Effect of geometry on void formation in commercial electroplating of thin strips to copper

Abstract

Three plating trials comprised of six samples were performed to evaluate the ability to attach thin strips of varying cross section to a copper substrate via commercial nickel electroplating in a nickel sulfamate bath. Nickel plated to the top, bottom, and sides of the nickel strips, as well as to the substrate. A significant void formed beneath rectangular nickel strips for all geometries studied, including strip widths of 500 µm to 5000 µm and gap thicknesses from 100 µm to 1500 µm. This is due to the starvation of ions when two regions of growing grains impinge and entrap a volume of electrolyte, surrounding it completely by deposited nickel. Impingement often occurs just past the edge of the strip which causes the void width to be greater than the strip width and form seams at the edge of the void. The rate of plating is greatest in areas where sharp corners exist, due to higher current density. More plating reaches under the strip as the width decreases and/or gap thickness increases. Thus, the void size decreases with decreasing aspect ratio, defined as the strip width over gap thickness. The use of a cylindrical strip produces sound plating with no voids. This is because the lack of corners allows impingement to occur first beneath the center of the circular cross section, so liquid is never surrounded. The results of this study demonstrate how to minimize or avoid void formation in commercial electroplating of suspended strips which is of great importance in the installation of sensor strips in the coating layer of continuous casting molds.