The microstructure, melting properties and wettability of Sn–3.5Ag–0.7Cu–xIn lead-free solder alloys

In this work, Sn–3.5Ag–0.7Cu-xIn (SAC357-xIn) solder alloys were prepared using a high-frequency induction melting furnace and then treated under air cooling and water quenching, respectively. The evolution of the microstructure of the Sn–3.5Ag–0.7Cu–xIn lead-free solder alloys under the addition of indium and its effects on melting point, hardness and wettability were investigated. The results show that the addition of indium can remarkably affect the microstructures and melting properties of SAC357 alloys. When the content of indium increases from 0 to 5 wt.%, indium is conducive to the formation and growth of Ag3(Sn, In) and Cu6(Sn, In)5. The intermetallic compounds (IMCs) of In4Ag9 are first detected in the microstructure of SAC357-5In alloy. When the content of indium is 20 wt.%, β-Sn phases disappear and In0.2Sn0.8 IMCs become the main phase. With the cooling rate increasing, the phase compositions of as-prepared alloys are almost not changed, but the solidification structures obviously refine. Meanwhile, the melting temperatures of as-prepared alloys decrease and the microhardness values of the alloys increase. SAC357-20In solder alloy prepared by water quenching obtains the lowest melting temperature of 186.2°C and the maximum spreading rate of 2.71 cm2·g−1 at the soldering temperature of 280°C.