Diffusion barrier and bonding characteristics of amorphous and polycrystalline electroless Co(W,P) layers (-Co(W,P) and poly-Co(W,P)) to SnAgCu (SAC) solder were investigated. In the SAC-Co(W,P) sample subjected to liquid-state aging at 250C, the spallation of intermetallic compound (IMC) into solder, a nano-crystalline P-rich layer at SACCo(W,P) interface, and the recrystallized Co(W,P) containing Co2P precipitates were observed. As to the SAC-Co(W,P) sample subjected to solid-state aging at 150C, a thick IMC layer neighboring to the P-rich layer formed at the solderCo(W,P) interface. Liquid-state aging resulted in an IMC mixture without spallation whereas solid-state aging induced a layer-like IMC in SACpoly-Co(W,P) samples. Amorphous W-rich layers were also observed in SACpoly-Co(W,P) samples and it could not inhibit subsequent alloy reactions in the samples. Analytical results indicated the -Co(W,P) is a sacrificial- plus stuffed-type barrier whereas the poly-Co(W,P) is mainly a sacrificial barrier. The activation energies of IMC growth were 110.7 and 81.8 kJmol for SAC-Co(W,P) and SACpoly-Co(W,P) samples, respectively. High P content in -Co(W,P) was found to degrade the bonding strength to the solder as revealed by the shear test and the control of P content would be a key issue for electroless plating layer applied to under bump metallurgy as the diffusion barrier.