Abstract :
Tbe 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-K~irm~n equations for large deflections
of plates with consideration of thermoelastic strains. The generalized von-Kfirm/m 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, fo r 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. © 1997 Elsevier Science Ltd
