Solid/Solid Interfacial Reactions in the Sn/C7025 and Sn-3.0 wt%Ag-0.5wt%Cu/C7025 Couples

Solid/solid interfacial reactions between two kinds of lead-free solder (Sn and Sn-3.0 wt%-0.5 wt%Cu) and C7025 alloy (Cu-Ni-Si-Mg alloy) at 125°C were investigated in this study. The intermetallic compound formed at both kinds of interface was separated into two continuous layers, the Ni-rich layer ((Cu, Ni)
6
Sn
5
) and the Ni-poor layer (Cu
6
Sn
5
) in both systems, by a line consisting of the voids. The total thickness was increased as time increased. It was thicker in the Sn-3.0 wt%-0.5 wt%Cu/C7025 reaction couple than in the Sn/C7025 case at the reflowing stage due to heterogeneous nucleation from Ag atoms. However, it was thicker in the Sn/C7025 couple than in the other couple after aging resulting from the presence of the Ag3Sn phase. The lines consisting of voids were believed to be the results of different values of coefficient of thermal expansion of different regions.