Dependence of Sn whisker nucleation and growth on stress: real-time experiments and development of a predictive model

非技术摘要：由于铅（Pb）的毒性，正在从电子制造业中淘汰。 然而，这在广泛用于保护电子元件的导电部件的锡（Sn）涂层中产生有害的副作用。 去除之前在这些涂层中使用的铅促进锡晶须的生长。 这些晶须是从涂层中自发生长出来的细丝，长度为毫米，并在许多关键系统中造成故障，包括卫星，导弹和核电站。该项目的目标是了解晶须生长背后的原因和机制，以便制定有效和可靠的策略来预防它们。 这项工作的重点是压力作为驱动力的影响，就像挤压牙膏管使牙膏流出来一样。 然而，在固体锡层中导致晶须生长的机制比这复杂得多。 进行良好的控制实验，以量化施加的应力和所得晶须形成速率之间的关系。 然后，实验结果被用来开发数学模型，可以解释晶须如何生长，并预测它们在不同条件下的形成。出版物和演示文稿让这些结果与制造界分享，合作让这些方法应用于新涂层的开发，以防止晶须。 该项目的一个外联部分将工程专业的本科生与当地高中配对，参加每周一次的课后活动，以促进技术领域的技能培养和参与。 博士研究通过积极招募代表性不足的群体的参与者，为本科生提供支持。 技术总结：锡须是一种细丝，从电子元件上的无铅锡涂层中自发生长，可能导致关键系统的故障。 压力被认为是驱动力，但这种关系是复杂的，机制还不清楚。 为了确定晶须形成的基本材料过程，进行了在测量晶须的同时控制层中的应力的实验。 在一组项目中，加热不同衬底上的Sn层用于产生热膨胀失配，并且由此产生的应力由衬底中的曲率确定。 在加热过程中，在扫描电子显微镜中实时测量相应的成核和生长动力学。 在其他实验中，通过使用独特的小型化系统施加压力来诱导应力，该系统可以在扫描电子显微镜内实时监测。这些方法用于研究应力对晶须动力学的影响，结合周围环境（湿度和腐蚀性元素）和不同的合金成分（纯Sn，Sn-Ag-Cu，Sn-In，Sn-Bi等）。 从这些实验中获得的数据被用来开发和细化晶须生长和成核过程的模型。 二维有限元分析模型包括幂律蠕变和应力诱导扩散到晶须基底的过程。 将模型扩展到3-D使晶界扩散和多晶化的影响被包括在内。 由模型得到的动力学参数可用于预测不同条件下的晶须生成。 了解条件和材料如何改变胡须生长过程可以指导无胡须涂层的开发。该奖项反映了NSF的法定使命，并且通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARY:Because of its toxicity, the element lead (Pb) is being eliminated from electronics manufacturing. However, this produces a harmful side-effect in tin (Sn) coatings that are widely used to protect the conductive parts of electronic components. Removing the lead that was previously used in these coatings promotes the growth of tin whiskers. These whiskers are thin filaments that grow spontaneously out of the coating to lengths of millimeters and have caused failures in many critical systems including satellites, missiles and nuclear power plants. The goal of this project is to understand the causes and the mechanisms behind whisker growth in order to develop effective and reliable strategies to prevent them. The work focuses on the effect of stress as a driving force, much like squeezing a toothpaste tube makes the paste come out. However, the mechanisms in the solid tin layer leading to whisker growth are much more complicated than this. Well-controlled experiments are performed to quantify the relationship between the applied stress and resulting whisker formation rates. The experimental results are then used to develop mathematical models that can explain how whiskers grow and predict their formation under different conditions. Publications and presentations let these results be shared with the manufacturing community and collaborations let these approaches be applied to the development of new coatings to prevent whiskering. An outreach component of the project pairs undergraduate engineering students with local high schools in a weekly after-school program to promote skill-building and participation in technical fields. The research of Ph.D. and undergraduate students is supported with active recruitment of participants from under-represented groups. TECHNICAL SUMMARY:Sn whiskers are thin filaments that grow spontaneously from Pb-free Sn coatings on electronic components and can lead to failures in critical systems. Stress is proposed to be the driving force, but the relationship is complex and the mechanisms are not yet understood. To determine the fundamental materials processes underlying whisker formation, experiments are performed that control the stress in the layer while measuring the whiskers. In one set of projects, heating Sn layers on different substrates is used to create thermal expansion mismatch and the resulting stress is determined from the curvature in the substrate. The corresponding nucleation and growth kinetics are measured in real-time in the scanning electron microscope during heating. In other experiments, the stress is induced by applying pressure using a unique, miniaturized system that can be monitored in real-time inside the scanning electron microscope. These approaches are used to study the effect of stress on whisker kinetics in conjunction with the ambient environment (humidity and corrosive elements) and different alloy compositions (pure Sn, Sn-Ag-Cu, Sn-In, Sn-Bi, etc.). The data obtained from these experiments are used to develop and refine models of the whisker growth and nucleation process. A 2D finite element analysis model includes processes of power-law creep and stress-induced diffusion to the whisker base. Extending the model to 3-D enables the effects of grain boundary diffusion and polycrystallinity to be included. Kinetic parameters obtained from the modeling can be used to predict whiskering under different conditions. Understanding how conditions and materials change the whisker growth process guides the development of whisker-free coatings.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.