Influence of nonuniform initial porosity distribution on adhesive failure in electronic packages

Adhesives in electronic packages contain numerous pores and cavities of various size-scales. Moisture diffuses into these voids. During reflow soldering, the simultaneous action of thermal stresses and moisture-induced internal pressure drives both pre-existing and newly nucleated voids to grow and coalesce, causing adhesive failure. In this work, a nonuniform initial porosity distribution in the adhesive is assumed. The entire adhesive is modeled by void-containing cells that incorporate vapor pressure effects on void growth and coalescence through an extended Gurson porous material model. Our computations show that increasing nonuniformity in the adhesive's initial porosity f₀ drives the formation of multiple damage zones. Under the influence of vapor pressure or residual stresses, interface delamination becomes the likely failure mode in low mean porosity adhesives with nonuniform f₀. For high mean porosity adhesives, the combination of vapor pressure and nonuniform f₀ distribution induces large-scale voiding throughout the adhesive. Residual stresses further accelerate voiding activity and growth of the damage zones, resulting in brittle-like adhesive rupture.