GOALI: Microstructural Evolution and Damage Nucleation Mechanisms during Thermomechanical Cycling in the Sn Phase of Lead-free Solder Joints

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TECHNICAL SUMMARY: As the electronics industry converts to lead-free solder worldwide, fundamental research on the physical and mechanical metallurgy (mechanisms of deformation, recovery, recrystallization) of tin (Sn) lags far behind its implementation. The substitution of Sn-based solders for Sn-Pb solder has led to failures in modes and locations never encountered before, and the fundamental cause of these failures is still not understood. Hence, lead-free solder joints lack a physically based (rational) foundation for reliability prediction. As the electronics infrastructure transitions to lead-free solder, high reliability products will become increasingly vulnerable. The research objectives of this program are to identify rules for operation of slip systems, hardening characteristics and recovery processes that can be implemented into crystal plasticity finite-element constitutive models. This analytical capability, combined with statistically large amounts of experimental characterization, will enable identification of fundamental microstructural evolution mechanisms and their interrelationship with dislocation generation and recovery in Sn based solders. A second major objective is to establish a database of observations on damaged and undamaged joints that also quantifies how locally active slip systems are correlated with crystal orientations, orientation gradients, grain boundaries, and recrystallization mechanisms so that interrelationships between these variables can be discovered. Success will be enabled by analytical capabilities present in the PI's group at MSU leveraged with Cisco's ability to provide high quality specimens manufactured in a repeatable industrially relevant process, augmented by the information gained in related research and development efforts. Insights gained will assist constitutive model development, and identify criteria that govern microstructural evolution and damage nucleation. These models will allow designers to evaluate worst-case microstructures in worst-case solder joint locations computationally, which cannot be done empirically.NON-TECHNICAL SUMMARY: Historically, more than half the failures in electronic systems can be traced to solder joints. Because fundamental research on the metallurgy of tin (the basis for lead-free solder) lags far behind its implementation, this failure rate will increase as the worldwide electronics industry transitions to environmentally friendly lead-free solder. At present, manufacturing decisions are based upon costly empirical studies that are limited to the product and service conditions tested. In this project undergraduate and graduate students will work together with industrial partners at Cisco Systems, Inc., to develop specimens, experiments, and analyze data that will enable physically-based 3-D material models to be built. Students and the PIs will work at Cisco via internships, and 2-4 weeks faculty visits in summers to develop synergistic research thrusts at the interface between industrial product development and basic science. Existing collaborations with colleagues at Max-Planck-Insititut fur Eisenforschung in Dusseldorf will contribute to analysis and modeling capabilities. These results will be publicized in archival journals, conferences, workshops, and K-12 outreach to help the public gain appreciation for how materials engineering affects electronic system reliability, and hence, modern life (why do electronics quit working?). As models are developed, they will be evaluated by electronic system design engineers at Cisco in order to establish the ability to computationally predict the likelihood of damage in particular designs.