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Stress relaxation in plastic molding compounds

Viscoelastic materials for plastic encapsulated molding compounds invariably exhibit a time-dependent stress response to an imposed constant strain, which is called stress relaxation. Stress relaxation tests with molding compounds used to encapsulate microcircuits have been performed to measure the time dependent non-linear constitutive relation between stress and strain as a function of temperature and imposed strain. In an effort to improve the design process, a methodology using short time stress relaxation tests can be used to provide long-time design information. The goal of this study is to characterize the non-linear viscoelastic behavior of the encapsulated molding compound during environmental conditions that microelectronics devices have possibly experienced in their lifetime in order to help in developing new design including material selection and process and to inform that it is necessary to include the effect of curing shrinkage as well as viscoelastic: behavior for better estimation for stress and deformation such as warpage in device.