Abstract
Electroless Ni-P (EN) has been popularly adopted and used as a diffusion barrier in the under-bump metallurgy (UBM) for flip-chip application. The EN with different P contents was first deposited on activated Cu/Al 2 O 3 substrates. To prevent the EN from oxidation, a thin Au coating was further plated on the EN/Cu/Al 2 O 3 substrates. Two types of solder alloys (63Sn-37Pb and 42Sn-58Bi) and two compositions of EN (Ni-5.5wt.%P and Ni-12.1wt.%P) were employed to investigate the interfacial reaction in the joint of solder/Au/EN/Cu. Occurrence of EN and intermetallic compound (IMC) stripping and dissolving was revealed. After annealing, Ni 3 Sn 4 and Ni 3 P formed between the solder and the EN in all joints. However, some of the Ni 3 Sn 4 IMCs stripped into the solder for a longer annealing time. The stripped EN was first observed in the Sn-Bi/Au/Ni-5.5wt.%P/Cu/Al 2 O 3 joints annealed at 185°C for 180 min. The stripped IMCs and the EN then dissolved in the solder and formed the Ni-P-Cu-Sn-Pb solid solution in the Sn-Pb/Au/Ni-5.5wt.%P/Cu/Al 2 O 3 joints annealed at 200°C or 240°C. The phenomenon of IMC stripping was found in all joints. However, both the stripping and dissolving of EN was only observed in joints with Ni-5.5wt.%P. The tendency of IMC stripping was related to the amount of IMCs, while the EN stripping corresponded to the surface condition of the EN.