Prediction of effect of heat sink adhesive on mechanical reliability of a wire bonded plastic ball grid array package

Stress analysis of a two-layer wire bonded plastic ball grid array (WB-PBGA) package has been carried out to study the effect of adhesively bonded heat sink. A three dimensional finite element model of the WB-PBGA package and the printed wiring board (PWB) is solved numerically to predict the stresses induced and assess their impact on the mechanical integrity of the die and package during cool-down of the assembly after adhesive cure. Stresses induced in the assembly have been computed for a wide range of values. of the heat sink adhesive mechanical properties and adhesive Young´s modulus. Adhesive Young´s modulus is in the range 0.7 to 34.5 GPa, and its coefficient of thermal expansion (CTE) is in the range 3 to 150 ppm/K covering most common adhesives, epoxies filled and unfilled and also eutectic lead-tin solder. From the results of the analyses stresses in the die and heat sink adhesive have been evaluated for cohesive failure of these components. Variation of interfacial peel and shear stresses at the interface between the mold compound and the adhesive and between the adhesive and the heat sink have also been examined.