In this paper, a damage mechanics-based continuum theory is developed to provide a theoretical framework for multi-field problems involving moisture diffusion, heat conduction, moisture evaporation, void growth, and material deformation in a temperature and time-dependent process in encapsulated microelectronics devices. The analysis of moisture diffusion using normalized moisture concentration is re-examined under various conditions, and the applicability of the thermal-moisture analogy is discussed. Effective stress concept is introduced to consider the effect of vapor pressure in the development of a continuum mechanics framework. It turns out that the volumetric strains consist of three parts: thermal expansion (or contraction), hygroscopic swelling, and vapor pressure-induced volumetric strains. Void volume fraction is introduced as a field variable to describe the damage progression. The evolution of void volume fraction is governed by the continuity equation. Vapor pressure is considered as another internal field variable, which is related to moisture evaporation. A complete set of multi-field governing equations are developed. A simplified process, which allows the coupled problem to be solved sequentially, is defined. A bi-material assembly is used to illustrate the multi-field solutions using ANSYS.
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