Abstract: Interfacial behavior between Ag-plated copper wire and Cu pads as well as Sn coated Cu pads on PCB (printed circuit board) were studied respectively during parallel micro gap welding process. Results show that Cu in the two sides of interfacial layer diffuses into Ag layer significantly at the interfaces of Ag/Cu joint. For Ag/Sn/Cu joint, the Sn distribution is influenced by Cu that diffuses into the Ag layer. Therefore, Sn concentration is higher at the top and the bottom of interface, but lower in the middle of interface. Although the Sn layer is very thin, it induces crack generation around the Ag-plated copper wire and decreases the bonding strength. The bonding strength of Ag/Sn/Cu joint is lower than that of Ag/Cu joint. For Ag-plated copper wires, the Cu pads are better for parallel micro gap welding rather than the Sn coated Cu pads on PCB.