WO1990004048A1

WO1990004048A1 - A method, bath and cell for the electrodeposition of tin-bismuth alloys

Info

Publication number: WO1990004048A1
Application number: PCT/US1988/003536
Authority: WO
Inventors: Harold P. Wilson
Original assignee: Atochem North America, Inc.
Priority date: 1988-10-14
Filing date: 1988-10-14
Publication date: 1990-04-19
Also published as: ATE105877T1; NO902630D0; HK103095A; BR8807847A; EP0397663B1; SE502520C2; LU87746A1; EP0397663A1; DE3889667T2; KR960008155B1; DK143590D0; KR900702085A; DK143590A; SE9002096D0; DE3889667D1; SE9002096L; NL194005C; NO902630L; NL194005B; JPH03503068A

Abstract

Baths are disclosed for electrodepositing bismuth-tin alloys of all ratios as shown by the Figure. The baths are of the acidic alkylsulfonate-type. Soluble bismuth anodes are used to replenish bismuth in the bath solution. The baths find particular use in electrodepositing eutectic compositions such as 58 % Bi-42 % Sn.

Description

A METHOD, BATH AND CELL FOR THE ELECTRODEPOSITION OF

TIN-BISMUTH ALLOYS

BACKGROUND OF THE INTENTION

This invention relates to an electroplating bath, an electroplating cell and a method for the electrodeposition of a wide range of tin-bismuth alloys onto a conductive substrate.

Alloys of tin and lead have been used in a wide variety of applications such as plating for circuit boards, as rust

inhibiting coatings on metals and as solder. Increased awareness of health and environmental hazards posed by lead has required handling and disposal precautions that often escalate the cost of handling or disposing of lead-containing materials including tin- lead alloys. In some situations, therefore, it would be desirable to replace tin-lead with alloys having acceptable characteristics but.that do not present health and environmental risks attendant to tin-lead alloys.

Moreover, tin-lead alloys have not proven entirely

satisfactory in some temperature sensitive applications where heat is undesirable. Tin-lead alloys have conventionally been used, for instance, as plating for multilayer circuit boards and as a eutectic solder used to bond together the layers of tin- lead plated circuit boards. The heat required to melt eutectic tin-lead alloy solder, however, is so high that it can damage the components of the circuit board or impair the conductivity characteristics of the circuit board.

Tin-bismuth alloys have characteristics that make them attractive replacements for tin-lead alloys for many purposes. Tin-bismuth alloys, for instance, do not present the health and environmental problems associated with lead containing alloys. Moreover, a tin-bismuth eutectic alloy has a melting point about 50°C lower than a tin-lead eutectic alloy, making the tin- bismuth eutectic alloy an attractive material for plating and soldering layered circuit boards. Indeed, the broad range of potential uses for tin-bismuth alloys may warrant production of tin-bismuth alloys having a bismuth content ranging from nearly

0% to nearly 100% bismuth. It is therefore desirable to develop a commercially acceptable electrolytic bath, cell and process capable of providing tin-bismuth alloys which may have any desired bismuth content ranging from just above 0% to just below

100% bismuth.

Conventional bismuth salts used in conventional electrolytes often are unstable and undergo undesirable hydrolytic

precipitation. To prevent or minimize this problem, additives which inhibit hydrolytic precipitation of bismuth in electrolyte baths are ordinarily used. Citric acid and chelating agents are examples of additives commonly used for this purpose.

Conventional baths containing such additives, however, may be difficult to maintain and do not provide versatile commercially satisfactory baths and cells for depositing bismuth-containing alloys of any desired bismuth content.

In some electroplating processes, small quantities of bismuth have been added to tin plate to retard the formation of tin pest and tin whiskers. From about 1% to 2% bismuth in the coplate is ordinarily adequate for this purpose. U.S. Patent No. 4,331,518 discloses an electroplating process which produces tin- bismuth alloys which may have as much as 10-14% bismuth in the co-plate. Soluble bismuth is provided in the electroplating bath as a chelated acid bismuth sulfate gluconate.

Optional use of small quantities of bismuth nitrate as an additive compound, in an electroplating bath containing alkyl sulfonic acid electrolyte, to aid in the deposition of tin-lead alloy onto a substrate is disclosed in U.S. Patent No. 4,565,610. The bismuth nitrate is said to lower current density of the bath or, when added in conjunction with an aromatic aldehyde and/or an alkylene oxide, to improve brightness of the tin-lead deposits.

The present invention provides a method, bath and cell for the electrodeposition of tin-bismuth alloys onto a conductive substrate so that the bismuth content of the coplate may range from greater than zero to less than 100% bismuth by weight of the electrodeposited alloy with the balance of the alloy being tin.

Practicing the present invention permits the electrodeposition of a fine grained alloy coating of bismuth and tin of any desired thickness and percent composition upon the immersed portion of a conductive substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The Figure is a correlation curve of percent bismuth in the coplate as a function of the weight ratio of bismuth to tin in an electroplating bath containing methane sulfonic acid electrolyte. Soluble bismuth was provided by bismuth methane sulfonate concentrate and a soluble bismuth anode while soluble tin was provided by stannous methane sulfonate concentrate. SUMMARY OF THE INVENTION

One embodiment of the present invention is an electroplating bath for electrodeposition of tin-bismuth alloy onto a conductive substrate comprising:

a) soluble bismuth in aqueous solution;

b) soluble tin in aqueous solution, wherein the soluble bismuth and the soluble tin are present in the bath in amounts sufficient to deposit a tin-bismuth alloy onto the conductive substrate and in a weight ratio relative to each other selected to provide a desired bismuth content of the tin-bismuth alloy on the conductive substrate; and

c) an alkyl sulfonic acid electrolyte in an amount

sufficient to inhibit hydrolytic precipitation of the soluble bismuth.

A further embodiment of the present invention is an electroplating cell for electrodeposition of tin-bismuth alloy onto a conductive substrate comprising:

a) an electroplating bath comprising

1) soluble bismuth in aqueous solution;

2) soluble tin in aqueous solution wherein

the soluble bismuth and the soluble tin are present in the bath in amounts sufficient to deposit a tin-bismuth alloy onto the conductive substrate and in a weight ratio relative to each other selected to provide a desired bismuth content of the tin-bismuth alloy on the conductive substrate; and

3) a lower alkyl sulfonic acid electrolyte in an amount sufficient to inhibit hydrolytic precipitation of the soluble bismuth;

b) an anode immersed in the bath;

c) a conductive substrate cathode immersed in the bath; and

d) a supply of electricity for electrodepositing tin and bismuth onto the conductive substrate.

The present invention also provides a method for the electrodeposition of a tin-bismuth alloy onto a conductive substrate comprising: a) providing an electroplating bath comprising:

1) soluble bismuth in aqueous solution;

2) soluble tin in aqueous solution wherein

the soluble bismuth and the soluble tin

are present in the bath in amounts sufficient to deposit a tin-bismuth alloy onto the conductive substrate and in a weight ratio relative to each other selected to provide a desired bismuth content of the tin-bismutha alloy on the

conductive substrate;

b) providing an anode immersed in the bath;

σ) supplying sufficient electricity to the bath

to electro deposit tin-bismuth alloy onto

the conductive substrate; and

d) immersing the conductive substrate into the

electroplating bath.

Additional advantages and embodiments of the invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention may be realized and attained by processes, materials and combinations particularly pointed out in the appended claims.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In the present application, the phrase "tin-bismuth alloy" is understood to mean an electroplated alloy coating of greater than 0% and less than 100% bismuth by total weight of the electro-deposited alloy coating with the balance of the

electrodeposited alloy coating being tin. Ordinarily, tin- bismuth alloy will have a minimum bismuth content of about 0.1% and a maximum bismuth content of about 99.9%.

It has been found that tin-bismuth alloy having nearly any desired bismuth content may be electroplated onto a conductive substrate from a relatively simple versatile electroplating bath which includes sufficient amounts of free alkyl sulfonic acid electrolyte. The deposition of alloys having a wide range of bismuth content from a bath of the present invention is possible, in part, because bismuth has been found to be more hydrolytically stable in the presence of sufficient amounts of free alkyl sulfonic acid, particularly methane sulfonic acid, than in conventional electrolyte solutions used for bismuth such as sulfuric acid or chloride-citrate. Using a sufficient amount of alkyl" sulfonic acid electrolyte therefore is one aspect of the present invention which makes possible the electrodeposition of tin-bismuth alloys having virtually any desired bismuth content.

Aqueous acidic electroplating baths of the present invention are therefore composed of alkyl sulfonic acids, and more

preferably lower alkyl sulfonic acids such as C_1-5 alkyl sulfonic acids. Methane or ethane sulfonic acid are the most preferred acids used in accordance with the present invention.

Lower alkyl sulfonic acid electrolytes useful may be purchased commercially from Pennwalt Corporation. Alternatively, alkyl sulfonic acids may be prepared by methods known to the art such as the methods described in U.S. Patent Nos. 774,049 and 2,525,942. The disclosures of U.S. Patent Nos. 774,049 and

2,525,942 are incorporated herein by reference. Hydrolytic precipitation in a multi-component electrolyte solution probably is quite complex, but in the case of bismuth, the following formula, in which soluble bismuth is provided by bismuth trimethane sulfonate, may represent the mechanism of hydrolytic precipitation in a simplistic manner: 3

Bismuthtrimethane Insoluble white Methane sulfonic sulfonate precipitate acid

Bismuth tri- hydroxide

To maintain stability of the bath and prevent hydrolytic precipitation of bismuth from the bath solution the amount of free alkyl sulfonic acid electrolyte in the aqueous acidic electroplating bath ordinarily ranges from about 100 grams per liter to about 400 grams per liter, preferably from about 150 grams per liter to about 300 grams per liter and more preferably 200 grams per liter to about 250 grams per liter. Generally, when the free MSA concentration in the plating bath is below 200 grams per liter, for example below about 150 grams per liter, the bath may undergo undesirable hydrolytic precipitation of bismuth after a period of time. When concentrations of about 200 grams per liter free MSA are maintained in the bath, hydrolytic precipitation seldom occurs and the degree of precipitation is very moderate. At concentrations of about 250 grams per liter free MSA, precipitation of bismuth ordinarily does not occur at observable levels.

Soluble bismuth available to form a tin-bismuth alloy on a conductive substrate may be provided in a bath of the present invention by adding a bismuth salt, preferably bismuth alkyl sulfonate, directly to the bath or by a soluble bismuth metal anode. Either source of soluble bismuth may be used without the other but they are frequently used together.

For most applications of the present invention, the amount of soluble bismuth in the aqueous acidic plating bath ordinarily ranges from about .05 grams per liter of the bath to about 150 grams per liter of the bath, preferably, from about .05 grams per liter of the bath to about 80 grams per liter of the bath.

Soluble bismuth is preferably provided in bath solutions of MSA initially in the form of bismuth trimethane sulfonate concentrate (alternatively referred to as bismuth methane sulfonate

concentrate).

Bismuth trimethane sulfonate concentrate may be prepared by reacting bismuth trioxide with 70% methane sulfonic acid. It has been found that a product bath solution of 200-225 grams of bismuth trimethane sulfonate per liter bismuth is stable only when there is at least 200 grams per liter free methane sulfonic acid in the solution. As the electroplating process proceeds, additional bismuth trimethane sulfonate concentrate may be added to maintain an adequate soluble bismuth content in the bath.

In one electroplating system of the present invention, a soluble bismuth anode is used to recharge the bismuth plating bath so that further additions of bismuth methane sulfonate are minimized or not required during operation. The bismuth anode preferably useful in the present invention is constructed of soluble bismuth metal, typically cast high purity bismuth. It has been found useful to bag the soluble bismuth anode with

polypropylene cloth. Other components of cells, such as the container for the electrolyte solution, are conventional. Those skilled in the art are well acquainted with electroplating cells and their assembly and would therefore be able to provide a cell in accordance with the teachings of the present invention.

When bismuth anodes are used to supply soluble bismuth, the immersion area of the anodes will have to be regulated to control the solubilization rate to meet plating demand and maintain solution concentration. Bismuth anodes generally produce tin- bismuth co-plates having clean grayish white satin finishes.

It has been found that bismuth anodes in plating baths containing about 250 grams per liter free methane sulfonic acid are very active and tend to increase the soluble bismuth

concentration, thereby slowly upsetting the bismuth/tin ratio in the co-plate to contain about 5% bismuth. This may be counteracted by adding additional tin to the bath. However, the soluble bismuth build-up is preferably minimized by controlling the immersion area of the bismuth anode to provide an anode current density near 110 amp/ft² where the anode current efficiency probably becomes sufficiently low to inhibit the solubilization rate of bismuth.

In the case of tin-bismuth co-plates containing 1-2% bismuth, useful for inhibiting the growth of tin whiskers and tin pest (allotropic transformation to alpha-tin, the gray cubic form, at 12°C to -70°C or lower) it may be preferable to add bismuth in chelated form to the bath rather than as a sulfonic acid concentrate. When this is done it is ordinarily possible to lower the concentration of free methane sulfonic acid to 150 or even 100 grams per liter.

When chelated bismuth is used in the bath as the source of bismuth ions, it also may be possible to substitute tin anodes for the bismuth anodes The following three chelating compounds may be used to provide chelated bismuth to the electrolyte bath:

(1) Tetraammonium bismuth dinitrilotriacetate Chelate.

(approx. 26% bismuth in water soluble transparent crystals)

(2) Diammonium bismuth diethylene triaminepentaacetate

Chelate. (approx. 325 grams per liter bismuth in concentrate)

(3) Trimethane sulfonic acid bismuth trimethane sulfonate trigluconate Chelate Complex, (approx. 200 grams per liter bismuth in concentrate)

The concentrations of these bismuth sources in the bath are calculated in the same manner as described for bismuth methanesulfonate except that the free acid concentration may be less

(say approx. 100-150 grams per liter, preferably about 150 grams per liter). These chelated compounds, particularly chelate No . 2, at high concentrations tend to saturate and cause a spontaneous breakdown precipitation in the MSA bath. The first chelate listed above is ordinarily the preferred chelate in MSA electrolyte baths of the present invention.

Processes for making the foregoing bismuth chelates are described in the ensuing paragraphs. Tetrammonium bismuth dinitrilotriacetate

Chelate.

Reaction Formulation

Component Amount

Bismuth (Bi) 20.0 g

Bismuth trioxide (Bi₂O₃) 22.2 g

Nitrilotriacetic acid (2m/m Bi) 36.6 g

Distilled Water 200 ml

Ammonium hydroxide (29% NH₃) 25 ml A slurry of Bi₂O₃ and nitrilotriacetate (NTA) is agitated and heated at 80°C for about one hour. The NH₄OH is added slowly to form a water clear solution. If crystals appear when the solution is cooled to room temperature a little distilled water is added to redissolve them. The solution is near saturation at about 200 grams per liter bismuth. The pH of the solution should be near 6.8 at 25°C. Any residue is removed by filtration through hardened ashless paper. The filtrate may be vacuum evaporated at 26-29 in. Hg and < 80°C to recover the crystals. The crystals may be vacuum dried at 29 in. Hg and < 50°C.

The bismuth content of the crystals is typically 26.4% by analysis. The crystals dissolve readily in water to form clear solutions that are stable at pH values at 7.0. Mildly alkaline

(pH 7.5 - 10) solution are somewhat unstable. However, increased alkalinity (pH 10+) restores stability.

Diammonium bismuth diethylene triaminepentaacetate

Chelate (DTPA).

A typical procedure for synthesis the diammonium bismuth diethylene triaminepentaacetate chelate is described below:

Reaction Formulation

Component Amount

Bismuth (Bi) 60 g

Bismuth trioxide (Bi₂O₃), 98.5% 67.9 g

Diethylene triaminepentaacetic acid

(DTPA) (1 m/m Bi) 116.0 g

Distilled or D.I. Water 200 ml

Ammonium hydroxide (29% NH₃) 39 ml

30% Hydrogen Peroxide 0.5 ml Reaction Procedure

The reactor is a 600 ml thick-walled borosilicate glass

(PYREX)^®beaker with a TEFLON^®encapsulated magnetic stirrer on a THERMOLYNE STIR-PLATE^®. A cover glass and a thermometer are available.

The water is added first and agitated. Then the DTPA is added to form a white slurry. The NH₄OH is added to dissolve the DTPA. Heating is started. When the solution is practically clear the bismuth trioxide is added to form a yellow slurry. After about 1.5 hours the solution temperature reaches about 90°C and the solution has a slight haze and a volume of about 300 ml. Then the cover glass is removed to promote evaporation to about 200 ml. The solution is cooled to room temperature and filtered at low vacuum through REEVE ANGEL 934 AH glass fibre paper to yield 175 ml clear yellow filtrate typically having a density of 1.54 g/ml at room temperature and analyzing 328 grams per liter bismuth. The product concentrate generally has a pH at room temperature of at least 6.0.

Trimethane sulfonic acid bismuth trimethane sulfonate

Triglutonate Chelate Complex.

Reaction Formulation

Component Amount

Bismuth (Bi) 20 g

Bismuth trioxide (Bi₂O₃), 98.5% 22.3 g

50% Gluconic Acid (3 m.gl.ac./m.Bi) 91.3 ml

Distilled or D.I. Water 30 ml

70% Methane Sulfonic Acid 58.1 ml

Reaction Procedure

The reactor is a 250 ml thick-walled borosilicate glass beakesr with a magnetic stirrer on a THERMOLYNE STIR-PLATE^®. A cover glass and thermometer are available The water is added first, followed by the gluconic acid. The solution is agitated and heating starts as the bismuth trioxide is added to form a slurry. The slurry is heated at about 94°C for nearly 3 hours. 70% MSA is then added in increments for another hour. When all of the MSA is added at 86.5°C the solution becomes clear dark red. Over another 2 hours the hydrogen peroxide is added drop wise to oxidize any bismuthite formed and the product solution is cooled to room temperature to yield about 125 ml of slightly viscous dark red solution. About 0.5 ml of the product solution is diluted 500/1 with D.I. water and shows no signs of hydrolysis or precipitation. Typically the product solution analyzes 174 grams per liter bismuth at a density of 1.518 g/ml at 25°C.

Soluble tin may be provided to a bath of the present invention by a salt of a tin compound or a tin soluble anode. Either source of tin may be used alone or they may be used together.

The preferred salts of a tin compound useful in the aqueous acidic electroplating bath of the present invention tin salts of alkyl sulfonic acids, preferably lower alkylsulfonic acids having 1-5 carbon atoms. The most preferred salt is stannous methane sulfonate.

The preferred amount of a tin salt, in terms of tin content in the bath of the present invention, ranges from about .05 grams of soluble tin per liter of the bath to about 80 grams of soluble tin per liter of the bath, preferably from about .05 grams of soluble tin per liter of the bath to about 50 grams of soluble tin per liter of the bath, A preferred range of stannous methane sulfonate is from about .13 grams per liter to about 208 grams per liter, more preferably from about 0.13 grams per liter to about 104 grams per liter. These amounts of stannous methane sulfonate provide, respectively, from about .05 grams to about 80 grams of soluble tin per liter of bath and from about .05 grams to about 50 grams of soluble tin per liter of bath. Generally, for most commercial purposes, soluble tin concentrations in the bath below about 5 grams per liter will seldom be practical.

Stannous methane sulfonate is preferably supplied in a concentrate containing about 300 grams per liter stannous tin and 10-30 grams per liter free methane sulfonic acid. Stannous methane sulfonate concentrate may be made by reacting stannous oxide with methane sulfonic acid. The concentrate may also be formed electrolytically using a tin anode in a membrane cell containing MSA.

As illustrated in the Figure, a correlation curve of percent bismuth in satin electroplate (tin-bismuth co-plate, sometimes referred to as alloy plate) as a function of the weight ratio of bismuth to total tin in the bath was derived from analyses of simultaneous samples of tin-bismuth co-plates and the plating bath over a wide range of alloys from about 10% bismuth to about 90% bismuth. The bismuth content of the tin-bismuth electro coplates on the cathode panels range from about 3.38% to about 98.48%. The weight ratio of bismuth to tin in the baths ranged from about 0.30 to about 9.50 and the weight concentration of total tin plus bismuth spanned about 27 to about 66.5 grams per liter. The correlation curve of percent bismuth in the tin- bismuth electro co-plate as a function of the weight ratio of bismuth to total tin in the bath is based on chemical analyses of samples taken through the above mentioned ranges. Plating variables other than soluble bismuth and tin content, of course, may be adjusted to provide a desired co-plate composition at the desired rate of electrodeposition. The plater could, for

instance, adjust current density to effect the desired average percent bismuth in the co-plate. Making such adjustments is within the ability of a person of ordinary skill in the art. The illustrated correlation curve is therefore an accurate guide for calculating both concentrations of tin and bismuth for the full range of tin-bismuth alloys and particularly for alloys having 10-90% bismuth in the co-plate.

A straight-line correlation of percent bismuth as a function of the weight ratio of bismuth to total tin in the bath has been found in the range 0-15% bismuth in the co-plate. In the range of 1-2% bismuth in the co-plate there may be another straight-line correlation with a higher slope than the slope appearing in the Figure as shown in U.S. Patent No. 4,331,518. The illustrated correctional curve, however is an useful guide for bath

composition even at the lowest and highest ranges of bismuth content.

The method for calculating the bath formulation of the present invention is based on the desired percentage bismuth in the tin-bismuth co-plate. The desired total bismuth content of the co-plate are selected. The correlation curve may be used to find the weight ratio of bismuth to tin corresponding to the percentage bismuth in the co-plate. A soluble tin concentration for the bath is selected and is multiplied by the weight ratio of bismuth to tin to provide the bismuth concentration needed for the bath. The volumes of bismuth methane sulfonate concentrate and stannous methane sulfonate concentrate for one liter of the bath may then be calculated according to the respective bismuth and tin analyses of the concentrates. Ordinarily, the free methane sulfonic acid contributed by the concentrates is subtracted from 250 grams per liter free methane sulfonic acid and the balance is used to calculate the volume of 70% methane sulfonic acid (say at 938 grams per liter 100% MSA) to add before the concentrates.

The bismuth content of the electroplate, of course, is determined by the weight ratio of bismuth to tin in the plating bath. If a broad range of tin concentration of 1-6 oz/gal or 7.5- 45 grams per liter is selected then the bismuth in the bath should range from 0.8-44 oz/gal or 6-330 grams per liter for 5- 58% bismuth in the co-plate. It is preferable to determine the best distribution of tin and bismuth concentrations in the plating bath for a given bismuth content in the electroplate according to the correlation curve.

For optimum bath stability and inhibition of hydrolytic precipitation of bismuth, when bismuth anodes are used and tin in the co-plate is replaced by adding acid stannous methane

sulfonate concentrate to the bath at frequent intervals, the minimum free methane sulfonic acid concentration is from about 200-250 grams per liter and the preferred concentration is near 250 grams per liter. The broad range of free methane sulfonic acid concentration however is about 100-400 grams per liter.

Electroplating baths of the present invention may contain conventional amounts of additives such as surfactants, grain refiners, primary and/or secondary brighteners. Modified aromatic aldehydes (or ketones) and/or modified alkylene oxides or their analogs. These additives may be components of a brightening and leveling system such as BRI-TIN^® and ULTRA STAN-100^® produced by M & T Chemicals, Inc., formerly the Vulcan Materials Company. The BRI-TIN^® additive system imparts a mirror bright finish to the tm-bismuth co-plate. ULTRA STAN-100^® i.s a system for promoting satin white tin plates having excellent reflowing and solderability characteristics in acid plating baths. It consists of two solutions, a Primary Addition Solution which is used mainly to make up the bath, and an Activator Solution which is added mainly to satisfy plating demand. These solutions contain the surfactants, brightness, levelers and enhancers necessary for promoting the desired satin white finish to the electro co-plate of tin-bismuth. The particular leveling and brightening system is an not essential feature, however, of the invention.

Different leveling and brightening systems may result in some alteration of the correlation of percentage bismuth in the co-plate as a function of the weight ratio of bismuth to total tin in the bath or cell. Thus, ULTRA STAN-100^® and BRI-TIN^® may result in correlation curves similar in form but different in curvature and having different correlation equation constants.

Such alterations, however, may readily be anticipated by and accounted for by a person skilled in the art.

The conductive substrate or cathode of electroplating cells of the present invention may be any object which is conductive of electricity. Frequently, such objects are composed of metals such as iron, nickel, stainless steel, zinc, copper, or combinations of metals. The foregoing metals are examples of conventional conductive substrates but the spectrum of conductive substrates which may be plated in accordance with the present invention is not limited to the listed metals.

The anode of electroplating cells of the present invention is preferably a soluble bismuth metal anode that functions as a source of soluble bismuth However other anodes useful in the present invention include tin metal anodes. Insoluble anodes, such as zircalloy, pyrolitic graphite and platinum, could be used in the present invention but are not preferred since they often provide poor quality in tin-bismuth electroplates and excessive oxidation of stannous tin.

The ratio of anode area to cathode area needs to be

regulated for the proper anode current density to control the rate of anode solubilization to meet the plating requirement and to prevent build up of the soluble bismuth concentration in the bath. Soluble bismuth build-up upsets the weight ratio of bismuth to tin to change the bismuth content of the co-plate. For 5% bismuth in the co-plate the ratio of bismuth anode area to cathode area probably would be about 1/7-8. For 58% bismuth in the co-plate the ratio of bismuth anode area to cathode area probably would be in the order of about 0.5/1. For higher bismuth content the ratio probably would be on the order of 2/1. Bismuth in the range of 1-2% in the tin-bismuth co-plate probably would require a ratio of bismuth anode area to cathode area in the order of 1/10. In that case plating performance might be better if high grade tin anodes are substituted for bismuth anodes, with bismuth added in concentrates according to plating demand and the concentration of free acid in the bath lowered to subtend anode activity.

In a typical process of the present invention, an aqueous acidic electroplating bath is prepared in an electroplating vessel known to the art and is circulated vigorously at room temperature (15°C to 25°C). An anode, preferably soluble bismuth metal anode, which can be wrapped or bagged in polypropylene, is immersed or placed into the bath and the current is turned on. A cathode current density from about 2 to about 40 amp/ft² should ordinarily be maintained. The conductive substrate with an anode area/cathode area ratio adjusted according to desired bismuth content of the tin-bismuth co-plate is then immersed into the aqueous acidic electroplating bath and reciprocated moderately.

The conductive substrate is immersed in the bath and remains immersed for a time sufficient to deposit a variable alloy coating of tin-bismuth of the desired thickness upon the

conductive substrate. The conductive substrate is subsequently withdrawn from the aqueous acidic electroplating bath.

It is beneficial to maintain the current in the bath until the conductive substrate has been completely withdrawn. This minimizes smutting of the plate caused by displacement of bismuth from the solution at high concentration by the substrate.

The plated conductive substrate should be washed thoroughly as quickly as possible to minimize staining.

This invention is further illustrated in the following Examples. It should be understood, however, that the invention is not limited to the specific details of the Examples.

PREPARATION OF THE CONDUCTIVE SUBSTRATE

Conductive substrates used for the electrodeposition of bismuth in Examples 1-4 were steel panels (25 cm² plating area) from Hull cell panels stripped of zinc electrocoat in 1:1 HCl and activated in 10% methanesulfonic acid at room temperature, with thorough washing with demineralized water after each treatment. The stripped panels then were electroplated with 0.15 - 0.25 ml copper in an acid cupric methane sulfonate bath as described in Table 1 before being electroplated with 0.1 to 1.0 ml bismuth. It was found that the adhesion of the electro copper plate to the steel panel was very much improved by a very short dip (e.g. 5-10 seconds) of the stripped panel in 20-50 grams per liter HNO₃ at room temperature and by very thorough washing before activation in 10% methanesulfonic acid.

Table 1 contains a listing of the bath composition for the electroplating of the Hull cell panels with copper and Table 2 contains a listing of the plating conditions and solution characteristics for the bath used to plate the panels with copper. Table 3 lists the plating conditions and solution characteristics that were common throughout Examples 1-4.

EXAMPLE 1

The panels electroplated in the electroplating bath of Table 4 resulted in a conductive substrate with an

electrodeposited alloy coating comprised of 95% Tin/5% Bismuth.

EXAMPLE 2

The panels electroplated in the electroplating bath of Table 5 resulted in a conductive substrate with an

electrodeposited alloy coating comprised of 90% tin/10% bismuth.

EXAMPLE 3

The panels electroplated in the electroplating bath of Table 6 resulted in a conductive substrate with an

electrodeposited alloy coating comprised of 42% tin/58% bismuth. This proportion of bismuth to tin comprises a eutectic coating.

EXAMPLE 4

The panels electroplated in the electroplating bath of Table 7 resulted in a conductive substrate with an electrodeposited alloy coating comprised of 14.5% tin/85.5% bismuth.

Tin-bismuth alloys that may be made in accordance with the present invention include: 1) 42% tin/58% bismuth which forms a eutectic material having a melting point of about 138°C,

approximately 50°C lower than tin-lead eutectic composition; and 2) 25/75 or 16/84 tin-bismuth alloys sandwiched in plastic sheets to make formable metallized plastic. Other tin-bismuth alloys may be expected to find utility in many applications previously filled by tin/lead alloys.

The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected, however, is not limited to the particular embodiments disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spritt of the invention.

Claims

WHAT IS CLAIMED IS:

1. An electroplating bath for electrodeposition of

tin-bismuth alloy onto a conductive substrate comprising: a) soluble bismuth in aqueous solution;

b) soluble tin in aqueous solution, wherein said soluble

bismuth and said soluble tin are present in said bath in amounts sufficient to deposit a tin-bismuth alloy onto said conductive substrate and in a weight ratio relative to each other selected to provide a desired bismuth content of tin-bismuth alloy on said conductuve substrate; and

c) an alkyl sulfonic acid electrolyte in an amount

sufficient to inhibit hydrolytic precipitation of said soluble bismuth.

2. An electroplating bath as defined in Claim 1 wherein said electrolyte comprises from about 100 grams ler liter to about 400 grams per liter of said bath.

3. An electroplating bath as defined in Claim 1 wherein siad soluble bismuth is provided by bismuth sulfonate salt.

4. An electroplating bath as defined in Claim 1 wherein said bismuth is provided by bismuth methane sulfonate concentrate and a solid bismuth anode.

5. An electroplating bath as defined by Claim 1 wherein said soluble tin is provided by stannous methane sulfonate concentrate.

6. An electroplating cell for electrodeposition of

tin-bismuth alloy onto a conductive substrate comprising: a) An aqueous electroplating comprising

1. a source of soluble bismuth in aqueous solution;

2. a source of soluble tin in aqueous solution wherein said soluble bismuth and said soluble tin are present in said bath in amounts sufficient to deposit a tin-bismuth alloy onto said conductive substrate and in a weight ratio relative to each other selected to provide a desired bismuth content of said tin-bismuth alloy on said conductive substrate; and

3. an electrolyte of free alkyl sulfonic acid in an amount sufficient to inhibit hydrolytic precipitation of said soluble bismuth;

b) an anode immersed in said bath;

c) a conductive substrate cathode immersed in said bath; and

d) a supply of electricity for electrodepositing tin and bismuth onto said conductive substrate.

7. A method for the electrodeposition of a tin-bismuth alloy onto a conductive substrate comprising: a) providing an electroplating cell comprising:

1. soluble bismuth in aqueous solution;

2. soluble tin in aqueous solution wherein said

soluble bismuth and said soluble tin are present in said bath in amounts sufficient to deposit a tin-bismuth alloy onto said conductive substrate and in a weight ratio relative to each other selected to provide a desired bismuth content of said tin-bismuth alloy on said conductive

substrate;

3. a lower alkyl sulfonic acid electrolyte in an amount sufficient to inhibit hydrolytic

precipitation of said soluble bismuth; and

b) providing an anode immersed in said bath;

c) supplying sufficient electricity to said electroplating cell to electro deposit a in-bismuth alloy onto said conductive substrate; and

d) immersing said conductive substrate into said

electroplating bath.

8. An electrodeposited tin-bismuth alloy having

substantially a eutectic composition.

9. An electrodeposited tin-bismuth alloy according to Claim 8 in which said eutectic composition comprises about 42% tin-58% bismuth by weight of the alloy and has a melting point of about 138°C.

10. An article comprising a conductive substrate and an electrodeposited eutectic tin-bismuth alloy thereon having a composition of about 42% tin-58% bismuth by weight of the alloy an a melting point of about 138°C.