Diffusion barrier and bonding characteristics of amorphous and polycrystalline electroless Co(W,P) layers (alpha-Co(W,P) and poly-Co(W,P)) to SnAgCu (SAC) solder were investigated. In the SAC/alpha-Co(W,P) sample subjected to liquid-state aging at 250 degrees C, the spallation of intermetallic compound (IMC) into solder, a nano-crystalline P-rich layer at SAC/Co(W,P) interface, and the recrystallized Co(W,P) containing Co(2)P precipitates were observed. As to the SAC/alpha-Co(W,P) sample subjected to solid-state aging at 150 degrees C, a thick IMC layer neighboring to the P-rich layer formed at the solder/Co(W,P) interface. Liquid-state aging resulted in an IMC mixture without spallation whereas solid-state aging induced a layer-like IMC in SAC/poly-Co(W,P) samples. Amorphous W-rich layers were also observed in SAC/poly-Co(W,P) samples and it could not inhibit subsequent alloy reactions in the samples. Analytical results indicated the alpha-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 kJ/mol for SAC/alpha-Co(W,P) and SAC/poly-Co(W,P) samples, respectively. High P content in alpha-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. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.004111jes] All rights reserved.