Investigations of laser soldered TAB inner lead contacts

The results of TAB (tape automated bonding) 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 were 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 of Ni inhibits this effect. 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. Laser soldering of Au-plated tapes to Au-Sn solder bumps is possible. The contacts show optimal pull forces and a minimal degradation after thermal aging. This is attributed to the formation of an intermetallic compound with a high stability