Pb-free solder-alloy based on Sn–Zn–Bi with the addition of germanium

We investigated the effect of Ge additions (0.1–0.5 at.%) on properties of Sn84Zn13Bi3 solder-alloys, specifically the anti-oxidation capability, electrical resistance, thermal expansion behavior, and mechanical strength. The Sn–Zn–Bi–Ge alloys have a melting point around 197 °C, and are not noticeably oxidized after soaking at 250 °C for 180 min under ambient air flow 100 ml/min. Coefficient of thermal expansion, tunable between 10 and 17 ppm/°C by the Ge addition, matches those of various metals such as Cu, Ni or lead-frames, leading to low thermal stress. Electrical resistivity of Sn–Zn–Bi–Ge solder-alloys, 10–15 μΩ cm, is much lower than that of Sn–37Pb solder. The micro-Vickers hardness, up to 400 MPa with 0.5 at.% Ge addition, is 150% that of Ge-free Sn84Zn13Bi3, or 300% that of Sn–37Pb solder. Moreover, it possesses better tensile strength, being 127% that of Ge-free Sn84Zn13Bi3, and 190% that of Sn–37Pb solder.