FRG: Modeling of Microstructure Evolution & Thermomechanical Fatigue in Lead-Free Solder Joints

本基金的重点是表征四种Sn-Ag基焊料的显微组织演变和随之而来的机械性能变化，这些变化是由热机械疲劳引起的。目标是开发基于微观结构的现象学模型，可用于预测故障的发生。实验计划使用组件级焊点样品，可以有效地监测微观结构和机械性能变化，作为实时热机械循环次数的函数。热机械循环包括在高温下的短时间和在低温下的长时间，以便评估低温蠕变的影响，这是电子工业中常用但非保守的加速测试实践所不考虑的条件。试样还将在工业加速试验设施中循环使用，以检验加热和冷却的不同速率和幅度如何影响结构损伤积累。取向成像用于评价晶体取向和错取向的结构变化。确定了第二相的尺寸分布和形状。机械性能的测量使用纳米压痕和破坏性测试技术，包括应力松弛、蠕变破裂和恒定应变率测试，以评估相应的失效应变。建模是基于Dorn/Garafolo sinh蠕变塑性方程与阈值应力的结合，阈值应力基于颗粒的生长速率动力学和由应力/应变历史修改的微观结构。为了解决环境问题，并适应几个国家正在实施的影响美国电子产品全球营销的政策，正在寻求在电子应用中使用含铅焊料的替代品。共晶锡银焊料是一种推荐的替代焊料，用于更高温度的使用条件。在恶劣的使用环境中使用的焊料，如汽车、航空航天和军事环境，可能会经历-50到180摄氏度的热漂移。由于焊接组件和基板之间的热膨胀不匹配，这种热循环会导致严重的热应力。这种热机械疲劳过程会损坏焊点，超过70%的电子系统故障是由于焊点失效造成的。＊＊＊

This grant focuses on characterizing the microstructural evolution and consequent changes in mechanical properties that result from thermomechanical fatigue for four Sn-Ag based solders. The goal is to develop microstructurally based phenomenological models that can be used to predict the onset of failure. The experimental plan uses component-scale solder joint specimens that allow effective monitoring of microstructural and mechanical property changes as a function of the number of real-time thermomechanical cycles. The thermomechanical cycling includes short times at elevated temperatures and long times at cold temperatures to allow the effects of low temperature creep to be evaluated, a condition not considered by the commonly used but non-conservative accelerated testing practice of the electronics industry. Specimens will also be cycled in industrial accelerated testing facilities to examine how different rates and magnitudes of heating and cooling affect structural damage accumulation. Orientation imaging is used to evaluate structural changes in crystal orientation and misorientation. The size distribution and shapes of second phases are also determined. Mechanical properties are measured using nanoindentation and destructive testing techniques that include stress-relaxation, creep-rupture, and constant strain rate tests to evaluate corresponding failure strains. Modeling is based upon a combination of the Dorn/Garafolo sinh equations for creep-plasticity coupled with threshold stresses based upon growth rate kinetics of particles and microstructure as modified by stress/strain history. %%%Alternatives to leaded solders used in electronic applications are being sought to address environmental concerns and to accommodate the policies unfolding in several foreign countries that will affect the global marketing of US electronic products. Eutectic Sn-Ag solder is a recommended alternative solder, aimed at higher temperature service conditions. Solders used in severe service environments, such as automotive, aerospace, and military environments, can experience thermal excursions that range between -50 to 180 degrees C. Such thermal cycles cause severe thermal stresses due to the thermal expansion mismatch between soldered components and the substrates. This thermomechanical fatigue process damages solder joints, and more than 70% of electronic system failures are due to failed solder joints. ***