Properties and microstructure of Sn–0.7Cu–0.05Ni lead-free solders with rare earth Nd addition

Abstract

The effects of rare earth Nd on the melting behavior, wettability, microstructure and mechanical properties of Sn–0.7Cu–0.05Ni solders have been investigated in this paper. The melting temperature was found to be slightly influenced with rare earth Nd addition. Results of wetting balance tests showed that adding a proper amount of Nd could substantially improve the wettability of Sn–Cu–Ni solders by decreasing the wetting time and increasing the wetting force. The morphology evolution of solder matrix indicated that Nd contributed to the refinement and uniformity of the microstructure when added within an appropriate range. It was because the fine NdSn3 particles formed primarily in the soldering process would act as heterogeneous nucleation sites during solidification. However, excessive addition of Nd was inadvisable due to the formation of large amounts of brittle and hard NdSn3 compounds with bulk shapes. Meanwhile, moderate amount of rare earth Nd could inhibit the growth of interfacial (Cu,Ni)6Sn5 IMCs during soldering. Results of micro-joints strength test also discovered that Sn–Cu–Ni–Nd soldered joints possessed sounder shear strength, which resulted from the refined microstructure and the improved solders/Cu interfacial morphology. These above results evaluated that the optimum content of Nd added into Sn–0.7Cu–0.05Ni solders was about 0.06 wt%.