Application of conservation integrals to cracks in the electronic packages

Abstract

At first, the popcorn cracking phenomena in plastic IC packages during reflow soldering are investigated by considering the heat transfer and moisture diffusion through the epoxy molding compound (EMC) along with the mechanics of interface delamination by assuming an inherent edge crack at the die pad/EMC interface and subsequent interface delamination under thermal and vapor pressure loadings. Heat transfer and moisture diffusion through EMC under die pad are analyzed by finite difference method during the preconditioning and subsequent reflow soldering process, and the amounts of moisture mass and vapor pressure at delaminated die pad/EMC interface are calculated as a function of the reflow soldering time. Using the finite element method and the methods of interface fracture mechanics, path independent energy release rate, stress intensity factor and phase angle are calculated and compared to the interface toughness which is assumed to be a function of the phase angle. Also, the effects of (i) thermal loading, (ii) crack face vapor pressure loading and (iii) mixed loading of both are estimated separately. The edge crack propagates toward the center leading to the delamination of the entire die pad/EMC interface most notably for the vapor pressure loading when thermal and vapor pressure loadings are applied simultaneously. The energy release rate increases parabolically with the crack length but proportionally with the vapor pressure while the interface toughness decreases with the crack length. Stress states near the crack tip were closer to mode II for thermal loading but closer to mode I for vapor pressure loading, and changed from mode II to mode I with the crack length for the mixed loading. Finally it was shown that thermal loading was the main driving force for crack propagation for small crack lengths, but vapor pressure loading played more significant role as the crack extended. Secondly, the interfacial fracture energies of the Cr/polyimide interface...