Zincation characterization for electroless Ni/Au UBM of solder bumping technology

A Zn layer is formed on the aluminum pad of an integrated circuit prior to the deposition of electroless Ni/P and immersion Au layers and the subsequent formation of a eutectic Pb-Sn solder bump from stencil-printed solder paste. Different zincation levels are observed from visual inspection of the bond pad. These zincation levels can be in evidence on the same wafer. As a result, it was necessary to determine the significance of the individual bond pad´s coloration/zincation level in order to guarantee the reliability of solder bumps. The integrity of the solder bumps was evaluated through contact resistance measurement, bump shear, and stud pull after thermal conditioning. The results show that, although the zincation level has measurable influence on the bump strength, both electrical and mechanical properties of the bumps are acceptable for all zincation levels