Effects of Cu,Bi, and In on microstructure and tensile properties of Sn-Ag-X(Cu,Bi, In) solders

Effects of minor additions of Cu, Bi, and In on microstructure, melting temperature, and tensile properties of Sn-Ag-based lead-free solders were investigated. It was found that the intermetallic compounds (IMCs) Ag₂In and Cu₆Sn₅ are formed in In- and Cu-containing solders, respectively. At low concentration, Bi dissolved in the Sn matrix and tended to precipitate pure Bi particles at the solubility limit of 4 wt pct Bi. The formation of large Ag₃Sn precipitates from the solder matrix was suppressed when alloying bismuth into the Sn-Ag alloy. The Bi addition resulted in a significant linear increase of the ultimate tensile strength (UTS) of solders, which is attributed to a solid-solution hardening mechanism. Solder strengthening due to In and Cu is less pronounced and attributed to a dispersion strengthening mechanism. The additions of Cu, Bi, and In all depressed the melting temperatures of Sn-Ag-based solders; however, In is the most effective one.