Effect of Heat Sink Assembly on Mechanical Reliability of a Wire Bonded Plastic Ball Grid Array Package

Since the advent of the transistor and integrated circuit, the performance of electronic equipment has increased significantly while footprint of systems at all levels continues to decrease. Recently, the number of transistors on a high end microprocessor has exceeded a billion. All of the above has necessitated considerable improvement in cooling technology and associated reliability. Heat sinks play an important role in cooling technology by providing an increased surface area to dissipate heat. While there has been a great deal of work related to the thermal performance of heat sinks, there has not been a corresponding effort in studying the mechanical related reliability issues of the heat sinks. Previous work has been done to study the effect of adhesively bonded heat sink on the mechanical reliability of a two-layer wire bonded plastic ball grid array (WB-PBGA) package. Stresses induced in the package have been computed for a wide range of values of the adhesive mechanical properties and Young’s modulus. 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 been qualitatively assessed for propensity of interfacial de-lamination along these two interfaces. In this paper a stress analysis of described package is carried out to study the effect of heat sink assembly on the mechanical reliability of the package. A three dimensional finite element model of WB-PBGA package and 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 due to the heat sink assembly. Die and C4 interconnect stresses are examined to evaluate package reliability. Stresses induced within the die and C4 interconnect are examined for different heat sink materials and variation of force developed by heat sink attachments such as clip and screw type has been examined. Finally recommendations will be made regarding choice of heat sink material and clipping force for overall heat sink assembly design. The modeling utilizes a solid model for geometry creation and a finite element commercial program for simulation.