Characterization of the Viscoelasticity of Molding Compounds 
in the Time Domain
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 in measuring these properties. However, it is necessary to consider the viscoelastic behavior when an accurate stress model is required. Viscoelastic properties of materials can be characterized in either the time or the frequency domain. In this study, stress relaxation experiments were performed on a molding compound in the time domain. A thermorheologically simple model was assumed to deduce the master curve of relaxation modulus using the time–temperature equivalence assumption. A Prony series expansion was used to express the material’s relaxation behavior. Two methods to determine the Prony pairs and shift factors were compared. After they were determined, the master curve at a reference temperature was shifted to every measured temperature for comparison with experimental data.