US2871171A - Method of electroplating copper on aluminum - Google Patents

Description

United States Patent METHOD OF ELE'CTROPLATING COPPER ON James Thomas Nesbitt Atkinson, Dartmouth, Nova Scotia, Canada, assignor to Her Majesty the Queen in the right of Canada as represented by the Minister of National Defence No Drawing. Application May 10, 1956 Serial'No'. 583,943

5 Claims. ('Cl. 204-33) This invention relates to new plating baths and to the plating of metals. It relates more particularly to'a new plating process whereby satisfactory copper plate may be deposited upon aluminium and its alloys using simpler techniques than hitherto thought possible.

Previously known methods for plating copper on aluminium or its alloys, with the exception of that described and claimed in my earlier copending application Serial No. 6'96,249,'filed November 10, 1955, have been very complicated and involve the use of a series of mechemical or chemical pretreatments by means of which the protective oxide coating is removed or replaced for instance with a zinc coating. While fairly satisfactory plates can be achieved,. the process is expensive and the attainment of uniform results requires a high degree of control of the various steps and the racking of the articles which are to be plated. Commercial quantity production on an economic basis such as can be achieved by the well known barrel plating process or by continuous wire or strip plating is generally unprofitable or impractical.

I have now found that it is possible to overcome the difficulty caused by the presence of the oxide film on aluminium or its alloys by electroplating copper from suitable mildly acid solutions following a simple alkaline degreasing operation only. Such solutions would be able components which will be reasonably effective in dissolving aluminium oxide film and be near the threshold of initiating attack on the underlying metal under conditions existing in the plating bath.

In addition, these solutions must alsopossess the well known properties required of any successful plating solution. It is, of course, not possible to list exhaustively all additions that could be made for improving the various properties of such a plating solution; those skilled in the art will, however, readily recognize the additions likely to have a further beneficial effect.

The essential components of the plating solution that I have discovered are a suitable form of copper in solution along with ammonium, oxalate, and pyrophosphate ions or complexes thereof, or the equivalents or substitutes for these as enumerated below.' The ammonium ions may be replaced in whole or in part or augmented pliates, The term condensed phosphates is hereused- ICC to mean materials which are derived from more than a single mole of anhydride (P and embraces themeta phosphates and tripolyphosphates as well as the pyrophosphates. Specifically the pyrophosphates may be, re-

placed wholly or partly by tripolyphosphates (corresponding to the acid H P O or partly by polymeta-- phosphates, (corresponding to the acids (HPO and particularly hexametaphosphates (corresponding, to the acid (HPO In practice the choice of the particular phosphate touse is governed largely by economic factors; consequently emphasis is placed on pyrophosphates as the presently cheapest possibility that appears to be fully satiscleaned should be treated with a suitable acid to removev factory. No evidence has been found to suggest that.

polyphosphaes, for example, are less desirable chemically than pyrophosphates. neither the polyphosphates nor polymetaphosphates are commercially available as pure chemical individuals; it would be desirable to use that particular chemicalin dividual which was most effective in this process if suchchemicals were available industrially.

Suitable proportions for the essential ingredients. ofthe plating bath are: pyrophosphate. or equivalent: about 100 grams per litre (as ammonium pyrophosphate); oxalic acid or equivalent: 60 grams per litre (as oxalic acid crystals); copper: 20 grams per litre (as cupric oxide). The plating solution however may be considerably more concentrated or dilute than this and satisfactory plates still obtained if suitable temperatures and current densities are employed as will readily be appar-- ant to a skilled plater. For instance solutions of'up to three times the above concentration have been tried out.

and used at higher temperatures of up to about C. and current densities of up to 30 amps. per square foot. Likewise solutions of half the above cencentration have been used at lower allowable current densities. Moreover, the proportions of the various essential ingredients may be varied considerably in many instances from those stated above, with the proviso that the amount of pyrophosphate or equivalent should be less than that required to fully complex the copper present, or in other words less than 2 mols of pyrophosphate per. mol of cupric oxide. In order to keep the copper in. solution at any pH within the range 5.0 to 6.5, other complexing.

agents for copper such as the oxalate ion, ammoniumion or triethylamine must of course be present in significant amounts. The temperature range is from about 30 to C. and withthe preferred bath compositions, temperatures are normally about 50 to 65 C. and the current density about 15 amps. per square foot of cathode surface.

. In addiionto the necessary components listed above, certain other additives such as fluorides or borates may be added to the bath in. small quantitiesv with beneficial results in certain cases as shown in the examples.

The process of the invention requires a simple degreasing operation only prior to plating.v If the work is heav-. ily soiled an initial vapour degreasing step may. be desirable, but a mild alkaline degreaser should be used. in any case. For example, immersion in an aqueous solution containing 3% eachof sodium carbonate and sodium phosphate for about two minutes at 60, C. is suitable for pure aluminium or the more corrosion resistant alloys. work in the alkaline solution for too long a time, particularly with the less corrosion resistant alloys; these lat ter tolerate shorter cleaning times only. The best general rule is to continue the cleaning until the appearance resembles that of a sample of 25 alloy cleaned for a two minute period in a freshly prepared cleaner as given above. Work which has been inadvertently over:-

Patented Jan. 27, 1959 I It should be noted that.

Some care should be taken, not to leave the 3 smut, following which it may be reprocessed in the alkaline degreaser.

Following the dcgreasing operation, the work should be rinsed and transferred to the plating bath, and should preferably be immersed for about half a minute before applying the plating current. Plating should then be performed at a current density of about amperes per square foot at the preferred operating temperature of -65 C. Normal rinsing and drying operations then follow.

Deposits of copper of up to about 0.0002 inch or so in thickness will have satisfactory adherence to aluminium alloys for work plated on racks or in bulk in a barrel or on a continuous wire plating machine or the like. Rack plated work shows adhesion in shear consistently exceeding the ultimate strength of tin-lead solder and is fully resistant to all adhesion tests based on bending. Barrel plated components show similar plating adhesion, with the adhesion of the coating probably exceeding the ultimate properties of the solder once again. Rivets so plated, for example, can have heads formed or malformed without disclosing any inadequacies in the plating Wire plated on a slow speed continuous plating machine on a laboratory scale showed adhesion probably of the order of the ultimate properties of the solder and was capable of sustaining severe bending without evidence of damage to the copper plate. Unfortunately, it is not possible to give precise adhesion values, as no generally applicable quantitative adhesion test has yet been devised.

If attempts are made to deposit copper plate thicker than a few tenths of a mil, or to use a deposit of copper produced by this process as a basis for the further deposition of other metals, certain precautions must be taken in order to develop maximum copper-aluminium adhesion. The reason for this is not definitely understood, but it seems to be due to the deleterious action of hydrogen on the copper-aluminium interface. It has been found that this difliculty may be at least partly overcome by means designed to exclude or remove hydrogen from this location, as follows: plating with a periodically interrupted direct current, so that the average current density can be kept low While retaining the advantages of good covering power associated with the higher current density actually used for plating; and/ or subjecting the partly plated work to a treatment designed to allow gases to escape from the metal, such as leaving the work for several days at room temperature, or heating for a few hours at 110 C. or for a shorter time at a higher temperature. Further techniques, such as properly controlled agitation etc. will be available in larger scale operation, as will be obvious to those skilled in the art.

In addition to the peculiar usefulness of the plating baths disclosed for plating on aluminium and alloys thereof, they can be used for plating on a variety of other metals among which are iron and zinc and their alloys. For this purpose they offer important advantages over the well known processes involving cyanides, as the new formulations are substantially less toxic than cyanides and they do not produce a dangerous gas on being mixed with acid solutions of the sort commonly found in a plating shop. They are particularly advantageous as a preliminary to high speed acid plating in continuous processes.

The process of the invention is illustrated by the following examples which it will be appreciated are exemplary only and are not intended in any sense to indicate the scope of the invention. The methods of formulating the various solutions are those which happened to be convenient at the time for maintaining precise composition control, and are not necessarily preferred commercial practice. In commercial practice the most convenient formulation procedures will depend on the starting materials available and will be obvious to those skilled in the art; g a Y Example I Grams Ammonium pyrophosphate 100 Oxalic acid crystals Cupric oxide 20 Water to make 1 liter.

The cupric oxide was dissolved in a hot solution of the oxalic acid and ammonium pyrophosphate, and the final value of the pH was brought to 5.6, using ammonia and/or pyrophosphoric acid for this adjustment.

Using a copper anode and a properly degreased aluminium cathode, a current of 15 amperes per square foot was passed through this solution for twenty minutes at 55 C. A fully adherent dull copper deposit was produced. Satisfactory plates were produced at average current densities ranging from about 5 to about 15 amperes per square foot. An uninterrupted current density of about 10 to 15 amperes per square foot is generally most satisfactory from an operating point of view.

Example ll As Example I but containing 10 grams of boric acid per liter in addition.

Example III As Example I but containing 0.2 gram of ammonium fluoride in addition.

Example IV As Example I but containing 50 grams of triethylamine per liter in addition. (This solution gives the best all round performance.)

Example V As Example I, but replacing pyrophosphoric acid and pyrophosphate by polyphosphoric acid and polyphosphates respectively throughout.

Example VI As Example I with grams of a 1:1 mixture of ammonium pyrophosphate and ammonium hexametaphosphate replacing the ammonium pyrophosphate in the solution. Similar results were obtained.

Repetitions of this experiment varying the range of this ratio showed that adhesion falls off unless about one quarter of the total phosphate is present as pyrophosphate.

Example VII The procedure of Example I was followed, but the composition of the bath was varied as indicated in the following table:

Prepared by evaporation of the solution prepared as in Example IV. The results were comparable with those obtained in Example IV.

Solution 3 was too close to the saturation limit for general use, though the more concentrated solutions, as expected, would permit the use of a higher current density (e. g. up to about'25 amperes per square foot) and necessitated the use of higher temperatures (e. g. up to about 85 C.).

Whatl claim as my invention is:

1. A method of plating copper on aluminum and its alloys comprising cleaning an aluminum cathode in a mildly alkaline cleaning bathyrinsing the cathode; immersing the cathode in an aqueous plating bath having a pH from about 5.0 to 6.5 consisting essentially of water containing about to 60 grams per liter of bivalent copper, calculated as cupric oxide, about 50 to 300 grams per liter of a compound selected from the group of soluble pyrophosphate, soluble polyphosphate and mixtures thereof, calculated as ammonium pyrophosphate, the total condensed phosphate being present in an amount which is less than that required to fully complex the copper, about 30 to 180 grams per liter of a soluble oxalate, calculated as oxalic acid, and about 25 to 150 grams per liter of a basic cation selected from the group consisting of ammonia and tertiary (lower alkyl) amines, calculated as triethylamine; and subjecting the cathode to a plating current of from about 5 to 30 amperes per square foot at a temperature of 30-90 C. until a copper plate has been deposited thereon up to a thickness of about 0.0002 inch.

2. The method set forth in claim 1 wherein the cathode is immersed in a plating bath for about one minute before being subjected to the plating current.

3. The method set forth in claim 1 wherein said plating bath additionally contains about 10 grams per liter of a soluble borate.

4. The method set forth in claim 1 wherein said platof a soluble fluoride.

5. A method of plating copper on aluminum and its alloys comprising cleaning an aluminum cathode in a mildly alkaline cleaning bath; rinsing the cathode, immersing the cathode in an aqueous plating bath having a pH of about 5.0 to 6.5 and consisting essentially of water containing about 20 grams per liter of cupric oxide, about 100 grams per liter of ammonium pyrophosphate, about 60 grams per liter of oxalic acid and about grams per liter of triethylamine; and subjecting the cathode to a plating current of about 15 amperes per square foot at a temperature of about C. until a copper plate has been deposited thereon up to a thickness of about 0.0002 inch.

References Cited in the file of this patent UNITED STATES PATENTS 1,322,494 Merritt Nov. 18, 1919 2,411,674 Wilson Nov. 26, 1946 2,437,865 Stareck Mar. 16, 1948 2,493,092 Stareck Jan. 3, 1950 2,766,195 Combs et al Oct. 9, 1956 OTHER REFERENCES Modern Electroplating, edited by A. G. Gray, John Wiley 8: Sons Inc., New York, 1953, pages 225-228.

Light Metal Age, June 1947, pages 8 and 9.

Metal Finishing, March 1946, pages -115.

The Monthly Review," American Elcctro Plates Society, March 1946, page 269.

Claims (1)

1. A METHOD OF PLATING COPPER ON ALUMINUM AND ITS ALLOYS COMPRISING CLEANING AS ALUMINUM CATHODE IN A MILDLY ALKALINE CLEANING BATH; RINSING THE CATHODE; IMMERSING THE CATHODE IN AN AQUEOUS PLATING BATH HAVING A PH FROM ABOUT 5.0 TO 6.5 CONSISTING ESSENTIALLY OF WATER CONTAINING ABOUT 10 TO 60 GRAMS PER LITER OF BIVALENT COPPER, CALCULATED AS CUPRIC OXIDE, ABOUT 50 TO 300 GRAMS PER LITER OF A COMPOUND SELECTED FROM THE GROUP OF SOLUBLE PROPHOSPHATE, SOLUBLE POLYPHOSPHATE AND MIXTURES THEREOF, CALCULATED AS AMMONIUM PYROPHOSPHATE, THE TOTAL CONDENSED PHOSPHATE BEING PRESENT IN AN AMOUNT WHICH IS LESS THAN THAT REQUIRED TO FULLY COMPLEX THE COPPER, ABOUT 30 TO 180 GRAMS PER LITER OF A SOLUBLE OXALATE, CALCULATED AS OXALIC ACID, AND ABOUT 25 TO 150 GRAMS PERLITER OF A BASIC CATION SELECTED FROM THE GROUP CONSISTING OF AMMONIA AND TERTIARY (LOWER ALKYL) AMINES, CALCULATED AS TRIETHYLAMINE; AND SUBJECTING THE CATHODE TO A PLATING CURRENT OF FROM ABOUT 5 TO 30 AMPERES PER SQUARE FOOT AT A TEMPERATURE OF 30-90*C UNTIL A COPPER PLATE HAS BEEN DEPOSITED THEREON UP TO A THICKNESS OF ABOUT 0.0002 INCH.