Packaging technologies such as BGA and CSP are now widely used to fabricate high-performance IC packages with ever-increasing degrees of miniaturization. In addition, electroless Ni/flash Au plating is now seeing wide-spread application because it offers improved solder wettability, etc. on electrodes. However, P tends to concentrate at the interface between the solder and Ni electrode, thereby reducing the joint strength. Despite several studies concentrating on plating methods and solder compositions, no solution to this problem has yet been found. However, through investigations on the use of various soldering fluxes, we successfully mitigated this decrease in joint strength by using a special flux containing Cu when joining solder balls to electroless Ni/flash Au-plated lands. Cross-sectional observation of the solder/electrode interface confirmed that the thickness of the concentrated P layer was reduced by the proposed method. We believe that the Cu ions contained in the flux acted as a barrier to the diffusion of Ni into the solder, thereby limiting the formation of a concentrated P layer.