The authors established a method to ensure uniform patterned copper plating layer thickness in semiconductor packages with high-density electrical circuits or ball grid arrays (BGA). A single BGA pattern unit was divided into small areas and for each, active area density was defined according to the area covered by the copper layer. Current distribution was determined for such an area by numerically solving Laplace and Butler-Voltmer equations, with consideration directed to active area density and plating solution conductivity. With the auxiliary grid electrode placed at a certain distance from the BGA so as to surround the latter by the former electrode, the current was found to be quite effectively prevented from becoming concentrated on sparsely dispersed circuit patterns generally situated near the edge of a unit BGA. Current distribution was noted to vary according to position and cross section size. Error could be made as little as 3.37% when this position was optimal.