Interdiffusion at the interface between Sn-based solders and Cu substrate

Abstract The migration of interphase interfaces in Sn-based solder joints was investigated by using the upper surface of unreacted Cu substrate (the original reaction interface) as a reference. Then a model was built to study the interdiffusion of Cu and Sn in Cu 6 Sn 5 and Cu 3 Sn layers. After being aged at 150 °C, both solder/Cu 6 Sn 5 and Cu 6 Sn 5 /Cu 3 Sn interfaces moved toward the solder matrix, and the Cu 3 Sn/Cu interface migrated toward the Cu substrate. Compared with Sn, Cu was found to be the faster diffuser in Cu 6 Sn 5 and Cu 3 Sn layers. The diffusion fluxes (DFs) of Cu and Sn in both intermetallic compound (IMC) layers decreased with the extension of aging period. The DF of Cu in Cu 3 Sn layer got close to that in Cu 6 Sn 5 layer. So did the DF of Sn. The diffusion of Cu and Sn can be suppressed by the addition of alloying elements (Ag 3.5 wt.% or Cu 0.7 wt.%), which can be indicated by various descending rates of the DFs of Cu and Sn in different joints.