Effect of expelling P content on Ni(P) dissolution and reaction with SnAgCu(Ni)

The present work studied the crystallinity effecting of Ni(P) layer on the interfacial reaction between SAC1205-0.05Ni solder and electroless-Ni(P)/electroless-Pd(P)/immersion-Au (ENEPIG) surface finish. By TEM analysis, two different Ni(P) crystallinities in ENEPIG surface finishes were confirmed, which are (1) nano-crystalline/amorphous and (2) polycrystalline microstructures. In the reflowing process, the actual Ni content in the molten solder is found to be dominated by the Ni(P) dissolution. Interestingly, we found that the Ni(P) dissolution of the Ni(P) layer with nano-crystalline/amorphous microstructure into the molten SAC1205-0.05Ni solder is less than that of the Ni(P) layer with a polycrystalline microstructure. We believe that the P content expelled from the Ni(P) layers would reside on the Ni(P) layer, which retards and defines the Ni(P) dissolution. Therefore, comparing to the Ni(P) layer with polycrystalline microstructure, more residual P would reside on the Ni(P) layer surface with nano-crystalline/amorphous, which would impede Ni(P) dissolution and Ni(P) consumption. Moreover, the phase and morphology of interfacial compound phase are greatly affected by the Ni amount in the molten SAC1205-0.05Ni solder. The more Ni content in the molten solder causes a high Ni content in the ternary (Cu,Ni)₆Sn₅ compound formed at the interface. Higher Ni content in the interfacial compound would convert needle-like (Cu,Ni)₆Sn₅ compound phase to long hollowed prismatic (Cu,Ni)₆Sn₅ rods.