The effect of Ag addition on the structures of the intermetallic compounds (IMCs) and the adhesion strength of the Sn-9Zn-xAg solder alloy/Cu interface has been investigated by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry, transmission electron microscopy and electron diffraction. The XRD patterns show that the eta-Cu6Sn5, eta'-Cu6Sn5 and Cu5Zn8 are found at the Sn-9Zn-xAg/Cu interface. The Ag3Sn formed at the interface when Ag addition is above 2.5 wt%. Monoclinic and hexagonal Cu6Sn5 intermetallic compounds (IMCs) are formed at the interface close to the solder alloy. A high-resolution lattice image also shows the same result and the bi-structural Cu6Sn5 grows as a twin-like crystallite along the boundaries. The adhesion strength has an opposite relationship to the crystallite size of the Cu6Sn5 layer. (C) 2003 Elsevier Science B.V. All rights reserved.