Effects of mechanical stress and moisture on packaging interfaces

When microelectronic packages fail in accelerated stress testing, it is often because of mechanical stress and/or moisture acting upon interfaces between polymeric adhesives or encapsulants and other package components such as solder interconnects, chip passivation, heat sinks, and chip carrier surfaces. Once the polymer loses adhesion, even if only in a small area, delamination at the interface can occur over time, leading to package failure. This paper describes an adhesion test methodology, using model materials and interfaces rather than actual packages, which has increased our understanding of the effects of mechanical stress and moisture and how they interact to induce adhesion failures. The effects of increasing severity of moisture exposures at elevated temperature and humidity conditions were measured using adhesion testing of epoxy/steel interfaces with and without adhesion promoters. An important aspect of this investigation pertained to the effect of the combination of mechanical stressing and exposure to moderate moisture conditions followed by solder reflow temperatures, again comparing the results for interfaces with and without adhesion promoters. Epoxy interfaces were weakened by the combination of mechanical stress and moisture exposure, thus allowing pockets of water to collect and cause delamination during subsequent solder reflows. Some insights are offered on how best to prevent this package failure mode, referred to as “popcorning,” caused by vaporization of moisture at the interface.