The solid-state reactions at the interface of Sn-9Zn-xAg lead-free solders/Cu have been investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and energy dispersive spectrometer (EDS). The XRD pattern shows that gamma-Cu5Zn8, Cu6Sn5, and Ag3Sn intermetallic compounds (IMCs) were formed at the Sn-9Zn-xAg/Cu interface and the quantity increased during long-term aging. The SEM, TEM, and EDS results indicate that Cu interdiffused to the solder and reacted with Sn to form Cu6Sn5 at the interface after aging at 150degreesC for 750 h. Cu3Sn IMC formed in the matrix of the Sn-9Zn-1.5Ag solder alloy after aging. (C) 2003 The Electrochemical Society.