Advanced Assessment Methodology of Lead-free Solder Joints under Multi-loads in Three-dimensional Electronic Packaging

本文提出了三维电子封装无铅焊点热疲劳可靠性评估的基本方法。特别是多载荷工况下的可靠性评估，其失效机理更为复杂。纳米压痕测试进行了铅或无铅焊料球，以评估多轴应力下的机械性能。压痕蠕变过程中出现了棘轮效应和蠕变机制的转变。在薄膜的情况下，如电镀完成，机械性能可能会改变，由于多轴应力。基于测量的性能，锡晶须引发进行了检查。从幂律蠕变到扩散蠕变的转变加速了原子的迁移速率，在局部区域原子聚集形成锡须。另外还讨论了连接器在多种载荷作用下的可靠性问题。通过三点弯曲试验研究了多载荷下的低周疲劳性能。非弹性应变范围是焊点疲劳寿命的关键，随着载荷的增加而增大。最后，提出了一种多载荷作用下连接件疲劳寿命的评估方法。

This paper reports a fundamental methodology of the assessment for thermal fatigue reliability of lead-free solder joints in three-dimensional electronic packaging. In particular, multi-loads condition provides complex failure mechanism to the assessment of reliability. Nanoindentation tests were carried out for lead or lead-free solder balls to evaluate mechanical properties under multi-axial stresses. Ratchet and transition of creep mechanism were found during the indentation creep. In the case of thin film such as plating finishes, mechanical properties may change due to multi-axial stresses. Based on measured properties, tin whisker initiation was examined. Transition from power-law creep to diffusion creep accelerates atoms migrating rate and then tin whiskers initiate due to the accumulation of atoms at the localized area. Another discussion is the reliability of connectors under multi loads. Three point bending tests were performed to evaluate the low-cycle fatigue under multi-loads. Inelastic strain range, which is the key to fatigue life of solder joints, increases with the multi loads. Finally, a methodology to assess the fatigue life of connectors under multi loads was developed.