Delamination control in electronic packaging using the energy method

Interfacial delamination, due to the presence of dissimilar material systems, is one of the primary concerns in electronic package design. The mismatch in the coefficient of thermal expansion between different layers in packages can generate high interfacial stresses when subjected to thermal loading during fabrication and assembly. Many of the failure criteria were developed to solve the problems with a precrack. However, in real electronic packages, the size and location of the cracks or/and delamination cannot be predicted. It is not easy to use the traditional fracture criteria to deal with complicated 3-D delamination problems. The delamination of copper leadframe/Epoxy Molding Compound (EMC) was selected in the study. The allowable stresses of the interface were evaluated by the Button Shear Test (BST). In this paper, the critical load acting on the upper part of the button shear sample was measured at a certain shear height and a finite element model was used to evaluate the interfacial stresses for different material system. An energy-based method is proposed by deriving the energy to initiate each of the tensile and shear modes of failure across the interfaces of the button shear test samples for the chosen interfacial material system. In order to benchmark the delamination failure criterion, two electronic packages, SOT #1 and SOT #2 were studied to investigate delamination in the soldering reflow process. Four kinds of interfaces in two packages were investigated under solder reflow respectively. The button shear test was done to obtain the critical force for each material system, and the allowable strain energy density UC of each material system was evaluated. By comparing the calculated interfacial energy density to UC of each material system, delamination at each interface can then be predicted. The predicted results were consistent with those from C-SAM.