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

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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.