EBSD原位观察法研究无铅焊点热-机械疲劳再结晶机制

Recrystallization behavior and microstructural changes are observed in Pb-free solder joints during thermomechanical fatigue (TMF). The orientations of solder joints can be changed significantly by recrystallization. The initiation and propagation of cracks are frequently accompanied with recrystallization until the final failure of Pb-free solder joints. However, the underlying formation mechanism of recrystallization and its role in the failure of Pb-free solder joints are still not well understood. Also, in-situ electron backscattered diffraction (EBSD) observation, an effective way to clarify the mechanism of recrystallization, has not been reported mainly due to its inability of pattern indexing from the cross-section contamination after TMF. The applicants found that a method of encapsulating cross-sectioned joint in glass tube could be applied to solve the problem. In this study, based on the in-situ EBSD observations and analysis, the underlying formation mechanism of recrystallization and its role in the failure of Pb-free solder joints are discussed, contributing to a better understanding of the reliability of Pb-free solder joints.

在热-机械疲劳(Thermomechanical Fatigue, TMF)条件下，无铅互连焊点会发生再结晶和微观组织演变，焊点局部再结晶区的晶体取向发生显著改变，裂纹在焊点的再结晶区萌生扩展直至最终失效。然而，目前关于无铅互连焊点的再结晶机制及再结晶行为对其失效模式的影响机制未有定论。而且，尚无针对焊点TMF晶体取向演变进行的电子背散射衍射(Electron Backscattered Diffraction, EBSD)原位观察实验的相关报道，这主要是由于在TMF后，无铅互连焊点横截面存在污染物，使得EBSD花样的标定变得很困难。申请者发现采用石英管封装焊点横截面的方法可以有效地解决这一问题。本项目基于EBSD原位观察及分析，开展无铅互连焊点再结晶形成机制及再结晶行为对其失效模式影响机制的研究，将极大程度地提高对无铅互连焊点可靠性的认识水平。

焊点是微电子互连中不可或缺的组成部分，起到了机械连接和电信号传输的作用。目前，微电子封装空间减小，芯片产热加剧，焊点所承受的电流密度不断增加，而且，由于封装材料间热膨胀系数(Coefficient of Thermal Expansion, CTE)的差异，电源的频繁开关和环境温度的变化使得焊点所承受的应力应变进一步增加。因此，焊点的可靠性很大程度上决定整个电子产品的可靠性和寿命。无铅焊点通常只由一个或三个β-Sn晶粒所构成，β-Sn具有体心四方(Body-Centered-Tetragonal, BCT)的晶体结构，而这种结构具有各向异性会严重影响焊点可靠性。申请人在研究过程中避免了对焊点横截面进行再次抛光，研究结果表明：无铅焊点的再结晶行为由亚晶旋转引起，亚晶在转动过程中的旋转轴为简单轴，表现为复杂轴的旋转行为可能是多次旋转的结果。申请人在重熔Sn3.0Ag0.5Cu (SAC305)焊点中首次发现了“双孪晶形核行为”，并且以此为基础，研究了SAC305+3.0POSS (Polyhedral Oligomeric Silsesquioxanes)和Sn3.0Ag3.0Bi3.0In (SABI333)钎料焊点的重熔形核行为，发现随着颗粒的添加，焊点的重熔取向有在双孪晶的基础上弥散开来的倾向。申请人进一步研究了SAC305+3.0POSS钎料BGA焊点的热疲劳行为，以及SABI333钎料线性焊点电迁移行为，发现焊点的可靠性得到显著提高，并且SABI333焊点在电迁移作用下会发生晶粒旋转行为。