The plating characteristics of an electroless cobalt-tungsten-phosphorus [Co(W,P)] layer and its capability to serve as a diffusion barrier of underbump metallurgy (UBM) for flip-chip Cu-IC are investigated. Increasing the pH of the plating solution in a range of 7.6-9.0 decreased the rate of deposition, increased the phosphorus content, altered the structure from polycrystalline to amorphous, and decreased the surface roughness of Co(W,P) films. The barrier capability of Co(W,P) to eutectic PbSn solder was evaluated by means of liquid- and solid-state aging tests. In specimens subjected to tests of these two types, a P-rich layer presented in between solder and unreacted Co(W,P), demonstrating the sacrificial barrier behavior of Co(W,P). Transmission electron microscopy (TEM) analysis of specimens subjected to liquid-state aging indicated the supersaturated P atoms, and the Co2 P consequently formed, might aggregate at grain boundaries of the P-rich layer, thus enabling the stuffed-type barrier capability of the Co(W,P) layer. TEM, scanning electron microscopy, and energy-dispersive spectrometer characterizations of specimens subjected to solid-state aging revealed that the reaction of solder and Co(W,P) implies a formation of CoSn2 and CoSn3 intermetallic compounds (IMCs) and a mixture of various IMCs in P-rich layer. Our experimental results demonstrated a satisfactory barrier property of electroless Co(W,P), heralding promising applications in UBM structures for flip-chip Cu-IC.