Although polymer-based materials are widely used in microelectronics packaging and viscoelasticity is an intrinsic characteristic of polymers, viscoelastic properties of polymeric materials are often ignored in package stress analyses due to the difficulty of measuring this property. However, it is necessary to consider the viscoelastic behavior when an accurate stress model is required. Viscoelastic properties of materials can be characterized either in the time domain or frequency domain. In this study, stress relaxation experiments were performed on a molding compound in the time domain. Prony series expansion was used to express the material’s relaxation behavior. Thermo-rheologically simple model was assumed to deduce the master curve of relaxation modulus using the time-temperature equivalence assumption. Two methods were compared to determine the Prony pairs and shift factor.

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