Vapor pressure and residual stress effects on failure of an adhesive film

Surface-mount plastic encapsulated microcircuits (PEM) are susceptible to temperature- and moisture-induced failures during reflow soldering. Adhesive failures in PEMs are studied using a model problem of a ductile adhesive joining two elastic substrates. The polymeric adhesive contains a centerline crack. The adhesive film is stressed by remote loading and residual stresses. Voids in the adhesive are pressurized by rapidly expanding water vapor. The computational study addresses three competing failure mechanisms: (i) extended contiguous damage zone emanating from the crack; (ii) multiple damage zones forming at distances of several film thicknesses ahead of the crack; and (iii) extensive damage developing along film–substrate interfaces. The second failure mechanism is found in low porosity adhesives, while the first is dominant in high porosity adhesives. The first is also the likely failure mode when voids in the adhesive are subjected to high vapor pressure. The third damage mechanism is operative in low porosity adhesives subjected to high residual stress. In general, both residual stress and vapor pressure exert pronounced effects on failure modes. Vapor pressure, in particular, accelerates voiding activity and growth of the damage zone offering insights into the catastrophic nature of popcorn cracking.