US2383895A - Method of depositing copper and baths for use therein - Google Patents

Description

proximate 5.0 electrometric.

Patented Aug. 28,1945

METHOD OF DEPOSITING COPPER AND BATES FOR USE THEREIN Jesse a. smack, Oakville, and Frank lassalacqua, Waterbury, Conn.,

now by Judicial change of name, Frank Passal, assignors to United Chromium, Incorporated, New'York, N. Y., acorporatlon of Delaware No Drawing. Application May 1, 1941, Serial No. 391,310

4 Claims. (Cl. 204- 52) This invention relates to the electrodeposition of copper, to electrolytic solutions for use there-- in, and provides improvements therein.

In the -art of electroplating, it is a desideratum to be able to electrodeposit a copper coating as smooth, fine-grained and ductile as possible and with as high a tensile strength as possible, within the shortest time, i. e., at the highest current density. These coatings of copper, which may range in thickness from a few ten-thousandths of an inch to several thousandths, should have structural characteristics which permit easy buffing, polishing. inding, turning and etching operations. The surface obtained by these operations should be essentially free of surface imperfections such as pits, non-uniformity of grain structure, etc. so as to make these coatings useful and satisfactory as a basis for subsequent coatings such as nickel, chromium, etc. and for use as printing forms, imprinting or embossing dies, building parts to size, etc. Processes looking toward these ends are known in the prior art, notably the difierent modifications of the copper sulfate-sulfuric acid bath which have long been used in the art of electroplating.

The present invention provides a'copper plating bath having as its important and primary constituents the bivalent copper, the ammonium and'the formate radicals, and adjusted to a pH value between 2.0 and 4.0 electrometric. It also provides a method of electrodepositing dense, fine-grained, and smooth deposits of copper, of high tensile strength, throughout a wide current density range. It also provides a method' of regulating and maintaining the bath for continuous operation.

We have found that a. copper salt such as copper sulfate, copper formate, etc. may be dissolved in water and sufiicient ammonium and formate added in the form of ammonium hydroxide and formic acid, or in the form of salts containing these radicals, either combined or separately, such as ammonium formate or ammonium sulfate and sodium formate, to give a clear dark-blue solution at pH values to ap- From the increase in the permissible total salt concentration and. from the deepeningof color-which occurs on the addition of an ammonium-containing compound to a formate-copper solution, it is believed that a complex of the double salt type exists in our solution. By reason of this double salt complex, the high ionic mobility of the formate radical and the high buflering effect obtained, deposition of smooth, fine-grained copper coatings on a variety of basis metal surfaces which have been suitably treated is obtained at high current densities. Certain non-ferrous metals such as brass andbronze may be plated directly, after a suitable cleaning cycle, with good adherence of the copper coating to the basis metal. With some other metals, such as nickel it is often necessary to activate them as by gassing cathodically in a solution of sulfuric acid before electrodepositing the copper, in order to obtain good adherence. With other metals, such as steel, cast iron, zinc-base die castings, etc. which have a tendency to displace metallic cop per from a bivalent solution in the acid pH range and form a loosely adherent film of copper, a fiash" or strike" in an alkaline copper solution, such as a cyanide copper or a pyrophosphate copper bath is applied, before electrodepositing copper from the present bath for obtaining good adherence. The. application of an adherent and continuous. copper coating to the basis metal surface in a suitable alkaline copper bath prior to immersing the surface to be plated in the acid copper bath will result in adherent coatings.

Specific examples of the preparation of two 'baths usable in our process are the following:

In preparing this solution the stated amount of copper sulfate was dissolved in 500 cc. of water. The stated volume offormic acid was volume of ammonium hydroxide. The resulting solution was then diluted to a volume of one liter with water, and the pH adjusted as hereinafter described. 5

EXAHPLI'Z Solution composition Copper formate (technical grade 32.4% cu) (47.1% formate) g./l 185 Ammonium formate (NH4OOC.H) g./l 87.5

in which the concentration of copper, formate and ammonium is as follows:

Copper (Cu) -i g./l so Formate (H.000) g./l.. 155 Ammonium (NH4) g./l

In preparing this solution, the stated amount of copper formate was dissolved in 750 cc. of water. To the resulting solution, the stated amount of ammonium formate was then added and the solution was stirred until this salt dissolved. The resulting solution was then diluted to a volume of one liter with water, and the pH 25 adjusted as hereinafter described.

The two specific examples of solution preparaiion cited represent solutions containing the primary constituents. Other constituents such as boric acid and borates, phenolsulfonic acid, gum arabic, gelatin, glue, dextrin, molasses, starch, casein, alum etc. may be added as brighteners or buffering agents.

In commercial operation of the bath, the pH is adjusted to a value of below 4.0 electrometric with a preferred optimum operating range of 2.5 to 3.5 electrometric. After the bath has been prepared in a manner similar to that cited in Examples #1 and #2, the pH of the resulting solution will have a value approximately within a range of 2.0-4.5 as determined with a glass electrode. If the pH is below the preferred range of 2.5 to 3.5, it-may be raised so as to come within this range by an addition of a basic compound, 1 such as copper carbonate, sodium hydroxide, sodium carbonate, ammonium hydroxide, etc. If th pH of the prepared bath is above the preferred range of 2.5 to 3.5, it may be lowered to a value within this range by the addition of a small volume of a concentrated sulfuric acid solution or of a formic acid solution.

During the commercial operation of the bath, the pH will-have a tendency to slowly drift either upward or downward, the rate of which will depend on such factors as bath temperature, degree, of agitation, rate of anode corrosion, anode and athode current densities, etc. If the pH of the bath is too low (below 2.0), the tendency will be to increase loss by volatilization of formate as formic acid and the formation of crystalline films on the anode. If the pH is too high (above 4.0), the tendency will be to co-deposition of cuprous cxide and basic salts of copper with metallic copper at the cathode, and also to the formation of a non-conducting film at the anodes.- To obtain a satisfactory-and practical working of the bath, and to avoid the consequences ofthe above stated tendencies, the pH thereof should be regulated and maintained between values of 2.0 and 4.0 as measured-with a glass electrode with a preferred optimum range of 2.5 to 3.5. Th relative concentrations of the bath constituents may be varied widely and the solution will still have satisfactory plating characteristics. However, we generally prefer a metallic copper I then added, followed by the addition of the stated concentration of 45 to 75 g./l., a'formate concentration of 75 to 155 g./l., and an ammonium concentration of 20 to 30 g./l. The copper may be conveniently added as copper sulfate, the formate as formic acid, and the ammonium as aqua ammonia solution. The concentrations of copper, formate and ammonium may be suitably adjusted to meet requirements for a particular operating procedure such as current density range desired, degree of agitation available, bath temperature, etc. Baths of exceptionally high copper concentration are obtainable by the present invention. Commercial cyanide copper baths have a copper concentration of about 22 to 30 g./l. (3 to 4 oz./gal.) acid copper baths of about 45 to 52 g./l. (6 to 7 oz./gal.); whereas commercial baths according to the present invention may well have 60 to '75 g./l. (5 to 10 oz./gal.).

The primary constituents of the bath herein disclosed are each chemically stable and will not appreciably decompose on being subjected to electrolysis. On electrolysis at elevated temperatures, some loss of formate by volatilization as formic acid will occur, resulting in a slow rise in pH. This necessitates periodic additions of ftrmlc acid to replace the loss of formate. Since the bath is extremely well buffered, the operating pH range is relatively wide, and the allowable formate radical concentration has relatively wide limits, the slow loss of formic acid will not present any serious problems in production operation of the bath.

' Under ordinary operating conditions, temperatures ranging from to F. or higher may be satisfactorily used, the particular temperature being chosen with reference to the speed of deposition desired, and other considerations usual with a plater. An increase in temperature will result in a corresponding increase in the maximum current density available. It is recommended, however, that the operating temperature be kept as low as production requirements permit so as to minimize the loss of formic acid by volatilization.

j Operating current densities of around 300 amperes per square foot are practicable, depending .on such factors as solution composition, operatplastic, acid-proof brick, etc. Copper anodes are Cu g./1 63 Formate g./l NR4 i g.'/1 24 pH (electrometric) 2.70

using a cathode current density. of 260 amperes per square foot and a bath temperature of 95 F.

The deposit thus obtained was suitable for rotog'ravure printing, being easily polished and etched; I

From the foregoing it will be perceived that there is provided a bath and a method by which copper deposits of good grain structure or texture are obtained at a high rate of deposition,

higher in fact for comparable quality of deposit than any previously known process.

What is claimed is:

1. An aqueous bath for electrodepositing cop-.

per, comprising essentially, in solution, 45 to 75 g./1. of copper, 75 to 155 g./l. of formate radicals and 20'to 30 g./l. ammonium radicals, and having a pH value between 2 and 4 electrometric.

2. An aqueous bath for electrodepositing copper, according to. claim 1, wherein said pH value is between 2.5 and 3.5 electrometrlc.

. cording to claim 3, wherein the pH value is maintained between 2.5 and 3.5.

JESSE E. STARECK. FRANK PASSALACQUA.