Reliable Au-Sn flip chip bonding on flexible prints

Au-Sn flip chip bonding is successfully introduced for the mounting of integrated circuits on flexible polyimide prints. Flip chip was used, since in most consumer electronics, and more specific for hearing instruments the useable volume is decreasing very rapidly. Since on the same flex print reflow soldering of other components is required, a high melting soldering process is preferred. An additional advantage of the Au-Sn 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 bondpads and are bonded to Cu tracks on a polyimide foil. The required Sn is either deposited on the bump or on the Cu tracks. Both Au-Sn soldering processes are performed by using pulsed heat thermode (gang) bonding. It is found that the quality of the bonds depends on the microstructure formed in the bonding region. EDX measurements indicate that for good quality bonds eutectic (80/20) Au-Sn or ζ´phases are required. To obtain these phases the temperature at the interface and the initial amount of Sn are optimized