Galvanic corrosion behavior at the Cu-Al ball bond interface: Influence of Pd addition and chloride concentration
Abstract The ball-bond interface between Cu wire and Al pad is a layered structure consisting of Cu, Cu9Al4, CuAl2, and Al. Bond failure often occurs between the Cu9Al4-CuAl2 layer due to the high galvanic corrosion rate and high concentration of interfacial voids. Pd-coated Cu wire and molding compound with low chloride concentration are two common practices used to enhance the bond reliability. However, the mitigating mechanisms of these two methods have not been fully revealed. In this study, electrochemical techniques including zero-resistance ammetry and polarization were used to characterize the effect of Pd addition and chloride concentration on the bond corrosion and failure behavior. Results show adding Pd and reducing chloride concentration reduced the galvanic corrosion rate between Cu and Cu9Al4. Moreover, the galvanic effect disappeared completely with a high amount of Pd addition such as 9 wt% in an electrolyte with a low chloride concentration such as 1 ppm NaCl. By reducing the chloride concentration, the galvanic corrosion rate between Cu9Al4 and CuAl2 was reduced due to a lower anodic dissolution rate of CuAl2. However, Pd addition increases the galvanic corrosion rate between Cu9Al4 and CuAl2 due to a higher cathodic activity of Cu9Al4. But, Pd is known to reduce the intermetallic growth rate and the associated internal stress buildup. Therefore, the concentration of voids at the Cu9Al4-CuAl2 interface was reduced and lead to lower bond failure rate.