Sn Whisker Nucleation and Growth: Fundamental Mechanisms Controlling Where, When and Why Whiskers Form on Sn Coatings

锡晶须成核和生长：控制锡涂层上晶须形成的地点、时间和原因的基本机制

• 批准号: 1206138
• 负责人: Eric Chason
• 金额: $ 38万
• 依托单位: Brown University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-09-01 至 2015-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1206138&HistoricalAwards=false
• 关键词: Sn; Whisker; Nucleation; Growth; Fundamental

TECHNICAL SUMMARYSn whiskers are a serious reliability problem in electronics manufacturing. Whiskers have been implicated in failures of pacemakers, nuclear power plants, satellites and other critical systems. Recently, the removal of Pb from manufacturing (for environmental reasons) has made the problem more severe since Pb had been used as an alloying element to prevent whisker formation. In previous work, whiskering has been shown to increase with the layer stress and finite element analysis (FEA) has been used to model how the intermetallic (IMC) growth at the Sn-Cu interface leads to stress in the layer. The new work will focus on the microscale features that determine why whiskers start to grow at specific sites, i.e., which grains will form into whiskers/hillocks and how is this influenced by the magnitude of the stress. Scanning electron microscopy/electron backscattering diffraction (SEM/EBSD) will be used to monitor the surface repeatedly. These images will be used to create videos of how whiskers/hillocks grow out of the surface. Correlation with the underlying grain structure will show which grain configurations are most likely to form whiskers. Synchrotron microdiffraction studies at the APS synchrotron will be used to measure the deviatoric strain as well as the grain structure in real time as whiskers form. Results of these studies will be used to determine what factors control where whiskers nucleate (e.g., local stress concentration, weak oxide, IMC accumulation, recrystallization, horizontal grain boundaries, etc.). The data will be interpreted using finite element analysis simulations, with the measured microstructure serving as a critical input for the calculations. Additional measurements during thermal cycling and under corrosive environments will be used to determine how these factors influence whisker formation. The knowledge gained from this work will be used to develop mitigation strategies (using layer structure and composition) to suppress whisker formation. NON-TECHNICAL SUMMARYTin whiskers are long thin filaments that grow out of tin coatings used in electronics manufacturing. These whiskers have caused failures in pacemakers, nuclear power plants, satellites and other critical systems. The problem has become more serious recently due to the removal of lead from manufacturing for environmental reasons. Lead-tin alloys had been used for the last 50 years to suppress whiskering and there is currently no alternative that prevents whisker formation as well as lead. The focus of this research is to understand why whiskers form where they do in order to develop schemes to prevent their formation. Stress in the layer is believed to be the driving force for making whiskers, but that does not explain why they form at specific sites on the surface. The researchers will use advanced characterization tools (electron microscopy) to repeatedly monitor the tin surface to observe the whiskers when they start to form. Videos made from these images will show how the whiskers grow out of the surface from individual sites. This technique also measures the orientation and shape of each tin grain in the layer which will enable the whisker formation to be correlated with the characteristics of the sites where they form. The researchers will also use a unique X-ray diffraction tool with microscopic spatial resolution to determine the stress in the individual grains in the layer. These measurements will provide input for mathematical modeling of the surface to understand how the stress develops in each grain and how that leads to whisker growth. Future work will include studying how humidity and temperature influence whisker growth. The results of these studies will be used to develop processing schemes to make layers that are resistant to whisker formation.

Sn晶须是电子制造中的严重可靠性问题。 胡须与心脏起搏器、核电站、卫星和其他关键系统的故障有关。 最近，从制造中去除Pb（出于环境原因）使得问题变得更加严重，因为Pb已被用作合金元素以防止晶须形成。在以前的工作中，晶须已被证明增加层应力和有限元分析（FEA）已被用来模拟如何在Sn-Cu界面的金属间化合物（IMC）的生长导致在层中的应力。 这项新的工作将集中在微尺度特征上，这些特征决定了为什么晶须开始在特定的位置生长，即，哪些颗粒将形成须状物/小丘以及这如何受到应力大小的影响。将使用扫描电子显微镜/电子背散射衍射（SEM/EBSD）反复监测表面。这些图像将用于创建晶须/小丘如何从表面生长出来的视频。 与底层晶粒结构的相关性将显示哪些晶粒配置最有可能形成晶须。 在APS同步加速器的同步微衍射研究将被用来测量偏应变以及晶须形成时的真实的颗粒结构。 这些研究的结果将用于确定哪些因素控制晶须成核的位置（例如，局部应力集中、弱氧化物、IMC积累、再结晶、水平晶界等）。 数据将使用有限元分析模拟进行解释，测量的微观结构作为计算的关键输入。 在热循环和腐蚀环境下的额外测量将用于确定这些因素如何影响晶须的形成。 从这项工作中获得的知识将用于开发缓解策略（使用层结构和成分），以抑制晶须的形成。非技术概述锡须是从电子制造中使用的锡涂层中生长出来的细长细丝。这些触须已经导致心脏起搏器、核电站、卫星和其他关键系统的故障。 最近，由于环境原因，铅从制造业中去除，这个问题变得更加严重。 铅锡合金在过去的50年里一直被用来抑制晶须，目前还没有替代品可以像铅一样防止晶须的形成。 这项研究的重点是了解为什么晶须形成的地方，他们这样做，以制定计划，以防止他们的形成。 层中的应力被认为是制造晶须的驱动力，但这并不能解释为什么它们在表面的特定位置形成。 研究人员将使用先进的表征工具（电子显微镜）反复监测锡表面，以观察晶须何时开始形成。 从这些图像制作的视频将显示胡须是如何从各个网站的表面生长出来的。 该技术还测量层中每个锡颗粒的取向和形状，这将使晶须形成与它们形成的位置的特征相关。 研究人员还将使用具有微观空间分辨率的独特X射线衍射工具来确定层中单个颗粒的应力。 这些测量将为表面的数学建模提供输入，以了解每个晶粒中的应力如何发展以及如何导致晶须生长。 未来的工作将包括研究湿度和温度如何影响晶须生长。 这些研究的结果将用于开发加工方案，以制造抗晶须形成的层。