Optimization of shrinkage and expansion properties of epoxy encapsulants

New resin and filler types now allow epoxy encapsulants to have higher filler loadings and lower expansion coefficients. Reducing encapsulant shrinkage in a wire bonded plastic package has the advantages of lower die stress and less warpage, but if shrinkage is too low, early corrosion failure may occur during humidity tests. To explain why, it has been proposed (1) that compression forces, due to encapsulant shrinkage, can prevent the water which is needed for corrosion from accumulating at the bond pad surface. The objective of this paper is to quantify this corrosion mechanism. The Nernst equation plus heat of adsorption data are used to predict that the minimum compression stress, normal to the bond pads, which is needed to prevent water accumulation and corrosion, should be at least 20 times the partial pressure of water vapor at the test temperature. For example, at 121°C/30 psia, the compression stress from the encapsulant should be at least 600 psi. Shrinkage and expansion data, measured for 5 epoxy encapsulants and 7 epoxy and polyimide die attach adhesives, were used to calculate die stress for each adhesive-encapsulant combination. All 12 materials had low ionic extractables (C1^-<20 ppm) but major differences had been found in failure times in pressure cooker tests. The corrosion results correlate with the calculated Z-axis stresses and support the prediction that the minimum stress should be ⩾20 P_w. Die surface stresses depend on the adhesive, adhesive thickness, adhesive cure and die size as well as on the encapsulant