Effect of indium addition on interfacial IMC growth and bending properties of eutectic Sn–0.7Cu solder joints

The microstructure and growth behaviors of interfacial IMCs between the Sn–0.7Cu–xIn (x = 0–5.0 wt%) solder and Cu substrate under solid aging were investigated. Bending test was conducted to evaluate the mechanical properties of Cu/Sn–0.7Cu–xIn/Cu solder joints. The needle-like Cu₆(Sn,In)₅ IMCs formed at interface instead of Cu₆Sn₅ in as-soldered In-containing solder joints. During solid aging, indium addition had little influence on the growth of Cu₆Sn₅ IMCs, but strongly suppressed the growth of Cu₃Sn IMCs. In Sn–0.7Cu–5.0In/Cu couple, the maximum solubility of indium in Cu₆Sn₅ and Cu₃Sn was about 4.9 and 3.1 at.% respectively which was independent of aging temperature. The average composition of Cu₆(Sn,In)₅ and Cu₃(Sn,In) were given as Cu₆(Sn_0.89,In_0.11)₅ and Cu₃(Sn_0.88,In_0.12). After indium addition, the diffusion coefficients were found to be lower than that of Sn–0.7Cu/Cu couple and the activation energy for the growth of Cu₃Sn increased with the increase of indium content. Doping indium into Sn–0.7Cu solder improved the bending properties of solder joints. Interfacial cracks were suppressed effectively after long time aging with indium addition.