US 3424597 A
Description (OCR text may contain errors)
United States Patent 3,424,597 ELECTROLESS NICKEL PLATING Charles R. Shipley, Jr., Newton, Mass, assignor to Shipley Company, Inc., Newton, Mass., a corporation of Massachusetts No Drawing. Filed Apr. 8, 1966, Ser. No. 541,059 U.S. Cl. 1061 7 Claims Int. Cl. C23c 17/02 ABSTRACT OF THE DISCLOSURE An electroless nickel plating composition of the nickel cation-hypophosphite anion type characterized by the addition of an effective amount of a solution soluble polymer. Nickel deposited from solution is readily plated without treatment to remove oxide coatings. It is believed that the addition to the plating solution provides a thin, protective layer over the nickel plated surface which is readily redissolved in a subsequent plating bath, thereby preventing oxidation of the nickel in water or in the atmosphere.
This invention relates to improved processes of electroless nickel plating of catalytic surfaces employing baths of the nickel cation-hypophosphite anion type, the nickel ions being reduced to metal by the hypophosphite ions. Electroless plating refers to plating by chemical reduction in the absence of an external electric source.
Electroless nickel deposition of the above type is well known and is disclosed for example in United States Patents Nos. 2,532,283, 2,762,723, 2,935,425, and 2,999,- 770, which disclose suitable materials, concentrations, operating temperatures, pH conditions and various additives such as buffers, complexing agents for the nickel ions, and stabilizers. Deposition occurs in the presence of a catalytic surface, generally a variety of metals as disclosed in the above patents. Also, non-catalytic surfaces, such as plastics, ceramics, or other dielectrics, may be sensitized with catalysts as disclosed in the above patents and as also disclosed in United States Patent No. 3,011,920.
Heretofore in the plating of nickel, whether electrolytically or by electroless deposition but especially the latter, the nickel surface rapidly becomes passive upon removal from the plating composition. The surface undergoes oxidation in water or in the atmosphere to form a tenaciously adherent oxidized surface resistant to further reaction. This surface resists overplating, which requires a substantially pure nickel surface, either with additional nickel or with dissimilar metals such as a protective coating of gold. Electroless nickel platings are particularly resistant to overplating, after storage, thought to be due to inclusions of impurities such as nickel phosphide. Electroless nickel plating obtained by the use of compositions such as described above become passive if stored for as little as 60 minutes. Storage overnight results in surfaces difiicult or impossible to activate, a condition which grows progressively worse with time.
It is accordingly the principal object of the present invention to provide electroless nickel compositions of the nickel cation-hypophosphite anion type which maintain an activated surface for periods of storage at least two weeks or longer such that they may be readily overplated with nickel or dissimilar metal.
In accordance with the present invention the improved electroless nickel compositions comprise aqueous solutions of a source of nickel ions in an amount sufiicient to provide a useful deposit and a source of hypophosphite ion in an amount sufficient to serve as a reducing agent therefor, characterized by the inclusion therein of an effective amount of a solution-soluble polymer which appears to provide on the surface a thin protective layer readily resoluble in the subsequent plating bath. The amount of the ICE addition is not critical, small amounts providing some improvement while large amounts short of undue increase in solution viscosity are tolerable. Amounts greater than incidental impurities up to about 5.0 grams per liter are preferred, and from about 0.1 to about 1.0 gram per liter are most preferred.
Suitable polymers include Gantrez M, a polyvinyl methyl ether; copolymers thereof such as Gantrez AN, believed to be a copolymer comprising alternating units of methyl vinyl ether and maleic anhydride; and Thickener L, a modified copolymer of Gantrez AN; all of the General Aniline & Film Corp; DX-840 Resin, believed to be a polyethylene maleic anhydride random copolymer of the Monsanto Chemical Company; SMA, a styrene maleic anhydride random copolymer of Sinclair Petrochemicals, Inc.; cellulose ethers such as CMC, carboxymethylcellulose; HEC, hydroxymethylcellulose; and CMHEC, carboxymethylhydroxyethylcellulose; available from Hercules Powder Company; and modified polyacrylamides available in various grades under the trademark Reten from the Hercules Powder Company. While the reason for the effectiveness of these materials is not completely understood, it is, as indicated above, believed to be related to the provision of a protective coating readily re-soluble in the subsequent plating composition. Molecular weight does not appear to be critical although intermediate grades are currently preferred.
The above polymeric additives are useful in substantially all of the nickel cation-hypophosphite anion types heretofore known. Illustrative examples are given below.
EXAMPLE I Ingredient:
NiSO 6H O g./l 20 NaH2PO H O g./l 30 Hydroxy-acetic acid g./l 28 Gantrez AN-139 g./l 0.2
NH OH, to pH 4.5-5.0. Water, to make 1 liter. PbC1- as stabilizer ppm. Pb 1 Platings made at 200 F. with the above composition over steel maintain an activated surface capable of reimmersion in the solution and additional adherent plating of nickel for a period of at least two weeks. Furthermore, the platings may be subsequently immersed, after storage for at least two weeks, in dissimilar plating compositions such as the replacement gold compositions disclosed in copending application Ser. No. 244,949, now US. Patent No. 3,266,929, and dissimilar metal plated thereover.
As additional examples, each of the following polymeric products or mixtures thereof can be substituted for the above polymer Gantrez AN-139, in a similar amount and with substantially the same result.
In the above examples nickel sulfate is the source of nickel cations, sodium hypophosphite the reducing agent, and hydroxyacetic acid a known complexing agent for the nickel ions. Lead chloride as stabilizer was used in accordance with US. Patent No. 2,762,723. Other suitable agents may be substituted and buffers employed where desired. The addition of polymer does not substantially change useful concentrations, pH ranges or operating conditions as heretofore known and as illustrated in the above patents. pH may range from about 3 to 11 but is preferably on the mildly acid side of neutrality.
It should be understood that the foregoing description is for the purpose of illustration and that the invention includes. all modifications falling within the scope of the appended claims.
1. In an electroless nickel plating composition comprising an aqueous solution of a source of nickel cations in an amout sufiicient to provide a useful deposit and a source of hypophosphite anions in an amount sufiicient to act as reducing agent therefor, the improvement comprising the addition of a polymer selected from the group consisting of polvyinyl methyl ether, a copolymer of methyl vinyl ether and maleic anhydride, a copolymer of polyethylene and maleic anhydride, a copolymer of polystyrene and maleic anhydride, carboxymethylcellulose, hydroxymethylcellulose, carboxymethylhydroxyethylcellulose, polyacrylamide, and mixtures thereof, said polymer being present in an amount effective to permit overplating with nickel after storage.
2. A composition according to claim 1 wherein said polymer is a copolymer of methylvinyl ether and maleic anhydride.
3. A composition according to claim 1, wherein said polymer is present in an effective amount greater than incidental impurities up to about 5.0 grams per liter.
4. A composition according to claim 1, wherein said composition has a pH between about 3 and 11.
5. The method of electroless plating which comprises contacting a catalytic surface with a composition according to claim 1 to deposit electroless nickel, and removing said surface from contact with said solution.
6. Method of plating according to claim 5, further characterized by the additional step of overplating said nickel after storage with nickel or gold.
7. A composition according to claim 2, wherein said polymer is present in an effective amount greater than incidental impurities up to about 5 .0 grams per liter.
References Cited UNITED STATES PATENTS JULIUS FROME, Primaly Examiner.
L. HAYES, Assistant Examiner.
US. Cl. X.R. 117-130,