FAILURE CRITERION FOR MOISTURE-SENSITIVE PLASTIC PACKAGES OF INTEGRATED-CIRCUIT (IC) DEVICES - APPLICATION OF VON-KARMAN EQUATIONS WITH CONSIDERATION OF THERMOELASTIC STRAINS

The maximum value of the von-Mises stress in the molding compound at t he chip corner is suggested to be used as a suitable failure criterion for moisture-induced plastic packages of integrated circuit (IC) devi ces. This criterion is able to reflect the role of various geometric a nd materials characteristics affecting the package propensity to moist ure-induced failures during high-temperature reflow soldering. It is s uggested also that the von-Mises stress be determined from the constit utive equations which are a generalization of the von-Karman equations for large deflections of plates with consideration of thermoelastic s trains. The generalized von-Karman equations are applied to the underc hip layer of the molding compound and consider thermal effects associa ted with the thermal expansion (contraction) mismatch of the materials in the package, as well as with temperature gradients. The predicted stresses are in good agreement with experimental observations. The cal culated von-Mises stresses can be used, particularly, for the developm ent of ''Figures of Merit'' that would enable one to separate packages that need to be ''baked'' and ''bagged'' (or ''rebaked'' and ''rebagg ed'') from those that supposedly do not. The calculated stresses can b e used also to judge whether the qualification test conditions for suf ficiently reliable packages (say, thick packages with small chips) cou ld be safely ''derated'' to an actual factory humidity profile. Finall y, the calculated von-Mises stress can be helpful in the selection of the most feasible molding compound for the given package design. A mor e reliable (and more expensive) material might be needed in the case o f a thin package with a large chip, while a low cost compound can be s uccessfully employed in the case of a thick package with a small chip. (C) 1997 Elsevier Science Ltd.