Interfacial versus Cohesive Failure on Underfill-Aluminum Interface-Effects of Interface Roughness

This paper is concerned with the interfacial adhesion and failure of underfill materials in flip-chip packages. Debonding of polymer-metal interfaces often involves both interfacial and cohesive failure. Since the cohesive strength of polymers is usually much greater than the polymer-metal interfacial strength, cohesive failure near the interface is usually desired to enhance the interfacial adhesion. Roughened surfaces generally produce more cohesive failure, therefore, are used commonly in practice to obtain better adhesion. In this paper a fracture mechanics model is developed that can.be used to quantitatively predict the amount of cohesive failure once the surface roughness data are given. An epoxy/Al interface was investigated using this fracture mechanics model. The predicted amount of cohesive failure as a function of surface roughness compares very well with the experimentally measured values. It is believed that this model can be extended to other polymer-metal interfaces.