The disturbed state concept (DSC) presented here provides a unified and versatile methodology for constitutive modeling of thermomechanical response of materials and interfaces/joints in electronic chip-substrate systems. It allows for inclusion of such important features as elastic, plastic and creep strains, microcracking and degradation, strengthening, and fatigue failure. It provides the flexibility to adopt different hierarchical versions in the range of simple (e.g., elastic) to sophisticated (thermoviscoplastic with microcracking and damage), depending on the user’s specific need. This paper presents the basic theory and procedures for finding parameters in the model based on laboratory test data and their values for typical solder materials. Validation of the models with respect to laboratory test behavior and different criteria for the identification of cyclic fatigue and failure, including a new criterion based on the DSC and design applications, are presented in the compendium paper (Part II, Desai et al., 1997). Based on these results, the DSC shows excellent potential for unified characterization of the stress-strain-strength and failure behavior of engineering materials in electronic packaging problems.
