The Effect of Temperature Cycling on Tin Whisker Formation
The Effect of Temperature Cycling on Tin Whisker Formation
复制标题
温度循环对锡晶须形成的影响
DOI：
--
复制
发表时间：
2003
期刊：
影响因子：
--
通讯作者：
V. Schroeder
中科院分区：
文献类型：
--
作者：
M. Dittes;P. Oberndorff;V. Schroeder
在某些条件下，元件表面的镀锡可能会长出晶须，这可能导致电子设备故障。虽然众所周知，锡和FeNi42的热失配是众所周知的，并且在这种材料组合的热循环后也有锡晶须的报道，但没有系统的研究热循环的影响。本文描述了不同循环条件对FeNi42上锡晶须生长速度的影响，并试图将这些试验与使用寿命条件联系起来。我们证明了晶须长度与∆T呈线性关系。此外，晶须生长速率随循环次数和/或晶须长度的变化而衰减。此外，在组装成印刷电路板的电镀元件上，最大晶须长度似乎减少了。最近，元件制造商推出了电镀纯锡作为无铅镀SnPb的替代品，用于引线框架器件的可焊面，这引起了人们对众所周知的晶须生长现象的新关注。由于大多数元件都采用铜基引线框架，因此对在铜基材料上形成的锡晶须进行了大量的研究。提出了锡晶须在铜基材料上生长的机理，并确定了可行的对策。对于在铜基材料上电镀锡，研究的重点是等温储存条件，以便研究在配送中心或组装前在最终客户处储存时晶须生长的倾向。据信，第二级组装将减缓晶须形成和生长的过程。虽然镀锡铜引线在等温储存条件下倾向于生长须，但镀亚光锡FeNi42引线在这种情况下通常不会生长须。FeNi42引线框架不像基于cu的引线框架那样流行，但它们仍然广泛应用于电子工业。在温度循环中，电镀在FeNi42上的锡通常形成晶须，而镀锡的铜在温度循环中通常表现出最小的晶须生长。对这种不同行为的解释可能与不同的应力诱导机制有关。在铜表面镀锡时，由于铜锡界面处金属间Cu6Sn5的过度不规则生长，在锡表面形成压应力。另一方面，对于在FeNi42上镀锡，FeNi42 （cte = 4.3 *10 K）和锡（cte = 23 *10 K）之间的热膨胀系数存在较大的不匹配，可能会引起应力。当温度循环应用时，这种不匹配会在受约束的锡层中引起应力。由于环境或操作引起的温度循环在大多数电子设备的使用寿命期间都会发生，因此应研究镀锡FeNi42基元件上晶须生长引起的故障风险。此外，不能假定在电路板组装过程中所有的镀锡熔断，因此部分电镀完成将在操作期间仍然存在。基于一些实验结果和上述考虑，本研究开始研究影响FeNi42电镀锡中晶须生长的各种参数。实验所用的测试包为全加工的TSOPII-66，内设存储芯片。所有包装来自同一生产批次，并在大批量生产的标准条件下电镀。x射线荧光分析测得镀层厚度为8.5±0.1 μm。锡表面的特点是哑光纯锡表面，平均晶粒尺寸为2 μm至6 μm。电镀过程的电解液是最新一代的基于msa的化学物质。电镀过程在连续运行的带式生产线中进行。镀相同类型的SnPb组件，并作为对照。这些Sn-Pb部件也是在标准生产过程中制造的，并经历了如下所述的温度循环和检查的相同条件。对照组的镀层厚度为8.2±0.1 μm，铅含量为10±1%。对于每个测试条件和测试间隔（例如250,500,1000循环），对两种电镀类型的五个组件进行了评估。因此，在每种环境条件和测试间隔下，对330根引线进行了检查。因此，如果没有明确提到，对图中的每个数据点检查了5个不同的组件。三种类型的温度循环室已被用于适用于各种条件。在大多数情况下，所谓的空气对空气或液体对液体的冲击条件下，使用了两室系统，以便在几秒钟内从热室转移到冷室，反之亦然。停留时间从组件从一个室转移到另一个室开始。温度循环小于10 K/min斜坡速率已通过使用单室系统进行。由于组件的热质量有限，测试样品直接跟随腔室的温度，停留时间在达到公差为5°C的温度极限后开始。首先用50倍放大的光学显微镜进行检查。之所以选择这种方法，是因为与扫描电镜不同，许多引线可以快速评估，并且光学显微镜测量的晶须长度与扫描电镜测量的晶须长度一致。因此，每个引线都在活虫的位置进行了全面检查，并确定了最长的须。在注意到最长晶须的位置后，在扫描电子显微镜（SEM）下检查每个组件，并在1000倍放大镜下从每个组件的最长晶须拍摄照片。为了比较的原因和更高的精度，这些晶须的长度再次在扫描电镜中测量。特定环境条件和测试条件下的最大须须长度是通过对暴露在相同条件下的5个组件中的每个组件测量的最大须须长度的平均值来计算的。此外，在未受修整和成形工具或任何弯曲影响的情况下，在300倍放大镜下从引线尖端拍摄该区域的照片，以获得晶须密度和长度分布的总体印象。在设计阶段，影响的参数确定为：•温度范围•绝对温度
Tin platings on component finishes may grow whiskers under certain conditions, which may cause failures in electronic equipment. Although the thermal mismatch of tin and FeNi42 is well known and tin whiskers have been reported after thermal cycling of this material combination, no systematic investigation on the effects of thermal cycling is available. In this paper we describe the influence of various cycling conditions on the whisker growth rate of tin on FeNi42 and attempt to correlate these tests to service life conditions. We demonstrate that the whisker length has a linear relationship with ∆T. In addition, the whisker growth rate appears to decay as a function of number of cycles and/or whisker length. Furthermore, the maximum whisker lengths appear to be reduced on plated components that have been assembled to a printed circuit board. Introduction Recent activities of component manufacturers to introduce electroplated, pure tin as the lead-free alternative to SnPb plating for the solderable finish on leadframe-based devices draws new attention to the well known phenomenon of whisker growth. Since most components have copper-based leadframes much effort has been put into investigations of tin whiskers formed on copperbased materials. A mechanism has been proposed for tin whisker growth on copper-based materials and viable countermeasures have been identified. For tin electroplating on copper-based materials, the focus has been on isothermal storage conditions in order to investigate the propensity for whisker growth during storage in distribution centers or at the end customer before assembly. It is believed that second level assembly will slow the process of whisker formation and growth. Although tin plated copper leadframes tend to grow whiskers under isothermal storage conditions, matt tin plated FeNi42 leadframes do not typically grow whiskers under these circumstances. FeNi42 leadframes are not as popular as Cu-based leadframes, but they are still widely used in the electronics industry. In temperature cycling, tin electroplate on FeNi42 typically forms whiskers, while tin plated copper typically exhibits minimal whisker growth during temperature cycling. The explanation for this different behaviour is presumably related to different mechanisms of stress induction. For tin plated on copper, it is believed that compressive stress is formed in tin finishes due to the excessive irregular growth of the intermetallic Cu6Sn5 at the copper-tin interface. On the other hand, for tin plated on FeNi42, stress may be induced by the large mismatch of the coefficients of thermal expansion between FeNi42 (cte = 4.3 *10 K) and tin (cte = 23 *10 K). This mismatch can cause stress in the constrained tin layer, when temperature cycling is applied. Since temperature cycles due to the environment or operation occur during the service life of most electronic equipment, the risk of failure caused by whisker growth on tin plated FeNi42 based components should be investigated. Further, it must not be assumed that all of the tin plating fuses during the board assembly processes, so portions of the aselectroplated finish will still exist during operation. Based on some experimental findings and the above considerations, the current study was initiated to address the various parameters that influence whisker growth in tin electroplated on FeNi42. Experimental The test packages used for the experiments were fully processed TSOPII-66 with a memory chip inside. All packages are from the same production lot and are electroplated in the standard conditions for mass production. The thickness of the tin finish has been measured by X-ray fluorescence analysis as 8.5 ± 0.1 μm. The tin finish is characterised as a matt pure tin finish with an average grain size of 2 μm to 6 μm. The electrolyte for the electroplating process was an MSA-based chemistry of the latest generation. The plating process was performed in a continuously operating belt line. SnPb plated components of same type were plated and used as a control. These Sn-Pb parts have also been made during standard production and underwent the same conditions of temperature cycling and inspection as described below. The plating thickness for the SnPb control group was 8.2 ± 0.1 μm and the lead content was 10 ± 1 %. For every test condition and test interval (e.g. 250, 500, 1000 cycles), five components of both plating types have been evaluated. Thus 330 leads are inspected for each environmental condition and test interval Hence – if not explicitly mentioned – 5 different components have been inspected for every data point in the diagrams. Three types of temperature cycling chambers have been used to apply the various conditions. In most cases with so called air to air or liquid to liquid shock conditions a two chamber system was used, so that the transfer from the hot to the cold chamber and vice versa occurs within a few seconds. The dwell time began with the completed transfer of the components from one to the other chamber. Temperature cycles of less than 10 K/min ramp rate have been performed by the use of single chamber systems. Due to the limited thermal mass of the components the test samples followed directly the temperature of the chamber and dwell time started after reaching the temperature limit with a tolerance of 5 °C. Inspection has at first been done with an optical microscope at a magnification of 50x. This method was chosen because many leads can be evaluated quickly, unlike in SEM, and the whisker lengths measured with an optical microscope are consistent with whisker lengths measured in SEM. Thus every lead was fully inspected in live bug position and the longest whisker identified. After noting the position of the longest whisker every component was inspected in scanning electron microscope (SEM) and a picture taken from the longest whisker of every component at 1000x magnification. The length of these whiskers have been measured in SEM again for comparative reasons and for higher accuracy. A maximum whisker length for a particular environmental condition and test condition is calculated by averaging the maximum whisker length measured on each of the 5 components exposed to the same condition. Additionally pictures of the area without influence of trim & form tools or any bending and from the tip of the leads at 300 x magnification were taken to get an overall impression of the whisker density and the length distribution. During the design phase the parameters of influence have been identified as • the temperature range • the absolute temperature