Sn基钎料互连焊点晶体取向和微观组织非均匀演变机理

One of the most important reasons for failure of solder interconnects in service is creep and fatigue caused by CTE mismatch between different packaging materials. Obvious evolution of crystal orientation and microstructure is frequently observed before the final failure of Sn-based solder interconnects, and this evolution is generally inhomogeneous, which is close related with recrystallization. Sn-based solder interconnects can recrystallize even at room temperature, and recrystallization only occurred at the local region of the solder interconnect, leading to the decreased hardness and strain concentration in the localized recrystallized region.Furthermore, the continuous grain boundary network formed after recrystallization facilitates grain boundary sliding, making the localized recrystallized region become the weakest link of the solder interconnect and leading to the initiation and propagation of cracks and final failure in this region.Aiming at the inhomogeous evolution mechanism of grain orientation and microstructure,this project intends to study the crystallographic relationship between the parent grain and recrystallized grains, and to discuss the deformation law and the localized recrystallization mechanism of Sn-based solder alloys. Also, the coarsening mechanism of microstructure in the recrystallized region was also studied. At the same time, it was discussed that what kind of crystal orientation, grain structure or microstructure contributes to increasing the properties of creep and fatigue,providing a new train of thought and data accumulation for inhibiting or postponing localized recrystallization, delaying the initiation and propagation of cracks, thus increasing the service life and reliability of solder interconnects.

不同封装材料间热膨胀系数失配导致钎料蠕变疲劳是服役焊点失效的重要原因之一。Sn基钎料互连焊点失效前常伴随有明显的晶体取向和微观组织演变，而且这种演变常常是非均匀的，这与再结晶密切相关。Sn基钎料焊点在室温下即可产生再结晶，而且只发生在焊点局部区域，使硬度明显下降，引起局部再结晶区的应变集中；而且再结晶后产生的连续晶界网络又促进了晶界滑移，从而使局部再结晶区成为互连焊点的薄弱环节，导致裂纹在此处萌生和扩展并最终失效。本项目针对焊点内晶体取向和微观组织的非均匀演化规律，分析原始晶粒和再结晶晶粒间的晶体学关系，探讨Sn基钎料本身的变形规律和局部再结晶行为产生机制，研究再结晶区内微观组织粗化机理。同时讨论何种晶体取向、晶体结构或微观组织更有利于提高焊点蠕变疲劳性能，为抑制或推迟焊点内部局部再结晶及其伴随的微观组织非均匀弱化，延迟裂纹萌生扩展，从而提高焊点服役寿命和可靠性提供新的思路和实验数据积累。

项目针对Sn基钎料互连焊点在正常服役条件下主要失效模式中存在的共性问题，即在应力应变作用下发生局部再结晶使互连焊点发生非均匀退化，研究了Sn基钎料互连焊点晶体取向和微观组织在焊点力学性能弱化和失效破坏中发生的非均匀演变规律及其所起的作用。提供了焊点从重熔到服役直至最终失效破坏过程中焊点微观组织演化的清晰脉络，从根源上挖掘Sn基钎料互连焊点变形和失效破坏背后更为深入的本质，揭示其对形变、断裂和可靠性影响。研究结果将为Sn基钎料互连焊点可靠性研究提供理论和数据上的支持，提高对互连焊点变形和失效模式等可靠性问题认识，同时为抑制局部再结晶及其引起的微观组织非均匀弱化、提高焊点可靠性探索新的思路。同时，项目还揭示了Sn基钎料产生局部再结晶弱化这一失效现象背后的根源性的问题是Sn基钎料的熔点低（217-221℃），导致产生局部再结晶的温度过低。经实验证明，交变载荷作用下，Sn基钎料即使在室温下也会产生再结晶（Sn在室温下的同系温度，即室温与Sn熔点的比值为：0.59（绝对温度）），蠕变会成为焊点失效的根本性原因。因此，本项目还初步探索了提高焊点熔点的方法，使回流后低熔点的Sn与Cu或者Ag等高熔点金属发生反应后完全消耗掉，形成高熔点的金属间化合物，从而使所形成焊点的熔点大幅提高，从而摆脱Sn熔点低的困扰，抑制了焊点的蠕变失效，从而进一步提高焊缝的服役可靠性。