Investigations of Laser Soldered TAB Inner Lead Contacts

Abstract

Tape Automated Bonding (TAB) is a modern technology which meets the requirements for micro‐connecting VLSI circuits. The limitations for gang bonding chips with high lead counts and reduced pitches are increased bond forces and induced mechanical stress. Laser soldering is an alternative for such contacts. Because microjoining of surfaces occurs via thermal energy from the laser beam, no mechanical pressure is necessary. Due to the optical properties of the laser beam and the possibility to reduce the laser spot, soldering of small pitches is possible. The results of TAB inner lead bonding with a pulsed Nd:YAG laser are presented. Tapes with three metallisations (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 optimised by variation of laser power and reliability investigations were performed. The metallurgy of laser soldering is different and more critical to long term reliability than that of gang bonded ILB‐contacts, even if identical tape and bump materials are applied. 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. But during thermal ageing 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 ageing. This is attributed to the formation of an intermetallic compound with a high stability. The Zeta phase acts as a diffusion barrier between the copper lead and the eutectic Au‐Sn solder.

Citation

Zakel, E., Azdasht, G. and Reichl, H. (1992), "Investigations of Laser Soldered TAB Inner Lead Contacts", Microelectronics International, Vol. 9 No. 1, pp. 7-13. https://doi.org/10.1108/eb044463