Due to the demand of cost reduction and higher input-output density in the integrated circuit (IC), Cu has become the material of choice over Au in the wire bonded electronic package. Although high-quality Cu wire bonds can be achieved by modifying the bonding conditions and process parameters, the long-term service reliability of the Cu-Al ball-bond interface in humid condition remains a major concern, especially to the automobile industry. At the Cu-Al ball-bond interface, two thin layers of intermetallics, Cu9Al4 and CuAl2, are sandwiched between the Cu wire and the Al pad. Accelerated humidity reliability tests showed Cu9Al4 disappeared, while Cu, CuAl2, and Al remained. The disappearance of Cu9Al4 cannot be explained based on the nobility ranking of the metallic entities present. Consequently, the corrosion-induced failure mechanism of the Cu-Al ball-bond interface remains unclear. This dissertation focuses on understanding the preferential attack on Cu9Al4 and its role in the ball-bond failure by using electrochemical and non-electrochemical techniques. Corrosion-mitigating mechanisms of current solutions can also be explained. Galvanic corrosion is considered as a major cause of bond failure due to the direct contact between dissimilar metallic entities at the ball-bond interface. Due to encapsulation, the galvanic corrosion should occur in a thin layer of the electrolyte at the mold-bond interface. The high ohmic resistance of this thin-layer electrolyte may constrain the galvanic corrosion at the contact interfaces between adjacent entities. Galvanic current density measurements showed the galvanic corrosion rate was higher between Cu9Al4 and CuAl2 than those between Cu and Cu9Al4 as well as between CuAl2 and Al. A crack should propagate faster along the interface between Cu9Al4 and CuAl2. A large area of residual alumina, which originates from the Al pad, is found between Cu9Al4 and CuAl2. Due to weak bonding between alumina and the intermetallics, the electrolyte can seep in more easily and accelerate the corrosion-induced crack growth. Due to the influx of Cu, CuAl2 is slowly transformed to Cu9Al4 during the annealing. Voids form within the residual alumina between Cu9Al4 and CuAl2 due to internal stress buildup. These voids between Cu9Al4 and CuAl2 can also facilitate the crack propagation. Therefore, the Cu-Al ball bond should fail at the interface between Cu9Al4 and CuAl2. The failure separates Cu and Cu9Al4 from CuAl2 and Al. For the Cu-Cu9Al4 couple, Cu9Al4 the anode should corrode much faster due to strong galvanic effect imposed by a larger area of Cu. For the CuAl2-Al couple, Al the anode should not be affected since the galvanic effect imposed by a small area of CuAl2 is weak. Therefore, Cu9Al4 appears to corrode faster than Cu, CuAl2, and Al.Pd-coated Cu wire and the "green" molding compound with a low chloride concentration are commonly used to mitigate the bond corrosion. Galvanic current density measurements showed that reducing the chloride concentration lowered the galvanic corrosion rate between Cu9Al4 and CuAl2 due to a lower anodic dissolution rate of CuAl2. On the contrary, Pd addition increased the galvanic corrosion rate between Cu9Al4 and CuAl2 due to a higher cathodic activity of Cu9Al4. But, Pd reduces the intermetallic growth rate and the associated internal stress buildup. The improved bond strength outweighs the increasing galvanic corrosion rate and leads to a lower bond failure rate.
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