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

本文报道了三维电子封装中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，为可靠性评估提供了复杂的失效机制。对含铅和无铅焊料球进行了纳米压痕试验，以评估其在多轴应力下的力学性能。在压痕蠕变过程中存在棘轮和蠕变机制的转变。在薄膜的情况下，如电镀饰面，机械性能可能会因多轴应力而改变。在测试性能的基础上，对锡晶须萌生进行了研究。由幂律蠕变向扩散蠕变转变，加速了原子迁移速率，原子在局部区域聚集，形成锡晶须。另一个讨论是连接器在多载荷下的可靠性。采用三点弯曲试验对多载荷下的低周疲劳进行了评价。非弹性应变范围是影响焊点疲劳寿命的关键，随载荷的增加而增大。最后，提出了一种评估多载荷作用下连接器疲劳寿命的方法。

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.