US2891871A - Tin immersion plating composition and process for using the same - Google Patents

Description

United States Patent '0 TIN 1M1VIERSION PLATING COMPOSITION AND PROCESS FOR USING THE SAME Myron Ceresa, Penn Township, Allegheny County, and

John R. vDrobne, Munhall, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application September 21, 1956 Serial No. 611,365

3 Claims. (Cl. 106-1) The present invention relates to plating tin on surfaces of copper and copper base alloys by chemical displacement. More specifically, the invention relates to the deposition of coatings of metallic tin on such cupreous surfaces by an immersion plating process without the use of an electric current.

In the past, coatings of tin have been deposited on articles by immersion plating techniques employing aqueous solutions containing strong liquid mineral acids such as hydrochloric acid and sulphuric acid. The use of such strong mineral acids in immersion plating as previously practiced, generally resulted in a pronounced attack on the copper or copper base alloys on which the tin was deposited. Moreover, the strong mineral acids appear to dissolve a portion of the tin coating deposited on the article being plated.

The object of the present invention is to provide a .nonmineral acid containing composition, adapted for mixing with water, from which aqueous solution tin may be immersion plated on to copper and copper base alloys without disadvantageous results obtained in prior art immersion plating techniques.

Another object of this invention is to provide a solution adapted for the immersion plating of tin on cupreous articles comprising a water solution of thiourea or derivatives thereof, at least one water-soluble stannous salt, and at least one normally solid, water-soluble, non-mineral acid.

Still another object of the present invention is to provide a method of depositing a coating of tin on cupreous metal by contacting said metal with an immersion plating solution for a period of time sufiicient to deposit a tin coating of desired thickness thereon, the solution comprising water, thiourea, or derivatives thereof, at least one water-soluble stannous salt, and at least one normally solid, water-soluble, non-mineral acid.

Other and further objects of this invention will, in part, be obvious and will, in part, appear hereinafter.

In accordance with the present invention and in the attainment of the foregoing objects, there is provided a composition adapted for mixing with water at time of use to provide an aqueous solution from which tin may be plated on to articles of copper and copper base alloys by immersing such articles in said solution, without the use of an electric current.

The composition, which is to be dissolved in water to form the immersion plating solution of this invention, is a mixture of dry components comprising (a) from 7.7% to 98.5% by weight of thiourea and certain water-soluble derivatives thereof, (b) from 0.2% to 64.5% by weight of at least one Water soluble stannous salt, and (c) from 0.9% to 90.5% by weight of at least one normally solid, water-soluble, non-mineral acid.

Bright, firmly adhering coatings of tin may be deposited on cupreous metal articles when the same are immersed in aqueous solutions of the above described mixture when said mixture is dissolved in water in proportions amounting to from about 2 to 35 ounces of said mixture per gallon of water.

It is preferred to use thiourea in the compositions of this invention. Satisfactory results are also obtainable using derivatives of thiourea or, more particularly, watersoluble compounds of the general formula it i! i wherein R represents at least one radical selected from the group consisting of hydrogen, an acetyl radical, and aliphatic hydrocarbon and aliphatic monohydroxy hydrocarbon radicals containing from 1 to 2 carbon atoms per radical. Examples of aliphatic hydrocarbon radicals include methyl, ethyl, methylene, ethylene, and the like and examples of monohydroxy hydrocarbon radicals include methylol, ethylol, and the like.

Satisfactory tin coatings may be applied on surfaces of cupreous articles using the aqueous immersion plating compositions of this invention by contacting said surfaces with the solution for a period of time sufficient to deposit a tin coating thereon of desired thickness. Immersion periods varying from 1 to 15 minutes have been found to provide bright firmly adhering tin deposits on cupreous surfaces. The thickness of the tin coating deposited on the cupreous surface treated with the plating solution of this invention depends on the particular concentration of solid components dissolved in the solution, the length of time the cupreous surface is in contact with the solution, and the temperature of the solution. The solution provides satisfactory plating at any temperature, i.e., from room temperature up to about the boiling point of the solution.

After the cupreous article has been immersed in the solution for a sufficient period of time and at a desired temperature to produce a tin deposit of desired thickness, the article .is removed from the solution and may be washed or rinsed with water. The tin deposit formed is bright, smooth, firmly adherent to the under lying cupreous surface, and free from pin holes or other defects.

Articles having a cupreous surface, which may be coated by immersing the same in the aqueous compositions of this invention, include articles of copper or cop per base alloys such as, for example, red brass, and bronzes, as well as members which have a surface layer of copper or a copper base alloy thereon, such as, for example, copper or brass plated steel-cored articles.

Tin is provided in the aqueous plating solutions of this invention in the form of at least one water-soluble stannous salt. Stannous chloride is particularly satisfactory and is a preferred source of tin in formulating the compositions of this invention. Other water-soluble stannous salts, such as, for example, stannous bromide, stannous iodide, and stannous sulfate, also may be employed with satisfactory results being obtainable. It is also possible to employ other tin salts such as, for example, sodium st-annite, which contains tin in the stannous condition. These tin salts may be used singly or in mixtures of two or more.

The acids employed in formulating the aqueous immersion plating compositions of this invention are normally solid, water-soluble, non-mineral or organic acids. Examples of suitable acids include citric acid, tartaric acid, sulfamic acid, malic acid, maleic acid and levulinic acid. 'l'lhese acids may be used singly or in mixtures of two or more.

A thiourea also is included in the compositions of this invention. Suitable thioureas are those selected from that class of compounds containing a bivalent sulfur atom both bonds of which are connected directly to a carbon atom and having at least one nitrogen atom connected directly to the carbon atom. Thioureas having the nucleus are particularly suitable. Thiourea, CS(NH is preferred since it is perhaps the most readily available compound of this class and its use affords excellent plating results.

In order to indicate more fully the advantages and capabilities of the present invention, the following specific examples are set forth. The percentages given are by weight unless otherwise indicated.

Example I Example I] An aqueous plating composition is prepared by dissolving a mixture comprising 58% thiourea, 8% stannous bromide and 34% or" tartaric acid in water, the solution comprising 10 ounces of the mixture of solids per gallon of deionized water. Excellent tin deposits result upon immersing cupreous articles in this plating composition.

Example III A tin immersion plating solution is prepared by dissolving a solid mixture comprising 80% of thiourea, 4% of stannous iodide and 16% of sulfamic acid in deionized water. Bright, firmly adherent tin coatings are deposited on cupreous metal articles when such articles are immersed in an aqueous solution containing 12 ounces of this mixture dissolved in one gallon of water. Comparable results are obtainable by substituting dimethyl thiourea for the thiourea.

Example IV A continuous coating of tin is deposited on a copper article by immersing such article in an aqueous solution containing 14 ounces per gallon of a solid mixture including 67% thiourea, 7% stannous iodide and 26% malic acid.

Example V An aqueous tin immersion plating solution is prepared by dissolving 14 ounces of the following mixture of solids in one gallon of Water.

Percent Thiourea 85 Stannous sulfate r- 3 Maleic acid 12 A smooth continuous coating of tin is deposited on copper articles when such articles are immersed in this composition at a temperature of 40 C. for 5 minutes. Equally satisfactory results are obtainable by substituting monomethylol thiourea for the thiourea and heating the aqueous solution to 60 C.

Example VI An aqueous tin immersion plating bath is prepared by dissolving 14 ounces of a solid mixture containing 40% thiourea, 15% sodium stannite and 45% levulinic acid in a gallon of water. A copper coated steel article is immersed in this bath for 15 minutes. It then is removed and rinsed with Water. A firmly adherent coating of tin is found to have been deposited on the copper surfaces.

The aqueous tin immersion plating compositions of this invention are more stable and capable of use for longer periods of time than the immersion plating solutions previously known in the art. It has been determined that there is an optimum concentration of the three essential solid ingredients of the compositions of this invention which permits plating the greatest number of cupreous articles Without the need of adding additional quantities of the three ingredients to compensate for their consumption during plating. Experience has shown that this optimum composition contains the three essential ingredients in substantially the following proportions.

Percent Thiourea 55 Water-soluble stannous salt 6 Non-mineral acid 39 The most satisfactory plating results are obtained when an aqueous composition is prepared by dissolving 12 ounces of the above mixture in one gallon of water, though from 10 to 20 ounces per gallon give excellent results.

It has been determined that cupreous articles having a greasy or otherwise dirty surface may be provided with a more uniform, bright and dense tin deposit if there is included in the immersion baths of this invention from 0.01% to 10% by weight of a wetting agent such as an alkyl aryl sulfonate.

While the present invention has been described with particular reference to preferred embodiments thereof, it will be understood, of course, that certain changes, substitutions, modifications, and the like, may be made therein without departing from its true scope.

We claim as our invention:

1. A solid composition adapted for mixing with water to provide a solution in which tin may be immersion plated onto copper and copper base alloys consisting essentially of from 7.7% to 98.5% by weight of a water soluble compound of the general formula wherein R represents at least one radical selected from the group consisting of hydrogen, an acetyl radical, and ali phatic hydrocarbon and aliphatic monohydroxy hydrocarbon radicals containing from 1 to 2 carbon atoms per radical, from 0.2% to 64.5% by weight of at least one water-soluble stannous salt selected from the group consisting of stannous chloride, stannous bromide, stannous iodide, and stannous sulfate, and from 0.9% to 90.5% by weight of at least one normally solid water-soluble non-mineral acid selected from the group consisting of citric acid, tartaric acid, sulfamic acid, malic acid, maleic acid, and levulinic acid, said composition being adapted for mixing with water to provide a tin immersion plating solution in which the components cooperate with one another to provide improved plating.

2. A solution adapted for immersion plating tin therefrom on copper and copper base alloys consisting essentially of a water solution of a mixture of from 7.7% to 98.5% by Weight of thiourea, from 0.2% to 64.5% by weight of stannous chloride, and from 0.9% to 90.5% by weight of at least one normally solid water-soluble non-mineral acid selected from the group consisting of citric acid, tartaric acid, sulfamic acid, malic acid, maleic acid, and levulinic acid, said solution consisting essentially of from 2 to 30 ounces of said mixture per each gallon of Water.

3. A method of depositing a coating of tin on a surface of a cupreous article which comprises contacting said article with an aqueous immersion solution for a period of time suificient to deposit a tin coating of desired thickness thereon, and thereafter removing said article from levulinic acid, said solution consisting essentially of contact with said aqueous solution, said aqueous solution 2 to 30 ounces of said mixture per each gallon of water. consisting essentially of a mixture of from 7.7% to 98.5%

by weight of thiourea, from 0.2% to 64.5% by weight References Cited m me of flns patent of stannous chloride, and from 0.9% to 90.5% by weight 5 UNITED STATES PATENTS of at least one normally solid water-soluble non-mineral 1,903,860 Gockel Apr. 18, 1933 acid selected from the group consisting of citric acid, 2,369,620 Sullivan et a1. Feb. 13, 1945 tartaric acid, sulfamic acid, malic acid, maleic acid, and 2,766,138 Talmey Oct. 9, 1956

Claims (1)

1. A SOLID COMPOSITION ADAPTED FOR MIXING WITH WATER TO PROVIDE A SOLUTION IN WHICH TIN MAY BE IMMERSION PLATED ONTO COPPER AND COPPER BASE ALLOYS CONSISTING ESSENTIALLY OF FROM 7.7% TO 98.5% BY WEIGHT OF A WATER SOLUBLE COMPOUND OF THE GENERAL FORMULA