Corrosion-Induced Mass Loss of Cu9Al4 at the Cu-Al Ball–Bond Interface: Explained Based on Full Immersion of Cu, Al, 
and Cu-Al Intermetallic Galvanic Couples
The disappearance of Cu9Al4 has been observed at the failed Cu-Al ball–bond interface in the standard humidity reliability test. Galvanic corrosion has been considered as the cause of the bond failure, yet no convincing argument has been provided to explain the preferential attack on Cu9Al4. Due to encapsulation, corrosion should proceed with the thin-layer electrolyte condition. The high ohmic resistance may constrain the galvanic corrosion between adjacent entities only. Thus, in this study, the galvanic corrosion between Cu and Cu9Al4, Cu9Al4 and CuAl2, and CuAl2 and Al were investigated using the zero-resistance ammetry. The results showed that the galvanic corrosion rate was the highest for the Cu9Al4-CuAl2 couple as compared with the other two galvanic couples. Also, the observed residual alumina between Cu9Al4 and CuAl2 and the associated internal stress build-up during the CuAl2-to-Cu9Al4 transition contributed to a higher crack propagation rate. Therefore, the failure first occurs at this interface. This failure then leads to a separation of Cu and Cu9Al4 from CuAl2 and Al. For the Cu-Cu9Al4 couple, Cu9Al4, the anode should corrode significantly faster due to the strong galvanic effect imposed by a larger surface area of Cu. In contrast, for the CuAl2-Al couple, the anodic corrosion rate of Al is slow as the galvanic effect imposed by the CuAl2 cathode is weak due to the small cathode surface area. As a result, Cu9Al4 disappeared from the failed ball–bond interface.