Thermodynamic characteristics, microstructure and mechanical properties of Sn-0.7Cu-xIn lead-free solder alloy

The microstructure and mechanical properties of Sn-0.7Cu solder alloy and Sn-0.7Cu with 1.0–5.0 wt% indium addition were investigated in this study. With indium added into Sn-0.7Cu solder, the melting temperature decreased and the reaction temperature increased. Coarse grains of Cu6(Sn,In)5 intermetallic compounds (IMCs) were formed in the alloy matrix when the indium content increased to 1.0 wt%. Indium was evenly distributed in the β-Sn phase and Cu6(Sn,In)5 IMCs in as-prepared In-containing solder alloys. After isothermal aging, indium accumulated gradually in the Cu6(Sn,In)5 IMCs while decreasing its content in the β-Sn phase. Tensile test results indicated that the ultimate tensile strength increased obviously and that the elongation decreased significantly with indium addition. Sawtooths were observed in the stress-strain curves of all aged In-containing alloys. A model describing a coherent relationship between the β-Sn phase and the Cu6(Sn,In)5 phase was proposed to explain this phenomenon. The fracture surface of the Sn-0.7Cu alloy exhibited a ductile fracture mode while a mixed ductile-brittle fracture mode occurred when excessive indium was added into Sn-0.7Cu alloys.