Interfacial reactions of Sn-19Bi-19In solder on OSP and ENIG surface finishes during thermal aging

This study investigates the interfacial reactions and mechanical properties of Sn-19Bi-19In (SBI) solder on OSP and ENIG finishes under isothermal aging at 85°C for up to 500h. The microstructure analysis of the SBI solders on the two surface finishes (OSP and ENIG) reveals that the intermetallic compound (IMC) morphologies of these SBI/OSP and SBI/ENIG joints are fundamentally distinct from those of conventional SAC solder. Our findings indicate that the high In concentrations of SBI solder induce the formation of In-containing ternary IMCs [Cu ₆ (Sn,In) ₅ for OSP and Ni(In _0.3 Sn _0.7 ) ₂ for ENIG], which strongly contribute to the IMC morphologies. For the SBI/OSP joints, Cu ₆ (Sn,In) ₅ IMCs form a rod-type structure, deviating from the typical scallop-type growth of SAC/OSP joints. Similarly, for the SBI/ENIG joints, Ni(In _0.3 Sn _0.7 ) ₂ IMCs develop a stacked cuboid morphology, also different from that of conventional SAC/ENIG joints. The shear strength data obtained from the SBI/OSP and SBI/ENIG joints confirm that the distinct morphological evolution of the In-containing IMCs has a significant influence on the shear strength variations with thermal aging.