Sn whiskers: fundamental mechanisms of nucleation and growth and applications to mitigation

非技术概述：锡晶须是在电子制造中使用的涂层和焊料中自发生长的长丝。它们是一个关键的可靠性问题，因为它们会产生短路，导致卫星坠毁、核电站关闭和医疗起搏器召回，以及其他许多故障。由于环境原因，铅（Pb）元素被限制生产，晶须的风险变得更大。在过去，添加这种元素有助于抑制晶须的形成。研究的重点是了解晶须形成的原因，以及开发和测试抑制它们的方法。这项工作使用了一种独特的系统，可以在测量涂层应力的同时监测涂层表面的晶须形成。由于应力被认为是晶须形成的原因，这使得应力和晶须之间的联系可以直接测量。除了基础研究外，还与工业伙伴合作研究与胡须预防有关的问题。在科学和技术教育方面，一个主要问题是保留这些领域的第一代和服务不足的学生。首席研究员正在通过团队指导努力来改善这一点，以支持新生，并让他们在学术生涯的早期接触实验室研究。技术总结：虽然应力被认为是晶须形成的驱动力，但人们还不清楚为什么晶须会从特定的位置形成（成核），以及是什么控制了它们的体积演变（生长）。本研究建立在开发一种独特的系统的基础上，该系统可以同时测量晶须密度和应力（当样品被加热时由热膨胀引起）。测量显示了应力如何改变成核速率，以及它在多大程度上降低了成核屏障。这些研究进一步提出了一种基于应力诱导晶界迁移的潜在成核机制。通过原位透射电镜和扫描电镜监测应力作用下晶粒的结构，可以捕捉到成核过程，进一步探索这一机制。电子显微镜对晶须体积演化的测量表明，只要高于临界阈值，生长速率与平均应力成正比。计算正在开发，使用有限元方法来模拟由于速率依赖的塑性和应力驱动的扩散造成的应力松弛之间的平衡。这些模型预测可以直接与生长测量进行比较，以改进模型参数并允许预测晶须生长速率。在与工业研究人员的合作下，实验能力正在扩展到测量腐蚀/湿度对晶须成核和生长的影响。一种利用机械压力诱导晶须的新技术正在被开发，以研究新的焊料配方，以确定它们抑制晶须的能力。

NON-TECHNICAL SUMMARY:Tin whiskers are long filaments that grow spontaneously out of coatings and solders used in electronics manufacturing. They are a critical reliability problem because they create short circuits that have led to satellite crashes, a nuclear power plant shutdown and a medical pacemaker recall, among many other failures. The risk from whiskers has become worse because the element lead (Pb) has been restricted from manufacturing for environmental reasons. In the past, addition of this element helped to suppress whisker formation. The research is focused on understanding the causes of whisker formation as well as developing and testing methods for suppressing them. The work uses a unique system that enables the surface of the coating to be monitored for whisker formation at the same time as the stress in the coating is measured. Since stress is believed to be the reason for whisker formation, this allows the connection between stress and whiskering to be directly measured. In addition to fundamental studies, there are collaborations with industrial partners to study issues related to whisker prevention. In science and technology education, a major problem has been the retention of first generation and underserved students in these fields. The principal investigator is improving this through team-based mentoring efforts to support new students and exposing them to laboratory research early in their academic careers.TECHNICAL SUMMARY:Although stress is believed to be the driving force for whisker formation, it is not understood why whiskers form from specific sites (nucleation) and what controls their volume evolution (growth). This research builds on the development of a unique system to simultaneously measure the whisker density and the stress (induced by thermal expansion when the sample is heated). Measurements show how the stress changes the nucleation rate and how much it decreases the barrier to nucleation. These studies further suggest a potential nucleation mechanism that is based on stress-induced grain boundary migration. In situ transmission and scanning electron microscope studies that monitor the grain structure under stress can capture the nucleation process to explore this mechanism further. Electron microscopy measurements of the whisker volume evolution show that the growth rate is proportional to the average stress, as long as it is above a critical threshold value. Calculations are being developed that use finite element methods to model the balance between stress relaxation due to rate-dependent plasticity and stress-driven diffusion. These model predictions can be compared directly with the growth measurements to refine the model parameters and allow prediction of whisker growth rates. In collaboration with industrial researchers, the experimental capabilities are being extended to measure the effect of corrosion/humidity on whisker nucleation and growth. A new technique for using mechanical pressure to induce whiskers is being developed to study new solder formulations to determine their ability to suppress whiskering.