Investigations of failure mechanisms of TAB-bonded chips during thermal aging

Eutectic soldering and thermocompression bonding were investigated with regard to thermal aging mechanisms in tape automated bonding (TAB) chips. These methods require two different metallurgies: Cu-Sn-Au and Cu-Au. Effects such as pore formation which are due to the Kirkendall effect and the formation of ternary phases were investigated. The influence of the lead roughness and of copper recrystallization was shown by the use of electrodeposited copper and rolled annealed copper. The influence of these parameters on long-term reliability is summarized. Results of the study include the following: the predominant failure mechanisms in a pull test was lead fracture. The results from the pull test show that the values are mainly affected by aging effects in the leads. Metallographic examinations show different aging mechanisms at the bonded interfaces. The Kirkendall effect and pore formation is accelerated by the accumulation of tin in the interface between copper and gold due to the high roughness of the tape. The formation of pores for eutectic soldering can be avoided by a tape with reduced roughness, which minimizes the accumulation of tin during the bonding process. Compared with other contact methods for chips, TAB technology shows high reliability, with very low degradation even after very severe thermal aging at 200°C