Phenomena of electroless Ni-P and intermetallic-compound stripping and dissolving in Sn-Bi and Sn-Pb solder joints with Au/EN/Cu metallization

Electroless Ni-P (EN) has been popularly adopted and used as a diffusion bar-bump metallurgy (UBM) for flip-chip application. The EN rier in the under with different P contents was first deposited on activated Cu/Al2O3 substrates. To prevent the EN from oxidation, a thin Au coating was further plated on the EN/Cu/Al2O3 substrates. Two types of solder alloys (63Sn-37Pb and 42Sn-58Bi) and two compositions of EN (Ni-5.5wt.%P and Ni-12.lwt.%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, Ni3Sn4 and Ni3P formed between the solder and the EN in all joints. However, some of the Ni3Sn4 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/Al2O3 joints annealed at 185degreesC 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/Al2O3 joints annealed at 200degreesC or 240degreesC. 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.