Vapor pressure and voiding effects on thin film damage

Plastic encapsulated microcircuits exposed to a humid environment are susceptible to thin film adhesive failures during reflow soldering. Recent computational studies on thin film adhesive failures have adopted void-containing cell elements based on an extended Gurson continuum constitutive model. Such an approach falls short of accurately replicating the ductile fracture process, particularly in cases where the film exhibits large elastic strains (e.g. polymeric materials). An alternative approach using discrete voids is capable of accurately resolving the growth and interactions of voids. In this work, thin film adhesive failures are studied using a model problem of a ductile adhesive joining two elastic substrates. The adhesive contains a centerline crack. Damage in the adhesive is confined to a single row of discrete voids ahead of the crack-tip. Under vapor pressure assisted voiding, large-scale adhesive damage is observed. Adhesive damage is further aggravated by the presence of large oblate voids. Our study offers some evidence of vapor pressure assisted void growth and coalescence as a key mechanism of popcorn cracking in IC packages.