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

这笔赠款的重点是表征四种锡-银基焊料的热机械疲劳引起的微观结构演变和机械性能的变化。其目标是开发基于微观结构的现象学模型，可以用来预测失败的发生。该实验计划使用组件规模的焊点样品，允许根据实时热机械循环的次数有效地监测微观结构和机械性能的变化。热机械循环包括高温下的短时间和低温下的长时间，以允许评估低温蠕变的影响，这是电子行业常用但非保守的加速测试实践不考虑的条件。试件也将在工业加速测试设施中循环，以检查不同的加热和冷却速度和大小如何影响结构损伤累积。取向成像被用来评估晶体取向和取向错误的结构变化。还确定了第二相的尺寸分布和形状。力学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、蠕变断裂和恒应变率测试，以评估相应的失效应变。模型基于Dorn/Garafolo Sinh蠕变塑性方程和基于颗粒生长速率动力学的门槛应力以及由应力/应变历史修正的微观结构的组合。%电子应用中使用的含铅焊料的替代品正在寻找，以解决环境问题，并适应几个国家正在实施的政策，这些政策将影响美国电子产品的全球营销。共晶锡银焊料是一种推荐的替代焊料，适用于较高温度的使用条件。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摄氏度之间的热漂移。这种热循环会由于焊接件和基板之间的热膨胀不匹配而导致严重的热应力。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故障造成的。***

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. ***