A study of the interactions of molding compound and die attach adhesive, with regards to package cracking

Large surface mount plastic packages are susceptible to a phenomenon known as "popcorn" cracking, in which the unit cracks during vapor phase or infrared solder reflow processes. In this evaluation, interactions between the die attach adhesive and molding compound were examined, concerning moisture absorption and external cracking. A large range of materials chemistries underwent testing: various molding compound systems (biphenyl-based, multi-functional, and epoxy cresol novolac), along with several adhesive systems. The results showed molding compound had the dominant effect in preventing package cracking, and biphenyl-based compounds had superior cracking resistance over conventional epoxy cresol novolac and multi-functional types. Many of these compounds were developed for thin (1.4 mm or thinner) plastic packages, but for this study showed these improvements were also applicable to thicker packages. Contradictory to recent studies, die attach adhesive did not have a significant effect in improving cracking resistance or reducing moisture absorption among the various compound systems. However, optical and scanning acoustic micrographs showed evidence of weakened die attach layers, and cracking originating from that interface. Trade-offs concerning molding compounds stress level will also be examined.<
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