US3575826A

US3575826A - Method and composition for electroplating tin

Info

Publication number: US3575826A
Application number: US768181A
Authority: US
Inventors: Kenneth P Bellinger; Joseph F Conoby
Original assignee: Conversion Chemical Corp
Current assignee: Conversion Chemical Corp
Priority date: 1968-10-16
Filing date: 1968-10-16
Publication date: 1971-04-20
Anticipated expiration: 1988-04-20
Also published as: DE1952218B2; GB1224966A; DE1952218C3; DE1952218A1; NL134759C; FR2020856A1; NL6915347A; JPS4924326B1

Abstract

AN AQUEOUS BATH FOR ELECTROPLATING TIN UPON METAL SURFACES CONTAINS STANNOUS ION, SULFATE RADICAL, A SURFACEACTIVE SULFATED POLYOXYALKYL CARBINAMINE AND AN IMIDAZOLINE DERIVATIVE. THE BATH HAS A HYDROGEN ION CONCENTRA TION OF ABOUT 0.9 TO 4.1 GRAMS PER LITER, AND IT OPERATES EFFECTIVELY EVER A WIDE RANGE OF CURRENT DENSITIES TO PRODUCE DESIRABLE DEPOSITS AT RELATIVELY HIGH PLATING RATES.

Description

United States Patent U.s. c1.204-s4 21 Claims ABSTRACT OF THE DISCLOSURE An aqueous bath. for electroplating tin upon metal surfaces contains stannous ion, sulfate radical, a surfaceactive sulfated polyoxyalkyl carbinamine and an imidazoline derivative. The bath has a hydrogen ion concentration of about 0.9 to 4.1 grams per liter, and it operates effectively over a wide range of current densities to produce desirable deposits at relatively high plating rates.

BACKGROUND OF THE INVENTION A variety of baths have been employed for electrodepositing tin upon metallic substrates, possibly the most widely used of which are the stannous fluoborate and caustic stannate solutions. Although these prior art baths have proven generally satisfactory and have been widely utilized, they are usually deficient in one or more desirable operating characteristics, and the deposits which they produce frequently fail to provide the levels of brightness, smoothness, ductility, adherance, solde-rability, porosity, or stability against ageing which are desired. Some of the baths are relatively expensive and many tend to be difiicult to maintain since fairly critical control of the concentrations of the components thereof is normally required. In addition, they often exhibit a tendency to be relatively slow and to operate properly only in fairly narrow ranges of current density.

Accordingly, it is an object of the present invention to provide a bath for producing smooth, adherent and relatively nonporous deposits of tin which may also be bright or semibright.

It is also an object to provide such a bath which is relatively inexpensive and convenient to maintain and which does not require a high level of control of the concentration of the components thereof within critical limits.

Another object is to provide a bath for electrodepositing tin at relatively high rates which is operable over a broad range of current densities.

A specific object is to provide a convenient and rapid method for electroplating tin upon metallic workpieces utilizing an improved sulfate plating bath to obtain highly desirable tin deposits at relatively low cost.

SUMMARY OF THE INVENTION It has now been found that the foregoing and related objects can be readily attained in an aqueous bath containing about 15.0 to 100.0 grams per liter of stannous ion, about 30.0 to 280.0 grams per liter of sulfate ion, about 0.5 to 6.0 grams per liter of a surface-active sulfated polyoxyalkyl carbinamine, and about 0.5 to 3.0 grams per liter of an imidazoline derivative. The hydrogen ion concentration of the bath should be maintained at about 0.9 to 4.1 grams per liter and the carbinamine and imidazoline are present in a ratio of their respective weights of about 0.753.0: 1.

The imidazoline derivative included in the bath corresponds to the formula:

wherein:

(1) R is an alkyl radical having 5 to 24 carbon atoms;

(2) G is a radical selected from the group consisting of OH, acid salt radicals, anionic surface active sulfate salt radicals and anionic surface active sulfonate salt radicals;

(3) Z is a radical selected from the group consisting of COOM, CH COOM, and

(4) M is a substituent selected from the group consisting of hydrogen, alkali metals and organic bases;

(5) Y is selected from the group consisting of OR' and (6) each R substituent is independently selected from the group consisting of hydrogen, alkali metals, and (CH COOM;

(7) A is an anionic monovalent radical;

(8) n is an integer from 1 to 4; and

(9) both the groups G and CH Z are present or absent.

In the method of electroplating tin, the aqueous bath is prepared and maintained at a temperature of about to Fahrenheit. A workpiece having a metallic surface and a suitable anode are immersed in the bath and a voltage providing a current density of about 150 to 2000 amperes per square foot at the surface of the workpiece is applied across the anode and workpiece to deposit tin upon the latter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The baths of the present invention require the stannous ion, the sulfate radical, a surface-active sulfated polyoxyalkyl carbinamine, and an imidazoline derivative for proper operation. The stannous ion and the sulfate radical may be furnished to the bath by any combination of compounds suitable for use in tin plating solutions, but to avoid the presence of unnecessary and possibly interfering ions, these components are desirably introduced in the form of stannous sulfate and sulphuric acid. As is apparent, the former compound will provide both the stannous ion and the sulfate radical, whereas the latter compound provides additional sulfate radical which is present in the bath. The use of sulphuric acid is also desirable from the standpoint of control of the hydrogen ion concentration, although other acids providing non-interfering radicals may also be used for that purpose. Tin compounds other than stannous sulfate, e.g., stannous fiuoborate, may be employed, but preferably they are used in conjunction with at least an equal weight of the sulfate compound.

Although the amount of stannous ion present in the baths may vary between about 15.0 and 100.0 grams per liter, it is preferably about 35.0 to 80.0 grams per liter. Similarly, although the concentration of sulfate ion may normally be between about 30.0 and 280.0, it is preferably about 50.0 to 150.0 grams per liter. Although a broad range of about 0.9 to 4.1 grams per liter is suitable for the hydrogen ion concentration, it is most desirably maintained at about 1.0 and 3.0 grams per liter. If the concentration of stannous ion in the bath is too low, the efiiciency of plating and the quality of the deposit produced will be unsatisfactory. On the other hand, although high levels of this ion permit desirable deposits to be obtained at higher current densities, too high a stannous ion concentration tends to result in the formation of an undesirable precipitate in the presence of relatively small amounts of sulphuric acid. Maintaining the sulfate radical and the hydrogen ion within the ranges of concentrations specified will ensure that the bath operates efiiciently to produce the type of deposits desired.

The surface-active carbinamine component In addition to the stannous ion and sulfate radical, the acid baths of the present invention require an effective amount of a combination of surface-active agents, one necessary member of which is an amphoteric sulfated polyoxyalkyl carbinamine which is stable under the conditions of plating. Generally, such carbinamines are produced by sulfating the terminal hydroxyl group on the polyoxyalkyl chain, and they may be in the form of an alkali metal salt thereof. Although the efiicacy of a particular member of this family should be tested to ensure its compatibility and effectiveness with the other components of the baths, various carbinamines having the following formula have been suitable:

in which R R and R are alkyl groups having a total of 7 to 23, and preferably 11 to 14, carbon atoms; m has a value of from 8 to 25, and preferably from 12.5 to 17.5; and X is a substituent selected from the class consisting of monovalent cations of hydrogen and of alkali metals. Surface active agents of this type are described in detail in U.S. Pat. No. 3,079,416, granted to Rohm & Haas Company on Feb. 26, 1963 as the assignee of John Dupre et al., and a particularly beneficial material is the sulfated polyoxyalkyl tertcarbinamine sold by Rohm & Haas under the trade name Triton QS-15.

Although it has been found that about 0.5 to 6.0 grams per liter of the carbinamine component may be used, the baths preferably contain about 0.75 to 3.0 grams per liter. Not only is there insufficient benefit to justify the cost of using more than about 6 grams per liter of this material, but also higher amounts tend to produce dullness and roughness in the deposit. Use of less than about 0.5 gram per liter is generally found inadequate to produce the results desired. It will be appreciated that a combination of such carbinamines may be employed in a single bath, or combinations of the type disclosed in Boettner Pat. No.

3,079,348, assigned to Rohm & Haas Company, may be r utilized benefically so long as the amount of the sulfated carbinamine is within the limits defined hereinbefore. Particularly good results have been obtained utilizing a combination of the sulfated polyoxyalkyl carbinamine (such as the Rohm & Haas QS15) and about 0.06 to 0.5 gram per liter of an amine polyglycol condensate (such as that sold by Rohm & Haas Company under the designation Triton CF-32). The indicated combination serves not only to provide a low foaming bath, but the bath is surprisingly effective in producing a tin deposit which is of increased brightness as compared to baths from which the amine polyglycol condensate is omitted.

The imidazoline derivative The second essential component of the combination of surface-active agents is an imidazoline derivative corresponding to the formula:

In the foregoing formula, R represents an alkyl radical having to 24 carbon atoms; G is OH ion, an acid salt radical, an anionic surface-active sulfate salt radical such 4 as preferably -OSO OR, or an anionic surface-active sulfonate salt radical; and Z is a --COOM, CH COOM,

radical. The substituent designated M in the foregoing formulae is a hydrogen atom, an alkali metal or an organic base, and that designated Y is either an -OR or N(R') A group. Each R substituent is independently selected from the group consisting of hydrogen, alkali metals, and (CH COOM; A represents an anionic monovalent radical and n represents an integer from 1 to 4. Use of the dotted line representation for the bonds connecting the substituents G and CH Z to the nitrogen atom indicates that the substituents are optionally present or absent, but it should be understood that'they are either both present or both absent. Desirable compounds are provided when, in accordance with the foregoing formula, G represents the radical OSO OR, particularly in which R is a C to C alkyl group, and Z is a COOM radical in which M is preferably an alkali metal cation.

Exemplary of the compounds corresponding to the foregoing formula which are satisfactory for use in the baths described herein are 2-alkyl-l-(ethyl-beta-oxypropanoic acid) imidazolines wherein the alkyl group is capryl, undecyl or a mixture of C -C chains, and the disodium salt of lauroyl-cycloimdinium-lethoxyethanoic acid-Z-ethanoic acid.

Although the amount of the imidazoline constituent may be about 0.5 to 3.0 grams per liter of solution, it is preferably about 0.75 to 2.0 grams per liter. Less than about 0.5 gram per liter of this material is found to be relatively ineffective, whereas more than about 3.0 grams is economically undesirable and tends to produce dull, rough deposits. Regardless of the specific amounts of the carbinamine and imidazoline derivatives which are utilized, the ratio of their respective weights should be about 0.75 to 3.0:1, and most desirably the ratio therebetween will be about 1.0 to 2.0:-l.0.

Other components In addition to the ions and compounds described here inbefore, other additives may be included to enhance the operation of the present baths, such as auxiliary surfaceactive agents and brighteners. However, one of the notable advantages provided by the present invention resides in the fact that very desirable deposits can be produced with no additive to the bath other than those necessary to furnish the stannous ion, the sulfate radical, the carbinamine, and the imidazoline derivative.

Exemplary of the brighteners which may be added are the aryl aldehydes, polyvinyl alcohol, gelatin, animal glue, evaporated milk, etc., and generally they will be added in a concentration of about 0.04 to 1.5 grams per liter. Brightness may also be enhanced by the inclusion of certain antioxidants, which also serve to reduce the tendency for insoluble precipitates to form in the bath. Such antioxidants include pyrocatechol, resorcinol and naphthol and may be employed in amounts of about 0.5 to 5.0, and preferably about 1.0 to 2.0 grams per liter. These additives are particularly effective in baths used at the higher plating rates in which the stannous ion concentration is relatively high.

In addition to the foregoing, buffers, chelating agents and other additives commonly employed in baths of this type may also be included if desired. Chelating agents such as citric acid, malic acid, and the aminopolyacetic acids (i.e., ethylenediamine tetraacetic acid, diethylenetriamine pentaacetic acid, nitriloacetic acid) are particularly beneficial to improve operation of the baths and to enhance the levels of brightness which may be obtained in the lower ranges of current density; they will normally be used in amounts of about 10.0 to 50.0 grams per liter. It should be noted that the stannous ion, hydrogen ion, and sulfate radical concentrations specified herein are based upon the use of stannous sulfate and sulfuric acid, and that complete disassociation thereof is assumed. These factors should be borne in mind when determining the necessary amounts of other compounds which may be used in lieu thereof.

As has been indicated hereinbefore, the baths of the present invention are operable over a wide range of cathode current densities, and more specifically this range is about 150 to 2000 amperes per square foot (a.s.f.). However, the baths are preferably operated at current densities of about 200 to 1200 a.s.f. and, will be more fully described hereinafter, the concentrations of the components of the the baths may be varied considerably so as to obtain optimum results at the various current densities employed. At the lower current densities, i.e., about 150 to 600 a.s.f., the rate at which the tin is deposited is about 0.2 to 1.3 mils per minute; at the higher current densities, i.e., about 600 and 2000 a.s.f., plating occurs at a rate of about 1.3 to 3.8 mils per minute. As a particular advantage, over the entire range of current density, plating efiiciency may be as high as 98 percent, based upon the theoretical rate of deposition.

Generally, the baths which provide the best results at the lower current densities contain a relatively low concentration of stannous ion as compared to baths best operated in the higher ranges of current densities. Exemplary of the baths which are most suitable for use at about 150 to 600 a.s.f. is one which contains about 50.0 grams per liter of stannous ion, about 65.0 grams per liter of sulfate radical, about 0.75 gram per liter of the carbinamine and about 1.0 gram per liter of the imidazoline derivative, the hydrogen ion concentration therein being about 1.0 gram per liter. As has been indicated previously, it is desirable to include a small amount of a chelating agent, such as EDTA, in the bath described to extend its effectiveness for producing relatively bright deposits to current densities at the low end of the range.

Exemplary of the baths which are most desirably operated at the higher current densities is one which contains about 75.0 grams per liter of stannous ion, about 110.0 grams per liter of sulfate radical, about 3.0 grams per liter of carbinamine, and about 1.5 grams per liter of the imidazoline derivative. This bath should have a hydrogen ion concentration of about 2.0 grams per liter; most desirably, it also contains about 2.0 grams per liter of pyrocatechol and about 0.06 gram per liter of an amine polyglycol condensate (such as Rohm & Haas Triton CF- 32). It should be noted that, in the baths containing the higher concentrations of stannous ion, care should be taken to avoid unduly high concentrations of sulfate radical because stannous sulfate is not soluble at all concentrations in baths of this type and high sulfate concentrations tend to produce undesirable precipitates therein.

The baths should be operated at temperatures between about 75 and 150 Fahrenheit, and preferably about 90 to 120 Fahrenheit. Operation below about 75 Fahrenheit tends to be inefiicient and to produce undesirable deposits, whereas temperature higher than about 150 Fahrenheit tend to produce dull, rough and generally unacceptable deposits. As a general rule, the higher the current density employed, the higher the temperature necessary for good results; by the same token, the current densities which are permissible are higher at the more elevated temperatures than at the lower temperatures. As regards the exemplary baths described hereinabove, the best temperature for operation of the bath of lower stannous ion concentration is about 100 Fahrenheit, whereas that for the bath of higher stannous ion concentration is about 120 Fahrenheit.

Although the practice is not absolutely essential, the best results are obtained with the present baths if the cathode (i.e., the workpiece) is moved relative to the bath at a high rate of speed. It will be appreciated that this may be accomplished by rotating the workpiece in the bath or by moving it linearly at a high rate of speed therethrough, such as in wire plating operations; it may also be achieved by agitating the bath or by rapid circulation thereof. Without such movement the deposits produced tend to be dark, spongy and poorly adherent, and, although the theory is not clearly understood, it is believed that this is due to the formation of a film about the cathode through which the stannous ion must diffuse. It is believed that the relative motion tends to decrease the effect of such a cathode film barrier by eliminating it or by decreasing its thickness, thus facilitating movement of the ions to the cathode. The rate of relative movement may vary considerably, but most desirably the workpiece is moved relative to the bath at a linear rate equivalent to at least about 200 feet per minute. Rates as high as 800 feet per minute, and even higher, may be employed depending upon the characteristics of the bath and the workpiece, and the type of apparatu employed. Any metallic substrate or metal-surfaced article which can be plated with tin using prior art baths may be coated in accordance with the present invention. For example, good deposits of tin may be produced upon articles of copper, nickel, iron and steel.

The best results are obtainable with these baths if relatively pure tin anodes are employed. One important feature with regard to the anode is its surface area relative to that of the cathode, since it has been found that current densities at the anode in excess of about a.s.f. cause excessive anode polarization with a resultant sharp decline in current efficiency. The conductivity of these baths is good and they are operative at relatively low voltages to produce good deposits. However, since these factors will vary somewhat according to the components and the concentrations thereof in the bath, some limited experimentation may be desirable to obtain optimum results for a given composition, installation and part, as is well known in the art.

Filtration of the bath is not essential but is normally beneficial particularly when contamination is encountered from air-borne impurities and by carry-over from other operations. For this purpose, various filter media including fabrics, porous stoneware and other conventional materials may be utilized. The need for correction of the depletion of the various components of the bath is best determined by a periodic quantitative analysis for the several components, the frequency thereof being a function of the size of the bath and the plating rates employed. The baths are relatively low in cost and utilize additives which may be used in relatively non-critical ranges of concentration, so that they may be readily maintained in condition for proper operation. Moreover, the baths and the components are easily stored as stable solutions, which may be readily prepared.

Illustrative of the efiicacy of the present invention are the following specific examples wherein all percentages are on a weight basis unless otherwise indicated.

EXAMPLE ONE An aqueous bath is prepared containing about grams per liter of stannous sulfate, about 45 grams per liter of sulfuric acid, about one gram per liter of 2-alkyl-1-(ethylbeta-oxypropanoic acid)-imidazoline, sold by Mona Industries, Inc. under the tradename Monateric CYA, and about 0.75 gram per liter of sulfated polyoxyalkyl tertcarbinamine, sold by Rohm and Haas under the tradename Triton QS-15; it has a hydrogen ion concentration of about 0.9 gram per liter. A cathode consisting of a length of copper wire and a pure tin anode are inunersed in the bath, which is heated to a temperature of about 80 Fahrenheit; the cathode is rotated in the bath at a rate equivalent to about 200 linear feet per minute. A voltage is applied across the anode and cathode to produce a current density of about 200 a.s.f. at the cathode and to cause tin to plate thereupon, the anode being of a size relative to the cathode sufficient to maintain the current density thereat below 80 a.s.f.

Thereafter, the same bath is heated to a temperature of 120 Fahrenheit and a second wire anode is immersed along with a cathode which is rotated at a rate equivalent to about 600 linear feet per minute. The voltage applied is sufiicient to produce a cathode current density of about 400 a.s.f. and the relative size of the anode and cathode are suflicient to maintain the anode current density below 80 a.s.f.

In both instances, the deposits produced exhibit excellent adherence to the copper wire, and the ductility, solderability and resistance to ageing thereof are found to be good. The workpiece plated at a current density of 400 a.s.f. is tested in accordance with ASTM Method B-33 and found to be nonporous.

EXAMPLE TWO A bath of the same composition as that prepared in accordance with Example One is prepared, with the sole exception that the 0.75 gram per liter of the carbinamine used therein is substituted with an equal weight of the imidazoline derivative specified so that the total amount of that compound in the bath amounts to 1.75 grams per liter. Plating with this bath under the conditions specified in Example One produces a deposit which is rough and streaked and which is generally unacceptable.

A second bath is prepared comparable to the bath of Example One, the sole difference being that the quantity of imidazoline used therein is substituted by an equal weight of the carbinamine, so that the total amount of carbinamine contained in the bath is 1.75 grams per liter. Use of this bath under the conditions indicated in Example One results in a deposit which is very coarse and completely unacceptable.

Accordingly, it is seen by a comparison of Examples One and Two that utilization of either the carbinamine or the imidazoline derivative alone produces unacceptable deposits. However, when these materials are used in combination in Example One in a total amount equal to the amount of each component used independently in Example Two, deposits of tin are produced which are highly desirable.

EXAMPLE THREE An aqueous bath is prepared as in Example One utilizing 120 grams per liter of stannous sulfate, about 45 grams per liter of sulfuric acid, about 1.0 gram per liter of the same carbinamine, and about 0.75 gram per liter of the same imidazoline derivative. This bath, maintained at a temperature of about 120 Fahrenheit and operated under cathode current densities of 1200 and 1800 a.s.f. produces semi-bright deposits upon a length of steel tubing one inch in diameter spun at 2250 rpm. during plating. Adding about 1.0 gram per liter of pyrocatechol to the bath increases the brightness of the deposit, and about 0.1 gram per liter of an amine polyglycol condensate (Rohm and Haas Triton (IF-32) enhances the brightness even further while also reducing the tendency for foaming the bath.

EXAMPLE FOUR Baths similar to those prepared in accordance with Example Three are prepared, substituting about 40 grams per liter of the stannous sulfate used therein with an equal amount of stannous fiuoborate and utilizing lauroyl cycloimidinium-l-ethoxy ethanoic acid-2-ethanoic acid-disodium salt (Miranol C2M sold by Miranol Chemical Company) instead of the imidazoline derivative specified. In all instances the deposits obtained are of comparable quality.

EXAMPLE FIVE An aqueous bath is prepared as in Example One by dissolving in water about 180 grams per liter of stannous sulfate, about 18 grams per liter of sulfuric acid, about 1.0 gram per liter of the same carbinamine, about 0.75

gram per liter of the same imidazoline derivative and about 1.0 gram per liter of pyrocatechol. The bath is heated and operated as in Example One, with the exception that the voltage applied produces current densities of 900 to 1200 a.s.f. at the cathode; in each instance the temperature employed is Fahrenheit. Although dull deposits are obtained at both plating rates, they are very smooth; increasing the concentration of pyrocatechol to 3.0 grams per liter produces semi-bright deposits under the same conditions.

Thus, it can be seen that the present invention provides a bath for producing a smooth, adherent and relatively non-porous deposit of tin which may in addition be bright or semi-bright. The bath is relatively inexpensive and is convenient to maintain since it does not require a high level of control of the concentration of the components thereof within critical limits. The bath may be used for depositing tin at relatively high rates and over a broad range of current density so as to aiford operating and economic advantages.

Having thus described the invention, we claim:

1. An aqueous acid bath for producing an adherent electroplated tin deposit comprising about 15.0 to 100.0 grams per liter of stannous ion; about 30.0 to 280.0 grams per liter of sulfate radical; about 0.5 to 6.0- grams per liter of a surface-active sulfated polyoxyalkyl carbinamine; about 0.5 to 3.0 grams per liter of a surface-active imidazoline derivative; about 0.9 to 4.1 grams per liter of hydrogen ion, said carbinamine and imidazoline derivative being present in a ratio of their respective weights of about 0.75 to 3.0: 1.0.

2. The bath of claim 1 comprising about 35.0 to 80.0 grams per liter of stannous ion, about 50.0 to 150.0 grams per liter of sulfate radical, about 0.75 to 3.0 grams per liter of said carbinamine and about 0.75 to 2.0 grams per liter of said imidazoline derivative, said ratio of carbinamine to imidazoline being about 1.0- to 2.0210, and the hydrogen ion concentration in said bath being about 1.0 to 3.0 grams per liter.

3. The bath of claim 1 wherein at least a portion of said stannous ion is provided by stannous sulfate and wherein at least a portion of said sulfate radical is furnished by sulphuric acid.

4. The bath of claim 1 wherein said carbinamine has has the formula:

1 Rg-()NH(C2H40) SO3X wherein R R and R are alkyl groups having a total of 7 to 23 carbon atoms, m is a number from 8 to 25 and X is a substituent selected from the class consisting of mono valent cations of hydrogen and of alkali metals.

5. The bath of claim 1 wherein said imidazoline derivative corresponds to the formula:

EEC-CH9, ICHZZ Nl-CHZOHEY wherein:

(l) R is an alkyl radical having 5 to 24 carbon atoms;

(2) G is a radical selected from the group consisting of OH, acid salt radicals, anionic surface active sulfate salt radicals, and anionic surface active sulfonate salt radicals;

(3) Z is a radical selected from the group consisting of COOM, CH COOM, and

(4) M is a substituent selected from the group con sisting of hydrogen, alkali metals and organic bases;

(5) Y is selected from the group consisting of OR and N(R') A;

(6) each R substituent is independently selected from the group consisting of hydrogen, alkali metals, and (CH COOM;

(7) A is an anionic monovalent radical;

(8) n is an integer from 1 to 4; and

(9) both the groups G and CH Z are present or absent.

6. The bath of claim 1 wherein said imidazoline derivative is selected from the group consisting of 2-alkyl-1- (ethyl-beta-oxypropanoic acid)-imidazolines wherein the alkyl group is selected from the class consisting of capryl, undecyl and mixtures of C C chains, and the disodium (salt) of lauroyl-cyclo-imidinium-l-ethoxy-ethanoic acid- 2- ethanoic acid.

7. The bath of claim 1 additionally containing about 0.5 to 5.0 grams per liter of an antioxidant selected from the group consisting of pyrocatechol, resorcinol and naphthol.

8. The bath of claim 1 additionally containing about 0.06 to 0.5 gram per liter of an amine polyglycol condensate brightening and defoaming agent.

9. The bath of claim 1 additionally containing about 10.0 to 50.0 grams per liter of a chelating agent selected from the group consisting of malic acid, citric acid and aminopolyacetic acids.

10. The bath of claim wherein said imidazoline derivative has a structure wherein G is an anionic surfaceactive sulfate salt radical having the formula OSO OR in which sulfate salt radical R is an alkyl group containing 10 to 18 carbon atoms, and wherein Z is a COOM radical in which M is an alkali metal cation.

11. In a method of electroplating tin, the steps comprising:

(A) preparing an aqueous acid bath comprising about 15.0 to 100.0 grams per liter of stannous ion; about 30.0 to 280.0 grams per liter of sulfate radical; about 0.5 to 6.0 grams per liter of a surface-active sulfated polyoxyalkyl carbinamine; about 0.5 to 3.0 grams per liter of a surface-active imidazoline derivative; and about 0.9 to 4.1 grams per liter of hydrogen ion, said carbinamine and imidazoline derivative being present in a ratio of their respective weights of about 0.75 to 3.0: 1.0;

(B) maintaining said bath at a temperature of about 75 to 150 Fahrenheit;

(C) immersing a workpiece having a metallic surface and an anode in said bath; and

(D) applying a voltage across said anode and workpiece to deposit tin on said metallic surface, said voltage providing a current density of about 150 to 2000 amperes per square foot at the workpiece.

12. The method of claim 11 wherein relative movement is established between said bath and the surface of said workpiece.

13. The method of claim 11 wherein said bath comprises about 35.0 to 80.0 grams per liter of stannous ion, about 50.0 to 150.0 grams per liter of sulfate radical, about 0.75 to 3.0 grams per liter of said carbinamine, about 0.75 to 2.0 grams per liter of said imidazoline derivative and about 1.0 to 3.0 grams per liter of hydrogen ion, said ratio of carbinamine to imidazoline being about 1.0 to 2021.0.

14. The method of claim 11 wherein at least a portion of said stannous ion is provided by stannous sulfate and 10 wherein at least a portion of said sulfate radical i furnished by sulphuric acid.

15. The method of claim 11 wherein said carbinamine has the formula:

wherein R R and R are alkyl groups having a total of 7 to 23 carbon atoms, m is a number from 8 to 25 and X is a substituent selected from the class consisting of monovalent cations of hydrogen and of alkali metals.

16. The method of claim 11 wherein said imidazoline derivative corresponds to the formula:

wherein:

(1) R is an alkyl radical having 5 to 24 carbon atoms;

(3) Z is a radical selected from the group consisting of COOM, CH COOM, and

(5) Y is selected from the group consisting of OR and N(R) A;

(7 A is an anionic monovalent radical;

(8) n is an integer from 1 to 4; and

(9) both the groups G and CH Z are present or absent.

17. The method of claim 11 wherein said voltage applied provides a current density of about 200 to 1200 amperes per square foot and wherein said temperature is maintained at about to Fahrenheit.

18. The method of claim 11 wherein tin is deposited upon said workpiece at the rate of about 0.2 to 3.8 mils per minute.

19. The method of claim 11 wherein said anode has a surface area relative to that of said workpiece such as to provide a current density at said anode less than about 80 amperes per square foot.

20. The method of claim 12 wherein said movement is created by rapid movement of said workpiece.

21. The method of claim 12 wherein said movement is at a rate equivalent to at least about 200 linear feet per minute.

References Cited UNITED STATES PATENTS 1/1968 Korpiun et al. 20454 5/1966 Hart 20454