Continuum Theory in Moisture-Induced Failures of Encapsulated IC Devices

Abstract

In this chapter, continuum theory and application to moisture-induced failures of encapsulated IC devices are reviewed. A single spherical void subjected to internal vapor pressure and thermal stress is investigated within the context of finite-deformation theory. There is a critical surface traction which defines the occurrence of unstable void growth and rupture. When elastic–plastic model is applied, the critical surface traction is about two to three times of the yield stress of the polymer material, which has an initial void volume fraction from 1 to 5%. When hyperelastic model is used, the critical stress is one order higher than that predicted by the elastic–plastic model. The theoretical results obtained from hyperelastic model are consistent with experimental observations. In addition, a model study is presented in this chapter on unstable void growth at interface. The problem is simplified to be equivalent to a void problem with increased initial void volume fraction. The increased initial void volume fraction, induced by the degradation of interface adhesion due to moisture, lowers significantly the critical stress. Therefore, the interfacial delamination might occur. For a soft film, however, it is possible that cohesive rupture forms first. Vapor pressure effects are incorporated into a continuum description of stresses and strains with the Gurson–Tvergaard’s model. The void evolution rate equations are extended to an interface with considerations of interface debonding due to temperature and moisture effects. A rigid-plastic model is introduced to analyze package bulge, and the limit pressure that leads package to crack is obtained. The relationship between vapor pressure, encapsulation thickness, and the length of die pad is established. Last, under the general framework of continuum mechanics using homogenization, the governing equations for a polymer with moisture are developed. The new framework provides a systematic approach to perform full-field analysis for moisture-induced failures of encapsulated IC devices.