Whisker Formation in Sn Coatings on Cu: Fundamental Mechanisms and Approaches to Mitigation

该奖项由2009年美国复苏和再投资法案(公共法律111-5)资助。技术摘要：锡基合金涂层多年来一直用于电子元件制造，以提高可焊性和防止腐蚀。最近的环境立法迫使这些地区消除了铅，这使得锡须重新出现，即从表面生长的细丝，最终可能导致短路。许多系统(卫星、航空系统、医疗设备等)都记录了由晶须引起的故障。并且对高可靠性系统的制造商来说是一个重大风险。尽管进行了大量研究，但晶须形成的许多基本方面仍然不清楚。部分困难在于多种材料过程相互作用以产生晶须(例如，互扩散、相变、应力产生和松弛等)。因此，很难确定潜在的机制。此外，许多工艺变量(薄膜厚度、晶粒度、镀膜条件等)已经被证明在胡须的形成中起到了作用。这使得很难比较在不同条件下进行的不同研究的结果。提出的研究计划旨在通过以下几个问题来确定控制晶须形成的基本机制：1.哪些过程控制着锡层中的应力？2.应力演变与晶须形成有何关系？3.是什么原因导致晶须在特定位置成核？4.添加铅如何防止晶须形成？5)我们如何利用我们对晶须形成的理解来防止它们的发生？实时薄膜诊断将被用来量化几个关键参数的同时时间演变，如薄膜应力、金属间化合物相形成和晶须密度，使用仔细控制的样品。这些动力学研究将得到电子显微镜研究的补充，以确定这些过程背后的潜在原子尺度机制以及晶须成核。铅锡合金的微观结构、膜厚和锡含量将被系统地改变，以分离和识别有助于晶须形成的不同动力学过程。分析模型将被用来解释测量结果并开发评估可靠性的预测能力。非技术摘要：锡须的形成，即从富锡无铅合金表面生长的纯锡细丝，在无铅电子制造中是一个严重的问题，因为它们最终会导致电子元件短路。拟议的研究计划旨在通过确定控制含锡合金富锡层中的应力的过程，了解应力如何演变及其与晶须形成的关系，找出是什么促进了晶须的形核，以及为什么添加铅阻止晶须的形成，从而确定控制晶须形成的基本机制。一个最终的目标是发现如何利用对晶须形成的了解来防止它们的发生，以便能够以可预测的可靠性制备新的涂层和富锡合金。与EMC公司的非正式合作将拓宽学生和行业参与者的知识。这些参与者将使学生接触到在现实世界环境中解决问题的机会，在现实环境中，经济等问题可能与技术问题一样重要。这项工作将包括对研究生和本科生进行实验和基本材料问题建模的教育。将通过招生和与佛罗里达州立大学-佛罗里达农工大学(HBCU)协会的合作学生交换，加强未被充分代表的少数族裔和女性的参与。将通过与教师合作开发用于布朗大学学校访问计划的薄膜压力模块来实现K-12教育外展。这些外展活动将通过与布朗MRSEC教育项目的协调来发挥作用。

This Award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).TECHNICAL SUMMARY:Sn-based alloy coatings have been used for many years in the manufacture of electronic components to enhance solderability and prevent corrosion. Recent environmental legislation has forced the elimination of Pb in these parts, which has enabled the re-emergence of tin whiskers, i.e. thin filaments that grow from the surface and can ultimately cause short circuits. Whisker-induced failures have been documented in numerous systems (satellites, aviation systems, medical devices, etc.) and represent a significant risk for manufacturers of high reliability systems. Despite significant research, many fundamental aspects of whisker formation are still not understood. Part of the difficulty is that multiple materials processes interact to create the whiskers (e.g. interdiffusion, phase transformations, stress generation and relaxation, etc.) so that it is difficult to identify the underlying mechanisms. Moreover, many processing variables (film thickness, grain size, plating conditions, etc.) have been shown to play a role in whisker formation. This makes it difficult to compare the results of different studies done under different conditions. The proposed research program is designed to identify the fundamental mechanisms that govern whisker formation by focusing on the several questions: 1. Which processes control stress in the Sn layer? 2. How is the stress evolution related to whisker formation? 3. What causes whiskers to nucleate at particular sites? 4. How does the addition of Pb prevent whisker formation? 5) How can we use our understanding of whisker formation to prevent their occurrence? Real-time thin film diagnostics will be used to quantify the simultaneous temporal evolution of several critical parameters such as film stress, intermetallic phase formation and whisker density, using carefully controlled samples. These kinetic studies will be complemented by electron microscopy studies to determine the underlying atomic-scale mechanisms behind these processes as well as whisker nucleation. Microstructure, film thickness, and Sn content in Pb-Sn alloys will be varied systematically to isolate and identify different kinetic processes that contribute to whisker formation. Analytical models will be used to interpret the measurements and to develop predictive capability for assessing reliability.NON-TECHNICAL SUMMARY:The formation of Sn whiskers, i.e. thin filaments of pure tin that grow from the surface of Sn-rich lead-free alloys, is a serious concern in Pb-free electronics manufacturing because they can ultimately cause short circuits in electronic components. The proposed research program is designed to identify the fundamental mechanisms that govern whisker formation by identifying the processes that control stress in Sn-rich layers in Sn-containing alloys, learning how the stress evolves and relates to whisker formation, finding out what promotes the nucleation of whiskers and why the addition of Pb prevents whisker formation. An ultimate objective is to discover how the acquired understanding of whisker formation can be used to prevent their occurrence so that new coatings and Sn-rich alloys can be prepared with predictable reliability. An informal collaboration with the EMC Corporation will broaden the knowledge of students as well as the industrial participants. These participants will expose the students to problem solving in a real-world environment, where issues such as economics can be as important as technical issues. This work will involve the education of graduate and undergraduate students in experiments and modeling of a fundamental materials problem. Involvement of under-represented minorities and women will be enhanced by recruiting and a collaboration-student exchange with the Florida State University-Florida A&M University (an HBCU) association. K-12 educational outreach will be achieved by working with teachers to develop modules on thin film stress to be used in school visit program at Brown University. These outreach activities will be leveraged through coordination with the Brown MRSEC educational programs.