Investigation of the influence of UBM on Lead Free Flip Chip Solder Fatigue Life by Explicit Finite Element Modeling of Intermetallic Layers

摘要

Lead free flip chip bumping for fine pitch applications necessitates the use of electroplated bumps. But the very nature of electroplating process makes it difficult to use the more common Sn/Ag or Sn/Ag/Cu lead free alloys. Electroplated pure tin bumping is relatively easy process and has provided with comparable results to eutectic Sn/Pb for thermal cycling reliability. However tin is extremely reactive and hence the choice of Under Bump Metallization (UBM) is very critical. Experimentally, it has been reported that a significantly higher (～40%) thermal cycle life is seen with the use of Cobalt UBM instead of Copper UBM for a flip chip device assembled on an alumina substrate. Under 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 means that larger amount of intermetallics are formed as a percentage of bump volume. The existing approach cannot account for the influence on the fatigue behaviour 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. The influence of the intermetallics properties on the stress and strain distributions and also on the accumulated inelastic strain have been explored and a reasonable correlation between the experimental and FEA results has been obtained. Both the experimental and FE results suggest that changing the UBM from copper to cobalt can improve the fatigue life by 20- 40%.