Role of Cu contents in the isothermal metallurgical reaction for the flip chip Sn-3.0Ag-[0.5 or 1.5]Cu joints with Cu/electroless Ni-P/immersion Au bonding pad during aging

Cu/electroless Ni/immersion Au metallization is usually used as bonding pad on the substrate side of flip chip solder joints. Sn-Ag-Cu solder is one of the promising candidates to replace the conventional Sn-Pb solder. Isothermal interfacial reaction in Sn-3.0Ag-(0.5 or 1.5)Cu solder joints with electroless Ni/immersion Au surface finish after aging at 150degC was investigated in this study. Intermetallic compounds (IMC) of (Cu_1-y,Ni_y)₆Sn₅ , (Ni_1-x,Cu_x)₃Sn₄ and P-rich layer formed between the solder and the EN layer in both Sn-Ag-Cu joints during aging. For the Sn-3.0Ag-0.5Cu joints after more than 2000 h aging, (Ni_1-x,Cu_x)₃Sn₄ IMC gradually grew. Through the investigation of cross-sectional microstructural observation and quantitative analysis, the interfacial microstructure and the elemental distribution between the solder and bonding pad could be clearly revealed. It was demonstrated that Cu content in the solders near the solder/IMC interface played an important role in the growth of (Ni_1-x,Cu_x)₃Sn ₄ and (Cu_1-y,Ni_x)₆Sn₅