Investigation of under bump metallization systems for flip-chip assemblies

The objective of this work was to investigate the quality of Under Bump Metallization (UBM) systems used for solder bump flip chip applications. These three commercial UBM systems were (i)Al/NiV/Cu UBM with print-and-reflow solder, (ii)Ti-W/Cu UBM with electroplated solder and (iii) Electroless Ni/Au UBM with print-and-reflow solder. The solder was of an eutectic Sn/Pb composition. UBM quality in terms of thermal stability, corrosion-resistance and thermal-mechanical reliability were investigated using various multiple solder reflow passes, high temperature storage, pressure cooker test and temperature cycling. Analytical tools such as Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray Spectrometry (EDS) were used to analyze the metallurgy reactions and failure modes. This study showed that none of the three systems investigated out-performed the other two in all aspects in the tests conducted.