高温机械循环下无铅焊料的力学行为和微观结构演变

M. A. Hoque, M. A. Haq, J. Suhling, P. Lall
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Cyclic stress-strain, monotonic stress-strain, and creep tests were then conducted on the prior cycled samples. The cyclic stress-strain curves obtained were studied to observe the degradation of hysteresis loop properties (peak stress, hysteresis loop area, and plastic strain range) with mechanical cycling. The monotonic stress-strain and creep test data were plotted, and several mechanical properties were characterized for various levels of cycling. Using the data from these tests, we have been able to characterize and quantify the cycling induced damage through the observed degradations of several mechanical properties (elastic modulus, yield strength, ultimate strength, and creep strain rate) with the amount of prior cycling. All of the mechanical cyclic testing in our prior work and the work of others has been performed at room temperature ($\\mathrm{T}=25\\ {}^{\\circ} \\mathrm{C}$), and thus cannot be easily extended to thermal cycling applications. 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As expected, the SAC305 samples cycled at elevated temperature demonstrated a lower peak stress and smaller loop area relative to the results for analogous specimens cycled at room temperature and the same strain range. Uniaxial tensile tests and creep tests at room temperature were also conducted on specimens that had been previously mechanically cycled for various durations (e.g 0, 50, 100, 300, 600 cycles) at high temperature. This allowed us to study the degradation of the constitutive behavior of the solder alloys that occurred during the high temperature mechanical cycling due to the fatigue damage that builds up in the specimens. In particular, the degradations of the elastic modulus, yield strength, ultimate strength, and the secondary creep strain rate with duration of high temperature mechanical cycling were evaluated. These values were compared to the analogous results obtained for room temperature cycling in our previous study. As expected, the SAC305 samples cycled at 100 °C proved to have larger damage accumulation and degradations in mechanical properties relative to the analogous samples cycled at room temperature. The degradations in the properties that occur during mechanical cycling were also correlated with the corresponding changes in the microstructure of the specimens. Rectangular cross-sectioned samples of the two lead free solder alloys were polished and selected regions indented to track the changes in the microstructure of a fixed region with mechanical cycling at $\\mathrm{T}=100\\ {}^{\\circ} \\mathrm{C}$. The observed microstructure evolution during cycling included IMC coarsening as well as micro crack initiation and growth. 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The monotonic stress-strain and creep test data were plotted, and several mechanical properties were characterized for various levels of cycling. Using the data from these tests, we have been able to characterize and quantify the cycling induced damage through the observed degradations of several mechanical properties (elastic modulus, yield strength, ultimate strength, and creep strain rate) with the amount of prior cycling. All of the mechanical cyclic testing in our prior work and the work of others has been performed at room temperature ($\\\\mathrm{T}=25\\\\ {}^{\\\\circ} \\\\mathrm{C}$), and thus cannot be easily extended to thermal cycling applications. In our current work, we have extended our prior study to examine mechanical cycling at elevated temperature. In particular, we have examined lead free solders subjected to high temperature mechanical cycling at $\\\\mathrm{T}=100\\\\ {}^{\\\\circ} \\\\mathrm{C}$ and $\\\\mathrm{T}=125$ °C. 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引用次数: 3

摘要

在波动的温度环境中,由于循环机械应力和应变,电子封装中的焊点经常经历疲劳失效。这些应力和应变是由热膨胀系数的不匹配引起的,并导致损伤积累,从而导致裂纹的萌生、扩展,并最终导致失效。在ECTC 2020上的上一篇论文中,我们研究了几种焊料材料(SAC305, $\text{SAC} +\text{Bi}$和$\text{SAC} +\text{Bi}-\text{Ni}-\text{Sb}$)在室温下机械循环过程中的损伤积累。圆形横截面焊料试样首先回流,然后使用微机械测试仪对这些试样进行机械循环。循环应力-应变试验、单调应力-应变试验、蠕变试验。通过对得到的循环应力-应变曲线进行研究,观察滞回线特性(峰值应力、滞回线面积和塑性应变范围)随机械循环的退化情况。绘制了单调应力应变和蠕变试验数据,并对不同循环水平下的几种力学性能进行了表征。利用这些试验的数据,我们已经能够通过观察到的几种机械性能(弹性模量、屈服强度、极限强度和蠕变应变率)随先前循环次数的下降,来表征和量化循环引起的损伤。在我们之前的工作和其他人的工作中,所有的机械循环测试都是在室温下进行的($\ mathm {T}=25\ {}^{\circ} \ mathm {C}$),因此不容易扩展到热循环应用。在我们目前的工作中,我们扩展了之前的研究,以研究高温下的机械循环。特别是,我们检查了在$\ mathm {T}=100\ {}^{\circ} \ mathm {C}$和$\ mathm {T}=125°C的高温机械循环下的无铅焊料。研究了两种钎料合金SAC305和$\text{SAC} +\text{Bi}$ (SAC_Q)。本文报道了SAC305在$\ mathm {T}=100\ {}^{\circ} \ mathm {C}$循环时的结果,并与$\ mathm {T}=25\ {}^{\circ} \ mathm {C}$循环时的类似结果进行了比较。最初,制备小的单轴圆柱形样品并在回流炉中回流。然后将这些试样在$\ mathm {T}=100$°C下进行不同时间的机械循环。然后利用测量的循环应力-应变曲线表征高温机械循环过程中滞回线性能(峰值应力、滞回线面积和塑性应变范围)的退化。正如预期的那样,与室温和相同应变范围循环的类似样品相比,在高温下循环的SAC305样品显示出更低的峰值应力和更小的环路面积。在室温下进行单轴拉伸试验和蠕变试验,也对先前在高温下机械循环不同持续时间(例如0,50,100,300,600循环)的试样进行。这使我们能够研究在高温机械循环过程中由于试样中积累的疲劳损伤而导致的焊料合金本构行为的退化。特别是,评估了弹性模量、屈服强度、极限强度和二次蠕变应变率随高温机械循环时间的退化情况。这些值与我们之前的研究中室温循环得到的类似结果进行了比较。正如预期的那样,与室温循环的类似样品相比,在100°C循环的SAC305样品具有更大的损伤积累和力学性能退化。力学循环过程中发生的性能退化也与试件微观结构的相应变化相关。对两种无铅钎料合金的矩形截面试样进行抛光处理,选定区域压痕,跟踪$\ mathm {T}=100\ {}^{\circ} \ mathm {C}$机械循环时固定区域的微观结构变化。在循环过程中观察到的微观组织演变包括IMC粗化和微裂纹的萌生和扩展。利用这项研究的结果,我们正在努力为受可变温度应用的无铅焊料制定更好的疲劳标准。
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Mechanical Behavior and Microstructure Evolution in Lead Free Solders Subjected to Mechanical Cycling at Elevated Temperatures
Solder joints in electronic packages often experience fatigue failures due to cyclic mechanical stresses and strains in fluctuating temperature environments. These stresses and strains are induced by mismatches in coefficients of thermal expansion, and lead to damage accumulation that contributes to crack initiation, crack propagation, and eventually to failure. In our previous paper at ECTC 2020, we investigated the accumulation of damage in several solder materials (SAC305, $\text{SAC} +\text{Bi}$, and $\text{SAC} +\text{Bi}-\text{Ni}-\text{Sb}$) during mechanical cycling at room temperature. Circular cross-sectioned solder specimens were first reflowed, and these samples were then mechanically cycled for various durations using a Micro-Mechanical tester. Cyclic stress-strain, monotonic stress-strain, and creep tests were then conducted on the prior cycled samples. The cyclic stress-strain curves obtained were studied to observe the degradation of hysteresis loop properties (peak stress, hysteresis loop area, and plastic strain range) with mechanical cycling. The monotonic stress-strain and creep test data were plotted, and several mechanical properties were characterized for various levels of cycling. Using the data from these tests, we have been able to characterize and quantify the cycling induced damage through the observed degradations of several mechanical properties (elastic modulus, yield strength, ultimate strength, and creep strain rate) with the amount of prior cycling. All of the mechanical cyclic testing in our prior work and the work of others has been performed at room temperature ($\mathrm{T}=25\ {}^{\circ} \mathrm{C}$), and thus cannot be easily extended to thermal cycling applications. In our current work, we have extended our prior study to examine mechanical cycling at elevated temperature. In particular, we have examined lead free solders subjected to high temperature mechanical cycling at $\mathrm{T}=100\ {}^{\circ} \mathrm{C}$ and $\mathrm{T}=125$ °C. Two solder alloys, SAC305 and $\text{SAC} +\text{Bi}$ (SAC_Q), have been investigated. In this paper, we report on the findings for SAC305 cycled at $\mathrm{T}=100\ {}^{\circ} \mathrm{C}$, and compare to the analogous results obtained for cycling at $\mathrm{T}=25\ {}^{\circ} \mathrm{C}$. Initially, small uniaxial cylindrical samples were prepared and reflowed in a reflow oven. These specimens were then mechanically cycled for various durations at $\mathrm{T}=100$ °C. The measured cyclic stress-strain curves were then used to characterize the degradation of hysteresis loop properties (peak stress, hysteresis loop area, and plastic strain range) with high temperature mechanical cycling. As expected, the SAC305 samples cycled at elevated temperature demonstrated a lower peak stress and smaller loop area relative to the results for analogous specimens cycled at room temperature and the same strain range. Uniaxial tensile tests and creep tests at room temperature were also conducted on specimens that had been previously mechanically cycled for various durations (e.g 0, 50, 100, 300, 600 cycles) at high temperature. This allowed us to study the degradation of the constitutive behavior of the solder alloys that occurred during the high temperature mechanical cycling due to the fatigue damage that builds up in the specimens. In particular, the degradations of the elastic modulus, yield strength, ultimate strength, and the secondary creep strain rate with duration of high temperature mechanical cycling were evaluated. These values were compared to the analogous results obtained for room temperature cycling in our previous study. As expected, the SAC305 samples cycled at 100 °C proved to have larger damage accumulation and degradations in mechanical properties relative to the analogous samples cycled at room temperature. The degradations in the properties that occur during mechanical cycling were also correlated with the corresponding changes in the microstructure of the specimens. Rectangular cross-sectioned samples of the two lead free solder alloys were polished and selected regions indented to track the changes in the microstructure of a fixed region with mechanical cycling at $\mathrm{T}=100\ {}^{\circ} \mathrm{C}$. The observed microstructure evolution during cycling included IMC coarsening as well as micro crack initiation and growth. Using the results of this study, we are working to develop better fatigue criteria for lead free solders which are subjected to variable temperature applications.
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