Thermal stress analysis of composite encapsulants with a spherical adhesive interface

Semiconductor devices packaged in a rigid epoxy often utilize an internal compliant coating to minimize stresses transferred to the chip. Thermal stress analysis of such a composite encapsulating system with a spherical adhesive interface has been performed. The compliant layer encounters the highest tensile stress level upon cooling from the epoxy molding temperature due to the mismatch of thermal expansion coefficients and elastic properties within the composite. Indeed, this stress may readily exceed the tensile strength of the inner coating resulting in its fracture. In order to facilitate a systematic search for compatible plastics which assure package integrity, a useful figure of merit which is suggested by the analysis, the composite modulus, has been devised.