Effect of Under Bump Metallization (UBM) Quality on Long Term Reliability

Ti/Cu/Ni Under Bump Metallization (UBM) structure is used for eutectic. i.e.. 63Sn37Pb. solder bump with Ti for adhesion and seal. Cu as current earning layer, and electroplated Ni as diffusion barrier. In this study, the effect of the integrity of UBM structure within a passivation via, particularly the integrity of Ni barrier layer, on the product long-term reliability has been investigated. The defective Ni plating within a passivation via is mainly due to the bumping pattern, the difficulty in monitoring Ni thickness and a non-appropriate Ni thickness specification. The reliability testing results indicate that the defective Ni plating has no effect on bump shear strength, temperature cycling and high temperature storage tests. But. it does cause a failure when an elevated temperature and current are applied such as temperature humidity with Bias Current test in tins study. Failure analysis was performed to understand the failure modes and failure mechanisms were also analyzed. The analysis results indicate that the Ni thickness, particularly at the corner of via is most critical to the reliability. In order to guarantee the long-term reliability, it is recommended that an appropriate Ni thickness specification be established based on quantitative analysis results by considering six major factors discussed in section eight.