With the advent of flip chip assembly, pads to be plated on printed circuit boards (PCBs) have shrunk dramatically to around 100 X 100 mu m(2) As a result, most commercial electroless nickel baths will fail to deposit nickel on such small micropads. In this paper. using a specially designed PCB layout, the effects of Pb2+ concentration and local environment are studied. A competition mechanism between two opposite actions, the adsorption from bulk solution by diffusion and the burying by nickel deposition is used to explain the effect of Pb2+ as a catalytic inhibitor. On a constantly growing surface, the dynamic density of adsorbed Pb2+ is the net result of these two opposite actions. Using this competition mechanism, the skipping problem of micropads is attributed to the combined result of nonlinear diffusion and high concentration of Pb2+ in the bulk solution. This competition mechanism also gives a new explanation for the effect of potential pinning. In the case of potential pinning, galvanic effect by electrons flowing from one area to another only exists during the initial period of nickel deposition. Once it is started, nickel deposition on any area is maintained by the electroless process, and hence, is actually self-supporting, By reducing the bulk concentration of Pb2+ to a lower range in a commercial bath, we have realized consistent nickel plating on micropads as fine as 50 mu m in diameter.