Solid-state interfacial reaction of eutectic Sn3.5Ag and pure tin solders with polycrystalline Cu substrate

Intermetallic compounds (IMCs) that form at eutectic Sn3.5Ag/Cu and pure Sn/Cu interfaces during solid-state aging are comparatively studied in terms of their respective morphological formations, orientation evolution behaviors, and growth kinetics. During solid-state aging, all the interfacial Cu ₆Sn₅ grains evolve into a layer-type morphology, except for select grains that experience abnormal growth. This abnormal growth is caused by the preferential growth of the Cu₆Sn₅ at the grain boundary in solder matrix. Meanwhile, textured growth occurs in the Cu₆Sn₅ layer formed at the eutectic Sn3.5Ag/Cu interface. The morphology of each texture is determined by the initial joint preparation conditions and affects the growth of interfacial IMCs. The results reveal that Sn diffusion occurs faster along the [0001] direction of the Cu ₆Sn₅ crystal than along angles from 25 to 50 relative to the [0001] direction. Additionally, the effects of solder composition on the interfacial IMC growth are evaluated. The results indicate that Ag addition retards IMC growth upon aging by inhibiting diffusion of Cu.