The effect of interfacial reaction of Ti/Ni(V)/Au under-bump metallisation (UBM) systems in Pb free solder had been investigated. The objective was to examine the microstructure change and intermetallic formation of Ni-based UBM during isothermal annealing in lead free solder as well as to understand its impact on the UBM failure mechanism. Studies revealed that after IR reflow, spalling of Ni-Sn compound from the UBM took place. A layer of Ni-Sn-V was found to have form in the UBM. The formation of Ni-Sn-V layer was believed to have an impact on the failure mechanism of the Pb free solder. Studies also showed that doubling the Ni layer thickness in this UBM system did not have significant improvement to the overall integrity of the solder joint. However, samples with electroless Ni(P)/Au UBM in lead free solder showed relatively good thermal properties. No major change in the intermetallic composition was observed. More details on the microstructure change during thermal aging of the UBM systems were presented in this article.