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

随着世界范围内电子工业向无铅焊料的转变，锡的物理冶金和机械冶金(变形、恢复、再结晶机理)的基础研究远远落后于其实施。锡基焊料取代锡铅焊料导致了以前从未遇到过的故障模式和位置，而这些故障的根本原因仍不清楚。因此，无铅焊点缺乏可靠性预测的物理(理性)基础。随着电子基础设施向无铅焊料过渡，高可靠性产品将变得越来越脆弱。该程序的研究目标是确定可用于晶体塑性有限元本构模型的滑移系统、硬化特性和恢复过程的操作规则。这种分析能力，结合大量的统计实验表征，将能够识别基本的微结构演变机制及其与锡基焊料中位错的产生和恢复的相互关系。第二个主要目标是建立一个关于受损和未受损关节的观察数据库，该数据库还可以量化局部活动滑移系统与晶体取向、取向梯度、晶界和再结晶机制之间的关系，以便发现这些变量之间的相互关系。成功将得益于密歇根州立大学PI小组的分析能力，利用思科的能力，提供以可重复的工业相关流程制造的高质量样品，并通过相关研究和开发工作中获得的信息加以增强。所获得的见解将有助于本构模型的发展，并确定管理微观结构演变和损伤成核的标准。这些模型将使设计者能够通过计算来评估最坏情况下焊点位置的微结构，这是经验上无法做到的。非技术摘要：从历史上看，电子系统中一半以上的故障可以追溯到焊点。由于对锡(无铅焊料的基础)冶金的基础研究远远落后于其实施，随着全球电子行业向环境友好型无铅焊料过渡，这种失败率将会增加。目前，制造决策是基于昂贵的经验研究，这些研究仅限于测试的产品和服务条件。在这个项目中，本科生和研究生将与思科系统公司的工业合作伙伴合作，开发样本、实验和分析数据，以便能够建立基于物理的3-D材料模型。学生和PI将通过实习在思科工作，并在夏季进行2-4周的教职员工访问，以开发工业产品开发和基础科学之间的接口协同研究。与杜塞尔多夫Max-Planck-Insiuitt Fur Eisenforschung的同事们现有的合作将有助于提高分析和建模能力。这些成果将在档案期刊、会议、研讨会和K-12外联活动中公布，以帮助公众了解材料工程如何影响电子系统的可靠性，从而影响现代生活(电子设备为什么停止工作？)在开发模型时，思科的电子系统设计工程师将对其进行评估，以建立通过计算预测特定设计中损坏可能性的能力。

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.