Bonding dynamics of compliant microbump during ultrasonic bonding investigated by using Si strain gauge

The bonding dynamics of a cone-shaped microbump during ultrasonic bonding are investigated by in situ measurements of the strain generated in a substrate using a piezoresistance strain sensor. The strain sensor is composed of a pair of p-and n-type piezoresistance gauges to extract strain components in the ultrasonic vibration along the plane parallel to the substrate surface and along the direction perpendicular to the surface. Flipchip bonding is performed at room-temperature. The time evolution of the strain generated in the substrate according to the load-up of pressing force and application of ultrasonic vibration is clearly detected. The softening of the bump metal during the application of ultrasonic vibration is clearly observed. Results of a comparative study between the bonding of a cone-shaped microbump and that of a flat-top microbump suggest mechanical stress concentration near the top end of the cone-shaped microbump, which results in the transformation of the crystal texture of the bump from grains to fine crystallites.