Characterization of toughness of epoxy based molding compound and its implementation in FEM code

Epoxy based molding compounds are typically used as an encapsulation material. Their role is to protect integrated circuitry from harsh environment. Typically, in standard IC packages we have materials with different coefficient of thermal expansion (CTE). While thermal cycling, the mismatch between the CTE of different materials can lead to delamination at the interface or crack in the bulk. In this study, we investigated experimentally the toughness of an epoxy based molding compound using compact tension specimen. Afterwards we explored different ways of modelling crack initiation and propagation using the two commercially available FEM codes Ansys and Abaqus. We looked at the accuracy of the prediction as well as the performance of each code. This is done to demonstrate the different characteristics of the programs and to get an overview of the applicability of the methods. Our methodology presented in this paper is divided into three parts. First, we experimentally characterized the toughness of the bulk of the epoxy based molding compound in the temperature range between -30°C until 150°C. Only tensile loading condition has been considered (mode I). Afterwards we have characterized the material by using standard analysis techniques such as TMA and DMA. This is required to create a material model for our simulations. Finally, we have created a FEM model and conducted numerical simulations using a 2D model. We investigated different techniques such as virtual crack closure technique (VCCT) and the extended finite element method (XFEM). Numerical simulations are conducted in both Abaqus and Ansys. In both codes the exact same mesh, loading and boundary conditions were used. Element topology in both codes are similar. We looked at the accuracy of the prediction (force-displacement graph), calculated fracture parameters, stresses in the area of the crack-tip and finally calculation times.