Effect of Bismuth Content on the Mechanical Cyclic Properties of SAC+Bi Lead Free Solders

M. A. Haq, M. A. Hoque, J. Suhling, P. Lall
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引用次数: 2

Abstract

During the transition from tin-lead solders to lead-free solders, Sn-Ag-Cu (SAC) alloys have become the most widely used lead free alloys for the various levels of interconnects in an electronic package. To improve the thermal cycling reliability of SAC alloys, new doped $\text{SAC} +\mathrm{X}$ alloys have been developed, were X = Bismuth is often added to provide enhanced strength and improved aging resistance. Our prior work has shown that the use of $\text{SAC}+\text{Bi}$ solders can improve the reliability of the electronic package. However, there have been only limited studies regarding the cyclic mechanical properties of lead free SAC solder alloys with added bismuth content. In this paper, the authors have quantified the evolution of the properties of $\text{SAC} +\text{Bi}$ lead free solder joints subjected to isothermal mechanical cycling. Various levels of bismuth content have been studied including 1%, 2%, and 3%, and the results have been compared to previous studies performed on SAC305. Cylindrical solder specimens were prepared and reflowed with a reflow profile similar to that utilized in industry SMT joints. The formed samples were then mechanically cycled at room temperature using five different total strain ranges including 0.003, 0.005, 0.007, 0.009, and 0.012. The obtained cyclic stress-strain curves were recorded up to the 20th cycle, where the hysteresis loops became somewhat stable from one cycle to the other. The obtained hysteresis loops were then analyzed to evaluate the plastic work accumulated, the peak stress, and the plastic strain range for each of the total strain ranges considered. The plastic work dissipated per cycle is often an important parameter in predicting solder joint reliability with models such as the Morrow/Darveaux models. For each solder alloy, the obtained plastic work was correlated with the applied total strain range to develop correlations between the two quantities. Using the obtained results, specimens of the various $\text{SAC} +\text{Bi}$ alloys were then mechanically cycled to failure using the same initial plastic work (hysteresis loop area) in the first cycle. Failure was considered to have occurred with a 50% drop of the applied load. The cyclic stress-strain loops obtained from the fatigue failure cyclic experiments were then studied to measure the change in the cyclic properties for different bismuth contents. This approach removed the limitations from previous fatigue life studies where a constant strain range was used across all the alloys, which resulted in various hysteresis loop sizes. Rectangular cross-sectioned and polished samples of all the alloys were also cycled at the same corresponding strain ranges to study the change in microstructure of the alloys with mechanical cycling. This has helped us gain a better understanding on how the bismuth percentage in an alloy affects the microstructure evolution during mechanical cycling.
铋含量对SAC+Bi无铅钎料循环力学性能的影响
在锡铅焊料向无铅焊料过渡的过程中,Sn-Ag-Cu (SAC)合金已成为电子封装中各级互连中应用最广泛的无铅合金。为了提高SAC合金的热循环可靠性,开发了新的掺杂$\text{SAC} +\ maththrm {X}$合金,其中X =铋经常被添加以提供增强的强度和改善的抗老化性。我们之前的工作表明,使用$\text{SAC}+\text{Bi}$焊料可以提高电子封装的可靠性。然而,关于添加铋的无铅SAC钎料合金的循环力学性能的研究非常有限。在本文中,作者量化了$\text{SAC} +\text{Bi}$无铅焊点在等温机械循环作用下的性能演变。研究了不同水平的铋含量,包括1%、2%和3%,并将结果与先前在SAC305上进行的研究进行了比较。制备了圆柱形焊料试样,并使用类似于工业SMT接头中使用的回流曲线进行回流。然后在室温下使用0.003、0.005、0.007、0.009和0.012五种不同的总应变范围进行机械循环。得到的循环应力-应变曲线记录到第20个循环,从一个循环到另一个循环,滞回曲线趋于稳定。然后对得到的滞回线进行分析,以评估所考虑的每个总应变范围的累积塑性功、峰值应力和塑性应变范围。每个周期消耗的塑料功通常是预测焊点可靠性的一个重要参数,如Morrow/Darveaux模型。对于每种焊料合金,获得的塑性功与应用的总应变范围相关联,以建立两者之间的相关性。利用得到的结果,在第一次循环中使用相同的初始塑性功(迟滞回线面积),对各种$\text{SAC} +\text{Bi}$合金试样进行机械循环直至失效。故障被认为发生了50%的下降所施加的负载。然后研究了疲劳破坏循环试验得到的循环应力-应变环,以测量不同铋含量下循环性能的变化。这种方法消除了以前的疲劳寿命研究的局限性,在这些研究中,所有合金都使用恒定的应变范围,导致各种迟滞回线尺寸。在相同的应变范围内,对所有合金的矩形截面和抛光试样进行循环,研究合金的微观组织随机械循环的变化。这有助于我们更好地理解合金中铋含量对机械循环过程中微观组织演变的影响。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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