The interfacial microstructures of Sn-3.0Ag-0.5Cu (SAC305) solder systems with thin Ni(P) mono-coatings and Cu-Ni(P) dual-coatings were investigated after reflowing and isothermal aging. The ultrathin mono-Ni(P) plating of the SAC305/Ni(P) solder joint was found to rapidly decompose and then transform into a Ni 2 SnP phase. An intermetallic compound (IMC) formed at the plating/substrate interface, indicating that the ultrathin mono-Ni(P) plating was not an effective diffusion barrier. However, only a single IMC layer ((Cu,Ni) 6 Sn 5 ) formed at the solder/plating interface in the SAC305/Cu/Ni(P)/Cu system. The (Cu,Ni) 6 Sn 5 IMC effectively suppressed atomic diffusion, protecting the Ni(P) plating and Cu substrate. Although P-Sn-O pores formed in the root of the (Cu,Ni) 6 Sn 5 IMC layer, the dual-Cu/Ni(P) plating protected the solder system for an extended period. The IMC growth rate constants of the SAC305/Cu, SAC305/Ni(P), and SAC305/Cu/Ni(P)/Cu solder joint systems were 0.180, 0.342, and 0.068  μ m/h 1/2 , respectively. These results indicate that the application of dual-Cu/Ni(P) plating can effectively hinder the growth of IMC.