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, Ni3Sn4 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, Cn(3)sn, Cu6Sn5, (Ni1-xCux)(3)Sn-4, and (Ni1-xCux)(6)Sn-5 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-Players, diffusion of Cu and Sri 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. (c) 2005 Elsevier B.V. All rights reserved.