AU632464B2 - A method, bath and cell for the electrodeposition of tin-bismuth alloys - Google Patents

Description

THE COMMISSIONER OF PATENTS 4786M/LPR 6- 1

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_ii> B.lI-IIB i l |M ]1r[ lI 1 I 1 OPI DATE 01/05/90 $PPLN. ID 27277 88 DC'1 S .I AOJP DATE 07/06/90 PCT NUMBER PCT/US88/03536 INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 4 (11) International Publication Number: "WO 90/04048 3/56 Al (43) International Publication Date: 19 April 1990 (19.04.90) (21) International Application Number: PCT/US88/03536 Published With interinal rch rort.

(22) International Filing Date: 14 October 1988 (14.10.88) 6 (71) Applicant: M&fT--HEMIAL-S--INC- [US/US]; One a Woodbridge Center, Woodbridge, NJ 07095 (US).

(72) Inventor: WILSON, Harold, P. 729 Gloucester Drive, Huron, OH 44839 E (74) Agents: MARCUS, Stanley, A. et al.; P.O. Box 1104, Rahway, NJ 07065 (US).

(81) Designated States: AT, AT (European patent), AU, BE (European patent), BR, CH, CH (European patent), DE, DE (European patent), DK, FR (European patent), GB, GB (European patent), JP, KP, KR, LU, NL, NO, SE, SU.

(54)Title: A METHOD, BATH AND CELL FOR THE ELECTRODEPOSITION OF TIN-BISMUTH ALLOYS 100o 0 0 80 80- W 2 0 I 2 3 4 5 6 7 8 9 WEIGHT RATIO B 1 /Sn IN THE BATH (57) 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.

WO 90/04048 PCT/US88/03536 A METHOD, BATH AND CELL FOR THE ELECTRODEPOSITION OF TIN-BISMUTH ALLOYS BACKGROUND OF THE INVENTION 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 tinlead alloys. In some situations, therefore, it would be desirable to replace tin-lead with alloys having accaptable characteristics but that do not present health and environmental risks attendant to tin-lead alloys.

Moreovez, 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 lower than a tin-lead eutectic alloy, making the tinbisuth eutectic alloy an attractive material for plating and rl WO 90/04048 PCT/US88/03536 soldering layered circuit boards. Indeed, the broad range of potential uses for tin-bismuth alloys may.warrant production of tin-bismuth alloys having a bisauth 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 tinbismuth 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 compourd, 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.

i 3 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 iniention 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.

RLF/02141 1 e RLF/02141 1 I 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 There is disclosed herein a method of depositing a tin-bismuth alloy on at least a portion of the surface of a circuit board to plate the board, the alloy being deposited from an electroplating bath comprising bismuth and tin ions in aqueous solution, in proportions adapted to deposit tin-bismuth alloy onto the circuit board in a relative weight ratio providing a controlled bismuth content of greater than lOwt% in the alloy, the bath containing an electrolyte of at least one alkyl sulphonic acid or salt thereof in an amount sufficient to inhibit hydrolytic precipitation of bismuth ion in the bath.

r RLF/02141 i

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i bg, i j i: c r i As There is further disclosed herein use of more than 200 g/1 of an alkyl sulphonic acid or salt thereof to inhibit hydrolytic precipitation of bismuth, in an electroplating process which electroplates tin-busmuth alloy of at least lOwt% bismuth from an aqueous electroplating bath containing soluble tin and bismuth species.

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 applicationi, 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 RLF/02141 i r 1 i i I i ~V1 1 WO 90/04048 PCT/US88/03536 electro-deposited alloy coating with the balance of the electrodeposited alloy coating being tin. Ordinarily, tinbismuth 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 slectrolytes 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.

L o i WO 90/04048 PCT/US88/03536 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: Bi (0--CH 3 3 3 H 2 0 Bi (OH) 3 3 HO-S-CH 3 I I 0 Bismuthtrimethane Insoluble white Methane sulfonic sulfonate precipitate acid Bismuth trihydroxide 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 i" i:

C.

or-r/i csfi/ftl:m WO 90/04048 rua,,o 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 25 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 S polypropylene clotlh. Other components of cells, such as the container for the electrolyte solution, are conventional. Those 1 I r~-rllIt00/MIei7 WO 90/04048 rL I uool uJ u 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 tinbismuth 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/ft 2 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 0 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.

r: 1 WO 90/04048 PCT/US88/03536 /0 The following three chelating compounds may be used to provide chelated bismuth to the electrolyte bath: Tetraammonium bismuth dinitrilotriacetate Chelate.

(approx. 26% bismuth in water soluble transparent crystals) Diammonium bismuth diethylene triaminepentaacetate Chelate. (approx. 325 grams per liter bismuth in concentrate) 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, Sat 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 (Bi 2 0 3 22.2 g Nitrilotriacetic acid (2m/m Bi) 36.6 g Distilled Water 200 ml Ammonium hydroxide (29% NH 3 25 ml SWO 90/04048 PCT/US88/03536 A slurry of Bi203 and nitrilotriacetate' (NTA) is agitated and heated at 80 0 C for about one hour. The NH 4 0H 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 sol-tion should be near 6.8 at 25°C. Any residue is removed bl filtration through hardened ashless paper. The filtrate may be vacuum evaporated at 26-29 in. Hg and 80 0 C to recover the crystals.

The crystals may be vacuum dried at 29 in. Hg and 50 0

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 triamineDentaacetate 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 (Bi20 3 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% NH 3 39 ml 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 WO W90/04048 PCT/US88/03536 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 NH 4 0H 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 Trimethane sulfonic acid bismuth trimethane sulfonate Triglutonate Chelate Complex.

O 0 II II 3 CH 3 -S-OH Bi (0-S-CH3) 3 (gluconate) 3 II II o 0 Reaction Formulation Component Amount Bismuth (Bi) 20 g Bismuth trioxide (Bi 2 0 3 98.5% 22.3 g Gluconic Acid (3 m.gl.ac./m.Bi) 91.3 ml Distilled or D.I. Water 30 ml Methane Sulfonic Acid 58.1 ml Reaction Procedure The reactor is a 250 ml thick-walled borosilicate glass beaker with a magnetic stirrer on a THERMOLYNE STIR-PLATE. A cover glass and thermometer are available.

*r i F- WO 90/04048 PCT/US88/03536

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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 slurry. The slurry is heated at about 94 0 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 0

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 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 a, i' WO 90/04048 PCT/US88/03536 about 104 grams per liter. These amounts of stannous methane sulfonate provide, respectively, from about .05 grams to about 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 puzpoies, 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 concentratiou of total tin plus bismuth spanned about 27 to about 66.5 grams per liter. The correlation curve of percent bismuth in the tin- Sbismuth electro co-plate as a function of the weight ratio of i 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 1 O' 0 PCT/US88/03536 WO 90/04048 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 S 25 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 i h 1 WO 90/04048 PCT/US88/03536 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 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 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-TII1 and ULTRA STAN-10 produced by M T Chemicals, Inc., formerly the Vulcan Materials Company.

t 11 t 1 IWO 90/04048 PCT/US88/03536 i WO 90/04048 PCT/US88/03536 /7 The BRI-TIN additive system imparts a mirror bright finish to the tin-bismuth co-plate. ULTRA STAN-100 is 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 RLF/02141 WO 90/04048 PCT/US88/03536 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 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 0 C to 25 0 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/ft 2 should i' t, RLF/02141 C j1 i a i:i:i Ei PCr/US88/03536 WO 90/04048 /9 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 cm 2 plating area) from Hull cell panels stripped of zinc electrocoat in 1:1 HC1 and activated in 10% methana:sulfonic 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 5-10 r r i

S

WO 90/04048 I PCT/US88,0353 6 seconds) of the stripped panel in 20-50 grams per liter HNO3 at room temperature and by very thorough washing before activation in 10% methaneoulfonic 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.

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i;i t I WO 90/04048 PCT/US88/0353 6 TABLE 1 Component Copper Free Methanesulfonic Acid Cupric Methane Sulfonate Concentrate (129 g/l Cu, 11 g/l Free MSA) 69.5% Methanesulfonic Acid (938 g/l 100% MSA) Concentration (g/1) 193.8 ml/l 40.4 ml/1 TABLE 2 Plating Conditions: Temperature Agitation Anode Ratio Anode Area to Cathode Area Cathode Current Density amp/ft 2 Cathode Current Efficiency Solution Characteristics: Clarity Color Residue 20 0 C-25°C (Room Temp.) None Rolled Electrolytic Cu 2:1 2-25 100% Water clear Slight yellow tint Practically none

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r WO 90/04048 PCY/US88103536 2Z TABLE 3 Plating Conditions: Temperature, °C Agitation solution circulation cathode reciprocation Anodes Ratio anode area to cathode area Room (20 C-25 0

C?

Vigorous Moderate Cast high purity bismuth bagged with polypropylene adjusted according to the desired percentage of bismuth in the tinbismuth co-plate.

Water clear Slight yellow tint Practically none Solution Characteristics: Clarity Color Residue 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.

i i f i .WO 90/04048 PCT/US88/03536 TABLE 4 ml/1 gq/1l Total Tin 15.0 Stannous Methane Sulfonate Concentrate 48.7 Bismuth 12.0 Bismuth Methane Sulfonate Coiicentrate 54.4 Free Methane Sulfonic Acid 250.0 69.5% Methane Sulfonic Acid 253.0 ULTRA STAN-100 Primary Addition Solution v/v) ULTRA STAN-100 Activator Solution v/v) Working Range of Cathode Current Density 2-40 amp/ft 2 Cathode Current Efficiency 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.

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1 1 1 1 1 1 1 WO 90/04048 i.

PCr/US88/03536 TABLE ml/1 a/l i Total Tin Stannous Methane Sulfonate Concentrate Bismuth Bismuth Methane Sulfonate Concentrate Free Methane Sulfonic Acid 69.5% Methane Sulfonic Acid ULTRA STAN-100® Primary Addition Solution v/v) ULTRA STAN-100 Activator Solution v/v) Working Range of Cathode Current Density Cathode Current Efficiency 15.0 48.7 24.0 109.0 250.0 241 2-40 amp/ft 2 EXAMPLE 3 The panels electroplated in the electroplating bath of Table 6 resulted in a conductive substrate with an electrodeposited lloy coating comprised of 42% tin/58% bismuth.

This proportion of bismuth to tin comprises a eutectic coating.

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I g ,WO 90/04048 Pc-/US88/03536 3~TA TABLE 6 ml/I a/I ml/l a/ Total Tin Stannous Methane Sulfonate Concentrate Bismuth Bismuth Methane Sulfonate Concentrate Free Methane Sulfonic Acid 69.5% Methane Sulfonic Acid ULTRA STAN-100® Primary Addition Solution v/v) ULTRA STAN-100 Activator Solution v/v) Working Range of Cathode Current Density Cathode Current Efficiency 24.4 55.0 275.0 250.0 200.0 35.0 40.0 2-20 amp/ft 2 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.

i N -i -'4 PcT/US88/03536 WO 90/04048 TABLE 7 ml/1 C/1 iii Total Tin Stannous Methane Sulfonate Concentrate Bismuth Bismuth Methane Sulfonate Concentrate Free Methane Sulfonic Acid 69.5% Methane Sulfonic Acid ULTRA STAN-100 Primary Addition Solution v/v) ULTRA STAN-100 Activator Solution v/v) Working Range of Cathode Current Density Cathode Current Efficiency Ratio of Anode Area to Cathode Area 6.4 20.8 49.6 228.0 250 200 35.0 40.0 2-20 amp/ft 2 2:1 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 1380C, 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

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Claims (7)

1. A method of depositing a tin-bismuth alloy on at least a portion of the surface of a circuit board to plate the board, the alloy being deposited from an electroplating bath comprising bismuth and tin ions in aqueous solution, in proportions adapted to deposit tin-bismuth alloy onto the circuit board in a relative weight ratio providing a controlled bismuth content of greater thar 10Owt% in the alloy, the bath containing an electrolyte of at least one alkyl sulphonic acid or salt thereof in an amount sufficient to inhibit hydrolytic precipitation of bismuth ion in the bath.

2. A method of plating a circuit board according to claim 1, in which the electroplating bath contains over 200 g/l of the alkyl sulphonic acid or salt thereof.

3. A method of plating a circuit board according to claim 1 or 15 claim 2, in which the resultant alloy has substantially the tin-bismuth eutectic composition.

4. A plated circuit board obtained by a method of any one of claims 1 to 3.

5. Use of more than 200 g/l of an alkyl sulphonic acid or salt thereof to inhibit hydrolytic precipitation of bismuth, in an electroplating process which electroplates tin-busmuth alloy of at least lOwt% bismuth from an aqueous electroplating bath containing soluble tin and bismuth species.

6. Use according to claim 5 in which the acid or salt thereof is methanesulphonic.

7. A method of depositing a tin-busmuth alloy on a circuit board, the method being substantially as hereinbefore described with reference to any one of the Examples. DATED this SIXTH day of OCTOBER 1992 Atochem North America, Inc. Patent Attorneys for the Applicant SPRUSON FERGUSON RLF/02141