The microvoids formation and morphology of intermetallic compounds formed at the Sn-9Zn/Cu interface with and without Ag addition have been investigated by scanning electron microscopy and energy dispersive spectrometry and pull-off test. The microvoids are formed at the Sn-9Zn/Cu interface after aging at 180degreesC for 100 h and at the Sn-9Zn-3.5 Ag/Cu interface after aging at the same temperature for 1000 h. The formation of the microvoids is inhibited by Ag addition. After aging at 180degreesC for 1000 h, the planar Cu6Sn5 and scallop-shaped Cu6Sn5 are observed at the Sn-9Zn/Cu and Sn-9Zn-3.5 Ag/Cu interfaces, respectively. The decomposition rate of the Cu5Zn8 and the growth rate of the Cu6Sn5 at 180degreesC at the Sn-9Zn/Cu interface are determined as 2.22 x 10(-9) and 6.80 x 10(-10) m s(-1/2), respectively, and those at the Sn-9Zn-3.5Ag/Cu interface are determined as 8.61 x 10(-10) and 5.40 x 10(-9) m s(-1/2), respectively. The adhesion strength of the Sn-9Zn-3.5Ag/Cu interface increases from 5.76 +/- 0.77 to 14.19 +/-0.75 MPa when aging time increases from 0 to 1000 h and shows a better solder joint reliability than the Sn-9Zn solder alloy. (C) 2003 Elsevier B.V. All rights reserved.