Influence of Intermetallic Properties on Reliability of Lead-Free Flip-Chip Solder Joints

Electroplated pure tin bumping as a lead-free alternative for ultra fine pitch applications is a relatively easy process and has provided us with comparable results to eutectic Sn/Pb for thermal cycling reliability. Experimentally, it has been reported that a significantly higher (~40%) thermal cycle fatigue life is seen with the use of cobalt under bump metallization (UBM) instead of copper UBM for a flip-chip device assembled on an alumina substrate. In the current approaches used to estimate fatigue life of solder joints, the solder joint is treated as a homogenous material and modeled as such. However, the smaller joint sizes and higher reactivity of Sn implies that a larger amount of intermetallics are formed as a percentage of bump volume. The existing approach cannot account for the influence on the fatigue behavior of these intermetallic layers within the solder joint. In order to investigate if a simplified engineering approach can provide some insight into this issue, we have attempted to explicitly model the intermetallics as a continuous but separate part of the solder joint. The main damage parameter investigated is the accumulated inelastic strain in a single thermal cycle. From the results, it is clear that the Young's modulus of the intermetallic layer plays an important role, more so when the ratio of intermetallic thickness to the solder joint standoff increases. Thickness of the intermetallic layer also influences the overall strain accumulation in the same manner. The CTE of the intermetallic layer has a relatively lesser influence on the strain accumulation. Both the experimental and FE results suggest that changing the UBM from copper to cobalt can improve the fatigue life by 20%-30%.