A Rubbery Cure Shrinkage Model for Analyzing Encapsulated Assemblies

Electrical component assemblies are encapsulated to provide delicate parts with voltage isolation and protection against damage. The stresses generated by cure shrinkage, although typically smaller than the thermal residual stresses produced by cooling, can cause delamination or fractures in an encapsulant. Mechanical failures can be reduced by optimizing the encapsulation process or altering the component geometry. This paper documents the performance of rubbery constitutive equations in analyzing the stresses and strains generated by isothermal curing. A rubbery cure shrinkage model was installed in a finite element program and material inputs were obtained by measuring the rubbery shear modulus and volumetric shrinkage of a gelled epoxy during isothermal curing at several temperatures. A test was performed to measure the structural response produced by curing, and the data were compared to finite element predictions. The rubbery model performs well when the glass transition temperature of the curing epoxy does not exceed the cure temperature.