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

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 Cu6Sn5 and Cu3Sn layers. After being aged at 150 °C, both solder/Cu6Sn5 and Cu6Sn5/Cu3Sn interfaces moved toward the solder matrix, and the Cu3Sn/Cu interface migrated toward the Cu substrate. Compared with Sn, Cu was found to be the faster diffuser in Cu6Sn5 and Cu3Sn 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 Cu3Sn layer got close to that in Cu6Sn5 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.

► The original reaction interface is used as the reference for interface migration.
► Cu is the faster diffuser in Cu6Sn5 and Cu3Sn layers.
► The addition of Ag or Cu into solder suppresses the interfacial diffusion of Cu.