The crystal growth, morphology and structure of the intermetallic compounds (IMCs) at the Sn-9Zn-xYAg/Cu interface have been investigated. During isothermal aging, the local strain and crystallite size of the IMCs vary with aging time due to the growth and decomposition of the IMCs. The Kirkendall voids form at the Sn-9Zn/Cu interface after aging at 180degreesC for 250 h but ate not observed at the Sn-9Zn-xAg/Cu interface at the same aging parameters. Ag is repelled from the monoclinic eta'-Cu6Sn5 which transforms to the hexagonal eta-Cu6Sn5 and reacts with Sn to promote the growth of the Ag3Sn. The diffusion coefficients of Sn in Cu6Sn5 are determined 5.76 x 10(-10) and 2.82 x 10(-10) cm(2)/s at 250degreesC when the Ag contents in the solders are 0.5 and 1.5 wt%, respectively. (C) 2003 Elsevier Science B.V. All rights reserved.