Delamination of copper/moulding compound interfaces in microelectronics packaging

A crucial aspect influencing the reliability of electronic devices is delamination occurring at the interface between different layers made of dissimilar materials. This type of failure is typically prompted by mechanical stresses arising because of the strong thermal gradients and a large mismatch in the thermal expansion coefficient of the materials involved. Device design may be greatly improved by providing reliable models, whose main challenge is the identification of suitable input parameters. In the present work we focused on the interface between copper and two different polymeric moulding compounds (MCs) in a power semiconductor package. MC is a composite material made of epoxy resin embedding a large volume fraction of silica micro-particles. A combined experimental/numerical method was developed to describe delamination. The surface and bulk mechanical properties of the constituents were first characterized experimentally by performing tensile, dynamic-mechanical analysis and scratch tests in the temperature range of interest. In the next step, bi-material laminae were tested using a fixed-arm peeling configuration at varying angles (45°, 90°, 135°). During testing extensive plastic deformation of the thinner Cu adherend occurred, whose contribution to the experimentally measured peeling force was dominant with respect to the interfacial fracture energy. A global energy balance approach, based on the testing protocol proposed by ESIS TC4 to become an ISO standard, was adopted to obtain the fracture energy associated with the peeling process, using the publicly available ICPeel software together with the previously obtained experimental data. As a result, consistent values for the fracture energy of both copper/MC interfaces were identified. These data were used to calibrate a 2D cohesive zone model implemented in the commercial finite element code Abaqus. The resulting FE model was successfully validated by comparing its predictions with additional fracture experiments performed on the same bimaterial laminae using a notched four-point bending configuration.