Abstract: In this paper, SEM was used to study the interfacial diffusion behavior of Cu/Sn5Sb1Cu0.1Ni0.1Ag/Cu and Ni/Sn5Sb1Cu0.1Ni0.1Ag/Ni solder joints under isothermal aging at 150 ℃. The results showed that the morphology of intermetallic compound (IMC) at Cu/Sn5Sb1Cu0.1Ni0.1Ag/Cu solder joint grew from thin needle-shaped to rod-shaped and the thickness of IMC increased with aging time increasing. The main composition of IMC was (Cu,Ni)₆Sn₅. The appearance of IMC at Ni/Sn5Sb1Cu0.1Ni0.1Ag/Ni solder joints changed from horst shaped to relatively granular shaped, and the thickness of IMC increased. The main composition of IMC was (Cu,Ni)₃Sn₄. The growth rate of IMC at interface of Sn5Sb1Cu0.1Ni0.1Ag/Cu solder joint was 7.39 × 10⁻² μm²/h, and that at interface of Sn5Sb1Cu0.1Ni0.1Ag/Ni was 2.06 × 10⁻² μm²/h. Nickel-plating significantly changed the morphology of IMC, reduced the growth rate of IMC, inhibited the growth of IMC, and significantly improved the aging resistance.