Abstract: The interfacial microstructure in Sn-Zn-Nd solder joint aged at 360 hours, 720 hours, and 3 000 hours was investigated respectively. The results indicated that the transformation of interfacial intermetallic compound layer was dominated by the atom diffusion. After long-term aging, the composition of intermetallic compound layer transformed from Cu5Zn8 into a mixed Cu-Zn-Sn compounds. During the aging process, the diffusion-induced stresses also accumulated at the interface between solder matrix and Cu pad. When it reached a certain value, voids and fractures was formed in the intermetallic compound layer, which deteriorated the soldered joint properties.