Effects of Molding Compound Cure on Warpage of Electronic Packages

Warpage is a critical issue for an electronic package molding process. Much work is done in the past to predict the warpage of a package after the cooling down from the molding process. However, there are many material models for the molding compound to predict warpage: Elastic and viscoelastic models are used, and there are even groups which use cure-dependent viscoelastic models. In this paper, we compare results obtained with the different models with each other. In our group we investigated the mechanical properties of a molding compound in detail. From this research it is concluded that the viscoelastic properties highly depend on the chemical conversion level. Furthermore, dilatometric studies showed that the bulk modulus and the coefficient of thermal expansion are independent on time. The thus obtained model which includes cure shrinkage is implemented in the finite element software ABAQUS by making use of user-subroutines (UMAT). The model is used in a theoretical study on the warpage of a double layered beam consisting out of a layer of molding compound and a layer of copper. This beam is cooled down form a temperature below T_g to room temperature with different cooling rates. Warpage of the beam is calculated with different models and a comparison is made. It turned out that there is a big difference between the different approaches.