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

技术总结锡晶须在电子制造中是一个严重的可靠性问题。胡须被认为与心脏起搏器、核电站、卫星和其他关键系统的故障有关。最近，由于铅已被用作防止晶须形成的合金元素，因此从制造中去除铅(出于环境原因)使这个问题变得更加严重。在以前的工作中，晶须生长随着层应力的增加而增加，有限元分析(FEA)已经被用来模拟在锡-铜界面上的金属间化合物(IMC)生长是如何导致层中的应力的。这项新的工作将专注于确定胡须为什么在特定位置开始生长的微观特征，即哪些颗粒将形成胡须/山丘，以及压力的大小如何影响这一点。采用扫描电子显微镜/电子背散射衍射仪(SEM/EBSD)对表面进行反复监测。这些图像将被用来制作胡须/山丘如何从表面长出的视频。与底层颗粒结构的关联将显示哪些颗粒构型最有可能形成胡须。APS同步加速器的同步加速器显微衍射研究将用于在晶须形成时实时测量偏差应变以及颗粒结构。这些研究的结果将被用来确定控制晶须形核的因素(例如，局部应力集中、弱氧化物、IMC积累、再结晶、水平晶界等)。这些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。在热循环和腐蚀性环境下的其他测量将被用来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被用来制定缓解策略(使用层结构和成分)来抑制胡须的形成。非技术性的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。这些胡须已经导致起搏器、核电站、卫星和其他关键系统出现故障。最近，由于环境原因，制造业中的铅被清除，这个问题变得更加严重。在过去的50年里，铅锡合金一直被用来抑制晶须的形成，目前还没有替代材料可以像铅一样防止晶须的形成。这项研究的重点是了解胡须为什么会在它们形成的地方形成，以便制定防止它们形成的方案。层中的压力被认为是形成胡须的驱动力，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)重复监测锡表面，以观察胡须何时开始形成。用这些图像制作的视频将展示胡须是如何从各个地点的表面长出来的。这项技术还测量了层中每个锡粒的方向和形状，这将使晶须的形成与它们形成的位置的特征相关联。研究人员还将使用一种独特的具有微观空间分辨率的X射线衍射工具来确定层中单个颗粒中的应力。这些测量将为表面的数学建模提供输入，以了解每个颗粒中的应力是如何发展的，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胡须的生长。这些研究的结果将被用于开发制造抗胡须形成的层的加工方案。

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.