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 Cu-6(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 Cu-6(Sn,In)(5) and Cu-3(Sn,In) were given as Cu-6(Sn-0.89,In-0.11)(5) and Cu-3(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.