Improvement on the microstructure stability, mechanical and wetting properties of Sn–Ag–Cu lead-free solder with the addition of rare earth elements

Ternary lead-free solder alloys Sn–Ag–Cu were considered as the potential alternatives to lead–tin alloys comparing with other solders. In this paper, microstructure and mechanical properties of Sn–2.5Ag–0.7Cu, Sn–3.5Ag–0.7Cu, Sn–3.5Ag–0.7Cu–0.1RE, and Sn–3.5Ag–0.7Cu–0.25RE alloys were researched. Coarse β-Sn grains were formed in Sn–2.5Ag–0.7Cu and Sn–3.5Ag–0.7Cu alloys and bulky Ag 3Sn intermetallics were found in Sn–3.5Ag–0.7Cu alloy. With the addition of trace rare earth (RE) elements the coarse β-Sn grains were refrained, at the same time, Cu 6Sn 5 and Ag 3Sn intermetallics were finer according to the adsorption affection of the active rare earth elements. Due to the fine and uniform microstructure, the tensile strength and elongation were improved. In addition, the wetting properties were also enhanced. All these results indicated that adding trace rare earth elements was an efficient way to develop new solders.