Investigations of laser soldered TAB inner lead contacts

The results of tape automated bonding (TAB) inner lead bonding with a pulsed Nd:YAG laser are presented. Tapes with three metallizations (Sn, Ni-Sn, and Au) were laser soldered to bumps consisting of gold and gold-tin. The pull strength of laser-soldered TAB contacts was optimized by variation of laser power and reliability investigations were performed. An accumulation of eutectic 80/20 Au-Sn solder in the bonded interface results in a strong degradation due to Kirkendall pore formation in the ternary Cu-Sn-Au system. The application of a tape with a diffusion barrier on Ni inhibits this effect. However, during thermal aging these contacts show a strong degradation of pull forces which is attributed to the formation of brittle intermetallic compounds of the elements Ni, Sn, and Au in the contact area. The contacts show optimal pull forces and a minimal degradation after thermal aging