无铅电子封装微互连焊点中的热时效和电迁移及尺寸效应研究
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摘要
随着现代电子产品向无铅化、微型化、多功能化方向发展,电子封装微互连结构的可靠性也面临更多问题。本文通过在模拟回流焊条件下制备不同尺寸的“铜丝/钎料/铜丝”三明治结构对接微互连焊点,系统地研究了无铅钎料Sn-3.0Ag-0.5Cu微互连焊点的热时效和电迁移及尺寸效应等可靠性问题。主要采用精密动态力学分析仪(DMA)对服役温度(100℃)下微焊点热时效和电迁移后的强度变化行为以及尺寸效应问题进行了研究,并运用光学显微镜、扫描电镜和能谱仪等方法分析了微焊点的显微组织变化、拉伸断口变化以及电迁移致微焊点极性效应等。
对微焊点在服役温度下的热时效研究表明:微焊点高度变化(100～300μm)及热时效时间改变(0～96h)对焊点准静态拉伸强度有明显影响。在未经时效时,高度为100、200和300μm微焊点的平均拉伸强度分别为53.75、46.59和44.38MPa,焊点高度减小导致微焊点内部力学拘束效应增强,因而微焊点强度提高;同时,热时效时间延长致使微焊点内钎料合金显微组织明显粗化,导致微焊点强度降低,经48h和96h热时效后不同高度的微焊点拉伸强度分别降至50.12、40.87、35.26MPa和44.13、38.38、33.48MPa;此外,96h较短热时间內的时效对微焊点界面IMC厚度无明显影响。
研究不同高度尺寸的微焊点在100℃环境温度下经施加电流密度为1×104A/cm2,通电时间为48h的直流电流作用后拉伸强度的变化结果表明,微焊点在经电迁移作用后拉伸强度明显退化,高度为100、200和300μm的微焊点其平均拉伸强度分别降至44.5、35.8和27.8MPa,相对于初始态分别下降了17.3%、23.2%和37.4%,强度退化程度随微焊点高度的减小而减弱,呈现出明显的尺寸依赖性。
研究进一步表明,电迁移还导致微焊点产生明显的极性效应,微焊点在经历较长时间电迁移后,阳极IMC明显比阴极厚。电迁移加速了微焊点阴极IMC的分解,导致阴极出现微空洞或裂纹,弱化了微焊点的阴极界面,使微焊点最终断裂位置处于钎料与铜丝界面的阴极,断裂模式也由未经电迁移的延性断裂变为延性与脆性相结合的模式。
With the electronic products moving towards lead-free, miniaturization and multi-functions, the reliability of electronic packaging interconnects has attracted more and more attentions. In this thesis, the typical copper-wire/Sn-3.0Ag-0.5Cu/copper-wire sandwich structured butt joints with various micro-scale sizes were prepared under the modeling reflow soldering condition. The isothermal aging and electromigration behavior of the micro-scale solder joints as well as the joint size effect were studied sysmatically. The tensile strength change of the micro-scale solder joints of different sizes after undergoing isothermal aging and electromigration were studied by a Dynamic Mechanical Analyzer (DMA) at the testing temperature of 100℃. The microstructure, fractographies and the electromigration induced polarity effect in micro-scale solder joints were characterized by an optical microscope (OM) and a scanning electronic microscope (SEM) equipped with energy dispersive spectroscopy (EDS).
The results of the influence of isothermal aging on the micro-scale solder joints show that both the joint thickness and isothermal aging time significantly affect the ultimate tensile strength of the joints under quasi-static micro-tension. Without isothermal aging, the tensile strength of the joints increases with decreasing the thickness of the joints, the average tensile strength of the solder joints with three different thicknesses of 100, 200 and 300μm are 53.75, 46.59 and 44.38MPa respectively; this is probably owing to the increase of mechanical constraint in the solder joints. It has also been shown that the tensile strength of the joint decreases with the prolongation of isothermal aging time which results in the coarsening of the microstructure in the solder joints. After aging for 48 hours, the average values of tensile strength of the above three types of joints decrease to 50.12, 40.87 and 35.26MPa repectively, and further decrease to 44.13, 38.38 and 33.48MPa after aging for 96 hours. However, the thickness of the intermetallic compounds (IMC) layer at the joint interface shows almost no change obviously even after aging for 96 hours.
Furthermore, the effects of electromigration on tensile strength of the micro-scale solder joints were also studied. The eletromigration tests were carried out with a constant current density of 1×104A/cm2 at 100℃for 48 hours. The experimental results show that the electromigration has brought about obvious degradation of tensile strength of the joints. The average values of tensile strength of the joints with the thickness of 100, 200 and 300μm decrease to 44.5, 35.8 and 27.8MPa after electromigration, i.e., decreased by 17.3%, 23.2% and 37.4% respectively. The degradation degree of the joint strength decreases with decreasing joint thickness and exhibits a clear dependence on the solder joints’size.
Moreover, it has been shown that the electromigration also induces obvious polarity effect in micro-scale solder joints. After undergoing a certain current loading, the IMC layer at the anode interface of the joints becomes thicker compared with that in the cathode. Clearly, the electromigration resulted in weakening of the solder/IMC interface in the joints’cathode and the decomposition of IMC at the cathode interface; subsequently the microviods and microcrack developed at the cathode interface of the joints, and finally the fracture occured at the cathode interface of the joints. Also, it has been seen that the electromigration leads to the transfer of fracture mode of the joints from the ductile to the combination of brittle and ductile.
对微焊点在服役温度下的热时效研究表明:微焊点高度变化(100～300μm)及热时效时间改变(0～96h)对焊点准静态拉伸强度有明显影响。在未经时效时,高度为100、200和300μm微焊点的平均拉伸强度分别为53.75、46.59和44.38MPa,焊点高度减小导致微焊点内部力学拘束效应增强,因而微焊点强度提高;同时,热时效时间延长致使微焊点内钎料合金显微组织明显粗化,导致微焊点强度降低,经48h和96h热时效后不同高度的微焊点拉伸强度分别降至50.12、40.87、35.26MPa和44.13、38.38、33.48MPa;此外,96h较短热时间內的时效对微焊点界面IMC厚度无明显影响。
研究不同高度尺寸的微焊点在100℃环境温度下经施加电流密度为1×104A/cm2,通电时间为48h的直流电流作用后拉伸强度的变化结果表明,微焊点在经电迁移作用后拉伸强度明显退化,高度为100、200和300μm的微焊点其平均拉伸强度分别降至44.5、35.8和27.8MPa,相对于初始态分别下降了17.3%、23.2%和37.4%,强度退化程度随微焊点高度的减小而减弱,呈现出明显的尺寸依赖性。
研究进一步表明,电迁移还导致微焊点产生明显的极性效应,微焊点在经历较长时间电迁移后,阳极IMC明显比阴极厚。电迁移加速了微焊点阴极IMC的分解,导致阴极出现微空洞或裂纹,弱化了微焊点的阴极界面,使微焊点最终断裂位置处于钎料与铜丝界面的阴极,断裂模式也由未经电迁移的延性断裂变为延性与脆性相结合的模式。
With the electronic products moving towards lead-free, miniaturization and multi-functions, the reliability of electronic packaging interconnects has attracted more and more attentions. In this thesis, the typical copper-wire/Sn-3.0Ag-0.5Cu/copper-wire sandwich structured butt joints with various micro-scale sizes were prepared under the modeling reflow soldering condition. The isothermal aging and electromigration behavior of the micro-scale solder joints as well as the joint size effect were studied sysmatically. The tensile strength change of the micro-scale solder joints of different sizes after undergoing isothermal aging and electromigration were studied by a Dynamic Mechanical Analyzer (DMA) at the testing temperature of 100℃. The microstructure, fractographies and the electromigration induced polarity effect in micro-scale solder joints were characterized by an optical microscope (OM) and a scanning electronic microscope (SEM) equipped with energy dispersive spectroscopy (EDS).
The results of the influence of isothermal aging on the micro-scale solder joints show that both the joint thickness and isothermal aging time significantly affect the ultimate tensile strength of the joints under quasi-static micro-tension. Without isothermal aging, the tensile strength of the joints increases with decreasing the thickness of the joints, the average tensile strength of the solder joints with three different thicknesses of 100, 200 and 300μm are 53.75, 46.59 and 44.38MPa respectively; this is probably owing to the increase of mechanical constraint in the solder joints. It has also been shown that the tensile strength of the joint decreases with the prolongation of isothermal aging time which results in the coarsening of the microstructure in the solder joints. After aging for 48 hours, the average values of tensile strength of the above three types of joints decrease to 50.12, 40.87 and 35.26MPa repectively, and further decrease to 44.13, 38.38 and 33.48MPa after aging for 96 hours. However, the thickness of the intermetallic compounds (IMC) layer at the joint interface shows almost no change obviously even after aging for 96 hours.
Furthermore, the effects of electromigration on tensile strength of the micro-scale solder joints were also studied. The eletromigration tests were carried out with a constant current density of 1×104A/cm2 at 100℃for 48 hours. The experimental results show that the electromigration has brought about obvious degradation of tensile strength of the joints. The average values of tensile strength of the joints with the thickness of 100, 200 and 300μm decrease to 44.5, 35.8 and 27.8MPa after electromigration, i.e., decreased by 17.3%, 23.2% and 37.4% respectively. The degradation degree of the joint strength decreases with decreasing joint thickness and exhibits a clear dependence on the solder joints’size.
Moreover, it has been shown that the electromigration also induces obvious polarity effect in micro-scale solder joints. After undergoing a certain current loading, the IMC layer at the anode interface of the joints becomes thicker compared with that in the cathode. Clearly, the electromigration resulted in weakening of the solder/IMC interface in the joints’cathode and the decomposition of IMC at the cathode interface; subsequently the microviods and microcrack developed at the cathode interface of the joints, and finally the fracture occured at the cathode interface of the joints. Also, it has been seen that the electromigration leads to the transfer of fracture mode of the joints from the ductile to the combination of brittle and ductile.
引文
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