Electroless Ni-P/Au plating is most common surface finishing for Cu pads or under bump metallization in ball grid array (BGA) packaging and chip scale packaging (CSP). Although the thickness of Au layer may affect the interfacial reaction between solder bumps and Ni-P/Au plating and thereby the reliability of the joints, the effects have not been clarified yet. In the present study, therefore, the interfacial microstructure and joint strength of Sn-Ag solder and Sn-Ag-Cu solder bumps reflowed on electroless Ni-P/Au plating with varying thickness of Au layer (0 to 500nm) were investigated. The effect of solder bump size on the joint properties was also evaluated. In the Sn-Ag solder joints, a P-rich layer, which resulted from reaction of Ni to the solder, formed at the interface between the solder and Ni plating regardless of thickness of Au coating. On the other hand, in the Sn-Ag-Cu solder joints, no P-rich layer formed at the joint interface except for the sample with 250nm and 500nm Au coating, in which a thin P rich layer was observed. This is caused by (Cu,Ni)6Sn5 layer formed at the interface, which suppresses diffusion of Ni into the solder. Since the P-rich layer was brittle and included defects, the joint strength degraded when thickness of Au layer was 250nm or more in both of the Sn-Ag and Sn-Ag-Cu solder joints, although the latter had higher joint strength than the former. When the size of the solder bumps reduced from 700ƒÝm to 300ƒÝm, the detrimental effect of the thick Au coating was more prominent. Thus, the thickness of Au coating should be controlled to be necessary minimum. As a result, Au coating thickness of 50nm provided the best joint strength in this work.