Effect of Au on interfacial reactions of eutectic SnPb and SnAgCu solders with Al/Ni(V)/Cu thin film metallization

Effect of various amounts of Au on the interfacial reactions of SnPb and SnAgCu solders and Al/Ni(V)/Cu underbump metallurgy were investigated after high temperature storage and multiple reflows. During high temperature storage, the presence of Au varied the formation of intermetallic compounds at solder/UBM interfaces from a binary Cu₆Sn₅ phase to a ternary Cu-Sn-Au or quarternary Cu-Sn-Ni-Au phase. The phase transformation was a diffusion controlled process, which was influenced by Au amount, aging temperature and solder composition. The effectiveness of the diffusion barrier layer of UBM was also weakened, since Ni and Sri could diffuse and react through a ternary or quarternary phase. Up to 500 hours at 150°C all samples showed a ductile failure inside solder under the ball shear test, which indicated a relatively good bonding between the solder and UBM. From these results it was concluded that detrimental effect of Au on the stability of Ni was not as significant as that of Ni/Au substrate metallization. Ni from substrate finish may also play an important role in the interfacial reaction between the solder and Al/Ni(V)/Cu UBM.