Stress relaxation test of molding compound for MEMS packaging

Polymer based materials are widely used in electronic packaging. The molding compound, in particular, comprises a significant portion of the package with the purpose of protecting the chips from the environment. Material characterization of molding compounds, therefore, has been a critical issue in predicting the thermo-mechanical behavior and reliability of electronic packaging. One of the distinctive features of polymers is viscoelasticity, which refers to an intermediate behavior between a solid and a liquid. To characterize time and temperature dependent characteristics of polymers, various test methods have been utilized. Among those methods, the stress relaxation test using dynamic mechanical analysis (DMA) is widely used. However, there are no standards or guidelines for performing stress relaxation test on molding compounds with DMA. In this study, DMA stress relaxation tests have been performed with the molding compound. The initial value of relaxation modulus from DMA was compared with the Young's modulus from tensile test. The temperature effect on the stress relaxation test was studied to determine the appropriate temperature profile. The sample thickness and strain dependency were also investigated. Finally, recommendations for proper future testing are proposed.