Studies on Microstructure and Mechanical Properties of Sn-Zn-Bi-Cr Lead-free Solder

In this paper, Traces of Cr were added into Sn-8Zn-3Bi solder alloys to study the influences of Cr on mechanical properties and high temperature reliability of solder alloys by room temperature tensile test and high-temperature aging treatment. With observation of microstructure and fracture surface analysis, the strengthening mechanisms of Cr in solder alloys were discussed. Experiments show that: the microstructure was refined after traces of Cr added and two kinds of intermetallic compound (IMC) formed. Therefore the addition of Cr improved the plasticity of Sn-8Zn-3Bi alloy. The elongation reached up to 50.96% after 0.1% Cr added; and the fracture mechanism converted from quasi-cleavage fracture into ductile fracture. But after more than 0.3% Cr added, the brittleness of the alloy increased. During 0, 4, 9, 16 days aging, the mechanical properties of Sn-8Zn-3Bi-0.3Cr alloy were improving slightly with aging time. Through the microstructure analysis it was found that the Sn-Zn-Cr phase was transited after aging and Zn in alloy was consumed, so that primary Zn phase was reduced and microstructure was improved.