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

非技术摘要：锡须是电子制造中使用的涂层和焊料自发生长出来的长丝。它们是一个关键的可靠性问题，因为它们会造成短路，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，铅元素(铅)被限制制造，因此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。这项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，使之能够在测量涂层中的应力的同时监控涂层表面的晶须形成。由于应力被认为是晶须形成的原因，这使得可以直接测量应力和晶须之间的联系。除了基础研究，还有与工业合作伙伴的合作，研究与胡须预防有关的问题。在科学和技术教育方面，一个主要问题是留住这些领域的第一代学生和服务不足的学生。首席研究人员正在通过基于团队的指导努力来改善这一点，以支持新学生，并让他们在学术生涯早期接触实验室研究。技术摘要：尽管压力被认为是胡须形成的驱动力，但尚不清楚为什么胡须形成于特定的位置(成核)，以及是什么控制了它们的体积演变(生长)。这项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(样品受热时热膨胀引起的应力)。测量结果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势垒。这些研究进一步提出了一种潜在的形核机制，即基于应力诱导的晶界迁移。利用原位透射和扫描电子显微镜研究应力作用下的颗粒结构，可以捕捉到形核过程，为进一步探讨这一机理奠定基础。晶须体积演化的电子显微镜测量表明，只要高于临界阈值，生长速度就与平均应力成正比。目前正在开发使用有限元方法来模拟速率相关塑性引起的应力松弛和应力驱动扩散之间的平衡的计算方法。这些模型预测可以直接与生长测量结果进行比较，以改进模型参数并预测晶须生长速度。在与工业研究人员的合作下，实验能力正在扩大，以测量腐蚀/湿度对晶须成核和生长的影响。一种使用机械压力诱导晶须的新技术正在开发中，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.