being created as there are being
destroyed and the lifetime of each dislocation contributes one increment of strain. During
steady-state creep, the dislocation density and the arrangement of the dislocations remains
essentially unchanged which results in a constant strain-rate [20]. In general, measurable
secondary creep-rates are found at stress levels greater than about 10'5 of the shear
modulus and at temperatures greater than 04me [24]. No changes in microstructure
will occur under steady-state conditions.

A constant strain-rate may occur during secondary creep, but there may be changes in
the microstructure such as local recrystallization near particle interfaces, etc. The linear
portion of secondary creep may be misleading. In order to demonstrate true linearity, the
strain-rate needs to be plotted against strain. The minimum strain-rate, which is
equivalent to the steady state or secondary creep-rate, represents a balancing condition
between the hardening and softening processes in the material. If the microstructure is
not in a state of equilibrium, steady-state creep is not occurring. In fact very few
materials undergo true steady-state creep, such as some pure metals and simple alloys.
There is little convincing experimental proof justifying the existence of a steady-state
creep component [16]. Unless evidence indicates the microstructure is unchanging, creep

occurring during Stage II is better referred to as secondary creep.

25

2. 4. 9 Secondary Creep-Rate Normalization
The way in which the creep-rate changes in respect to temperature can be shown by the
relationship, as = A0" exp— (QC / RT), which is known as ‘power-law creep’.

Considering secondary creep behavior, different values of n and Q, will be associated
with different regimes of stress and temperature. It is possible to condense all of the
different temperature creep data into one set through a normalization process. When

secondary creep behavior can be described with the power-law, if both sides of the power

law equation are multiplied by the factor, exp(Qc /RT), the result is,

é, exp(Qc / RT) = A0”. The left side of this modiﬁed power law equation is referred to

as the ‘temperature compensated secondary creep-rate’ [18]. A similar equation can be
used for shear stress deformation. This modiﬁed function allows each log éx/logo
relationship obtained from different temperatures to be superimposed onto a single line.
The stress is also normalized with respect to temperature, 0' /E, where ‘E’ is the value of

Young’s Modulus at the appropriate creep temperature [18].

26

 

2.5 C.

The pr<
im'olx'c'
llrese

sliding

exponc

2.5.1 1

Disloc
therme

mecha

roughl

dislocz
dislocz
They
compe
rearrar
Wt
creep,
dominz
elevate

allowir

2.5 CREEP MECHANISMS

The process of creep has considerable theoretical controversy as to the exact mechanisms
involved. There exists at least four different types of creep deformation mechanisms.
These include dislocation glide, dislocation creep, diffusion creep, and grain boundary
sliding. A chart in Appendix A summarizes the creep mechanisms, creep and grain size

exponents represented by values of n and p respectively, and activation energies, Q.

2.5.1 Dislocation Controlled Creep Mechanisms

Dislocation glide occurs when dislocations moving along slip planes overcome barriers
thermally assisted mechanisms involving the diffusion of vacancies or interstitials. This

mechanism occurs at relatively high stresses as compared to other creep mechanisms,

roughly 10‘4 <%<10‘2. The creep-rate is established by the ease with which

dislocations are impeded by obstacles including precipitates, solute atoms, and other
dislocations [18]. The basis for dislocation creep was developed by Orowan and Bailey.
They concluded that the steady state creep-rate represents a balance between the
competing factors of the strain hardening rate and the rate of thermal recovery by the
rearrangement and annihilation of dislocations.

Weertmann was responsible for the development of the earliest models of dislocation
Creep. The models were based on a mechanism in which dislocation climb is the
dominating factor. If an obstacle inhibits the movement of a gliding dislocation at
elevated temperatures, a small amount of climb may allow it to overcome the obstacle,

allowing it to glide to the next set of obstacles and the process is repeated. Almost all of

27

 

the strain is produced by the glide, however, the climb step controls the rate. Since
diffusion of vacancies or interstitials is necessary for dislocation climb, the rate limiting
factor is atomic diffusion.

This model predicts an equation for the rate of creep where the stress exponent equals
three, however, creep experiments with a range of metals show that the exponent on
stress varies from 3 to 8, with a value of 5 being the average [21]. Thus for a range of
medium to high stress levels at temperatures above half of the melting temperature, the
steady state creep-rate is characterized by a power-law relation. When more than one
mechanism is functioning, the creep behavior is controlled by the fastest mechanism. If
there are mechanisms operating in sequence, the slowest mechanism will control creep

deformation.

2. 5.2 Diffusion-Controlled Creep Mechanism

Diffusional creep occurs under low stress, high temperature conditions. Difﬁrsional creep

involves the ﬂow of vacancies and interstitials through a crystal under the inﬂuence of
. . 0' _
applied stress. Diffusron creep occurs for stress levels around G <10 4. Nabarro-

Hening and Coble creep are included under this category [18]. When a polycrystalline
metal creeps by the diffusion of atoms through the crystal lattice, the process is known as
Nabarro-Herring creep [25]. When diffusion of atoms occurs along the grain boundaries,
the process is known as Coble creep [26]. Although support for the idea of diffusional
creep has been offered through experimentation and can be found throughout the

literature [27], it is still a controversial topic. There is an ongoing argument concerning

28

the validity of Nabarro-Herring creep as well as a controversy over ‘denuded zones’

which were offered as the ﬁrst immediate microstructural proof for diffusional creep [28].

29

2.6 CREEP-FATIGUE INTERACTIONS

Solder joints used for both mechanical and electrical contacts in electronic packaging
devices normally do not fail as a result of a single high stress event [29]. Usually failure
is a result of repeated or prolonged load applications that result in damage accumulation
via creep-fatigue failure of the solder. The nature of the environment most solder joints
are exposed to involves a variety of deleterious factors. Automotive applications provide
an excellent test of the solder joint. Temperature ﬂuctuations in the range of -40°C to
150°C are possible in automotive environments, along with mechanical vibrations in a
range from 20 to 2000 Hz [30]. Fatigue can arise due to both the mechanical vibrations
and ﬂuctuations in temperature. As discussed previously, because of high homologous
temperatures, fatigue behavior of solder joints is related to creep as well as the process of
stress relaxation [31].
Solder joints can be considered as structural materials since they function not only as
electrical interconnects, but also as mechanical bonds. In electronic devices, solder
joints oﬁen constrain materials of differing coefﬁcients of thermal expansion (CTE) that
upon an increase in temperature due to internal heat dissipation from an electrical current
or simply from environmental changes, will result in plastic strains [32]. Plastic strains
are caused by CTE mismatch. Depending upon the size of the joint, even minute changes
in temperature can have a drastic effect upon the solder and the component(s) it connects
[32]. If the connected components have matched CTE’s, there is still a danger of plastic
Strain due to differential rates of heating and cooling (temperature gradients) in localized

regions [33]. When these electronic devices are repeatedly turned on and off, or

30

 

IE

temperature cycles are created by the device itself, cyclic strains can result. Typically a
sequential development [29] of local stressing, microcrack formation followed by crack
initiation which may result from the accumulation of creep cavitation damage [31].
Crack propagation then ensues resulting in the failure of electrical solder joints. Evidence
of creep fatigue interactions include fatigue straiations (fatigue) as well as intergranular
cracking and voids on some of the grain surfaces (Creep) [34].

In order to improve upon the design and characteristics of solder joints, it is necessary
to gain a better understanding of the mechanical and fatigue properties of solder alloys
used in electronic components [35]. Currently, for a variety of solder alloys, fatigue data
are available in the literature. However before applying this to the solder at hand,
consideration must be given to solder composition, microstructure, design of the
specimen, testing conditions, as well as the author’s deﬁnition of the fatigue. Plastic
deformation occurs in many solder joint designs since the accumulated strains exceed the
elastic limit of the solder [31]. If these factors are ignored, assessment of the fatigue life
may be incorrect [3 5]. Another pertinent issue involves the duration of the actual fatigue
lifetime. The actual lifetimes required for ﬁeld use greatly exceed plausible time for
testing. As a result, service reliability must be estimated through fatigue models [31]. In

other words, reliable accelerated testing procedures must be developed.

31

2. 6.1 Stress Relaxation

Most solders are characterized as being very compliant. When stress is applied to a
solder joint, especially at elevated temperatures, the solder will yield in a manner to
reduce the applied stress [36]. Stress relaxation and creep are two very closely related
phenomena differing by which factor is held constant. Creep occurs when stress is held
constant and the strain changes with time. However, when the strain is held constant, i.e.,
the material’s dimensions are unchanging, and the stress decreases either by way of static,
dynamic, or creep conditions, the load it supports decreases with time. This process is
known as stress relaxation [35, 16]. From a microstructural standpoint, the processes are
virtually the same, and the proﬁles of strain-rate vs. stress for both processes are identical
[36]. A further term, known as creep relaxation, refers to the time-dependent changes
simultaneously occurring in both strain and stress [3 6].

A simpliﬁed model of stress relaxation was provided in a paper by Hall. When
thermal stresses arise in a soldered assembly under conditions previously described, the
components which are responsible for the applied stress on the solder are usually only

stressed within their elastic range. A stiffness value, k, can be associated with this
. dF . .
assembly 1n the form k = —E [36]. If the solder yrelds (creeps) by a distance Ax, then

the force on the joint will be decreased by a value of kAx, [36]. The equation,

(a, —aC)

Ay = T LAT — k—H [36], combines the shear strain, shear stress, and temperature

components into one relation. The variable F is the force associated with each solder

joint. The coefﬁcients of thermal expansion of the solder and the component being

32

 

 

SO

W

It

[In

soldered are given by as and ac respectively. The height of the solder is given by H, and
2L is the in-plane dimension of the substrate. The effect described by the equation is easy
to visualize, for example, if the temperature is held constant, the force will be related to
the shear strain in a linear manner with the value of the slope being -kH [36]. Conditions
will approach stress relaxation if the value of k is large or stiff, while low values of k
imply creep processes [36]. Strain is associated with a compliant solder at high
temperature conditions, as well as smaller forces since relaxation due to creep can occur
almost instantly. On the other hand, stress is high with a stiff solder at low temperatures
[36].

The accumulation of damage will be different for different parts of the thermal cycle
since stress relaxation during the heating and cooling parts is not constant [31]. The
incorporation of highly ﬂexible leads with solder joints can greatly reduce creep strains
developed as a result of cyclic loading conditions. A shearing of the solder joint will
generally result when conditions do not allow for the use of ﬂexible leads, such as in
surface mounting [37]. However, an investigation by Ross et al. [17] indicates that the
use of ﬂexible leads can cause more damage than relief if certain criteria are not met.
Findings indicated that lead stiffness must be at least 1000 times more ﬂexible than the

solder [17]. The process of stress relaxation is indicated in Figure 2.

2. 6.2 Fatigue

Probably the most recognizable model for the prediction of fatigue life, Nf, is the Cofﬁn-
Manson relationship. The equation is based upon an empirical relation and can be found

in a variety of forms. In order to better predict the fatigue life of solders, Cofﬁn-Manson

33

 

We

lead

SITE

to t

et.

lit:

(13

fa

type relations have been adjusted to account for temperature and frequency effects [3 8].
In order for this model to be utilized to predict the lifetimes of components with ﬂexible
leaded parts, the strain range must be the total plastic strain range. This must also include
strain associated with creep deformation which must be determined accurately and added

to the total plastic strain [17].

 

A8 -A A0 ‘8
The form included here, N f = [ g P] + {—0—} , was provided in a paper by Tien,
f

0

et. al. The equation can be divided into two parts, low cycle and high cycle fatigue. The

—A
A6
ﬁrst term, [jg—p] , describes low cycle fatigue which is controlled by the rate of crack
,-

growth [38]. The plastic strain range Asp which is raised to the -A power is related to the

-8
A0
damage stored in each cycle. The second term, {-0—} , is described as the high cycle

fatigue component. During high cycle fatigue, the component spends most of its life
initiating cracks and the damage stored in each cycle is best described by the stress range
Ao raised to the -B power [38]. Fatigue life in the range of up 104 cycles is associated
with low cycle fatigue, whereas fatigue life above 106 cycles is associated with high cycle
fatigue [3 8]. The exponents A and B are respectively the low and high cycle fatigue
Cofﬁn-Manson variables. The fracture strain is given by 8f, while 00 is the stress
associated with high cycle fatigue [38]. Vaynman, et al. found this model to be
acceptable at strains above 102, when subjected Pb-Sn solders to isothermal fatigue.
However, at strains below 102, the relationship failed due to strong creep fatigue

interactions during cycling [35].

34

2. 6.3 Fatigue of Solder Joints

Fatigue in solder joints generally results from repeated thermal cycles caused by turning
on and off electrical devices. The temperature ﬂuctuations can induce cyclic strains as a
result of thermal expansion mismatches between the solder and components, as
previously mentioned, and result in the eventual failure of the component [32].
Prediction of the fatigue life of solder joints is a difficult task as a result of the time and
temperature dependent characteristics of creep behavior [39]. A recent investigation into
fatigue modeling has illustrated that creep is the dominating factor in the determination of
fatigue life [40]. When considering fatigue life, there are a variety of relevant factors
which must be considered including the range of temperature ﬂuctuations, average
temperature, dwell times, dwell temperature, rates of loading and unloading, joint
geometry, solder microstructure, and the presence of alloying elements and intermetallics
[39].

Fatigue damage is usually the result of dislocation pile-ups which cause stress
concentrations. The complex microstructure of as-solidiﬁed eutectic solders as well as its
instability at high temperatures make fatigue prediction models complicated [3 7].
Currently, no reliable predictive theory for thermal fatigue under creep conditions exists
[37]. Recent attempts at modeling the complex interactions between creep and thermal
cycling have indicated that the deformation and localized accumulations of strain
produced during thermal cycling are not reversible [40]. This results ﬁom distortions
occurring which change the stress distribution in the joint. Subsequent thermal cycles
then have an altered impact on the state of stress and strain [30]. Experimental evidence

indicates that strain is not reversible since continued thermal cycling results in

35

System is in Equilibrium

06, Component TReﬂow
ins—0t. ,
or, PWB
Component Reﬂow Soldered to Printed Wire Board
0t1>0t2>0t3

Removed from Reﬂow Furnace

or, Compression <——————

 

<1— orI . - -
0% Tensron —————>

PWB Shrinks Faster than Solder and Component
Shear Stresses Result from CTE Mismatch Causing Shear Strains

 

Resting Outside of Reﬂow Furnace

 

. TRoom
or, Compressron <~———
<»—Ot] .
<— Olz Tensron —*

Stress is Reduced as Shear Strain Increases
Stress Relaxation Occurs

Figure 2. The Process of Stress Relaxation

36

fi

It
in;
C0

0C-

deg

permanent shape change. The process of thermo-mechanical fatigue is illustrated in
Figure 3.

Inaccuracies result when accelerated therrno-mechanical fatigue tests are performed.
It has been shown [41] that under accelerated thermo-mechanical fatigue testing, the
majority of plastic deformation occurs during loading of the specimen, whereas, in a test
conducted under realistic heating and cooling rates, most of the plastic deformation
occurs as a result of stress relaxation. During the cold part of the cycle, stress relaxation
may not be as signiﬁcant as during the warm part of the cycle since creep has a high
dependence upon temperature [34]. As a result, the effects of creep are greatly ignored

under accelerated test conditions. The differences are illustrated in Figure 4.

37

Low Temperature Dwell '

  
  

 

Figure 3. The Process of Thermo—Mechanical Fatigue

38

High Temperature Dwell

 

High Temperature Dwell

—j +1000 psi

   
    

-0.005

\ + -1000 psi

Low Temperature Dwell

(a)

- High Temperature Dwell

    
    

+1000 psi —'—

—,- -2000 psi

Low Temperature Dwell

(b)

Figure 4. Differences Between Normal (a) and Accelerated (b) Thermo-Mechanical
Fatigue Testing Adapted From Darveaux et al.

39

2.7 CREEP IN SOLDER JOINTS

Recently, there has been much work done concerning the creep of solder. It has also been
acknowledged in a variety of papers that there is a signiﬁcant difference between testing
easily fabricated bulk specimens of solder and testing actual solder joints. The
discrepancies are related to a smaller grain size due to faster cooling rates during
fabrication as well as the lack of substrate constraint in bulk specimens which eliminates
the formation of intermetallics near and at the substrate/solder boundary. Both of these
effects are believed to be responsible for the signiﬁcant increase in strength of solder
joints compared to bulk specimens.

It has been noted [5] that solder containing silver (Ag) offer some of the highest creep
resistances. This also results in reducing the strain observed during creep. Raeder, et al.
[5] found in a study comparing Sn-Ag and Sn-Bi solder joints that at strain-rates greater
than 10'4 s", Sn-Bi has a higher ﬂow stress, and at strain—rates lower than 104 s", Sn-Ag
solder has a higher ﬂow stress. Raeder also noted signiﬁcant creep differences between
bulk solder and solder joint specimens.

Studies on composite solders have also been conducted within the last several years
[10, 15, 41]. In all instances, the addition of a dispersed phase signiﬁcantly increased the
creep resistance of the solder. It has also been noted that increased porosity may result

from these additions.

40

CHAPTER 3

EXPERIMENTAL PROCEDURE

3. 1 SAMPLE PREPARA T I ON

3.1.1 Joint Fabrication

The substrate material used in the fabrication of the solder joints consisted of copper dogbone
halves, fabricated via electrical discharge machining (EDM). The substrate dimensions are
shown in Figure 5a. In order to achieve a solder cross-sectional area of approximately 1 mm, a
solder mask was applied with a ﬁne brush, approximately 1 mm from the narrow end of the
copper substrate. The freshly masked copper substrate halves were then heated at a temperature
of 150 °C for one hour.

Before the solder joints could be fabricated, the Cu substrates were chemically cleaned with a
solution of 50% Nitric acid and 50% water. Once the Cu substrates were cleaned and dried, a
ﬁne drop of Alpha 200-L ﬂux was applied to the narrow end of the dogbone shaped substrate.
Half of the Cu substrates were then placed, ﬂux side up, into a specially machined alruninum
ﬁxture shown in Figure 5a. Two pieces of solder foil, approximately 1mm2 in area and 30
microns in thickness, were sandwiched between the two Cu substrates as shown in Figure 5a. A
thermocouple was connected to the aluminum ﬁxture which was then placed directly onto a
preheated hot plate. The ﬁxture was heated to a temperature of 270°C on a hot plate and then
quickly removed and placed on an aluminum chill block to simulate a cooling temperature

history of industrial practice for reﬂowed solder joints. Once the aluminum ﬁxture reached

41

 

£1.30
. mm

P:
5111'?) I

 

 

 

F1
1Sure
g

  
   

Solder Foil

Alpha 200L Flux

   
  

Solder Mask Thermocouple Wires

0.26 mm

 

58.0 mm

Heated Aluminum Fixture

Figure 5a. Assembly of the Solder Joint.

 

   

 

 

 

 

Figure 5b. Finished Solder Joint with Magniﬁed View of Joint Area.

42

 

ambient
The ima

mmSO

Which \1
Nodm
characte
iis’lngu
mama

Visible z

ambient temperature, the solder joints were removed and gently inspected by hand for integrity.
The intact joints as seen in Figure 5b were then polished. The solder joints ranged in thickness

from 50 to 100 pm.

3. 1.2 Solder Microstructures

Two types of lead-free solder were used in this investigation. Solder joints were fabricated from
eutectic tin-silver solder (96.58n—3.5Ag) with and without 20v% 5-8 pm CuGSns intermetallics
which were added as reinforcements. Scanning electron microscope (SEM) micrographs of the
two different solder microstructures are shown in Figure 6. The non-composite solder is
characterized by a colony type microstructure. The colonies which are only slightly visible, are
distinguished by minute Ag38n intermetallics visible in the grain boundaries. A layer of Cu6$n5
intermetallics which formed at the solder-substrate interface during the soldering operation is
visible at the top of the micrograph. In the composite solder, Cu6$n5 intermetallics are clearly
visible throughout the matrix. Naturally forming Cu68n5 intermetallics are also visible along the

solder-substrate interface in the upper right comer.

3.1.3 Polishing

During the polishing procedure, the solder joints were held with a special grip designed to
properly support the specimen as shown in Figure 7. Wet/dry sandpaper (240 and 600 grit) was
the ﬁrst medium employed in polishing the solder joints. Next, a 0.5 micron polishing solution
used with a standard metallurgical polishing cloth adhered to a narrow aluminum block was used
to further polish the specimen. The specimen was then ﬁnished with a 0.05 micron SiOz

colloidal suspension in the same manner.

43

Cu substrate “.1.
C r168n5

lllltlmcldlllc’s

I 96 556 3 5Ag s...
' 1' Solder

 

Figure 6a. Eutectic 96.SSn-3.5Ag Non-Composite Solder Micrograph.

 

Figure 6b. Eutectic 96.SSn-3.5Ag Composite Solder Micrograph.

 

 

< 50.0 mm 7- ,

Polishing Cloth Aluminum Bar

 

Figure 7. Specimen Grip and Polishing Bar.

45

In order to monitor displacement of the solder joints during creep testing, it was
necessary to intentionally scratch the polished surface across both Cu substrates and the
solder area. This is illustrated on page 53 with Figure 11. Several methods were used
during this investigation. The ﬁrst method was to rub the polished surface over a coarse
polishing cloth. This method worked well for images taken with a standard optical
microscope. However, the resulting scratches were too ﬁne to be adequately monitored
with the current image capturing system which incorporated a traveling microscope with
a digital camera mounted on a tripod. A single-sided razor blade lightly pressed across
the polished surface seemed to give the best results for this investigation. The scratch
appears relatively large under a standard optical microscope, but when used in the current

image capturing system, it provided sufﬁcient results.

46

3.2 CREEP TESTING

In order to subject the miniature solder joints to creep deformation at room and elevated
temperatures, it was necessary to develop suitable testing equipment. The method chosen
for free strain measurement required that the digital camera have an unobstructed view of
the solder joint. This resulted in a variety of difﬁculties. Further problems were
encountered when it was necessary to supply heat for elevated temperature creep tests.
The devices and methods employed for this experiment underwent several stages of

development until satisfactory testing methodology was achieved.

3. 2.1 Miniature Creep Frame

The miniature creep frame underwent signiﬁcant development throughout this
investigation. The ﬁrst design (Figure 8a) worked relatively well, but did not allow
incorporation of the miniature furnace. It was simply an aluminum machined plate
connected an I-beam. It consisted of one vertical slot in front which provided clearance
for the suspended solder joint and another narrow slot traversing the top which allowed
for the solder joint to be suspended with a piece of piano wire. The next version depicted

in Figure 8b was developed to incorporate miniature furnace.

3. 2.2 Miniature Electrical Resistance Furnace

A miniature electrical resistance furnace was constructed for use with the shear-lap
soldered specimens. Nichrome, an alloy commonly used for electrical heating elements,
due to its strong oxidation and electrical resistance, was used in wire form to create

heating coils. In order to achieve even spacing of the coils, the Nichrome wire was

47

 

(b)

Figure 8. Development of the Miniature Creep Frame.

48

wound around a 0.5 inch threaded bolt. The wound wire was then covered with a thin
layer, approximately 1/4 inch, of kaowool, for thermal insulation. The assembly was then
wound with ﬁber glass and coated with resin. In order to keep the ﬁber glass windings
secure, three pieces of copper wire were fastened around the middle of the assembly and
near the ends in a manner similar to the steel rings found on a wooden barrel. Excess
copper wire was allowed on the two outer windings to be employed as electrical leads.
The terminal ends of the Nichrome wire were fastened to the two outer copper wire
windings. The furnace is shown in Figure 9.

A variable voltage power supply was used to provide current for the miniature
furnace. The temperature was monitored with a thermocouple. Teﬂon® coated type K
thermocouple wires were fastened to the shear-lap specimen. The wires were twisted
onto the copper substrate as near as possible to the solder joint without blocking the view
of the camera. Coated wires were employed to ensure no electrical contact with the
Nichrome wire coil. A heavier piece of copper wire was wrapped around the center of the
fumace and secured onto the I-beam with a machine screw. The copper wire is strong
enough to support the light weight furnace and also ﬂexible so the fumace can be easily

aligned with the hanging shear-lap specimen. See Figure 10.

3. 2.3 Deformation of Solder Joints

The miniature solder joint was placed in the creep frame which was equipped with an
electrical resistance heater (when necessary) and designed for dead load creep testing.
The microscope and ﬁber optic lighting system were then positioned for optimrun

resolution of details. The scratch must be clearly visible before testing begins, because as

49

Kaowool Nichrome Wire Coil

Resin Coating
Fiberglass Wrapping

 

Figure 9. Miniature Electrical Resistance Furnace.

50

     

Support
Solder Joint
Length Extension

Thermocouple Leads

Electrical Resistance Furnace

 

 

Figure 10. Creep Frame Set Up for Elevated Temperature Testing.

51

the creep test progresses, the scratch becomes less distinguishable (Figure 11). If the
resistance ﬁrmace was used, the length of the solder joint was extended at the suspended
end in order to allow exposure of the joint area to the microscope. This was done by
sandwiching the end of the solder joint between two small lengths of metal and fastening
the assembly with wire. Prior to loading, an image of the undisplaced scratch position
was captured using a traveling microscope and a digital camera as a basis for comparison
to subsequent images taken during the creep test. The specimen identiﬁcation and load
used were also recorded. Immediately aﬁer loading the specimen with the desired weight,
another image was captured. Care was taken to accurately record the time associated with
each image. Depending upon the weight used, reliable experiments lasted anywhere from
10 minutes to more than three months, so this made accurate time measurements
imperative for the short duration experiments.

The frequency of image capturing depended upon the load used. Usually 10 to 30
data points are sufﬁcient for generating an accurate creep curve. Variance in temperature
as well as safety precautions during elevated temperature testing required the operator to
remain present so that adjustments in temperature could be performed.

The creep tests used for this investigation were performed at 25°C, 85°C, and 125°C
with temperature ﬂuctuations less than 7 degrees throughout the duration of each elevated
temperature experiment. Both longer duration (low stress) and short duration (high
stress) creep tests were performed at room temperature, but only short duration tests were
performed for the elevated temperature creep. The short duration or accelerated tests
were conducted in a range from 30 minutes to approximately 24 hours, while the long

duration creep tests usually lasted for 2 to 3 months.

52

 

    

'11 Before Creep

 

 

 

 

 

 

 

 

(a) (b)

Figure 11. Displacement of Scratch Due to Creep Shown as
(a) Raw Captured Image in Negative Format.
(b) Enhanced Image for use in Datathief.

53

 

 

3.3 DATA EXTRACTION TECHNIQUE

3.3.] Image Enhancement

After an image has been captured, it was edited in Adobe Photoshop®. Several tasks were
performed on each photo before they were ready for analysis. Each digitally captured
image was ﬁrst inverted to a negative format. A white image with a black scratch was
more convenient for analysis in Datathief than a black image with a white scratch. As the
creep test progressed, the scratch became more difﬁcult to detect asillustrated in Figure
11. A scratch which was only slightly visible in Photoshop was almost impossible to
distinguish when opened in Datathief It then becomes necessary to highlight or trace the
scratch with the line tool in Photosh0p®. This is illustrated in Figure 11. Most of the
error involved in these creep tests probably occurs during this procedure, since the
resolution of the scratch decreases with increasing displacement.

In Figure 11, images in the left column are examples of digitally captured scratch
displacement images (negative format) from a composite solder creep tested at 125°C.
Time of capture is noted next to each image. The images in the right column have been
enhanced in Dat'athz'ef as described previously. The images used in Datathz'ef must be of
uniform dimensions. This is accomplished by establishing a uniform canvas size for all
enhanced images. This eliminates the possibility of non-uniform image distortion in
Datathief Both solder/substrate interfaces can be seen in the enhanced images and are
highlighted in the image captured after 20 minutes.

If the full raw image is opened in Datathz'ef, the area of interest is too small for

accurate analysis. For these reasons each image is carefully cropped. However, as the

54

creep experiment progresses, the area of interest from each image increases vertically. By
changing the canvas size or image border, the dimensions of the images stay the same
while the surrounding ‘canvas’ is enlarged. Since subsequent images only grow in the
vertical direction, the simplest process is to edit the ﬁnal, tallest image from the creep
experiment ﬁrst. Sufﬁcient borders are required on every image for the ease of data
extraction. By examining the ﬁnal image, appropriate dimensions can be determined and

applied to all the previous images, with the ﬁrst image having the largest vertical border.

3. 3.2 Data Extraction

Datathief is a data digitizing shareware package available only for the Macintosh
platform, which allows the user to extract data points from a digitized image by
establishing an x-y coordinate system directly onto the image. Since the displacement of
the scratch must be measured by extracting data from many images, it was necessary to
establish the same x-y coordinate positions on each subsequent image. Since shear
displacements were measured, a normalized dimensional scale based on the thickness of
the joint was used as a basis for computing shear strains (Figure 12). The displacement of
the scratch was modiﬁed or scaled with respect to the thickness in order to obtain a value
of zero for the maximum displacement. See Appendix B for a detailed example of the
data extraction process. The axes must be established in a manner so they can easily be
accurately transferred to each subsequent image. The technique developed in this
instance, was to use an 8.5 “ by 11” transparency overlaid onto the computer monitor.
Several images from the same creep test, from beginning to the end, were then opened in

the Datathief program and analyzed for similarities in an attempt to ﬁnd an identifying

55

II"

Proﬁle of a shear-lap solder joint
fabricated from Cu dog—bone halves

I] Cu substrate ‘SnrAgm Cu substrate
6 0:5 "1

 

 

Normalized position across solder joint.

Figure 12. Normalized Position Across the Thickness of a Solder Joint.

 
 

 

' 3'; me can options - ' ‘

The x and y axes
were established
using a pre-drawn
coordinate system on
a transparency which
was overlaid onto the
computer monitor.
This allowed for the
same coordinates to

 

 

 

 

 

 

 

 

 

 

 

gm. be established for
:5 1i each enhanced image.
iii. Consistent units must
we “Mm “memmm
Image from Figure 11b image.
(42 minutes elapsed time)
opened in Datathief for
data extraction.
x = 0 ”at

 

 

 

Figure 13. Screen Capture of Data Extraction Using Datathief

56

mark visible on each image near the stationary interface which could be used as a
landmark throughout the data extraction procedure. This could be anything from a
scratch to a small pore, but it must be visible on each subsequent image. A marker pen
was then used on the overlaid transparency to trace this landmark as well as the left and
right interfaces. These highlights can then be conﬁrmed by opening different enhanced
images, as seen in Figure 11b, in Datathief and checking for alignment of the
transparency and the image. Once this was carried out, axes could then be drawn onto the
transparency with the aid of a ruler. The longer the axes, the less error was involved due
to the angle of View and parallax distortion inherent in the computer monitor. Figure 13
is a screen capture of an enhanced image seen, in Figure 11b, opened in Datathief.

Once the axes are established, an x-y coordinate window in Datathief displays the
current position of the cursor. The cursor was traced along the length of the enhanced
scratch via the mouse. The mouse was then clicked at even increments, usually every
centimeter or two on the monitor according to the pop up x-y coordinate window which
indicates position on the image (Figure 13). The exact x—coordinate positions were noted
and used on each subsequent image. The process was then repeated for the remaining
images. Since it was impossible to determine the duration of each creep test beforehand,
images were captured frequently, although well deﬁned creep curves could be produced
without extracting data from every image. Once the coordinate axes were established,
about 10-20 datum points were extracted from the scratch, at the same x-coordinate

positions for every image.

57

3. 3.3 Reproducibility of Data Extraction

In order to determine how sensitive the data were to human interpretation, comparisons
were made of a set of data extracted from the same set of images several months apart
using the same procedure. The error between the two measurements of the same average
strain increased with strain from about 1 to 10%, due to the degraded clarity of the

captured image at larger strains.

58

3.4 DATA PROCESSING

From the extracted data, a single plot can be produced which illustrates the entire scratch
displacement history on one specimen (Figure 14). This is similar to overlaying all of the
captured images onto one image, but in the form of a plot. Global creep strain is deﬁned
as the average creep strain across the thickness of a specimen. Measured global strains
generally result in creep curves of the classic sense as shown in Figure 1. This
designation is made since a comparison between the two forms (global and localized) is

I'ICCCSSEII'y.

3. 4.1 Global Creep Strain Measurement

The ﬁrst step in calculating the global creep strain was to measure the shear displacement
of one side of the specimen with respect to the other. Displacement during creep
deformation occurred only at one solder/substrate interface, while the other interface
remained stationary. A coordinate system was established by designating the end of the
scratch at the stationary solder/substrate interface as a reference origin.

The next step was to calculate the net shear displacement. In order to account for the
non-horizontal shape of the initial scratch, the difference between the data for the initial
scratch position and a simpliﬁed horizontal origin was subtracted from the subsequent
scratch position data derived from images captured during the creep test. The resulting
global net shear displacement after 8520 seconds was plotted with position across the
thickness in Figure 15, where the bolder lines represent the net shear displacement. From
this ﬁgure, the global creep strain was computed by dividing the total net displacement by

the thickness.

59

Non-Composite Sn-Ag at Room Temperature, 21.4 MPa
Scratch Displacement Due to Creep (Sample 039)
‘r I l l i

i T T I l l

 

I I I i

1.2_rrur

 

 

 

 

 

Normalized Position

. ' g E 5 .
.' - i ‘ i :

' I ‘ ,
_ r~ . ; , .
/ ........... .._;
0.4 ,' - 1 J
;

 

 

 

 

, . g : Before Creep - - -"..;=- - 4080 see
,I’ /' g 3 —B— 240 sec - O— 5640 see
E] ___________________ g ________________________ 3 ____________ -l— 600 sec --z’ 7140 see ,
0 2 1’. x /' 3 —V— 1260 see - -ir— 8520 sec
E, E r --V--3120 sec
0 - . . . l r . r . . . . l r i . l . r i
O 0.2 0.4 0.6 0.8 1

Normalized Position Through Solder Joint Thickness

Figure 14. Scratch Displacement History.

 

Global Net Shear Displacement (Sample 039)

 

 

 

 

 

 

 

 

 

1 .2 l j W ! I I I I I l T I I l ‘ f l I J
b ' i - ; 4;
1 :_ ................................................................... i .......................................... . ............. J val-E
.............. - .9‘

f _______ . ............. + --------- 9 ' " g ”ﬁlo-J" :
2% BE ------ . .....:::'.:’ _
$08 T """"""""""" """" [,LQQ‘i'Z""'i """"""""""" j
o e ‘ ‘ §_.-- ' ’ I
S — : i ,.--";" g _
3 0.6 ~ ------- , - --------- ~ ------------- ; -------- _ .. ------------------------ 5 rrrrrrrrrrrrrrrrrrrrrr a
D ’— ' ...o". , ’ I i -
U .-.n T I I f . a
o -. . ,.- , , , . . -
.5 3 .e-tf.-_____ x _________ . . l
a 0-4 j“ ' " ' ' ' “ ' ' ' " ' ’ ";”_j_;.;é" , , ’ g Before Creep (Simpliﬁed) -
E ’ , ’ ’ 3 0 Before Creep (Localized) -
g — _____ 3 , , , ’ § m-E- Before Creep (Global) ~

0.2 3‘ - a" """""""" ’“y‘f' """"""""""""" 3' """"""" —-V-— 8520 see (Localized)
i , , ’ 3 § - G - 8520 sec (Global) 3
_ , , ' ’ i 3 --------- 8520 see Net Shear Displacement
0 l r r l i 1 l L l l f x I r r x r 1 1 1
0 0.2 0.4 0.6 0.8 1
Normalized Position Across Solder Joint Thickness

Figure 15. Difference Between Global and Localized Deformation.

60

3. 4.2 Localized Creep Strain Measurement

The local shear strain was determined by ﬁtting a cubic polynomial function using
Kaleidagraph® software to the shape of each scratch. The same procedure for
determining the net shear displacement was employed. This resulted in the localized net
shear displacement shown in Figure 16. By differentiating the resulting cubic
polynomials, the local SIOpe of each scratch, which is the local shear strain, was
computed. Note that in some locations reverse (negative) strains were observed, such as
position 0.7. The degree of accuracy of each ﬁtted curve was indicated by the coefﬁcient
of determination, R2, where a value of 1 indicates a perfect ﬁt. The average value of R2
for all of the curve ﬁtting procedures in this experiment was 0.99.

A plot indicating the creep strain history for different positions across the solder joint
thickness (indicated by normalized position) was then produced from this information as
illustrated in Figure 16 (strain-time history at selected illustrative positions is shown, note

that at position 0.7, the strain rate is negative at later times).

61

1.2

Normalized Displacement

The slope can be computed Simpliﬁed Origin
0.2 i __________ for any posmon 'x' once the ______ + Before Creep (Fitted)
net displacement has been ——B—- 8520 SEC (Fitted)
r.” COmPutéd --------- 8520 see Net Shear Displacement
0 l 1 L i 1 1 1 1 1 L l I 1 1 1 I l I l
0 0.2 0.4 0.6 0.8 1
Normalized Position Across Solder Joint Thickness
Figure 16. Cubic Polynomial Curve Fit.
25°C, 21.4 MPa, Non-Composite Sn-Ag, Polynomial Fit
4 Localized Strain vs. Time (Sample 039)
1 I 1 f T I I I T I I I I r T
E + 0 0.6 Normalized ' § j
r 1} 0.1 A 3‘." Position : q
0.2 08- Through -
3 ‘ v: 0.3 />- ~ 0.9 Solder Joint """""""""""""""""""""""""""" g """""" j
04 to Thickness Er E
(>- 05 Global 5 , / ’g ~
. , a ;
2 __ ................................................................................. ’./ ............................... _.
l i _
t: r . ’ r
E * i ‘
1 _ . V y _
0 _ . ...................... ‘ — ——————————— CT
I A A A _
_1 l r l i l 1 l l l l 1 l 1 1 l l 7
0 2000 4000 6000 8000

Figure 17.

Localized Net Shear Displacement (Sample 039)

 

 

 

 

 

 

I I j l I I F l I I I I I I I I I I

.. . . : j
: I ‘

_ 5 2|

W ‘“

_ ................................................ nee-.22---.-..--.---.--ff'.ft: ...... --.r ______ “

. . a

4; —
- u

.........................................................

 

 

 

 

 

 

 

 

 

    
  

 

 

 

 

 

 

 

 

Time (sec)

Localized and Global Creep Strain.

62

 

3. 4.3 Strain-Rate, Position, and Time

In order to examine how the strain-rate varied with time and normalized position, three-
dimensional plots were produced ﬁ'om the localized strain history plots. Polynomial
curves of order 3-7 were used to ﬁt the strain time curves (Figure 17) at each position.
The derivative was calculated with respect to position to obtain the strain-rate, and plotted
using Deltagraph® 3.0 software as illustrated in Figure 18. The 3-D surface is
represented as a contour plot on the top of the plotting space. The shape of the 3-D
surface varied from specimen to specimen. The example shown in Figure 18 illustrates

the variability in the detailed strain-rate history in the measured part of the specimen.

63

3x10

Strain ZXIO

Rate
(1/5)

1x10’4’

—4

/

4/

z

/

 

Oz
1

Normalized

Figure 18a.

Figure 18b.

Position

 

 

 

 

 

14.42
8.42
2 42 Time
0 ' (Klloseconds)

3x104

2110‘1

1x104

 

 

 

0.00015

0.0001

0.00005

0

 

 

Variation of Strain-Rate to Time and Normalized Position for a Composite
Solder Creep Tested at 25°C.

 

Normalized 0-5
Position

     

(Seconds)

 

 

0.0004

0.0003

0.0002

0.0001

-0.0001

 

 

 

-0.0002

 

 

Variation of Strain-Rate to Time and Normalized Position for a Non-
Composite Solder Creep Tested at 85°C.

64

3. 5 SEM OBSER VA T I ONS

3. 5.1 Determination of Stress

After the specimen failed, the fractured halves were examined optically or with a SEM,
and low magniﬁcation images were taken of the entire fracture surface, as shown in
Appendix C. All specimens exhibited porosity, and the area of pores larger than 500 ,um2
were subtracted from the entire surface area, using NIH image software, in order to
determine the actual cross-sectional area of the soldered region. The true stress operating
on the cross sectional area was computed using this reduced area. Due to the long
moment arm effect of the two copper tabs, strain in the joint was nominally uniform in
the straining direction even though porosity present throughout the solder joint could lead

to asymmetry in load carrying area with respect to the center-line.

3. 5.2 Initiation of Fracture

The method employed for creep testing the miniature solder joints did not allow for
detection of crack initiation. In order to determine the location of fracture initiation four
solder joints, two fabricated from composite solder and two from non-composite solder
were subjected to high stress dead load conditions. The solder joints were prepared in the
same manner as described in Section 3.1.3. Visual records of the polished surfaces were
recorded prior to loading with the use of a SEM. The solder joints were strained using
the miniature creep frame. Each solder joint was initially subjected to a weight low
enough to avoid rupture, approximately 2.27 Kg. (5.0 1b.). The load remained on the
solder joint for roughly 10 seconds and was then removed. The solder joint was then

removed from the creep frame and examined under an optical microscope. If no signs of

65

crack initiation were visible, the process was repeated with load increments of 0.23 Kg.
(0.5 1b.). This procedure resulted in differing strain-rates depending upon each specimen.
The strain-rates were not measured. When signiﬁcant changes in the polished solder joint

were detected, examination was performed with an SEM.

3. 5.3 Location of Fracture

Examination of the fractured solder joints was performed to determine the location of the
fracture across the polished thickness of the solder joint. These images were compared

with the fracture surfaces images in order to better characterize the mode of fracture.

66

CHAPTER 4

RESULTS

4.1 COMPARISON OF EXPERIMENTAL CREEP DA TA T 0 DA TA
FOUND IN THE LITERATURE

In order to determine the reliability of the testing methods used in this investigation, a
comparison of the creep data acquired was compared to data found in the literature. The
data used for the comparison came from an investigation by Darveux et al. [41]. They
subjected aged, realistically sized solder joints (ceramic chip carriers soldered together
with eutectic Sn-Ag) to creep deformation at temperatures of 131°C, 77°C, and 27°C.
Solder joints used in this study were not subjected to aging processes. The data of
Darveaux et al. was normalized with respect to temperature in a manner described in
Section 2.4.9. The normalized secondary (or minimum) creep strain-rate was plotted
against normalized shear stress. Darveaux et al. used a ‘trial and error method’ to produce

a best ﬁt curve to represent the normalized creep data. Values determined by Darveaux et
al. for G = G, + G1T(°C) , where Go= 2.8x106 psi and G1 = 1.0x104 psi/°C were assumed

for this investigation. In order to simplify the comparison, a single plot comparing the

data from this investigation to the data of Darveau et al. was produced (Figure 19).
Another reason for this comparison was to examine the effects that the intentionally

added Cu68n5 particles had upon creep behavior at room and elevated temperatures. From

this comparison, it is apparent that the composite solder is more creep resistant than the

67

 

 

Normalized Creep Strain vs. Normalized Shear Stress

   
 
 
 

. ' ' l g
103 E —— Darveaux ........................................................................ i
E A 25°C Composite ................................................................... a
101 E ‘ 25°C Non-Composite ....................................................................... a.
E 85°C Composite ....................................................................
101 i I 85°C Non—Composite ------------------------------------------------------------------ E

._\
O.
or

 

125°C Composite
125°C Non—Composite

 

 

y(TlG)exp(QlkT)

_\
0.

(It
Ill] 1

 

_X
0.
\I

Figure 19.

 
   

.................................................................................................................

 

6104 810'4 10'3
rIG

Comparison of Secondary Creep-Rate Data.

68

 

 

Normalized Creep Strain vs. Normalized Shear Stress

 

 

 

 

 

   
 
  
 
   

 

103 E Darveaux I __________________________________________ 2
E A 25°C Composite ....................................................................... g
A 10‘ E ‘ 25°C Non-Composite ------------------------------------------------------------------------ E
E E 85°C Composite """"""""""""""""""""""""""""""""""""""""""""""""""" “E
g 10'1 E ' 85°C Non-Composite """""""""""""""""""""""""""""""""""" j:
g ‘3 E 125°C Composite """""""" """" v """""" A """""""""" 1
610 E 1250C Non-Composite EEEEEEEEEEEEE v EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
- F A ..........................
3:;- 10'5 e ----------------------------------------------------------------------------------------------- a
210‘1 4104 6104 810“1 103
13,6

Figure 19.

Comparison of Secondary Creep-Rate Data.

68

non-composite solder, at room temperature and 85°C, but the composite solder joints are

comparable in creep resistance to the non-composite at 125°C.

4.1.1 Room Temperature Composite Data

The composite solder joints tested at room temperature exhibited superior resistance to
creep as compared to the non-composite solder joints. The strain-rate of the solder joint
tested at the lowest stress was near 3.5 orders of magnitude less than the trend established
by Darveaux et al. As stress increased this difference decreased by an order of magnitude
on average. A trend different from that established by Darveaux et al. for the non-
composite data appears to be present for the composite data resulting from this

experiment.

4.1.2 Room Temperature Non-Composite Data

Room temperature (25°C) creep data obtained in this study was in relative agreement with
the trend established by Darveaux, et al. The three solder joints tested at higher stresses
were all characterized by strain-rates within an order of magnitude of Darveaux’s best ﬁt
curve. Two of the points were in such proximity that only one was visible. Of the two
solder joints tested at lower stresses, both were characterized by strain-rates which were
lower than the established trend. One solder joint had a strain-rate within an order of
magnitude of the established trend, while the other specimen had a strain-rate more than

an order of magnitude less than the established trend.

69

4.1.3 85‘C Data

The two composite solder joint tested at lower stresses agreed well with the best ﬁt curve.
Both were characterized by strain-rates well within one order of magnitude of the
established trend. As stress increased, the margin between the data and the best ﬁt curve
increased to around one order of magnitude. Non-composite solder joints tested under

this temperature environment exhibited similar results to the composite data.

4.1.4 125‘C Data

All of the composite solder data was characterized by strain-rates within one order of
magnitude of the trend established by Darveaux et al. Two of the solder joints tested
exhibited higher strain-rates than the established trend. The other two specimens were
characterized by strain-rates which were equal or almost equal to the established trend.
Both of the non-composite solder joints tested at this temperature had strain-rates nearly
equal to the established trend and their data points fell almost directly upon the best ﬁt
curve. These samples appeared to have slightly better creep resistance than non-
composite solder joints tested under lower temperature conditions. However, this
apparent trend should be disregarded due to the inherent scattering of data associated with

creep.

7O

4.2 GLOBAL VS. LOCALIZED CREEP STRAIN

The global strain-time history usually exhibited classical primary, secondary and tertiary
strain behavior as indicated by the heavy lines in Figure 20b. The vertical scale of a
global creep curve became ﬂattened when plotted with the localized creep curves from a
single specimen since a wider range of values were present. As a result, plots of
individual global creep curves are available in Appendix D. Characterization of the local
strain history of all samples included in this investigation is provided in the following
sections. As mentioned previously, these specimens include composite and non-
composite solder joints deformed at room temperature, 85°C, and 125°C. Since there are
more than 20 solder joints tested, descriptions will be limited to the most representative
specimens. The associated ﬁgures include a scratch displacement history (a), a strain

history (b) and strain-rate history with respect to position (c).

4.2.1 Room Temperature Composite Creep Data

Figure 20 illustrates creep history of a composite solder joint subjected to creep
deformation under a stress of 26.2 MPa for about 4.5 hours (16.1 Ks). This specimen had
highest localized strain accumulation near the interface at position 1.0. Identiﬁcation of
the beginning of global tertiary creep was near a shear strain of 0.5. However on a local
scale, tertiary creep appeared to start at a larger shear strain of 1.2 near the interface. The
secondary creep-rate was measured to be 4.2x10'5 5".

The 3-D plot indicated a sharp initial decrease in the strain-rate near the interface

(positions 0 and 1.0), with a much lower initial strain-rate in the center of the specimen.

71

25°C, 26.2 MPa (Sample 053)
Composite Sn-Ag

25°C, 26.2 MPa (Sample 053)
Composite Sn-Ag
Localized Strain vs. Time

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

            
      
 

 

 
    
   

 

 

 

 

 

 

 

 

 

 

 

 

2.5 ' _ r 1 i
I 02 vNormalized 3
.‘ 04 Posmon ,7
E 0-8 Elapsed 2 ,1 06 Through ..9
o ‘ - ’ ' Solder Joint ,/1
E Tlme y 08 _ I, I
a) . . (Sec) ,_ Thickness .
L; g 2 —D—0 1.5 -,.,--- 1.0 ------------- w -----
u, 5 /n" _ ,‘O —B— 420 .5 Global
"504 "";i"“ """ r " 2340 g . ;
'o ,..(; o , v 5580 a;
3 La ....... 7500
'7; 0'2 ” .0 + 9660
E . 11580
g o --------------------------
. . <> . 16140 I -I -i i i
_o'2 llilllil lll]|| 0 - llllllllllll v v
C O O O *0 O O O
O 0.2 0.4 0.8 1 o 8 8 8 8 ._ c: z 8
Normalized Position Through N " ‘° °° ‘— x— .4 .—
Solder Joint Thickness Time (sec)
(at) (b)
/ ﬁe 0.00035
// \E/W 0 0003
/ \ '
0.30 , A ><\ \’ 0.30
. \
’ ' \ \\, 0.00025
\
\\
Strain 0.201 \\ \r0v20 0.0002
Rate \\
(US) l \\ \' 0 00015
\ l ‘
0.10 ’ 7C0 10
, 0.0001
0 E , 0 0.00005
1
14.42
Normalized 0
Position 8.42
Time
0 2'42 (Kiloseconds)
(C)
Figure 20. Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)

Histories for Composite Solder at 25°C.

 

ra

pC

Fi

de

5P

ho

fof

CR

512

be

be

P0

AI

Val

Cnc

The strain-rate decreased with time in a normal way to reach a minimum strain-rate in the
range of 2-8 x 10'5 s". The strain-rate then gradually increased to reach higher values at
the two interfaces prior to rupture. The highest strain-rate (3.3 x 10'5 s") was observed at

position 1.0.

4.2.2 Room Temperature Non-Composite Creep Data

Figure 21 illustrates creep history of a non-composite solder joint subjected to creep
deformation under a stress of 11 MPa for a duration near 2 months (6.0 Ms). This
specimen had highest localized strain accumulation near the interface at position 0,
however the specimen deformed in a homogeneous manner with the localized strains
following closely to the trend of global strain. Identiﬁcation of the beginning of tertiary
creep is near a global shear strain of 0.15. Near the interface, tertiary creep appeared to
start at a slightly higher shear strain near 0.18. The secondary creep-rate was measured to
be 3.0x10'8 s“.

The 3-D plot indicated a sharp initial decrease in the strain-rate near the interface
(positions 0) and to a much lesser extent position 1. The strain-rate initially peaked

between positions 0.3 and 0.5 peaks at a value near 5.0x10'8 s".

The strain-rate at
position 0 remained constant throughout the test after a slight increase near the beginning.
At the other interface, position 1, the strain-rate rapidly increased where it peaked at a
value of 7.5x10'8 5'1 aﬁer 2 Msecs. It then decreased, followed by a ﬁnal increase at the

end of the test to a value of 2.0x10'7 5". From approximate positions in the range 0.8 to

0, the strain-rate remained relatively constant until the end of the test, where an increase

73

25°C, 11 MPa (Sample 042)

Non-Composite Sn-Ag

 

.0
A

.0
w

.........

.0
N

Normalized Displacement
O

 

liillllllllllllllll

O

O 0.2 0.4 0.6

Solder Joint Thickness

(a)

 

 

0.
Normalize
Position

     
 
    
 
    
 

 

0.8 1
Normalized Position Through

25°C, 11 MPa (Sample 042)
Non-Composite Sn-Ag

 

     
  
 

 

 

 

  

0Spratch Displacement Due to Creep Localized Strain vs. Time
. lllIlllllllllllE‘lTl 03 . r1]r1rr_
. , z._. 2320:?“ s a
0.6 ******************************** 0.25 “EH ‘0’: Through /
Elapsed , 09 Solder Joint I / ;
0.5 ---------------------------- Time 02 1.0 Thickness , —
(K390) Global 5 ”1

............................

llllllllll

 

 

 

 

.\;\c
o r -
_005 Plll|llllllllllllllllllillllli_
(D (D (D ‘0 (D ‘D
O O O O O O
O 1- ‘— ‘— v— ‘— x—
F N ('0 V 10 L0

Time (sec)

0))

 

0.0000002
0.00000015
0.0000001
;. 0.00000005

000000005
00000001
000000015
0.0000002
000000025
00000003

 

 

 

 

(Kiloseconds)

(C)

Figure 21.

Displacement (a), Creep Strain (b), and Strain-Rate (c) Histories for Non-

Composite Solder at 25°C.

74

 

[E

\\

re
51:
CC

th

F1",
de

SD

in strain was experienced across the entire solder joint with the maximum strain—rate at

position 1.

4.2.3 85 °C Composite Creep Data

Figure 22 illustrates creep history of a composite solder joint subjected to creep
deformation under a stress of 13.2 MPa for about 1.3 hours (4.6 Ks). This specimen had
highest localized strain accumulation near the interface at position 1.0. Identiﬁcation of
the beginning of tertiary creep was near a global shear strain of 0.5 but near the interface,
tertiary creep appeared to start at a larger shear strain of 0.75. The secondary creep-rate
was measured to be 9.8x10'5 3".

The 3-D plot indicated a slight initial decrease in the strain-rate near the interface at
position 0, with a gradual increase in strain-rate towards position 1. The strain-rate
remained relatively constant along position 0 except for a slight increase and decrease in
strain-rate at the start and ﬁnish of the test respectively. At position 1, the strain-rate
continually increased to peak at a value near 8x10'4 3". The strain-rate remained constant
throughout the duration of the test at position 0.2 and increased towards position 1

throughout the test.

4. 2.4 85 °C Non-Composite Creep Data

Figure 23 illustrates creep history of a non-composite solder joint subjected to creep
deformation under a stress of 19.6 MPa for about 34 minutes (2.0 Ks). Unlike the prior
specimen, the largest strains were observed in the center of the specimen near position

0.5. Identiﬁcation of the beginning of tertiary creep is near a global shear strain of 0.24

75

2%

 

Fi

85°C, 13.2 MPa (Sample 033) 85°C, 13.2 MPa (Sample 033b)
Composite Sn-Ag Composite Sn-Ag
Localized Strain vs. Time,

 

    
   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ngratch Displacement Due to Creep 1 5
' : : : I ; ' + 0 Normalized ' I i L i
: Position , 4
«0'7 : l ' y H7; 8; Through / . «l
506 g .[ i _.x’_’,-,:@; 1 v 00 S°'der A , %
GE, ' ; . ' ' 31 0.4 . g 4
. v .. 2P 0.6 , ; 1
163-05 (E 7": . ,; o 0.8 ~ 3
~ 1 5 ..5; ~-— 1.0 .. 3:7...
5,-0.4 1— V V ' =0.5 Global ' _. c" . 1
1: ’__ - - , 1 i 5 A ‘
“0.2 ' o i . i -~1
E :L ‘ .0 E : ' 1 : :1
20.1 t , ----- ~ ‘ 5 3 : i
' 0’ 1 . a I ; . i 4
0 i Yer-1' L i‘l‘l‘ ‘ I ‘ _05 WW
0 0.2 0.4 0.6 0.8 1 0 § § § § §
Normalized Position Through T N " “’
Solder Joint Thickness Time (see)
(a) (b)
6x10“ 6x104
4x104 1 4x104
Strain ,
Rate 4 4
(us) 2x10 1 2x10
0 ’ 0
-2x104 , -2xl0“
1
4820
Normalized 0.5 3120
Position 1820 Time
0 (Seconds)
(C)

Figure 22. Displacement (a), Creep Strain (b), and Strain-Rate (c) Histories for
Composite Solder at 85°C.

76

85°C, 19.6 MPa (Sample 025) 85°C, 19.6 MPa (Sample 025

 

 

   
   

 

 

 

 

 

 
   
   
    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Non-Composite Sn-Ag Non-Composite Sn-Ag
Scratch Displacement Due to Creep Localized Strain vs. Time
0.35 TIVITTTIIlTTITTllTI 0.5 1W7:
: ‘ 3 ——a—— 05 v o 9 f}? E
.. 03 ________________ _- 0.4 —a— 0.7 ~<> 1.0 w ------ —_
c ' I 0 8 Global . ‘
i” 3 0 3 _ ......................... . ............. _‘
§025 ' g ; :F 5
(I j 2 :_ ................. . . . . . . ..... L
E. 0 2 S Elapsed 0 : : {/21 3
. _ _ c , a
g - Time '3 0 1 _ ............. . ..... .;
1 (Sec) h ' 3 , v i
0.15 a : ‘ ' ' V v . _
E -e~ 1080 a) 0 ”$14): Y...‘ r; v.1 ' 'V i
= 01 _ —e— 120 --|— 1320 3 ; ”x j I
N - t 9' 480 . 1680 -01 .— """ ’" " -~>\-..\~;» i
E H _ v 660 e 1860 g ; . a :
z°0.05 ------ +760—0—2040 -o_2 5 ------- fie —:
' —t—- 900 Z : 5 1 ‘ :
0 111 '0.3 ’llllll lI_L_LAlillllel 1‘
o o o o o
0 0.2. 0.4 0,-6- 0.8 1 o 8 g g g g
Normallzed Posmon Through “ “ N N
Solder Joint Thickness Time (see)
(a) (b)
=§§é '
ain‘t
6x104 1 gal! 6x104
- Elli
.1- “>39. 4x104
4X10 -i=‘
Ill? .
2x104 ‘ "' 2x10
Strain .. :
Rate 0 . 0
(Us)
-2x10'4 ‘ -2x10“
-4xl0““ 4x104
Normalized
Position 0 520 Time
(Seconds)
(6)

Figure 23. Displacement (a), Creep Strain (b), and Strain-Rate (c) Histories for Non-
Composite Solder at 85°C.

77

 

 

 

 

 

Fig
def
ach
ldel
Spe.
the

Cl'Cc'

Posi

but near the center, tertiary creep appears to start at a slightly larger shear strain of 0.3.
The secondary creep-rate was measured to be 1.2x10'4 3".

The 3-D plot indicated a slight initial decrease in the strain-rate near the interface at
positions 1 and a higher initial strain-rate in the center of the specimen. Unlike the prior
specimen, the strain-rate remained the highest at the center of the joint. At position 1 the
strain decreased until specimen fracture. Then strain-rate near position 0 increases until
an elapsed time of 1400 seconds where it attained a maximum strain near 2.5 x 10‘1 s'1
and began a decrease in strain-rate until the end of the test. The highest strain-rate (5.0 x
10‘1 3") developed in the center of the specimen between positions 0.3 and 0.5 at the end

of the test.

4. 2.5 125 °C Composite Creep Data

Figure 24 illustrates creep history of a composite solder joint subjected to creep
deformation under a stress of 11.6 MPa for about 18 minutes (1.1 Ks). This specimen
achieved highest localized strain accumulation near the interface at position 1.0.
Identiﬁcation of the beginning of tertiary creep for this specimen was not obvious. The
specimen behaved in a tertiary manner after a global shear strain value near 0.1, but near
the interface, tertiary creep appeared to start at a larger shear strain of 0.3. The secondary
creep-rate was measured to be 5.8x10'4 5".

The 3-D plot indicated a sharp initial increase in the strain-rate near the interfaces
positions 0 and 1.0, with a much greater increase near position 1 (5x10'3 3"). A sharp
decrease in initial strain-rate initially existed at the center of the specimen with a value of

-2x10'3 5". Near position 0, the strain-rate remained relatively constant until a decrease

78

“cartoon-00.0 UeNszscz

 

125°C. 11.6 NIP-at (Sample 028) 125°c, 11.6 MPa (Sample 028)
CompOSIte Sn-Ag Composite Sn-Ag
(Spratch Displacement Due to Creep Localized Strain vs. Time
.ir 1 . .

 

    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

.. 2...”; ; . l‘
5 0'6 [E W E ' : ' ' 1 Elapsed 0.8 --: -* , i
g - 1' " ‘ Time i
005 (See) ,’ , , .
g
- —0—0 3
3'04 >B— 120 .E x
Q 480
u 0.3 720 g 0'4 ﬂ
3 . 840 i
= + 900
E 0'2 960 ° 2 4
I— 1020
‘2’ 0.1 1080 0 , . : _i
1140 1 : - . .
0@;_o 11_l lli‘lllllllllllllJ_1_LJll
0 0.2 0.4 0.6 0.8 1 0 § § § § § §
Normalized Position Through _ “ "
Solder Joint Thickness Tlme (see)
(a) (b)
7.
<\
\.
‘ Z>r
0004* : \\w0.004
.i/Hpv \ \\r
0.0025 \\— 0.002
Strain 7 \-
Rate ’
(Us) 0 —o
a \\.L
0002' \\~-0.002
' 1020
Normalized 0.5 620
Position 320 Time
0 (Seconds)

(C)

Figure 24. Displacement (a), Creep Strain (b), and Strain-Rate (c) Histories for
Composite Solder at 85°C.

79

 

If

before p r0 ryh A6 €hy cyl eyr au€hy pa’cyr jacl e, ehy pirdcl -rdey C1 crydpyC ‘hr ra’ jh’y d
third of the test duration, remained constant for another third, and then increased again to
terminate in a manner completely opposite of the initial strain-rate. The entire solder
joint behaved in an opposite manner as it did at the beginning of the creep test achieving a

1

maximum strain-rate of 1.5x10'3 5’ . The strain-rate near position 1, sharply decreased

and then behaves erratically until a continual decrease half way through the test.

4. 2. 6 125 °C Non-Composite Creep Data

Figure 25 illustrates creep history of a composite solder joint subjected to creep
deformation under a stress of 12.7 MPa for about 50 minutes (3.1 Ks). This specimen
achieved highest localized strain accumulation near the interface at position 0.
Identiﬁcation of the beginning of tertiary creep was near a global shear strain of 0.6, but
near the interface, tertiary creep appeared to start at a larger shear strain of 0.9. The
secondary creep-rate was measured to be 1.3x104 s'l.

The 3-D plot indicated a relatively constant strain-rate across the thickness of the
solder joint with a slight decrease at position 0. The strain-rate remained constant
throughout most of the test except at position 0 where the strain-rate appears to follow a
sinusoidal pattern until rupture. After an elapsed time near 2600 seconds, the strain-rate
sharply increased in a uniform manner, except near position 1, with the maximum value
of 2.2x10-3 s-l at the center of the solder joint. The strain-rate near position 1 remained

relatively unchanged throughout the duration of the test.

80

125°C, 12.7 MPa (Sample 027)
Non-Composite Sn-Ag
Scratch Displacement Due to Creep

.-E1—"’—'

    
 
     
   
 

 

.. i 1.2
50.8 Elapsed

E ' I Time 1
8 1 1 (Sec.)

(5 ’ j

30.6 . V i ......

'2 1 'li ,

o .. . ,1‘

30.4 - g , ......

5 ‘ ,' :

Tu '

E02 . 5v .........

O ,7 . .

2 I, :

 

 

 

 

 

 

 

125°C, 26.2 MPa (Sample 027)
Non-Composite Sn-Ag

 

Localized Strain vs. Time

—D—0 -—*——0.7
—B— 0.1 .,... 0.8
0.2 ...i 0.9

10

 
 
  

 

 

 

Global '1

  

 

 

 

 

 

0 0.20.40.60.81 o § § § § § §
Normalized Position Through " " N N "’
Solder Joint Thickness Time (sec)
(a) (b)
//
0.0025 - —0.0025 0 0025
><
’ 0.002
//
0.0015“ . 0.0015
// 0.0015
Strain
Rate / 0.001
(l/s) - , 0.0005
0 . _ 0 0.0005
-0 0005 — - 00005 0
3080
Normalized
Position Time -0.0005
0 (Seconds)
(C)
Figure 25. Displacement (a), Creep Strain (b), and Strain-Rate (c) Histories for

Composite Solder at 85°C.

81

 

 

\‘2
(11

ii

4.3 POSITION 0F MAXIMUM LOCALIZED CREEP STRAIN

It is evident through determining the localized creep strain, that strains of much higher
magnitudes than the global strain are present at different positions across the thickness of
the solder joints. The room temperature non-composite solder joints creep tested at
higher stresses all exhibited maximum localized creep strains along the interface. The
non-composite solder deformed at lower stresses yielded higher localized strains near the
middle of specimen.

Non-composite solder tested at 85°C resulted in two samples exhibiting maximum
localized creep strain at the interface, and the other sample very near the interface.
Composite specimens tested at 85°C yielded mixed results with half achieving maximum
localized strains at the interfaces, and half near the middle of the solder joint.

Non-composite solder tested at 125°C all exhibited maximum localized creep strain
values along the interface. The composite specimens again yielded mixed results. Two
of the samples behaved in an extremely homogeneous manner with the localized creep
values very similar to the global creep values. One of the two remaining samples
demonstrated maximum localized strain values at the interface, while the other sample

yielded maximum localized creep strain values near the middle of the specimen.

82

 

 

 

 

 

4.4

Mic:
0011'
0001

W35

r00
sur
ho:
ele
so

SO

4.4 FRACTURE SURFACES

Micrographs of the fracture surfaces were taken using an SEM. An attempt to correlate
outlying positions on the Dom plot by analyzing the fracture surface was made but
conclusive evidence was not found. However, a temperature/composition related trend
was apparent. Fracture surfaces (Figure 26a) from non-composite solder joints creep
tested at room temperature and low stresses, appear to be relatively smooth and ﬂat
(planar) with one or two big steps. With increasing stress, the fracture topography took
on a rougher appearance with many levels of fracture surface elevation (Figure 26b). The
room temperature composite samples (Figure 260) at higher stresses possessed smooth
surfaces which appeared to be stippled rather than smeared, suggesting that a more
homogeneous failure process occurred. Fractures surfaces of all specimens tested at
elevated temperatures (Figure 26d), were similar to the room temperature non-composite
solder at lower stresses. The ﬁ'acture occurs mostly on one plane, but the surface was
somewhat rougher, and more pronounced smearing was present than the non-composite

specimens crept at 25°C.

83

 

 

Fl

Fig

 

21.7 MPa. (Sample 038)

25°C, I =

Non-Composite Fracture Surface, T

Figure 26a.

of.”
a
. M.

 

23.5 MPa. (Sample 040).

25°C, 1 =

Composite Fracture Surface, T =

Non-

Figure 26b.

84

 

 

 

Fi

Fig

 

Figure 26c. Composite Fracture Surface, T = 25°C, 1: = 32.2 MPa. (Sample 052).

 

Figure 26d. Composite Fracture Surface, T = 125°C, 1 = 9.1 MPa. (Sample 036).

85

4.5 RESULTS OFLOCATION 0F FRACTURE EXAMINATION

The solder joints subjected to creep deformation in this investigation underwent fracture
in a consistent manner. In general, two types of fracture occurred during this
investigation, fracture through the middle of the specimen and fracture near one of the
interfaces. Fracture appears to be dependent upon strain-rate and the amount of porosity
present in the solder joint. If a large pore was present in the solder joint, approximately
20% of the fracture surface area, fracture occurred through the center or diagonally across
the thickness from one interface to the other. Solder joints which lacked porosity
fractured along or near one of the interfaces when subjected to high strain-rates (104 5").
However, when subjected to low strain-rates (10'8 s'l) fracture occurred through the
middle of the solder joint for the specimens examined. No correlation between
composite and non-composite solder was noticed, besides the fact that the composite
solder generally possessed a greater amount of porosity. Results for the different testing
conditions are indicated in Appendix E along with SEM micrographs of the fracture and

fracture surfaces.

86

4.6 RESULTS OF CRACKINITIATION TESTS

The results of this investigation were similar to the results in Section 4.5. In general,
fracture initiated along or near the interface. Three of the four solder joints tested
indicated an initiation at the interface. The fourth solder joint, fabricated from composite
solder, demonstrated crack initiation near the center of the solder joint. Upon rupture of
the specimen a large pore was present in the center of the joint, roughly 20% of the
fracture surface area in size. It should be noted that all tests in this particular
investigation were performed at high strain-rates, greater than 10'3 5". SEM micrographs
for these tests are included in Appendix E.

Small circular areas (Figures 27 and 28), varying in diameter from approximately 10
to 70 ,um were noticed to form near an interface after some deformation. These area were
present on one solder joint prior to testing and are believed to be the results of strain
during polishing. The areas appear to separate themselves from the rest of the strained
matrix. Upon further deformation, the areas behave in two different manners. In some
instances the areas tend to collapse into the matrix Figure 27c. The areas have also been
observed to ‘pop’ out of the matrix as indicated in Figure 27b and Figure 28. Although
these areas are a result of deformation, the exact cause is currently unknown to this
investigator. Since they tend to form near the interfaces, they may be linked to the

Cu68n5 intermetallics which naturally form there.

87

 

(C)

Figure 27. Formation of Circular Areas Under High Stress Conditions.
(a) Formation of Circular Areas.
(b) ‘Pop’ Out Upon Further Deformation.
(c) Collapse of Circular Area.

88

 

(b)

Figure 28. Solder Joint Micrographs Before (a) and After (b) Further Deformation.

89

CHAPTER 5

DISCUSSION

5.1 CREEP BEHA VIOR

A summary of all creep tests is provided in Appendix F. The creep behavior of the solder
joints is best summarized in the Dorn plot in Figure 19, which shows the normalized
shear strain-rate vs. the normalized shear stress. The solid line describes the non-
composite Sn—Ag data of Darveaux et al. and it is apparent that most of the present data
are in agreement with the trend established by this best ﬁt curve.

The addition of the Cu6Sn5 particles as matrix reinforcements signiﬁcantly improved
creep behavior at room temperature conditions. The room temperature composite data
were as much as 1000 times more creep resistant than the non-composite solder. At
85°C, the composite data were in close agreement with the Darveaux data, but the non-
composite creeped at least 10 times faster than the composite. At 125°C, the creep
resistance of the non-composite and composite solders were similar. In addition to
increased strength the composite solder appeared to deform in a more homogeneous

manner

90

5.2 HOMOGENEI T Y 0F DEFORMA TION

Determination of homogeneity of deformation can be examined using visual and
analytical methods. Observations of the creep strain-time history plots conveyed a good
sense of whether or not the sample deformed in a homogeneous manner. Local tertiary
creep generally commenced only in a limited part of the specimens where the strains are
highest. Analytically, by taking the ratio of the onset of global tertiary creep to the onset
of localized tertiary creep, a value resulted representing the homogeneity of deformation.
The closer the value was to l, the more homogeneously it deformed, since a ratio of 1
implies that the onset of local and global tertiary creep occurred at the same strain, which
in turn implies that local and global strain-time histories are the same.

Figure 29 shows how the onset of local tertiary creep varied with secondary strain-
rate. Room temperature creep of the non-composite solder at higher stresses tolerated
high local shear strains near a value of 3 before tertiary creep commenced, as compared to
the composite solder, where the beginning of tertiary creep occurred at local shear strains
in the range of 0.2-1.2. At lower stresses, the non-composite specimen exhibited local
tertiary creep at a strain very close to the global tertiary creep strain. Since the composite
specimens generally exhibited a local strain for the beginning of tertiary creep close to the
global strain at the beginning of tertiary creep, this indicates that the strain evolved more
homogeneously in the composite solder. It may also be an indication that strain evolves
with lower stress in the non-composite solder, as indicated by the lines that suggest these
trends on the various strain history plots. At elevated temperatures, localized tertiary

creep began at strains between 0.2 — 1.5 for all specimens.

91

Figure 30 illustrates how the strain homogeneity ratio varies with strain-rate. A trend
which suggests that homogeneity of deformation decreases with increasing creep-rate is
indicated for the room temperature non-composite data. For room temperature and
125°C deformation, the composite solder exhibited higher levels of homogeneity than the
non-composite solder. At 85°C, the trend is not as easily identiﬁed. The most
homogeneous deformation occurred in the composite at 125°C, but the onset of local
tertiary creep was at a lower strain than most other specimens as shown in Figure 30.
The large amounts of strain tolerated by the non-composite samples under a high rate of
deformation (lxlO'4 or greater) is indicative of the inaccuracies involved in accelerated
testing. The toleration of such a large amount of strain before the onset of tertiary creep
is unrealistic.

A possible relationship between porosity and tertiary creep was also investigated. A
plot was produced which compared homogeneity of deformation to the average percent
porosity present in each fractured soldered half (Figure 31). The plot suggests no
dependency upon porosity. Therefore, porosity was not a major variable on damage in

this Study,

92

Onset of Local Tertiary Creep vs. Secondary Creep Rate

 

 

 

    
  
 
 

 

 

 

  

 

 

 

3 r, I I l I I I I I I I I l I I I I I f I I I I I I I ! T I I T r I ‘1 I I I i l T I Id
5 r A Composite 25°C 1
to C , .......................................................................... j
(.3 2-5 L ‘ Non-Composnte 25°C .
3 : Composite 85°C j
g 2 _ ﬂ .......................................................................... z
to 3 I Non-Composite 85"C .
E; ; Composite 125°C 1
t L4 .. K 1' . ...... __
'93 1'5 r ° Non-Composite 125°C . j
_ t . A ; ,.
8 L ' i a
.._ , f g f
o _ f # , , ,; -— ._ :
£05 r #,##~0 """"" 3.; """""" V:
O : A 3 A V :
0 i— l l l l l I l Ii 1 l l l l l J_1i l l l l I l I | l L 41 J._l 1 L4 Ii 1 l l J l I l l-
10‘8 10‘7 10's 10'5 0.0001 0.001
Global Secondary Creep Rate
Figure 29. Onset of Local Tertiary Creep vs. Global Secondary Creep-Rate.
1 Homogeneity of Deformation vs. Secondary Creep Rate .

.o .o
(D (O
IIIIIIIII

.0
\l

lIIIIIIIIl

IIII]

.0
01
l

I I

Onset of Tertiary Creep Ratio
(Global I Local)
O
0)

 

I I I IITIIr I I IIIIII T I ITITIII
v
I l

   
 
 
   
 

 

Composite 25°C
Non-Composite 25°C
Composite 85°C

 

 

 

 

 

0.4 r """"""""""""""""""""
: V Non—Composite 85°C

0.3 E— 5.- Composite125°C
T , 0 Non-Composite 125°C ‘3 j

0'2 '— I lillllli 1 LIIXIF 1 r LJIIIII l I lllllli l 1L41J_L1~

10'8 10'7 10'6 10'5 10'4 10'3
Secondary Creep Rate
Figure 30. Strain Homogeneity Ratio vs. Global Secondary Creep-Rate.

93

Homogeneity of Defamation vs. Average “lo Ffprosity
I l 1 -\..ai T I

 

 

 

 

 

 

 

 

1 f T I T I ﬁI I I I I T‘T I I I T T F
.2 i v 2
‘5 ........... A ............ . ......................... Y ............................................................. _.
tr 0-3 , 5 , r) ,
§= * : A ‘
cg z ........................................ L .................... .2
if 06 r 02 ; A 025°C -
.5; : 0 V g 5 A NC25°C § _, Z
”'8 i—A. ....................... I ................... ‘ o .-..-.--..- ........... .4
T5 0.4 . 3 A ~ C850 ;
Ev . 9‘ ‘ § V NC85°C ‘ 1
o L I ' O o _l
g 02 __ .................. ' ....................... . .................... C125C L ....................... _
o . 3 g 0 NC125°C i

0 L 4 A; L I I L l I l J l i J I 1 I i l 1 l 1%]

0 0.05 0.1 0.15 0.2 0.25

Average % Porosity on Load Bearing Area

Figme 31. Homogeneity of Deformation vs. Average Percent Porosity.

94

5.3 ANALYSIS OF FRAC T URE

The solder joints subjected to higher strain-rates (10'4 3") all exhibited fracture along the
interface, unless porosity was present in the amount of at least 20% of the fracture surface
area. It has been observed that Cu6$n5 intermetallic particles naturally form at the Cu
substrate-solder interface (Figure 6). The presence of these unavoidable intermetallics
would have an embrittling effect on the microstructure as compared to the compliant
solder matrix. As a result, the solder joint is weaker at the interface and will tend to
fracture there unless signiﬁcant porosity is present throughout the solder matrix or a much
lower strain-rate is experienced. This manner of solder joint fracture was consistent with

expected results mentioned in Section 2.3.

95

CHAPTER 6

CONCLUSIONS

A criteria for damage based on the onset of tertiary creep was examined in shear creep
deformation of miniature solder specimens made from eutectic Sn-Ag solder with and
without 20v% 5-8 pm Cu68n5 reinforcements. The strain-time history was measured both
globally and locally and the range of local strain variation was quantiﬁed. The composite
solder exhibited a more uniform deformation, but tertiary creep commenced at a lower
creep strain than non-composite solders at room temperature. The local shear strain
where the onset of tertiary creep was observed increased with strain-rate for both solder
types, and the fracture surface features became more rough. At elevated temperatures,
tertiary creep commenced at a similar strain for composite and non-composite specimens,
and the fracture surfaces were similar. The composite material was more creep resistant
at lower temperatures, but it is not yet clear whether this beneﬁt outweighs the fact that
less strain is tolerated locally before tertiary creep commences. Porosity appeared to have

no effect upon homogeneity of deformation.

96

APPENDICES

97

APPENDIX A

CREEP MECHANISMS

98

 

 

 

 

 

 

 

 

 

 

Stress Regime n T p Q Mechanism
1 High 2 QL Naborro-Herring
Low 1 Low 3 QGB Coble
1 High 0 Q, Harper-Dom
5 High 0 QL Dislocation Climb (M-Type)
Intermediate 7 Low 0 Q j Dislocation Climb (M-Type)
3 High 0 Q1 Dislocation Viscous Glide (A-Type)
5 Low 0 Q i Dislocation Viscous Glide (A-Type)
High * H/L O Q l or Q, Power Law Breakdown

 

Adapted from Darveaux et al., page 539, and Yang et al., page 1137.

Deﬁnitions

 

Lattice Self Diffusion

 

L
QoB Grain Boundag Diffusion

 

 

Qj Dislocation Pipe Diffusion

 

 

1 Inter-Diffusion of Solute Atoms

 

 

Table l. Creep Mechanisms

99

 

APPENDIX B

DATA REDUCTION

100

Table 2 represents extracted data from an enhanced scratch displacement image which
was digitally captured via a traveling microscope. The data extraction was performed via
the Datathief shareware program. The ﬁrst column, ‘x position’, represents the thickness
of the solder joint, from one interface to the other, as it appeared on the computer monitor
under the Datathief program. Except for the ﬁrst and last values, the data are extracted at
equal intervals. The units are centimeters, i.e., the thickness of the solder joint as it
appeared on the computer monitor is represented by a value of (15.78 cm - 5.11 cm =-
10.67 cm). The actual thickness of the solder joints ranged from 50 to 100 m.

The remaining columns represent the position of the scratch line prior to loading of
the solder joint and during the creep experiment. For example, the column labeled ‘2940
sec’ represents data extracted from the image captured 2940 seconds into the creep
experiment. On each captured image, the data was extracted at the same x position
established in the ﬁrst column. It can be seen that as time progressed, the position of the
scratch remained relatively constant for the ﬁrst row of data, labeled ‘5.1 l ’, but changed

drastically in the last row, labeled ’15.78’.

101

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

102

x position Before Greg) 1080 sec 1920 sec 2940 sec 4020 sec 5700 sec
5.11 8.81 8.77 8.64 8.64 8.72 8.61
5.98 8.87 8.80 8.33 8.01 7.77 7.86
6.99 8.98 8.71 7.81 7.14 6.81 6.56
7.99 9.12 8.57 7.26 6.37 5.69 4.94
9.00 9.26 8.37 6.72 5.53 4.59 3.12
10.04 9.40 8.13 6.11 4.66 3.40 0.84
11.04 9.55 7.90 5.56 3.94 2.27 -l.16
12.05 9.69 7.64 5.10 3.19 1.29 -2.72
12.99 9.83 7.38 4.64 2.41 0.22 -4.48
13.99 9.94 7.12 4.27 1.84 -O.50 -5.63
14.97 10.11 6.92 3.84 1.32 -1.17 -6.73
15.78 10.19 6.74 3.55 0.89 -1.51 -7.28

Table 2. Scratch Displacement Data.

 

In order to simplify the data reduction process, the data representing the thickness of
the solder joint was normalized. The thickness of the solder joint was normalized by
subtracting the origin, 5.11, from each value in the ﬁrst column and then dividing each of
those values by the established solder joint thickness (15.78 - 5.11). This resulted in a
normalized solder joint thickness ranging from 0 to 1.

The scratch displacement data was modiﬁed or sealed according to the thickness of
the solder joint. For simplicity, the maximum displacement of the scratch, represented
here by a value of -7.28, was shifted to a value of 0 and then sealed to the solder joint
thickness. This was done by adding a value of 7.28 to every scratch position value, i.e.,
every number in every column with exception of the ﬁrst column. Each modiﬁed value

was then divided by the solder joint thickness (15.78 - 5.11). The results are seen in

Table 3.

103

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

104

x position Before Creep 1080 see 1920 see 2940 see 4020 see 5700 sec
0.000 1.507 1.504 1.492 1.492 1.499 1.489
0.082 1.512 1.506 1.462 1.432 1.409 1.418
0.176 1.523 1.498 1.413 1.351 1.320 1.296
0.271 1.536 1.484 1.362 1.278 1.214 1.145
0.365 1.550 1.465 1.311 1.200 1.112 0.974
0.462 1.563 1.444 1.254 1.119 1.000 0.760
0.556 1.576 1.422 1.203 1.051 0.895 0.574
0.651 1.589 1.398 1.160 0.981 0.803 0.427
0.739 1.602 1.373 1.117 0.908 0.703 0.262
0.832 1.613 1.349 1.082 0.854 0.635 0.154
0.924 1.629 1.330 1.041 0.805 0.573 0.051
1.000 1.636 1.313 1.014 0.765 0.540 0.000

Table 3. Normalized Position and Modiﬁed Scratch Displacement Data.

 

APPENDIX C

DISPLACEMENT, STRAIN, AND STRAIN-RATE HISTORIES

105

25°C, 31.1 MPa (Sample 052) 25°C, 31.1 MPa (Sample 052)
Composite Sn-Ag Composite Sn-Ag

 

      
    
 

 

 

 

 

 

 

 

 

 

 

Scratch Displacement Due to Creep 0 4 Localized Strain vs. Time
0.35 I I I I I I 1 I I I ' I I I I I I I I : a A I ‘ _ I . /
lion-Illaui'l ...F-l' 0419
E 0 3 Elapsed 0.3 .
Time ;
gozs , (s...) 0.2
«I . 7
- 0.2 «1- _.._o
3 ’ — a— 240 0.1
O 2880 .5
o 15 .
:9: v 4860 g o ' ‘
:g- 0.1 ’0’" 3::
2 0.05 . - 10380 '0'1
~- — 10920
0- ' .02
o 0.2 0.4 0.6 0.8 1 o
Normalized Position Across
Solder Joint Thickness
(a) (b)

 

  
       
  

 

  
 

  
 
      
 
          
     
      
       
      

 
 

   

    
   

 

 

 

 
 
 

‘ 0.0004
4x104 . 41‘104 0.0003
g 0 0002
2x104 ’ 2"o4 4 I
_ - 0.0001
Strain " » .3.
Rate "L. 0 2:: 0
(US) ’\ 00001
-2x10'4 “>’\\>\ -2x10“ '
‘ "% II 00002
.4x10“ “‘M l -4x10" -0.0003
‘Q 4 -0.0004
-6x10“‘ ” ” ¢ \\ 6x104
’ ’ < -0.0005
1 ” Q 10540
’ -0.0006
0 5 ‘ 6540
- < 2540 Time
”3335113" 0 (“m“)
(C)

Figure 32. Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 25°C.

106

25°C, 28.9 MPa (Sample 053) 25°C, 28.9 MP3 (Sample 053)
Composite Sn-Ag Composite Sn-Ag
Localized Strain vs. Time

 

 

   
  
 
  

 

  
  

 

 

 

 

 

 

 

 

 

 

Scratch Displacement Due to Creep 2 5
1 157 . ' ‘ ' ' ' ' i. ' ' ~ I 02 Normalized I I i ‘0 I
.' : __E,__ 0:4 Position g/‘(g
H 0.8 n_ Elapsed 2 : 06 .Through _ ”.711 ..... 3
5 Time - . 0’8 Solder Jomt // 5
g ‘ -i 0.9 Thickness ,/ i
EDS (Sec.) L gjzi 1'0 : : : "
g 1'5 Z Global """
Q — . .
l) 0.4 c — ‘
o E 1 ..........
'O u
.2 0.2 ‘0
:1:
3 0.5 ---------------------
E 0 ..........................
_0-2 I l i l 1 1 i l 1 l i l l 0 I v v
O O O O ‘70 O O O
0 0.2 0.4 0.6 0.8 1 o 8 8 8 8 ‘— 5 v (0
Normalized Position Across N " ‘° °° ‘— 1— ‘— ‘—
Solder Joint Thickness Time (sec)
(a) (b)
0.00035
0.0003
3x10-4 , , 3x104
0.00025
Sn . 2x10“1 , 2x104 0.0002
am . ,
Rate 1
(1 /S) 0.00015
1x104 ’ , 1x104

 

 

0.0001

 

 

0.00005

0

 

 

 

Normalized
Position

 

Figure 33. Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 25°C.

107

25°C, 25.8 MPa (Sample 054)
Composite Sn-Ag

 

()Sgratch Displacement Due to Creep

Modified Displacement
.0 9 .o .o .o 9 .
N (a & OI

_\

 

 

 

 
  
 
 

 

Empsed

Time

(KSec.)

—D—o

-B— 207.6
9441

v 1244

w‘ 1478
—t— 1590
r7 1753

 

 

  

1

0 0.2 0.4 0.6 0.8 1
Normalized Position Across
Solder Joint Thickness

2x105”

a

IMO”

I

Strain
Rate 0 ‘
(l/s)

‘1

 

-lx105’
1

Normalized 0_ 5
Position

Figure 34.

 

 

 

 

 

25°C, 25.8 MPa (Sample 054)
Composite Sn-Ag

+0
-------- 0.2
03

V 10

 

 

 

 

Global

 

Localized Strain vs. Time

Normalized 4
Position , 1
Through , j
Solder Joint 3

Thickness ******

§ :8:

2000

Time (ksec)

O

0

ID

>.
sift“ 2x105
1x105
0
-1x105
1634
1034
o (Kiloseconds)

(C)

108

(b)

 

20000
1 5000
1 0000

i 5000

-5000

-1 0000

 

 

 

 

Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 25°C.

25°C, 21.9 MPa (Sample 040) 25°C, 21.9 MP3 (Sample 040)

 

       

 

 

 

 

 

 

 

 

 

  
 
 

 

 

Non-Composite Sn-Ag Non-Composite Sn-Ag
1Sgratch Displacement Due to Creep 5 Localized Strain vs. Time
. I l l I l l l I l I l I | l I I l | l '_ 1 . I :
E i 4 f E 5 2 —:
g 1-2 i : i f 3 :
.1. » 3 — -------- 1 --------- l ........ ' C ............. _
g C E 1' :
3 0.8 ------- c 2 r ; , i
Q ._ I I 3 I
-o 0.6 E ; .;
a: H 1 , .. I
5.3 . . U) 2 3/1. - "' I
'80'4 --------------------- 3% —u—o1 ~o 0.8 D 3
5 0 .— —B— 0.4 + 0.9 'i
0-2 """"""" : 0 5 . 1.0 :
r V 0.7 Global _,_,_‘
0 -1
o o o o o
0 0.2 0.4 0.6 0.8 1 o 8 8 9, 8 8
Normalized Position Across ‘— " N “'
Solder Joint Thickness Time (sec)
(a) 0))
0.0025
. 2
0.002 x 00.002 0 00
0.0015
, -0001 i .
0.001 . 0.001
Strain
Rate
(Us) 0.0005
0 r , 0
-o 0005’ , 00005 0
1
Normalized '0-0005

 

 

 

 

Position ' Time

 

(C)

Figure 35. Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 25°C.

109

25°C, 19.9 MPa (Sample 038)
Non-Composite Sn-Ag
Sgratch Displacement Due to Creep

(I‘illIIllIllIiIlI

 

Elapsed
Time
(Sec)

 

 

1560
2820
4680
6660
7860
' ' l r' 8520

Q'.-{.

   

Modified Displacement
§

 

 

 

o r._1. @7g14@.$4

0 0.2 0.4 0.6 0.8 1
Normalized Position Across
Solder Joint Thickness

(a)

##I—LJ

 

 

Normalized
Position

Figure 36.

 

 

 

   
  
   
  

25°C, 19.9 MPa (Sample 038)

Composite Sn-Ag

chalizjegcl

1 Normalized 1
Position '
Through
Solder Joint
Thickness

 

 

 
 

Histories for Non-Composite Solder at 25°C.

6000
8000

Strain vs. Time 88

 

 

 

 

Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)

 

25°C, 19.8 MPa (Sample 039) 25°C, 19.8 MPa (Sample 039)

 

   
   

 

    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Non-Composite Sn-Ag Composite Sn-Ag
1Sgratch Displacement Due to Creep 4 Localized Strain vs. Time
_ FlllIlllIlllIllIIIl’T_
E 1
E El d 3
apse
8 0-8 _ Time
<0
3- (366.) C 2
.2 0-6 '8
Q h
.5 m
.9 0.4 1
t:
1:
2° 02
. 0 ,
0 ”Lillililililllii‘
0 0.2 0.4 0.6 0.8 1 o 8 8 8 8
Normalized Position Across N v ‘° °°
Solder Joint Thickness Time (sec)
(60 (b)
1‘
/ '\E
/j’
’ X/ i > r"
\\ ﬂ
8x104’ > ’ z8x104
/ \
/ /‘ “7
/ \ '
4x104’ /
/
Strain , 5
Rate
(Us)
0 ’0
-2x10“‘ , '2x‘04
1 8240
0.5 5240
Time
Normalized 0 2240 (Seconds)
Position
(C)

Figure 37. Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 25°C.

111

25°C, 10.5 MPa (Sample 009)

 

 

 

 

25°C, 10.5 MPa (Sample 009)

 

 

 

 

 

 

 

 

 

Non-Composite Sn-Ag Non-Composite Sn-Ag
0Sgratch Displacement Due to Creep 0 4 Localized Strain vs. Time
_ I l I I I I I I I I I I I I I I I I I - _‘
C 2
11.1 _
E . 2
0 _
U _
E -
Q .4
.9 r
O _—
'U _
.92 -
t _.
'U ' ‘
0 ﬂ
5 Lu» :
-o‘1 0 1 I L l J i l l l l 1 l l l 1 l 1 l 1 i l l l | —
0 0.2 0.4 0.6 0.8 1 o “:9 “’9 ”$3 ”52 .08
Normalized Position Across ‘- N N v “7
Solder Joint Thickness Tlme (590)
(a) (b)
0.00000012
7, I 7
12X10 12X10 0.0000001
8x108” ’ 8x108 3., 0.00000008
0.00000006
4xl0's ’ ' 4"“) 8 0.00000004
Strain 000000002
Rate 0 / , 0
(Us) 0
-4x10'8 / —-4x10"‘ 000000002
4377
. -0.00000004
Normalized

 
 

POSi‘lO" 1377 Time

 

 

Figure 38. Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)

Histories for Non—Composite Solder at 25°C.

112

 

25°C, 11 MPa (Sample 042)

 

 

 

 

25°C, 11 MPa (Sample 042)

     

 

 

 

  

 

 

    

 

 

  

 

 

 

Non-Composite Sn-Ag Non-Composite Sn-Ag
Scratch Displacement Due to Creep o 3 Localized Strain vs. Time
0.7 I I I I I I I I I I I I l' I I I T u 0 Normalized 0 19 a
_B__ 0.3 Posmon /
.. 0.6 . 0-25 A 08 Across , )L
5 v 09 Solder Joint : ,x;
E 0.5 0.2 r 1 0 Thickness j
s . 3
a ............... . __.
_ 0.4 0.15 _
.3 7': 2
90.3 ..0.1 “i
g a :a :
5'. I?» L
:3 0-2 0.05 _ _. . , :
o : :
5 0.1 0 7 ¥(:2 """"" 1 """"""" . """ 1
o -o.05 I—IlllIllllIll l'I11_.l_LIllllIJ_lllA
o 0.2 0.4 0.6 0.8 1 o °2 '9 ”$2 “’2 “’2 “’2
Normalized Position Across " N _ m " "’ ‘°
Solder Joint Thickness Time (890)
(a) (b)
m 0% 'mo 7 0.0000002
0.00000015
1x10'7‘ -1x10'7 0.0000001
Strain 000000005
Rate 0 ” 0 0
(Us) I,
-1x10'7‘ “1x104 -0.00000005
‘ 00000001
-2x10 7‘ 2-2x10 7 000000015
-0.0000002
.7_ _ _7
'3‘”) '3X10 000000025
I 00000003
- 0.5 2x103
Normalized Time
Posmon (Kiloseconds)
(C)
Figure 39. Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)

Histories for Non-Composite Solder at 25°C.

113

85°C, 10.8 MPa (Sample 033)
Composite Sn-Ag
0Sgratch Displacement Due to Creep
. IIIIIIIIIIIIIIIIIII"

Li

 

   

 

 

 

 

 

 

0.7 """""""""" r """ 2,2
_, ; Elapsed
E .- ' ‘ Time
80.5 7‘ —D—0
5! ,1 .2 —a— 120
30.4 r “r“- ';I-~g y‘_ 420
5 2 Of ’7 .-’f I ' 1 v 950
130.3 -.’- 9". - o 1680
3.3 . ' " ; —a2- 2100
502 — ..... ,2 . 2700
2° I . ,0 . I , .n 3240
0.1 ‘ ' r j , 1 gear,
‘70- : e 4620
o £4.‘9‘_I;L_L_L_L_J_1_LI__LL_1_I_LL_J_3
0 0.2 0.4 0.6 0.8 1
Normalized Position Across
Solder Joint Thickness
(a)
6xlO‘4
4x104
Strain

Rate 4
(“5) 2X10

-2x104
1

Normalized 05
Position

(C)

Figure 40.

85°C, 10.8 MPa (Sample 033b)
Composite Sn-Ag

  
 

  

--------10

        

‘ l . l
m; l. 1 r J 51.4 smleﬁle¥LJ_LLi_L

 

 

 

 

 

 

 

 

 

 

 

0.5 - 5'.
5 Global
E .
° fol/0‘0 a
I i 0.46 f e
_o.5 TILLIIJLLJI1114I111L11111
Time (sec)
09)
5.
= 0.0007
5 0.0006
6x10“1
0.0005
4x104 0.0004
0.0003
.4
2"”) 0.0002
0 0.0001
0
.4
'2‘” -0.0001
4820
-0.0002
3120
1820 Time
(Seconds)

Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)

Histories for Composite Solder at 85°C.

114

85°C, 10.4 MPa (Sample 0345) 85°C. 10.4 MPa (Sample 0345)
Composite Sn-Ag Composite Sn-Ag

Localized Strain vs. Time

I l I I I I l I I I I I I I I I "I if I

g070 3 59

 

 

S1cratch Displacement Due to Creep

IIIITIII'IIIII

 

Modified Displacement

 

 

 

 

 

 

o 0.2 0.4 0.6 0.8 1
Normalized Position Across

 

 

 

 

 

  

Solder Joint Thickness
(a) , (b)
0.003
0003
0002 0.002
2 0.001 0.001
Strain I,
Rate 0’ '0
( l/s) - 0
2-0001
-0.001
'-0002
- -0.003 -0-002
1680
Normalized '0'003

 

 

 

 
 

 

Position

0 (Seconds)

(C)

Figure 41. Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Composite Solder at 85°C.

115

85°C, 12.3 MPa (Sample 018b)
Non-Composite Sn-Ag
2Sfcratch Displacement Due to Creep

 

Modified Displacement

 

l I

IIIIIIIIIIII

  
 

III]

Elapsed
Time
(Sec)

 

 

—D—0
-El— 360
780
——t-— 1800
O , 1920
— - I-— 1980

 

 

 

 

0

0.2

0.4 0.6

0.8

1

Normalized Position Through
Solder Joint Thickness

 

Figure 42.

 

 

 

Normalized

Position

A
32

l\

\ \//X \\\

 

 

 

 

  
  

 

\

‘KVIA-V..- \\\

 

. wart-Ilse

 

(C)

85°C, 12.3 MPa (Sample 018b)
Non-Composite Sn-Ag
Localized Strain vs. Time

 

 

  

 

 

 

 

 

3 I
2.5
2
1.5 E
.E :
g 1 :— ------------------
‘D E
0.5 E.
0
—0.5 :—
_1 :ALI l l I l l I l I | l 1 I I I l l |
O O C) O
0 g; 8 8 8
1— ‘— N
Time (see)
(b)

  

 
 

(Seconds)

 

$20005

20.004

 

, 0.003

 

 

 

 

 

Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 85°C.

116

85°C, 16.3 MPa (Sample 025)
Non-Composite Sn-Ag

0Sagratch Displacement Due to Creep
_ IIIIIIIIIW’TITIIIWI

 

p placement

Modified Dis

0

 

Figure 43.

 

 

 

 

 

0.2 0.4 0.6 0.8 1
Normalized Position Through
Solder Joint Thickness

 

Normalized
Position

\

r

A\

V

‘VA‘IIA'!

1'1‘I1‘\\\Ii
\I 11““ ‘\\\

'I” ‘I‘.

i"

‘-‘"I-\-L‘\\V.\\

 

'C

III-I

i
I.

.I'AII'

i

 

(C)

85°C, 17.3 MPa (Sample 025)
Non-Composite Sn-Ag
Localized Strain vs. Time
'"Tﬁﬁ—‘T—I—T‘ WIﬁ—r—T—I—I—F‘TT—ﬁ‘ﬁI A

 

 

 

 

 

 

 

 

 

 

(Seconds)

Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)

Histories for Non-Composite Solder at 85°C.

117

 

85°C, 16.3 MPa (Sample 026)
Non-Composite Sn-Ag
Scratch Displacement Due to Creep

0-5 W i 1 W71

.0
5

Modified Displacement
.0
N

0
0 0.2

 

   
  

1.2
La_
1 ‘9—
03 2.
_,_
0.6

 

 

0.4 0.6 0.8 1 § §

Normalized Position Through
Solder Joint Thickness

 

 

 

 

 

 

 

 

 

 

 

 

Normalized

Through /

Thickness //
0555,13;
: -' :1

. 1‘13,..:'; ........

   

.. .0
SolderJoint gf/ j
U3

Time (sec)

 

Position .. . j/'_

85°C, 16.3 MPa (Sample 026)
Non-Composite Sn-Ag
Localized Strain vs. ije

 

 

 

 

 

 

(a) (b)
>e
0.012’< \ F0012
/
A < p 0.01
/
0.008 1 \ \2 0.003
/
A
\ \2 0.006
0.004 ’/ \ K, 0.004
Strain /
\,
Rate I 0.002
(Us) 0 '0
-0.02 ‘ ”0.002
1 430
Normalized 5 330
Position ' 230 Time
(Seconds)
0
(C)
Figure 44. Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)

Histories for Non-Composite Solder at 85°C.

118

 

125°C, 9.4 MPa (Sample 028) 125°C, 9.4 MPa (Sample 028)

 

 

          
 

 

 

 

 

 

 

   
   

 

 

 

   

 

 

 

Composite Sn-Ag Composite Sn-Ag
(Spratch Displacement Due to Creep 1 Localized Strain vs. Time
. I I I I | l l I I l I I l I I l I I
a 0.6 0.8
C
o
E 0.5
§ 0.6 —_
7g“ .s - :
._ (6 . ’ _ _
0 Q3 5 0.4 _ ; . , ' _
1:1 . .2 : . ~
u— ........................ , 2 , , _ . i . . A
:6 0 2 0.2 , 1’): ‘ ii . : I _
g 7,1? I L 7
0.1 I , . . _ 1 j I . ..
03° E E 0 0"”5" :' """" . "T
OI'IIOIIIIIIIIIJ_I_I_1|1' lllilliIiiIIlllIlllIlJi
o 0.2 0.4 0.6 0.8 1 o 8 8 8 8 § §
. . . N v to co ‘_ ‘_
Normalized POSltlon Across '
Solder Joint Thickness Time (sec)
(a) (b)
0.005
0004-” ,0_004 0.004
0.003
0.002— - 0.002 0.002
Strain
Rate 0 < 0 0.001
(l/s) ’ '
0
-0 002’ _. _0_002 -0.001
-0.002
Normalized -0.003
Position 320 Time
0 (Seconds)
(C)

Figure 45. Displacement (a), Localized Creep Strain (b), and Strain—Rate (c)
Histories for Composite Solder at 125°C.

119

125°C, 9.9 MPa (Sample 030)
Composite Sn-Ag
{Spratch Displacement Due to Creep
- E . . I’D—F

_|

 

        
     

  

a
E
g 7 El d
apse

3 0‘8 Time
2 (See)
a
D — a— 2340
'5 2820
é’ 0'4 ,5 3300
E , - ~- > . 3840
20.2 . {2 —:—4°2°

i I. - 1 ; e- 4140

0 :{LLLAIA l ‘._._._;_..I.1#,I I—ATTﬁi
0 0.2 0.4 0.6 0.8 1
Normalized Position Across
Solder Joint THickness
(a)
12x10 3’ ‘
> 4

  

\“‘

 

 

Normalized
Position

Figure 46.

 

125°C, 9.9 MPa (Sample 030)

 

 

Composite Sn-Ag
1 Localized Strain vs. Time
, WWFW ‘
_ 0 Normalized I.
1 -3— 03 Position
. 0:4 Across
. 05 Solder Joint _' i
0.8 4:2- 2 0.7 Thickness """ i 1 " 4
+ 0.9 ' ' ‘
Global

 

 

 

(b)

 

 

 

 

 

 

Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)

Histories for Composite Solder at 125°C.

120

125°C, 11 MPa (Sample 036)
Composite Sn-Ag

125°C, 11 MPa (Sample 036)

Composite Sn-Ag

Localized Strain vs. Time

 

S1c2ratch Displacement Due to Creep

  
 

 

' ' _ ' I I I i I I I 1'
Normalized ; -
Position
Across

l l |

 

 

 

 

 

 

Global Solder Joint ..l..j ..L.

Thickness

|1||Il

| 1 i l

 

 

 

 

 

 

_.D—0
I. --------- 0.3
E 1 . . Elapsed 15 7 '
g _ . . : HT Time '
80's --::— ----- - C3, ----- (390-) :
E - , . 3 \ ‘k — —u—0 1:
9- _\ —E— 1740 ._
£06 — E a? E i \t‘ 2220 g
1, Z 5 5 RR: v 2520 w
a, O 4 __ _ - _ _____ L0 A ______ X "WHO”... .. 2880
SE ' - ' * o - \—' —4—3120 0.
‘8 i ‘0‘ i 2072 3420
50.2 —— ~I— 3600
: 3 § 5 : . 3660
- i i i i 0, e 3720
O I 1 ll II] III III
0
0 0.2 0.4 0.6 0.8 1 o 8

Normalized Positioin Across
Solder Joint Thickness

0.0015 ’
Strain
Rate
(l/s)

0.0005 ’

 

 

 

 

 

Normalized
Position

  
 
 
 

0000
0000
O
0')

 

’ -0.0005

00

0
L00
(‘0‘?

Time (see)

(b)

 

 

 

 

 

Figure 47. Displacement (a), Localized Creep Strain (b), and Strain—Rate (c)

Histories for Composite Solder at 125°C.

121

125°C, 8.1 MP8 (Sample 037) 125°C, 8.1 MPa (Sample 037)

    

 
  
   

 

 

 

  
    

 

 

 

 
 
 
 
 
 
  

Composite Sn-Ag Composite Sn-Ag
Sgratch Displacement Due to Creep 2 5 Localized Strain vs. Tlme
— I I I I ~ -
..- , _ Elapsed 2
c _ II I . . I - Time .......
8 —D—O - .
2 —E— 600 1.5 . “E """" " I
n. .. 1080 E 3 '
g 1 , - v , , 1500 E
.0 : 2% 1920 a 1
q, : —t— 2520 ‘ - .
E ; , ,, 2940 : ' " 4 5
‘8 0.5 """""""""""""""" g . - g— 3350 0 5 3 _ : :
E It ' 4020 ' — ' g r \U
3 ~--o~~474o I . 5 § 1101
i I I 1 5700 — i l l l I i l I J 1 l l l i [ll l i l ‘
o ‘ I_l l l J i l l l i l l l I o l l A I l i
0 0.2 0.4 0.6 0.8 1 0 § § § § § §
Normalized Position Across " N m " ‘° ‘°
Solder Joint Thickness Time (see)
(a) (b)
0.0016- EE 0.0016 0.0016
‘ i 0.0014
0.00l2- \"0'0012 0.0012
_. \—
0.001
0.0008 - \— 0.0008 I
\ 0.0008

0.0006

 

 

 

 

    

 

 

 

0~°°°4‘ ‘ r0.0004
5"“ 0.0004
R ,
(13:0 0.0002
0
-0 0004 —-0.0004
-0.0002
Normalized .
Position 0 2080 Tim -0.0004
(Seconds)
(C)

Figure 48. Displacement (a), Localized Creep Strain 0)), and Strain-Rate (c)
Histories for Composite Solder at 125°C.

122

125°C, 9.7 MPa (Sample 024) 125°C, 9.7 MPa (Sample 024)

       
   
 
     
   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

      

 

 

 

Non-Composite Sn-Ag Non-Composite Sn-Ag
(Sgratch Displacement Due to Creep 1 2 Localized Strain vs. Time
' - - . 'i——. '_5 j"? ‘ D_ Normalized
. if?» 5:0,), f/ EIapIsed _ B— 8 1 POSlthl‘l A
H I '7 ,, "I l " '5’ ‘ Tlme 1 02 Across ___.r:,._|:l__
5 0.4 """"" (Sec) V 0 3 Solder Joint I
E —o— o o 0 5 Thickness -
3 —El— 420 0'8 —-— 1.0 """ F """ H ’T
¢_u 0.3 -------------- 720 ‘ ,1 _
a 1200 1: g
1’ 1440 g 0‘6 —
g 02 ____________________ —-— 1680 5 j
a, 2040 0 4 _
E l— 2340 _
1:: 2530 :
£04 """"""""""""""""" e~~2640 02
—.— 3000 3
. . ~ 3240
0 1 l l i l I l i l l l i 1 1 I 0 '
o o o o o o o
0 0.2 0.4 0.6 0.8 1 o 8 8 8 8 8 § §
Normalized Position Across ‘— " N N
Solder Joint Thickness Tlme (sec)
(a) (b)
0.0012 ’ ' 0.0012
0.0008 ’ \, 0.0008
0.0004 ‘ , 0.0004
Strain
Rate
(“8) 0 , -0
—0 0002 - L-0.0002
Normalized
Position 920 Time
0 (Seconds)
(c)

Figure 49. Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 125°C.

123

125°C, 10.9 MPa (Sample 027) 125°C, 10.9 MP3 (Sample 027)

  
 

 

  
  

 

 
    

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

    

 

 

 

Non-Composite Sn-Ag Non-Composite Sn-Ag
Scratch Dplaceent Due to Creep 1 4 Localized Strain vs. Time
’9/li' ’D—— Jig-=5" . I
" 1.2 . '0'77708 g;
20.8 -- ~ 0.2 --I— 0.9 - 3
a: lo ; a
E 1 Global " 1
8 . .4
20-6 0.8 ,_‘1
0. c _(
.2 '5 I ‘
a “0.6 L’
130.4 5 A
d) I
g 0.4 -/ _
o ,2 ~~~~~~~~~~~~~~~~~~~~ . ------- 5”
50 ‘- ’9' 3 2 3 0.2
x 1 E E «n ‘27:- .' 3 3 3.:
Orllllilllilllilll o lllilllIiIlllilLllillllillll'ila
o 0.2 0.4 0.6 0.8 1 0 § 3 § § 3 §
Normalized Position Across ‘— " N N ‘°
Solder Joint Thickness Time (sec)
(a) (b)
0.0025- ’ 7), 0.0025 0,0025
0.002
0.0015' \, 0.0015
0.0015
Strain
Rate 0.001
(l/s) , 0 0005
- 0 0.0005
-0 0005' — -0 0005 0
3080
Normalized
Position Time "0‘0005
0 (Seconds)

Figure 50. Displacement (a), Localized Creep Strain (b), and Strain-Rate (c)
Histories for Non-Composite Solder at 125°C.

124

APPENDIX D

GLOBAL SECONDARY STRAIN-RATE DATA

125

25°C, 31.1 MPa, Composite Sn-Ag (Sample 052)
Global Strain History
l ' l ' l ' I '

 

0.35 l I l l l r r
—e— Global Strain

 

I l l l l l l l
. I

 

 

0.3

0.25

0.2

0.15

Strain

0.1

0.05

 

Ill1lllllllllllllTITllllllll

 

 

 

o 2000 4000 6000 8000 1 104 1.2104

Time (sec)

Figure 51. Global Secondary Strain Rate for 25°C, 31.1 MPa, Composite Solder

25°C, 28.9 MPa, Composite Sn-Ag (Sample 053)
Global Strain History
.1...!l,.,ll

 

1 T T l I l l l I l I

~ —e—Global Strain f § § 5 g 5 _

 

 

 

 

Strain

 

 

 

 

0 2000 4000 6000 8000 1104 1.2104 1.4104 1.6104

Time (sec)

Figure 52. Global Secondary Strain Rate for 25°C, 28.9 MPa, Composite Solder

126

25°C, 25.8 MPa, Composite Sn-Ag (Sample 054)
0 3 Global Strain History
, , , , ,,,,,, I ,,,,,,,, ﬂ , , ,

 

 

T l i T T ‘Y T i Y T I T

 

 

 

 

 

 

 

 

Figure 53. Global Secondary Strain Rate for 25°C, 25.8 MPa, Composite Solder

25°C, 21.9 MPa, Non-Composite Sn-Ag (Sample 040)
Global Strain History
T I I , , , , 1 ,

 

 

 

 

 

 

Strain

l
..........................................................................................................

 

 

' Q
’— l I : a
’- I I I
0 2 l 1 l 1 l 1 l 4 1 i l l l l i 1 1 l l i l 1
O

 

Time (sec)

Figure 54. Global Secondary Strain Rate for 25°C, 21.9 MPa, Non-Composite Solder

127

25°C, 19.9 MPa, Non-Composite Sn-Ag (Sample 038)
Global Strain History
. T r ﬂ , I T . . l . I . q

 

T

 

1.6 l r I l

 

 

 

Strain

 

 

 

 

Time (sec)

Figure 55. Global Secondary Strain Rate for 25°C, 19.9 MPa, Non-Composite Solder

25°C, 19.8 MPa, Non-Composite Sn-Ag (Sample 039)
Global Strain History
, I ﬂ , , I l , a : , ,

 

 

 

 

 

 

 

 

009 T V T I l T
0.8 —9— Strain _
.E
E
U)
o 2000 4000 6000 8000 1 10‘
Time (sec)
Figure 56 Global Secondary Strain Rate for 25°C, 19.8 MPa, Non-Composite Solder

128

25°C, 10.5 MPa, Non-Composite Sn-Ag (Sample 009)
Global Strain History
r . I I a T a l .

 

 

0_3 i T i l I T r

 

 

 

0.25

0.2

0.15

Strain

0.1

0.05

 

ill llTl llll [ill llll llll ‘T‘
l l l l l

 

 

 

_0 05 1 1 l 1 i l 1 1 1 i J 1 m 1 i 1 I l 1 i l l 1 l

O
—L
_x
O
a:
N
.1.
O
C»
00
_L
O
a)
A
_x
O
o:
01
_L
O
a)

Time (sec)

Figure 57. Global Secondary Strain Rate for 25°C, 10.5 MPa, Non-Composite Solder

25°C, 11 MPa, Non-Composite Sn-Ag (Sample 042)
Global Strain History

 

 

0.25 l l T T J I l l r l

 

 

 

0.2

0.15

Strain

0.1

0.05

 

 

 

 

 

o 1106 2106 3106 4106 5106 6106

Time (sec)

Figure 58. Global Secondary Strain Rate for 25°C, 11 MPa, Non-Composite Solder

129

85°C, 10.8 MPa, Composite Sn-Ag (Sample 033b)
Global Strain History
. . I 1 , a f ! .

 

0.8 l T a T rT
—e—Strain '

 

l l ﬁi i l l r

J11!

 

 

0.7

 

0.6

WTfIllll

0.5

ﬂfi

l

Strain

0.4
0.3

—1

0.2

rjlrlllilIlTTTrT

 

 

 

 

0.1
2000 3000 4000 5000

O
_x
O
O
0

Time (sec)

Figure 59. Global Secondary Strain Rate for 85°C, 10.8 MPa, Composite Solder

85°C, 10.4 MPa, Composite Sn-Ag (Sample 034b)
Global Strain History
l T l I l i 17

l i T T l

 

0.7 l l l i i 1

 

 

 

l
lllll

Strain

 

 

 

 

O 500 1000 1500 2000

Time (sec)

Figure 60. Global Secondary Strain Rate for 85°C, 10.4 MPa, Composite Solder

130

85°C, 12.3 MPa, Non-Composite Sn-Ag (Sample 018b)
Global Strain History
a r , I Y , ,

 

I I T T

 

2.5 f l T I l r
l +suam § 3 g (

 

 

 

Strain

 

 

 

 

 

0 500 1000 1 500 2000

Time (sec)

Figure 61. Global Secondary Strain Rate for 85°C, 12.3 MPa, Non-Composite Solder

85°C, 17.3 MPa, Non-Composite Sn-Ag (Sample 025)
Global Strain History
, T I , , . , 1 1

 

 

0.35 , T T 1 ,
—e—Straln ‘

 

 

 

 

............................................................................................................

..................................................................................................................

........................................................................................................

 

 

 

Time (Sec)

Figure 62. Global Secondary Strain Rate for 85°C, 17.3 MPa, Non-Composite Solder

131

85°C, 16.3 MPa, Non-Composite Sn-Ag (Sample 026)
Glob

 

0.5 r

l T

i l I l l ' l l l I l l l l I l

 

 

aI Strain History
1 l ' l ' l l l

I. 17 l

 

 

 

‘ —e— Strain

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

.E
Q
5
100 150 200 250 300 350 400 450
Time (sec)
Figure 63. Global Secondary Strain Rate for 85°C, 16.3 MPa, Non-Composite Solder
125°C, 9.4 MPa, Composite Sn-Ag (Sample 028)
Global Strain History
0.5 T 1 T i , I . 7 , . , I , , 1‘ r 4
-—e— Strain ' :
.s _:
g T ‘ _
5 f ............................................... _‘
0 : l i L 1 i 1 i 1 i i l 1 L l :
0 200 400 600 800 1000 1200
Time (sec)
Figure 64. Global Secondary Strain Rate for 125°C, 9.4 MPa, Composite Solder

132

Strain

Figure 65.

Strain

Figure 66.

1.2

0.8 _

0.6

0.4

0.2

500

1.2

0.8

0.6

0.4

0.2

125°C, 9.9 MPa, Composite Sn-Ag (Sample 030)
Global Strain History

 

 

i l i I I I I l I I i I l I I I i i I i I I I I
1 ' l

‘ —e— Strain

l l l i I T l l i I T i l T
I

 

 

 

............................................................................................................

....................................................................

: § j is = 1.7'7e-04

 

 

l
lIlll4lllillli|iiJLil llillliillll

lei

 

 

 

2000 2500 3000 3500 4000

Time (sec)

1000 1500

Global Secondary Strain Rate for 125°C, 9.9 MPa, Composite Solder

125°C, 11 MPa, Composite Sn-Ag (Sample 036)
Global Strain History

4500

 

l T

—e— Strain

I 7 l T i l l I i l 7 l I l i I T I I T l i l l l l I T l T l I T l T |

 

 

 

 

......................................................................................................

..............................................................................................

...................................................................................................................

 

 

i I l i l I l l i l l l 1 i i 1 l l l i l l I i l l l l l i l l 1 CL

 

 

2000 2500 3000 3500

Time (sec)

500 1000 1500

Global Secondary Strain Rate for 125°C, 11 MPa, Composite Solder

133

4000

 

2 T

125°C, 8.1 MPa, Composite Sn-Ag (Sample 037)
Global Strain History
1 ' ‘ ' ‘ l ‘ ' ' ‘

T T j T l T T I l T T T I l l

 

—e— Strain

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

.s
92
5
0 1000 2000 3000 4000 5000 6000
Time (sec)
Figure 67. Global Secondary Strain Rate for 125°C, 8.1 MPa, Composite Solder
125°C, 9.7 MPa, Non-Composite Sn-Ag (Sample 024)
Global Strain History
0.5 I ' I i T I T l ’- 1 I l T l l I T l T l I l T T l I T i T i _I T T T j A
j —e— Straln ' ‘
0.4 - ------------------------------------------------------
03 ; ..................................................................................
.5 t
E _
(D 0.2 L
01 f .. . .............
O :_ 1 I i L 1 1 i..._1___1_ 1. 1 i l 1 1 1 i_1_..1___1_.___1__i 1 l 1 l '1 l 1 1 .1- ~
0 500 1000 1500 2000 2500 3000 3500
Time (sec)
Figure 68. Global Secondary Strain Rate for 125°C, 9.7 MPa, Non-Composite Solder

134

125°C, 10.9 MPa, Non-Composite Sn-Ag (Sample 027)
Global Strain History
W,,..

 

1 l i l l I I I l l I
. -9-— Strain 3 g i : _

l i I T r ‘r T I T T T V

 

 

 

 

Strain

 

...........................................................................................

 

 

 

0 500 1000 1500 2000 2500 3000 3500

Time (sec)

Figure 69. Global Secondary Strain Rate for 125°C, 31.1 MPa, Composite Solder

135

APPENDIX E

FRACTURE SURFACES AND PROFILES

136

 

 

 

 

 

 

 

 

 

 

 

 

Stress OGTC %
Sample Type °C (MPa) 78 (3'1) OGTC OLTC OLTC Porosity
052 Composite 25 31.1 1.33E-05 0.16 0.19 0.84 0.09

 

Figure 70. Fracture Surfaces and Proﬁle for 25°C, 31.1 MPa, Composite Solder.

137

 

 

 

 

 

Stress OGTC %
‘Sample Type °C (MPa) is (5'1) OGTC OLTC OLTC Porosity

 

 

 

 

 

 

 

 

 

 

 

053 Composite 25 28.9 4.15E-05 0.52 1.2 0.43 0.025

 

Figure 71. Fracture Surfaces and Proﬁle for 25°C, 28.9 MPa, Composite Solder.

138

 

 

 

Stress OGTC %
Sample Type °c (MPa) ,5 E1) OGTC OLTC OLTC Porosity
| 040 Non-Compos'_ite 25 21.9 4.85E—04 0.9 3 0.3 0.13

 

 

 

 

 

 

 

 

 

Figure 72. Fracture Surfaces and Proﬁle for 25°C, 21.9 MPa, Non-Composite Solder.

139

 

~

ill .i‘l.\ , ~ILI

OGTC OLTC OLTC Porosi

s")

(

 

Figure 73. Fracture Surfaces and Proﬁle for 25°C, 19.9 MPa, Non-Composite Solder.

140

 

 

 

Stress OGTC %
iSample Type °c (MPa) 75(s'1) OGTC OLTC 0ch Porosity
039 Non-Composite 25 19.8 8.42E-05 0.79 3 0.26 0.064

 

 

 

 

 

 

 

 

 

 

Figure 74. Fracture Surfaces and Proﬁle for 25°C, 19.8 MPa, Non-Composite Solder.

141

 

‘0 yr“, ,.

ca 1 i A" .

 

Sample Type

71. (8'1)

OLTC

 

 

Porosity

 

 

I 009 Non-Composite

 

 

 

7.13E-08

 

 

0.25

 

 

0.02

 

Figure 75. Fracture Surfaces and Proﬁle for 25°C, 10.5 MPa, Non-Composite Solder.

142

 

 

 

 

 

Stress OGTC %
Sample Type °C (MPa) 7-5 (s'1) OGTC OLTC OLTC Porosity
033 Composite 85 10.8 9.75E-05 0.5 0.9 0.56 0.21

 

 

 

 

 

 

 

 

Figure 76. Fracture Surfaces and Proﬁle for 85°C, 10.8 MPa, Composite Solder.

143

 

 

OGTC OLTC TC Porosity

 

Figure 77. Fracture Surfaces and Proﬁle for 85°C, 10.4 MPa, Composite Solder.

144

 

 

 

 

Stress OGTC %
Sample Type °c (MPa) ,5 (s‘) OGTC OLTC OLTC Porosity
018 Non-Compositg 85 12.3 2.82E-04 1.3 1.67 0.78 0.036

 

 

 

 

 

 

 

 

 

Figure 78. Fracture Surfaces and Proﬁle for 85°C, 12.3 MPa, Non-Composite Solder.

145

 

 

 

 

Stress OGTC %
Sample Type °c (MPa) ,5 (s‘) OGTC OLTC OLTC Porosity
[ 025 Non-Composite 85 17.3 1.24E-04 0.24 0.3 0.8 0.11

 

 

 

 

 

 

 

 

 

Figure 79. Fracture Surfaces and Proﬁle for 85°C, 17.3 MPa, Non-Composite Solder.

146

 

 

 

 

Stress OGTC %
Sample Type °c (MPa) ,5 (s4) OGTC OLTC OLTC Porosity
| 026 Non-Composite 85 16.3 8.4E-04 0.22 0.5 0.4 0.043

 

 

 

 

 

 

 

 

 

Figure 80. Fracture Surfaces and Proﬁle for 85°C, 16.3 MPa, Non-Composite Solder.

147

 

 

 

 

 

 

 

 

 

 

 

 

 

Stress OGTC %
Sample Type °c (MPa) ,5 (3'1) OGTC OLTC OLTC Porosity
028 Composite 125 9.4 3.92E-04 0.28 0.3 0.93 0.24

 

Figure 81. Fracture Surfaces and Proﬁle for 125°C, 9.4 MPa, Composite Solder.

148

 

 

 

 

 

 

 

 

 

 

Stress OGTC %
Sample Type °c (MPa) ,5 (s4) OGTC OLTC OLTC Porosity
030 Composite 125 9.9 1.77E-04 0.3 0.3 1 0.19

 

 

 

 

 

 

 

 

 

Figure 82. Fracture Surfaces and Proﬁle for 125°C, 9.9 MPa, Composite Solder.

149

 

 

 

 

Stress OGTC %
Sample Type °C (MPa) ,5 (3'1) OGTC OLTC OLTC Porosity

036 Composite 125 11 2.26E-04 0.11 0.15 0.73 0.14

 

 

 

 

 

 

 

 

 

 

 

Figure 83. Fracture Surfaces and Proﬁle for 125°C, 11 MPa, Composite Solder.

150

 

 

.‘4
with.

'l"

*5
*w

Jr.
-{'

i "r .
«rut

1
l

";‘-d..

 

 

 

 

 

 

Stress OGTC %
Sample Type °C (MPa) ,5 (3'1) OGTC OLTC OLTC Porosity
037 Composite 125 8.1 2.27E-04 1.1 1.29 0.85 0.024

 

 

 

 

 

Figure 84. Fracture Surfaces and Proﬁle for 125°C, 8.1 MPa, Composite Solder.

151

 

‘ . 5
_t-rr_ \\E' .

_-'IIIIF_ '

q

 

 

Stress OGTC %
Sample Type °C (MPa) ,5 (S1) OGTC OLTC OLTC Porosity

 

 

 

 

L 024 Non-Cothe125 9.7 1.0E—04 0.24 0.64 0.38 0.046

 

 

 

 

 

 

 

Figure 85. Fracture Surfaces and Proﬁle for 125°C, 9.7 MPa, Non-Composite Solder.

152

OGTC OLTC Porosity

 

Figure 86. Fracture Surfaces and Proﬁle for 125°C, 10.9 MPa, Non-Composite Solder.

153

APPENDIX F

SOLDER JOINT CREEP DATA

154

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Stress Secondary OGTC % Porosity of
Sample Type °C (MPa) Creep-Rate OGTC OLTC OLTC 1333““
ll ace
052 C 25 31.1 1.33E-05 0.16 0.19 0.84 0.091
053 C 25 28.9 4.15E-05 0.52 1.2 0.43 0.025
054 C 25 25.8 1.28E-07 0.14 0.3 0.47 NA
040 NC 25 21.9 4.85E-04 0.9 3 0.30 0.128
038 NC 25 19.9 8.54E-05 0.86 4 0.22 0.053
039 NC 25 19.8 8.42E-05 0.79 3 0.26 0.064
009 NC 25 10.5 7.13E-08 0.2 0.25 0.80 0.02
042 NC 25 11 3.14E-08 0.15 0.19 0.79 NA
033a C 85 7.4 3.05E-06 NA NA NA 0.205
033b C 85 10.8 9.75E-05 0.5 0.9 0.56 0.205
(+ 0.5 lb.)
034a C 85 8.1 4.71 E-06 NA NA NA 0.222
034D C 85 10.4 1.46E-04 0.4 0.7 0.57 0.222
(+ 0.7 lb.)
0183 NC 85 9.8 1.39E-05 NA NA NA 0.036
018b NC 85 12.3 2.82E-04 1.3 1.67 0.78 0.036
(+0.75 lb.)
025 NC 85 17.3 1.24E-04 0.24 0.3 0.80 0.11
026 NC 85 16.3 8.40E-04 0.22 0.55 0.40 0.043
028 C 125 9.4 3.92 E-04 0.28 0.3 0.93 0.242
030 C 125 9.9 1.77E-04 0.3 0.3 1.00 0.187
036 C 125 11 2.26E-04 0.11 0.15 0.73 0.14
037 C 125 8.1 2.27E-04 1.1 1.29 0.85 0.024
024 NC 125 9.7 1.03E-04 0.24 0.64 0.38 0.046
027 NC 125 10.9 1.37E-04 0.46 0.97 0.47 0.034
Deﬁnitions
+ () lb. Increase in load upon establishing a secondary creep-rate
C Composite Solder
NC Non-Composite Solder
OGTC Onset of Global Tertiary Creg)
OLTC Onset of Local Tertiary Creep

 

 

% Porosity of
Fracture Surface

 

Average percent of the two fractured load bearing cross-

sectional areas characterized by voids and porosity.

 

 

Table 4.

Solder Joint Creep Data.

155

 

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