Abstract :
The maximum value of the von-Mises stress in the molding compound at the chip corner is suggested to be used as a suitable failure criterion for moisture-induced plastic packages of integrated circuit (IC) devices. This criterion is able to reflect the role of various geometric and materials characteristics affecting the package propensity to moisture-induced failures during hightemperature reflow soldering. It is suggested also that the von-Mises stress be determined from the constitutive equations which are a generalization of the von-Kirmin equations for large deflections of plates with consideration of thermoelastic strains. The generalized von-Kimman equations are applied to the underchip layer of the molding compound and consider thermal effects associated 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 calculated von-Mises stresses can be used, particularly, for the development of “Figures of Merit” that would enable one to separate packages that need to be “baked” and “bagged” (or “rebaked” and “rebagged”) from those that supposedly do not. The calculated stresses can be used also to judge whether the qualification test conditions for sufficiently reliable packages (say, thick packages with small chips) could be safely “derated” to an actual factory humidity profile. Finally, the calculated von-Mises stress can be helpful in the selection of the most feasible molding compound for the given package design. A more reliable (and more expensive) material might be needed in the case of a thin package with a large chip, while a low cost compound can be successfully employed in the case of a thick package with a small chip.
