Abstract: 0.2% Zn element was added into Sn-3.5Ag Pb-free solder, and the interfacial microstructure at solder/Cu has been studied under as-soldered and as-aged condition. During soldering, the Cu-Sn intermetallic and its scallop-type structure are not changed by the small amount addition of Zn. However, in the Zn-contained solder joint, the scallops are found to be smaller but more compared with Zn-free solder joint. After thermal aging, the growth thickness, compositions and morphology of the IMCs at the interface are affected by the 0.2% Zn addition. In the Sn-3.5Ag-0.2Zn/Cu joints, the growth of IMC layer is evidently retarded and the Cu₃Sn IMC layer is completely inhibited. Furthermore, compared with the change from scallop-type to flaky-type morphology observed in Zn-free solder joints, scalloptype structure is still the predominant morphology in Zn-contained solder joints during solid-state aging. The inhibitation of small amount of Zn addition on the IMC growth and change of morphology is attributed to the retarding effect of interdiffusion coefficient of the IMCs.