Abstract: Improved through-hole plating of printed circuit boards, wherein the ratio of the printed circuit board thickness to the diameter of at least one through hole is greater than 3 to 1, is achieved by a high-throw acid copper plating bath of this invention. The high-throw acid copper plating bath comprises an aqeuous solution of (A) copper sulfate, (B) sulfuric acid, (C) chloride ion, e.g., hydrochloric acid, (D) a carrier, (E) a brightener, and (F) an alkali metal salt; wherein the plating bath has a pH of not greater than about 1, the concentration of the hydrochloric acid is from about 6.0.times.10.sup.-4 to about 1.8.times.10.sup.-3 moles/liter, and the mole ratio of copper sulfate:sulfuric acid is not greater than about 1:25. Unlike previous high-throw acid copper plating baths, the bath of this invention contains an alkali metal salt. The use of the alkali metal salt makes it possible to maintain a lower acid concentration resulting in easier maintenance and more consistent plating.

Abstract: For acid copper plating baths containing a brightener used to produce smooth copper coatings of high brilliancy, it has been found that the "break-in" period normally needed after a brightener is added to the plating bath, has been virtually eliminated by use of the brightener of this invention. This brightener consists essentially of a peroxide oxidation product of a dialkylamino-thioxomethyl-thioalkanesulfonic acid wherein each alkyl and alkane group individually contains 1 to 6 carbon atoms and wherein the peroxide oxidation of the dialkylamino-thioxomethyl-thioalkane-sulfonic acid is carried out in an acid, aqueous medium having a pH of not more than about 1. In an added embodiment of this invention the acid copper plating bath also contains hydrolysis products of the peroxide oxidation product of a dialkylamino-thioxomethyl-thioalkanesulfonic acid.