Effect of the macro- and microthrowing power of the electrolyte on the uniformity of distribution of electroplated copper in through-holes for PCB

Abstract

This article studies the effect of macro- and microdistribution factors on the uniformity of electroplated copper coatings in through-holes of printed circuit boards. A computer program has been developed for simulating the primary potential distribution field and for calculating the local current densities for a two-dimensional model of electrodes with arbitrary shapes and mutual arrangement. The cross-sectional profile of a through-hole with a channel width equal to the hole diameter was taken as the two-dimensional model. A calculation of the primary potential distribution field predicts that the current density in wide through-holes decreases significantly and electroplating in narrow holes is nearly blocked. As the diameter of a through-hole decreases, the macrothrowing ability decreases in accordance with the decrease in the Wagner number, while the nonuniformity of the copper deposit thickness inside the holes increases, as confirmed by micrographs of cross sections of samples obtained in the electrolyte with and without a set of additives. The latter comprised leveling agents based on polyalkylene glycols and nitrogen-containing heterocyclic compounds, along with brightening organic sulfur-containing additives. Analysis of data on the distribution of copper on the cross sections of samples treated in stagnant electrolytes and with stirring, other conditions being equal, reveals a significant effect of uneven microdistribution in narrow holes. Addition of the set of leveling and brightening compounds to the electrolyte provides a small positive leveling power, which makes it possible to obtain uniform smooth coatings without a local thickness decrease at the orifice. The noticeable increase in the copper layer thickness in the middle of narrow holes in the presence of additives observed under conditions of diffusion limitations cannot be explained solely in terms of the effect of primary and secondary distribution. The effect of the increase in d1/2 may be due to the depolarizing effect of the brightening sulfur-containing components of additives inside the holes.