Polymers are being widely used in microelectronic packages and viscoelasticity is one of the important characteristics of polymers. In practice, most package stress studies arbitrarily neglect the polymer viscoelasticity and use elastic models although viscoelastic constitutive relation is different from commonly used elastic-plastic behavior due to the fact that the characterization of viscoelasticity is complex and time-consuming. With this in mind, special response of thermal-mismatch stress induced by temperature excursion within polymers are expected to be very different than the linear elastic response, and better knowledge for thermomechanical stress behavior in polymers needs to be developed. In this paper, a bi-material plate consisting of a polymer film on an elastic substrate was used to demonstrate the significance of polymer viscoelasticity. The constitutive relation for a linear viscoelastic thermorheologically simple material (TSM) was adopted for the polymer layer. The effect of polymer viscoelasticity on stress evolution under different temperature profiles has been investigated.

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