Microstructural investigations were performed on the interfacial reactions between eutectic SnIn solder and Cu substrate during reflowing at 433 K and solid-state aging at 373K. Cu2(In,Sn) was identified as the only intermetallic compound at the interface, which consists of two sub-layers with different morphology, a fine-grained sub-layer at the Cu side and a coarse-grained sub-layer at the solder side. During solid-state aging, voids were found between these two Cu2(In,Sn) sub-layers but not at the substrate interface, which was also attributed to the Kirkendall effect considering the different diffusion fluxes of Sn or In and Cu atoms in different sub-layers.

Intermetallic Compound Identification and Kirkendall Void Formation in Eutectic SnIn/Cu Solder Joint during Solid-State Aging. P.J.Shang, Z.Q.Liu, D.X.Li, J.K.Shang: Philosophical Magazine Letters, 2011, 91[6], 410-7