The bonding interface features in thermosonic flip chip (FC) bonding are of interest to researchers in microelectronics packaging. In this study, a die with Al pads and eight gold bumps was bonded to a silver-coated pad on our lab test bench. The interface of the sample was analysed by using a high-resolution transmission electron microscope (HRTEM). For FC bonding parameters (e.g. ultrasonic power 2 W, bonding time 350 ms, heating temperature 150 degrees C, and bonding force 3.2 N), the thickness of atom diffusion at the Au-Ag interface is about 200 nm and that at the Au-Al interface is about 500 nm. In addition, ultrasonic vibration of FC bonding leads to the growth of the dislocation density in bonded materials and the formation of a cluster of dislocations at the interface. Therefore, short circuit diffusion plays a major rule during ultrasonic bonding when the temperature rise is relatively low. These observations will be helpful for further analysis. (C) 2007 Elsevier B.V. All rights reserved.