Wire-bond failure mechanisms in plastic encapsulated microcircuits and ceramic hybrids at high temperatures

Ceramic hybrids are the preferred solution when long-term high-temperature reliability is required, but standard plastic encapsulated microcircuits (PEMs) are an interesting alternative due to low price and high availability. Test vehicles with standard PEMs were subjected to thermal ageing at 150–175 °C. Six of eight vehicles failed after only three weeks at 175 °C, and the cause of failure was found to be microcracking at the interface between gold ball and aluminium bond pad giving rise to resistance increase. The intermetallic region was formed during high-temperature lead soldering and continued to develop during thermal ageing. The high-temperature performance of aluminium wire bonding to a selection of thick film metallizations on ceramic substrate was also investigated. Gold–palladium has previously been reported as a high-temperature solution, but we found that the mechanical strength of aluminium to gold–palladium (AuPd) degraded seriously at temperatures above 200 °C due to intermetallic formation. Aluminium to silver thick film plated with copper and nickel showed good mechanical strength and unaltered electrical resistance after four weeks thermal ageing at 250 °C.