Viscoelastic Characterization and Simulation of Thermal Interface Materials

The thermal interface material (TIM) located above the chip in a package is a key factor in determining the heat dissipation of a chip. The deformation or warpage of the package significantly affect the performance of the TIM, which induces stress that causes failures such as cracking and interface delamination. Therefore, a detailed analysis of the stress state of the TIM in the package is essential for the thermal management of the chip. Most polymer materials used in package are viscoelastic, showing obvious correlation with temperature and time. However, in most finite element analysis (FEA) the polymer materials are usually treated as linear elastic, which may lead to the deviation of simulation from the actual situation. In this paper, we use inverse analysis method to obtain the viscoelastic constitutive parameters of TIM based on the Prony series derived from the experimental stress relaxation test. The viscoelastic constitutive of the TIM is applied to FEA for studying the cooling process in a flip-chip package. We find that the viscoelastic TIM may absorb more stress than the elastic material does, leading to less deformation from the bottom side of package transferring to the upper lid due to stress relaxation. In addition, the stress and strain distribution in the plane of viscoelastic TIM also shows characteristics different from that derived using linear properties. This work will help to improve the understanding of the actual mechanical behavior of TIM during the deformation of a package.