Prediction of delamination in a bi-material system based on free-edge energy evaluation

Adhesively bonded assemblies are widely used in micro- and opto- electronics. The mismatch in the coefficient of thermal expansion between the dissimilar material layers in the assembly can generate high thermally induced interfacial stresses during the fabrication and assembly process. These stresses concentrate at the assembly ends and often cause delaminations. Delaminations in plastic packages of IC devices typically occur at the interfaces between the epoxy molding compound (EMC) and copper lead frame. The most effective method for measuring the adhesion strength between the two materials is the "Button Shear Test"(BST), which is widely used by industry. In this analysis, the BST approach was employed to evaluate the interfacial adhesion strength between the EMC and the copper lead frame under loading applied to the EMC at different distances from the interface. The critical shear forces (forces-at-failure) were recorded and the measured data were used as input information for theoretical evaluations. Both the analytical and finite element models were developed to analyze the stress distribution at the interface. A simplified two-dimensional model for an approximate evaluation of the elastic interfacial stresses in a bi-material assembly is considered in the study. All the materials are assumed to be elastic, homogeneous and isotropic, and the engineering theory of bending of plates is used to evaluate the stresses and displacements. The analytically predicted peeling stress showed good agreement with the finite element results, while shear stress was reasonably consistent with the finite element data. The area occupied by the elevated peeling stress increases with the distance between the applied shear force and the interface ("shear height"), while the maximum value of the. peel stress decreases. The location of the positive and negative peel stress regions changes with the "shear height". The dominant area for the button shear test with different shear heights can be evaluated using these analytical results. The critical interfacial energy then can be evaluated as a material constant by using the dominant area and interfacial tensile stress. The interfacial energy can be used as a parameter to predict delamination occurring at the interface of the EMC and copper lead frame.