Abstract: Effects of rare earth element Pr on the intermetallics(ZMC) between Sn0.7Cu0.05Ni-xPr lead-free solder and Cu substrate were studied in this paper.And the possible influence mechanism was initially introduced.The experimental results showed that the interface morphology was greatly modified and became more uniform and flat due to the addition of Pr,and the effect of 0.05% Pr addition was more remarkable than that of 0.15% Pr.Adding Pr could reduce the driving force and time of the interfacial reaction to restrain excessive IMC growth.It was concluded that there was a synergistic effect on the interfacial reaction between Pr and Ni.The strength and ductility of the solder joint were greatly improved due to the refined microstructure because of the pinning effect on the migration of grain boundaries brought by adding Pr,and precipitation hardening strengthened by PrSn₃ particles serving as the precipitated phase.