Interfacial microstructure evolution between eutectic SnAgCu solder and Al/Ni(V)/Cu thin films

The evolution of interfacial microstructure of eutectic SnAgCu and SnPb solders on Al/Ni(V)/Cu thin films was investigated after various heat treatments. In the eutectic SnPb system, the Ni(V) layer was well protected after 20 reflow cycles at 220 °C. In the SnAgCu solder system, after 5 reflow cycles at 260 °C, the (Cu, Ni)₆Sn₅ ternary phase formed and Sn was detected in the Ni(V) layer. After 20 reflow cycles, the Ni(V) layer disappeared and spalling of the (Cu, Ni)₆Sn₅ was observed, which explains the transition to brittle failure mode after ball shear testing. The different interfacial reactions that occurred in the molten SnAgCu and SnPb systems were explained in terms of different solubilities of Cu in the two systems. The dissolution and formation of the (Cu, Ni)₆Sn₅ phase were discussed on the basis of a Sn-Ni-Cu phase diagram. In the solid-state aging study of the SnAgCu samples annealed at 150 °C for up to 1000 h, the Ni(V) layer was intact and the intermetallic compound formed was Cu₆Sn₅ and not (Cu, Ni)₆Sn₅, which is the same as was observed for the eutectic SnPb system.