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The interface microstructure interaction between SnCuAg Pb-free solder and three different Ni-based alloys under bump metallization (UBM) systems are studied using transmission electron microscopy (TEM). Ni reacts with Sn-rich Pb-free solder swiftly and is consumed readily by Pb-free solder. Thermally stable intermetallic compounds at the solder/UBM interface such as Cu6Sn5 or Ni3P are necessary to protect Ni from being penetrated by Pb-free solders. Interface microstructure study reveals the potential process and reliability issues when applying Pb-free solders on existing UBM systems. Cu/Ni(V)/Al, can be used for Pb-free solder with marginal thermal budget. Au/Ni(V)/Ti/Al, is not suitable for Pb-free solder. And Au/eNi(P)/Al, is the most suitable for Pb-free solder and shows best thermal stability.