The crystal growth of intermetallic compounds at the interface of Sn-9Zn-xAg/Cu (x = 0, 0.5, 1.5, 2.5 and 3.5 wt%) lead-free solders has been investigated by optical microscope, X-ray diffractometer, scanning electron microscope, and energy dispersive spectrometer. The results show that the more Ag is added to the Sn-9Zn alloy, the more Ag-Zn compounds form in the solder matrix in which Zn reacts with Ag and hinders the diffusion of Ag. Ag dissolves in CU6Sn5 and changes the lattice constant Of CU6Sn5 as formed in the liquid-solid reaction. After long-term aging, the dissolved Ag is repelled from Cu6Sn5 and forms Ag3Sn with Sn at the interface, but the crystallite size Of CU6Sn5 decreases from 301.2 to 189.5 nm when the Ag content increases from 0 to 3.5 wt%. (C) 2002 Elsevier Science B.V. All rights reserved.