| Recent design of multilayer printed circuit boards in the electronic industry requires a precise control of plating processes in high aspect ratio through-holes to produce a uniform copper deposit. In order to investigate the uniformity of the deposit, current distribution in a through-hole electrode has been calculated and measured under DC, pulse and reversed pulse plating conditions. For the latter two conditions, the main focus is placed on the effect of the periodic operating variables. Agreements between experimental and theoretical current distribution are generally satisfactory.;In high aspect ratio through-holes, the uniformity in current distribution is most likely dominated by ohmic resistance. As mass transfer resistances near the end regions are alleviated in pulse electrolysis, the current distribution approaches the secondary current distribution, which is always less uniform than DC electrolysis under the same plating speed. This effect is more pronounced at higher frequencies and lower duty cycles. However, the overall plating speed can be increased significantly.;In reversed pulse electrolysis, the uniformity in current distribution is generally better or comparable to DC electrolysis, but the overall plating speed is low. Operating parameters governing the uniformity are found to be duty cycle and the ratio of anodic to cathodic applied potential. An approximate criterion for uniformity is derived as a function of applied potentials, duty cycle and kinetic parameters. The uniformity is generally improved at higher duty cycles, but insensitive to the frequency of the cycle. |
