Effect of Ni-P thickness on solid-state interfacial reactions between Sn-3.5Ag solder and electroless Ni-P metallization on Cu substrate

Solid-state interfacial reactions between Sn-3.5Ag solder and electroless Ni-P metallization on Cu substrate were investigated for three different Ni-P thicknesses. It was found that during interfacial reactions, Ni ₃Sn₄ intermetallic grows at the Sn-3.5Ag/Ni-P interface along with the crystallization of electroless Ni-P layer into Ni₃P compound. Additional interfacial compounds (IFCs) such as Ni-Sn-P, Cu ₃Sn, Cu₆Sn₅, (Ni_1-xCu _x)₃Sn₄, and (Ni_1-xCu _x)₆Sn₅ were also found to grow at the Sn-3.5Ag/Ni-P/Cu interfaces depending upon the Ni-P thickness. In the sample with thin Ni-P layer, formation of these IFCs appeared at lower aging temperature and within shorter aging duration than in the samples with thicker Ni-P. The complete dissolution of electroless Ni-P layer into Ni₃P and Ni-Sn-P layers was found to be the main cause for the growth of additional IFCs. Across the Ni₃P and Ni-Sn-P layers, diffusion of Cu and Sn takes place resulting in the formation of Cu-Sn and Ni-Cu-Sn intermetallics. It is shown in this paper that multi-layered IFC growth at the Sn-3.5Ag/Ni-P/Cu interfaces can be avoided by the selection of proper Ni-P thickness.