Mcgean-rohco, Inc.

Randal D King

Americus C Vitale

John R Dodd

Eda R Montgomery

Anthony J Arduengo, Iii

Carl W Reinbold

An efficient spray displacement tin plating process of copper printed circuit innerlayers is disclosed for the manufacture of multilayer printed circuit boards. The displacement tin plating solution is stabilized and replenished by incorporating free tin metal in the plating solution reservoir. Unwanted tin(IV) ions which are produced by aerial oxidation during the spray process, are reacted with tin metal in the reservoir to produce sufficient tin(II) ions to compensate for losses due to plating, oxidation and the like. Not only is the plating bath stabilized, but its useful life is extended dramatically.

Stabilized spray displacement plating process

A stabilized spray displacement tin plating process of copper printed circuit innerlayers is disclosed for the manufacture of multilayer printed circuit boards. During prolonged use of spray tin plating, the plating solution becomes saturated with cupurous thiourea complex which precipitates interfering with spray nozzles and other mechanical components. The tin plating solution is stabilized by removing portions from the reservoir before saturation is reached, selectively precipitating the thiourea complex, and returning the remaining solution back to the reservoir. The precipitated cupurous thiourea complex is redissolved in acid and either disposed of by conventional waste treatment or the acid solution can be electrowinned to reclaim copper and oxidize thiourea to form a more acceptable acid solution for waste treatment. Preferably, the thiourea is isolated from the anode during electrowinning, so that the acid thiourea solution formed may be used to partially replenish the tin plating solution in the reservoir. By such periodic removal of the complex, the plating bath is stabilized and its useful life is extended dramatically.

Process for extending the life of a displacement plating bath

An environmentally innocuous effective replacement for thiourea is disclosed for use as a complexing agent in displacement plating processes in which the plating solution is applied to the substrate surface to be plated by immersion or by spraying, cascading, pouring and the like. The replacement complexing agent is an imidazole-2-thione compound having the formula: ##STR1## wherein A and B are the same or different --R--Y groups, wherein R is linear, branched or cyclic alkenyl group containing 1 to 12 carbon atoms and Y is a hydrogen, halogen, cyano, vinyl, phenyl, or ether moiety. Of this class of compounds, 1-methyl-3-propyl-imidazole-2-thione is preferred for immersion tin plating. This class of complexing agents is particularly useful in spray displacement tin plating for the manufacture of printed circuit boards wherein free tin metal is added to the plating solution.

Complexing agent for displacement tin plating

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. In addition this high throw process makes it possible to obtain in a substantially shorter time, plated circuits and through-holes with an acceptably high aspect ratio.

High-throw acid copper plating using inert electrolyte

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.

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Wilmington, DE, United States of America

Randal D King