Reliable Au-Sn flip-chip bonding on flexible prints

Flip-chip bonding, with an AuSn metallization system, has been successfully introduced for the mounting of integrated circuits (IC´s) on flexible polyimide prints. Since in most consumer electronics, and more specifically for hearing instruments, the usable volume is decreasing very rapidly, maximum miniaturization is achieved by using flip-chips. In order to avoid open circuits during reflow soldering of all other components, a high melting soldering process is required for the bonding of the IC´s. An additional advantage of the AuSn process is that the bumps do not completely melt, and a certain stand-off height is guaranteed. The bumps are deposited on top of the band pads and are bonded to copper tracks on a polyimide foil. The required tin is either deposited on the bump or on the copper tracks. Both AuSn soldering processes are performed by using pulsed heat thermode (gang) bonding. It has been found that the quality of the bonds depends on the microstructure formed in the bonding region. Energy dispersed X-ray analysis (EDX) measurements indicate that eutectic (80/20) Au-Sn or ξ´ phases are required for good quality bonds. To obtain these phases, the temperature at the interface and the initial amount of tin are optimized. As a consequence of a large thermal mismatch and a small stand-off height of the IC, the number of cycles to failure during temperature shock experiments is limited. The results are remarkably improved (by a factor of 20) by using an epoxy-based underfill material