Microstructural modeling of cyclic creep damage in tin-lead eutectic solder

A cyclic creep damage model for eutectic alloys is developed from a micro-structural perspective, in order to provide a rational methodology for predicting creep-fatigue behavior based on the material morphology. This study focuses on Tin-Lead eutectic solder, but the formulation is generic enough to be easily extended to other emerging lead free and high temperature solder materials.; One of the major hurdles in creating an analytical micro-structural damage model for heterogeneous materials like solder is the lack of an appropriate model to relate the overall or macro-scale far-field stress fields to local stresses at sites of micro-structural damage (i.e. void nucleation and growth sites). In this dissertation, an analytical micro-macro stress transition model is developed which takes into account the local microstructure, interactions with second phase particles, grain boundary sliding in the Tin matrix and its blocking by equiaxed second-phase Lead particles, and diffusional relaxation for typical micro-structures encountered in Sn-Pb eutectic solder. Wherever possible, a mechanistic approach is followed. The micro-macro stress transition model is implemented not only for static creep conditions, but also for cyclic loading. New insights into creep-fatigue behavior of solder are obtained by combining the micro-macro stress transition model with appropriate void nucleation and growth models (based on the mechanisms of grain boundary diffusion and power law creep). In particular, microstructural considerations are approximately incorporated in a model for void growth due to grain boundary sliding (as a suitable model is not available for this mechanism).; Most researchers use a-priori failure criteria or void coalescence rules as inputs to their damage law. In this dissertation, the failure criterion is mechanistically derived from principles of cavitation instability, and based on micro-structural processes such as degradation of material strength due to grain coarsening, constitutive softening and changes in yield strength due to increase in void fraction (due to cavitation).; The applicability of the complete micro-structural cyclic creep damage model for Pb-Sn eutectic solder model is demonstrated for a simple thermo-mechanical example representative of actual solder joint behavior in real life applications. New insights into the dependence of damage behavior of solder on the micro-structural features of are obtained from systematic parametric studies. Such information provides materials engineers with crucial information for optimizing the microstructures of new solders.; Based on the developed micro-structural cyclic creep damage model, suggestions are provided for the development of better macro-scale or phenomenological models which can be used for engineering applications, such as designing robust solder interconnects for micro-electronics applications; virtual qualification of reliability through simulation and predictive analysis; better design of accelerated life tests; and life consumption monitoring.