Effects of rare earth Ce on microstructures, solderability of Sn–Ag–Cu and Sn–Cu–Ni solders as well as mechanical properties of soldered joints

In order to improve the properties of Sn–3.8Ag–0.7Cu and Sn–0.5Cu–0.05Ni solders, 0–0.1 wt.% of rare earth Ce was added to the base alloys, and the microstructures were studied. Meanwhile, the solderability of solders on Cu substrate was determined by the wetting balance method, the mechanical properties of soldered joints were investigated by the pull-force test and the fracture morphologies were analyzed. It is found that the new solder alloys possess better solderability than the base solder alloys, and the greatest improvement to the solderability of Sn–3.8Ag–0.7Cu–xCe is obtained with around 0.03–0.05 wt.% Ce, while the optimal Ce content is in the range of 0.05–0.07 wt.% for Sn–0.5Cu–0.05Ni–xCe solders. The results also indicate that the mechanical properties of soldered joints are improved with increasing Ce addition, with peak mechanical properties at Sn–3.8Ag–0.7Cu–0.03Ce and Sn–0.5Cu–0.05Ni–0.05Ce. The grain sizes of solder alloys are refined due to the addition of Ce, thus the mechanical properties of soldered joints are improved simultaneously.