Photoelastic and numerical investigation of thermally induced restrained shrinkage stresses in plastics

Photoelastic analysis has been used to measure the stress level within a polymeric material when stresses result from restrained shrinkage. These measurements indicate that for the particular epoxy used in this study up to two-thirds of the total birefringence that develops during cooling from the mold temperature (which is above the glass transition temperature) is due to frozen-in molecular orientation and does not represent Hookean elastic stresses within the polymer. Comparison of finite-element simulations with experimental results suggests that encapsulated microelectronic devices experience a lower level of stress than one would predict with a standard linear elastic analysis when the encapsulant has a glass transition temperature below the molding (or postcure) temperature. It is suggested that this is easily explained by the exponential decrease in modulus above the glass transition temperature and the possibility of stress relaxation.<>