A study of delamination growth in the die-attach layer of plastic IC packages under hygrothermal loading during solder reflow

Abstract

This paper describes a comprehensive numerical study of the effects of many parameters on the propagation of delaminations in the die-attach layer using a finite element linear elastic fracture mechanics approach. An 80-pin plastic quad flat package of dimensions 20 mm × 14 mm × 2.7 mm was considered. The parameters investigated include the coefficient of thermal expansion (CTE) of the die-attach material, the thickness of the die-attach layer, the CTE of the pad material, and the size of the initial delamination (defect). The effect of the location of the initial delamination was studied by considering delaminations at the center, edge, and quarter-point locations along the pad/die-attach interface. The effect of swelling of the encapsulant due to moisture absorption during preconditioning at 85°C/85%RH was also considered. The combined effect of the hygrostress and thermal stress developed during solder reflow was obtained by suitably superimposing the respective stress intensity factors. The effect of the pressure of water vapor at the delamination was also investigated and found to be negligible for small delaminations. This study has shown that the propagation of delamination from an initial defect located at the edge of the pad/die-attach interface is unstable. Once delamination propagates, it will continue until the entire interface is completely delaminated.