Thermal cycling simulation in electronic packages using molecular dynamic method

Abstract

Reliability under thermal cycle conditions is one of the main concerns in electronic packaging design. The cause of this concern arises from the mismatch in coefficient of thermal expansion between the different layers in a package leading to high interfacial stresses induced by thermal loading during fabrication and assembly. If these stresses exceed the limiting value, delamination will occur. The present study is focused on the reliability of the interfacial failure between conductive polymer (silver epoxy) and gold coated on sample die. The thermal cycling test was conduced with a given thermal profile. The adhesion strength between epoxy and gold-coated die subjected to at different thermal cycles was evaluated using the button shear test (BST). A simple molecular model of a bi-material system, which consists of epoxy and gold, was built to evaluate the interfacial energy of the gold-epoxy system. In order to dramatically reduce the computational time, the system was modeled with a limited number of atoms. A preset strain was applied to the model representing a forcing step as the epoxy material was pulled away from the gold substrate. Equilibration was conducted to relax the whole system before the next strain step proceeded. The procedure was repeated using different strains. The interfacial energy induced in the interface under different thermal cycles was evaluated. The trend of the interfacial energy indicates the change of the adhesion strength between epoxy and gold during the thermal cycling test. The simulation results can be benchmarked by the results of BST subjected to the same number of thermal cycles. The simulation result demonstrated that the basic molecular model could be used to predict reliability in the package design.