Abstract: The influences of Cu-particle addition in Sn-9Zn lead-free solder powder(composite solder)on the interfacial reaction of Sn-9Zn/Cu joint were investigated under extended reflowing time conditions.The results show that Cu-particle addition in Sn-9Zn lead-free solder decreases dramatically the growing rate of the interfacial intermetallic compound (IMC) in Sn-9Zn/Cu joint, consequently which results in the reduction of the interfacial IMC layers and Kirkendall defect in the interfacial IMC layers.The thickness of IMC layers increases with increasing reflowing time and decreases with increasing Cu-particle content in Sn-9Zn lead-free solder.EDX analysis of IMC layer indicates that it is only composed of Cu -Zn intermetallic compounds there is no Cu-Sn intermetallic compounds to befound under existing test conditions.