CA1333377C - Tin-bismuth alloy coated article - Google Patents

Abstract

The invention comprises an article of manufacture comprising at least one printed wireboard (PWB) with at least one conductive surface, having a tin-bismuth alloy deposited on at least a portion of the surface, the alloy being deposited from an electroplating bath comprising bismuth and tin ion in aqueous solution, wherein the bismuth and the tin ion are present in the bath in amounts sufficient to deposit the tin-bismuth alloy onto the conductive surface of the PWB in a weight ratio relative to each other selected to provide a controlled bismuth content of the tin-bismuth alloy on the conductive surface of the PWB in an amount of greater than about 10 wt.% bismuth, the bath having an electrolyte comprising at least one alkyl sulfonic acid or salt thereof in an amount sufficient to inhibit hydrolytic precipitation of the bismuth ion.

Description

BACKGROUND OF THE INVENTION
This lnvention relates to an electroplatlng bath, an electroplatlng cell and a method for the electrodeposltlon of a wlde range of tln-blsmuth alloys onto a conductlve substrate.
Alloys of tin and lead have been used ln a wlde varlety of appllcatlons such as platlng for clrcult boards, as rust lnhlb-ltlng coatlngs on metals and as solder. Increased awareness of health and envlronmental hazards posed by lead has requlred hand-llng and dlsposal precautlons that often escalate the cost of handllng or dlsposlng of lead-contalnlng materlals lncludlng tln-lead alloys. In some sltuatlons, therefore, lt would be deslrable to replace tln-lead wlth alloys havlng acceptable characterlstlcs but that do not present health and envlronmental rlsks attendant to tln-lead alloys.
Moreover, tln-lead alloys have not proven entlrely satlsfactory ln some temperature sensltlve appllcatlons where heat ls undeslrable. Tln-lead alloys have conventlonally been used, for lnstance, as platlng for multllayer clrcult boards and as a eutectlc solder used to bond together the layers of tln-lead pla-ted clrcult boards. The heat requlred to melt eutectlc tln-lead alloy solder, however, ls so hlgh that lt can damage the compo-nents of the clrcult board or lmpalr the conductlvlty character-lstlcs of the clrcult board.
Tln-blsmuth alloys have characterlstlcs that make them attractlve replacements for tln-lead alloys for many purposes.
Tln-blsmuth alloys, for lnstance, do not present the health and envlronmental problems assoclated wlth lead contalnlng alloys.
Moreover, a tln-blsmuth eutectlc alloy has a meltlng polnt about 50C lower than a tln-lead eutectlc alloy, maklng the tln-blsmuth eutectlc alloy an attractlve materlal for platlng and solderlng ~F

layered clrcult boards. Indeed, the broad range of potentlal uses for tln-blsmuth alloys may warrant productlon of tln-blsmuth alloys havlng a blsmuth content ranglng from nearly 0% to nearly 100% blsmuth. It ls therefore deslrable to develop a commerclally acceptable electrolytlc bath, cell and process capable of provl-dlng tln-blsmuth alloys whlch may have any deslred blsmuth content ranglng from ~ust above 0% to ~ust below 100% blsmuth.
Conventlonal blsmuth salts used ln conventlonal electro-lytes often are unstable and undergo undeslrable hydrolytlc pre-clpltatlon. To prevent or mlnlmlze thls problem, addltlves whlchlnhlblt hydrolytlc preclpltatlon of blsmuth ln electrolyte baths are ordlnarlly used. Cltrlc acld and chelatlng agents are examples of addltlves commonly used for thls purpose. Conven-tlonal baths contalnlng such addltlves, however, may be dlfflcult to malntaln and do not provlde versatlle commerclally satlsfactory baths and cells for deposltlng blsmuth-contalnlng alloys of any deslred blsmuth content.
In some electroplatlng processes, small quantltles of blsmuth have been added to tln plate to retard the formatlon of tln pest and tln whlskers. From about 1% to 2% blsmuth ln the co-plate ls ordlnarlly adequate for thls purpose. U.S. Patent No.
4,331,518 dlscloses an electroplatlng process whlch produces tln-blsmuth alloys whlch may have as much as 10-14% blsmuth ln the co-plate. Soluble blsmuth ls provlded ln the electroplatlng bath as a chelated acld blsmuth sulfate gluconate.
Optlonal use of small quantltles of blsmuth nltrate as an addltlve compound, ln an electroplatlng bath contalnlng alkyl sulfonlc acld electrolyte, to ald ln the deposltlon of tln-lead alloy onto a substrate ls dlsclosed ln U.S. Patent No. 4,565,610.

1 3~3377 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.
Practising 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
The invention provides an article of manufacture comprising at least one printed wireboard (PWB) with at least one conductive surface, having a tin-bismuth alloy deposited on at least a portion of the surface, the alloy being deposited from an electroplating bath comprising bismuth and tin ion in aqueous solution, wherein the bismuth and the tin ion are present in the bath in amounts sufficient to deposit the tin-bismuth alloy onto the conductive surface of the PWB in a weight ratio relative to each other selected to provide a controlled bismuth content of the tin-bismuth alloy on the conductive surface of the PWB in an amount of greater than about 10 wt.% bismuth, the bath having an electrolyte comprising at least one alkyl sulfonic acid or salt thereof in an amount sufficient to inhibit hydrolytic precipitation of the bismuth ion.
In preferred embodiments the bismuth content of tin/bismuth alloy is greater than about 20 wt.%, preferably between about 20 wt.% and about 80 wt.%. In a particular embodiment the alloy comprises the tin/bismuth eutectic. The article may comprise a plurality of PWBs or the alloy may be deposited on a plurality of surfaces.
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 3a to each other selected to provlde a deslred blsmuth content of the tln-blsmuth alloy on the conductlve substrate; and c) an alkyl sulfonlc acld electrolyte ln an amount sufflclent to lnhlblt hydrolytlc preclpltatlon of the soluble blsmuth.
A further embodlment of the present lnventlon ls an electroplatlng cell for electrodeposltlon of tln-blsmuth alloy onto a conductlve substrate comprlslng:
a) an electroplatlng bath comprlslng 1) soluble blsmuth ln aqueous solutlon;

2) soluble tln ln aqueous solutlon whereln the soluble blsmuth and the soluble tln are pre-sent ln the bath ln amounts sufflclent to deposlt a tln-blsmuth alloy onto the conduc-tlve substrate and ln a welght ratlo relatlve to each other selected to provlde a deslred blsmuth content of the tln-blsmuth alloy on the conductlve substrate; and 3) a lower alkyl sulfonlc acld electrolyte ln an amount sufflclent to lnhlblt hydrolytlc pre-clpltatlon of the soluble blsmuth;
b) an anode lmmersed ln the bath;
c) a conductlve substrate cathode lmmersed ln the bath; and d) a supply of electrlclty for electrodepositlng tln and blsmuth onto the conductlve substrate.
The present lnventlon also provldes a method for the electrodeposltlon of a tin-blsmuth alloy onto a conductlve sub-strate comprlslng:

.~ 4 a) provldlng an electroplatlng bath comprlslng:
1) soluble blsmuth ln aqueous solutlon;
2) soluble tln ln aqueous solutlon whereln the soluble blsmuth and the soluble tln are present ln the bath ln amounts sufflclent to deposlt a tin-blsmuth alloy onto the conduc-tlve substrate and ln a welght ratlo relatlve to each other selected to provlde a deslred blsmuth content of the tln-blsmuths alloy on the conductlve substrate;
3~ a lower alkyl sulfonlc acld electrolyte ln an amount sufflclent to lnhlblt hydrolytlc pre-clpltatlon of the soluble blsmuth;
b) provldlng an anode lmmersed ln the bath;
c) supplylng sufflclent electrlclty to the bath to electro deposlt tln-blsmuth alloy onto the conduc-tlve substrate; and d) lmmerslng the conductlve substrate lnto the electroplatlng bath.
Addltlonal advantages and embodlments of the lnventlon wlll be set forth ln part ln the descrlptlon whlch follows, and ln part wlll be apparent from the descrlptlon, or may be learned by practlce of the lnventlon. The advantages of the lnventlon may be reallzed and attalned by processes, materlals and comblnatlons partlcularly polnted out ln the appended clalms.

DESCRIPTION OF ~K~ KK~ EMBODIMENTS OF THE INVENTION
In the present appllcatlon, the phrase "tln-blsmuth alloy" ls understood to mean an electroplated alloy coatlng of greater than 0% and less than 100% blsmuth by total welght of the electrodeposited 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 Cl 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.

Hydrolytlc preclpltatlon ln a multl-component electro-lyte solutlon probably ls quite complex, but in the case of blsmuth, the followlng formula, ln which soluble blsmuth is provided by bismuth trimethane sulfonate, may represent the mechanlsm of hydrolytic precipitation ln a simplistlc manner:
O O
Il 11 Bi(O-S-CH3)3 + 3 H2O ~ Bl(OH~3 + 3 HO-S-CH3 O O
Blsmuthtrimethane Insoluble white Methane sulfonic sulfonate precipitate acid Bismuth tri-hydroxide To maintain stablllty 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 electro-platlng bath ordlnarlly ranges from about 100 grams per llter to about 400 grams per liter, preferably from about 150 grams per llter 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 ln the platlng bath ls below 200 grams per liter, for example below about 150 grams per liter, the bath may undergo undeslrable 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, pre-clpltatlon of blsmuth ordlnarily does not occur at observable levels.
Soluble blsmuth avallable to form a tln-blsmuth alloy on a conductlve substrate may be provlded ln a bath of the present lnventlon by addlng a bismuth salt, preferably bismuth alkyl ~- 7 ':

sulfonate, dlrectly to the bath or by a soluble blsmuth metal anode. Elther source of soluble blsmuth may be used wlthout the other but they are frequently used together.
For most appllcatlons of the present lnventlon, the amount of soluble blsmuth ln the aqueous acldlc platlng bath ordl-narlly ranges from about .05 grams per llter of the bath to about 150 grams per llter of the bath, preferably, from about .05 grams per llter of the bath to about 80 grams per llter of the bath.
Soluble blsmuth ls preferably provlded ln bath solutlons of MSA
lnltlally ln the form of blsmuth trlmethane sulfonate concentrate (alternatively referred to as blsmuth methane sulfonate concen-trate).
Blsmuth trlmethane sulfonate concentrate may be prepared by reactlng blsmuth trloxlde wlth 70% methane sulfonlc acld. It has been found that a product bath solutlon of 200-225 grams of blsmuth trlmethane sulfonate per llter blsmuth ls stable only when there ls at least 200 grams per llter free methane sulfonlc acld ln the solutlon. As the electroplatlng process proceeds, addl-tlonal blsmuth trlmethane sulfonate concentrate may be added to malntaln an adequate soluble blsmuth content ln the bath.
In one electroplatlng system of the present lnventlon, a soluble blsmuth anode ls used to recharge the blsmuth platlng bath so that further addltlons of blsmuth methane sulfonate are mlnl-mlzed or not requlred durlng operatlon. The blsmuth anode prefer-ably useful ln the present lnventlon ls constructed of soluble blsmuth metal, typlcally cast hlgh purity blsmuth. It has been found useful to bag the soluble blsmuth anode with polypropylene cloth. Other components of cells, such as the contalner for the electrolyte solutlon, are conventlonal. Those skllled in the art are well acqualnted wlth electroplatlng cells and thelr assembly and would therefore be able to provlde a cell ln accordance wlth the teachlngs of the present lnventlon.
When blsmuth anodes are used to supply soluble blsmuth, the lmmerslon area of the anodes wlll have to be regulated to con-trol the solublllzatlon rate to meet platlng demand and malntaln solutlon concentratlon. Blsmuth anodes generally produce tln-blsmuth co-plates havlng clean graylsh whlte satln flnlshes.
It has been found that blsmuth anodes ln platlng baths contalnlng about 250 grams per llter free methane sulfonlc acld are very actlve and tend to lncrease the soluble blsmuth concen-tration, thereby slowly upsettlng the blsmuth/tln ratlo ln the co-plate to contaln about 5% blsmuth. Thls may be counteracted by addlng addltlonal tln to the bath. However, the soluble blsmuth bulld-up ls preferably mlnlmlzed by controlllng the lmmerslon area of the blsmuth anode to provlde an anode current denslty near 110 amp/ft2 where the anode current efflclency probably becomes suffl-clently low to inhlblt the solublllzatlon rate of blsmuth.
In the case of tln-blsmuth co-plates contalnlng 1-2%
blsmuth, useful for lnhlbltlng the growth of tln whlskers and tln pest ~allotroplc transformatlon to alpha-tln, the gray cublc form, at 12C to -70~C or lower) lt may be preferable to add blsmuth ln chelated form to the bath rather than as a sulfonlc acld concen-trate. When thls ls done lt ls ordlnarlly posslble to lower the concentratlon of free methane sulfonlc acld to 150 or even 100 grams per llter.
When chelated blsmuth ls used ln the bath as the source of blsmuth lons, lt also may be posslble to substltute tln anodes for the blsmuth anodes.

The followlng three chelatlng compounds may be used to provide chelated blsmuth to the electrolyte bath:
(1) Tetraammonlum blsmuth dlnltrllotrlacetate Chelate.
(approx. 26% blsmuth ln water soluble transparent crystals) (2) Dlammonlum blsmuth diethylene trlamlnopentaacetate Chelate. (approx. 325 grams per llter blsmuth ln concentrate) (3) Trlmethane sulfonlc acld blsmuth trlmethane sulfo-nate trlgluconate Chelate Complex. (approx. 200 grams per llter blsmuth ln concentrate) The concentratlons of these blsmuth sources ln the bathare calculated ln the same manner as descrlbed for blsmuth methane-sulfonate except that the free acld concentratlon may be less (say approx. 100-150 grams per llter, preferably about 150 grams per llter). These chelated compounds, partlcularly chelate No. 2, at hlgh concentratlons tend to saturate and cause a spon-taneous breakdown preclpltatlon ln the MSA bath. The flrst chelate llsted above ls ordlnarlly the preferred chelate ln MSA
electrolyte baths of the present lnventlon.
Processes for maklng the foregolng blsmuth chelates are descrlbed ln the ensulng paragraphs.
Tetraammonlum blsmuth dlnltrllotrlacetate Chelate.
Reactlon Formulatlon comPonent Amount Blsmuth (Bl) 20.0 g Blsmuth trloxlde (B1203) 22.2 g Nltrllotrlacetlc acld (2m/m Bl) 36.6 g Dlstllled Water 200 ml Ammonlum hydroxlde (29% NH3) 25 ml ,~
~

A slurry of B12O3 and nltrllotrlacetate (NTA) ls aglta-ted and heated at 80C for about one hour. The NH40H ls added slowly to form a water clear solutlon. If crystals appear when the solutlon ls cooled to room temperature a llttle dlstllled water ls added to redlssolve them. The solutlon ls near satura-tlon at about 200 grams per llter blsmuth. The pH of the solutlon should be near 6.8 at 25C. Any resldue ls removed by flltratlon through hardened ashless paper. The flltrate may be vacuum evaporated at 26-29 ln. Hg and < 80C to recover the crystals.
The crystals may be vacuum drled at 29 ln. Hg and c 50C.
The blsmuth content of the crystals ls typlcally 26.4%
by analysls. The crystals dlssolve readlly ln water to form clear solutlons that are stable at pH values at 7Ø Mlldly alkallne (pH 7.5 - 10) solutlon are somewhat unstable. However, lncreased alkallnlty (pH 10+) restores stablllty.

Dlammonlum blsmuth dlethylene trlamlnePentaacetate Chelate (DTPA).
A typlcal procedure for synthesls the dlammonlum blsmuth dlethylene trlamlnepentaacetate chelate ls descrlbed below:
Reactlon Formulatlon Component Amount Blsmuth (Bl) 60 g Blsmuth trloxlde (B12O3), 98.5% 67.9 g Dlethylene trlamlnepentaacetlc acld (DTPA) (1 m/m Bl) 116.0 g Distllled or D.I. Water 200 ml Ammonlum hydroxlde (29% NH3) 39 ml 30% Hydrogen Peroxlde 0.5 ml Reactlon Procedure The reactor ls a 600 ml thlck-walled boroslllcate glass (PYREX)~ beaker wlth a TEFLON~ encapsulated magnetlc stlrrer on a , ~

THERMOLYNE STIR-PLATE~. A cover glass and a thermometer are avallable.
The water ls added flrst and agltated. Then the DTPA ls added to form a whlte slurry. The NH40H ls added to dlssolve the DTPA. Heatlng ls started. When the solutlon ls practlcally clear the blsmuth trloxlde ls added to form a yellow slurry. After about 1.5 hours the solutlon temperature reaches about 90C and the solutlon has a sllght haze and a volume of about 300 ml. Then the cover glass ls removed to promote evaporatlon to about 200 ml.
The solutlon ls cooled to room temperature and flltered at low vacuum through REEVE ANGEL 934 AH glass fibre paper to yleld 175 ml clear yellow flltrate typlcally havlng a denslty of 1.54 g/ml at room temperature and analyzlng 328 grams per llter blsmuth.
The product concentrate generally has a pH at room temperature of at least 6Ø

Trlmethane sulfonlc acld blsmuth trlmethane sulfonate Trlglutonate Chelate Complex.
O O
Il 11 3 CH3-SI-OH Bl (O-S-CH3)3 (gluconate)3 Reactlon Formulatlon Components Amount Blsmuth (Bl) 20 g Blsmuth trloxlde (B12O3), 98.5% 22.3 g 50% Gluconlc Acid (3 m.gl.ac./m.Bl) 91.3 ml Dlstllled or D.I. Water 30 ml 70% Methane Sulfonlc Acld 58.1 ml Reactlon Procedure The reactor ls a 250 ml thlck-walled boroslllcate glass beaker wlth a magnetlc stlrrer on a THERMOLYNE STIR-PLAT~ . A
cover glass and thermometer are avallable.
Trade-mark The water ls added flrst, followed by the gluconlc acld.
The solutlon ls agltated and heatlng starts as the blsmuth trl-oxlde ls added to form a slurry. The slurry ls heated at about 94C for nearly 3 hours. 70% MSA ls then added ln lncrements for another hour. When all of the MSA ls added at 86.5C the solutlon becomes clear dark red. Over another 2 hours the hydrogen per-oxlde ls added drop wlse to oxldlze any blsmuthlte formed and the product solutlon ls cooled to room temperature to yleld about 125 ml of sllghtly vlscous dark red solutlon. About 0.5 ml of the product solutlon ls dlluted 500/1 with D.I. water and shows no slgns of hydrolysls or preclpltatlon. Typlcally the product solu-tlon analyzes 174 grams per llter blsmuth at a denslty of 1.518 g/ml at 25C.
Soluble tln may be provlded to a bath of the present lnventlon by a salt of a tln compound or a tln soluble anode.
Elther source of tln may be used alone or they may be used to-gether.
The preferred salts of a tln compound useful ln the aqueous acldlc electroplatlng bath of the present lnventlon tln salts of alkyl sulfonlc aclds, preferably lower alkylsulfonlc aclds havlng 1-5 carbon atoms. The most preferred salt ls stan-nous methane sulfonate.
The preferred amount of a tln salt, ln terms of tln content ln the bath of the present lnventlon, ranges from about .05 grams of soluble tln per llter of the bath to about 80 grams of soluble tln per llter of the bath, preferably from about .05 grams of soluble tln per llter of the bath to about 50 grams of soluble tln per llter of the bath. A preferred range of stannous methane sulfonate ls from about .13 grams per llter to about 208 grams per llter, more preferably from about 0.13 grams per llter to about 104 grams per llter. These amounts of stannous methane sulfonate provlde, respectlvely, from about .05 grams to about 80 grams of soluble tln per llter of bath and from about .05 grams to about 50 grams of soluble tln per llter of bath. Generally, for most commerclal purposes, soluble tln concentrations ln the bath below about 5 grams per llter wlll seldom be practlcal.
Stannous methane sulfonate ls preferably supplled ln a concentrate contalnlng about 300 grams per llter stannous tln and 10-30 grams per llter free methane sulfonlc acld. Stannous methane sulfonate concentrate may be made by reactlng stannous oxlde wlth methane sulfonlc acld. The concentrate may also be formed electrolytlcally uslng a tln anode ln a membrane cell contalnlng MSA.
As lllustrated ln the Flgure, a correlatlon curve of percent blsmuth ln satln electroplate (tln-blsmuth co-plate, sometlmes referred to as alloy plate) as a functlon of the welght ratlo of blsmuth to total tln ln the bath was derlved from analy-ses of slmultaneous samples of tln-blsmuth co-plates and the platlng bath over a wlde range of alloys from about 10% blsmuth to about 90% blsmuth. The blsmuth content of the tln-blsmuth electro co-plates on the cathode panels range from about 3.38% to about 98.48%. The welght ratlo of blsmuth to tln ln the baths ranged from about 0.30 to about 9.50 and the welght concentratlon of total tln plus blsmuth spanned about 27 to about 66.5 grams per llter. The correlatlon curve of percent blsmuth ln the tln-blsmuth electro co-plate as a functlon of the welght ratlo of blsmuth to total tln ln the bath ls based on chemlcal analyses of samples taken through the above mentloned ranges. Platlng ~ 333377 variables other than soluble blsmuth and tin content, of course, may be ad~usted to provlde a deslred co-plate composltlon at the deslred rate of electrodeposltlon. The plater could, for lnstance, ad~ust current denslty to affect the deslred average percent blsmuth in the co-plate. Making such ad~ustments is withln the ablllty of a person of ordlnary sklll ln the art. The lllustrated correlatlon curve ls therefore an accurate gulde for calculatlng both concentratlons of tln and blsmuth for the full range of tln-blsmuth alloys and partlcularly for alloys havlng 10-90% blsmuth ln the co-plate.
A stralght-llne correlatlon 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 lllustrated correctlonal curve, however, ls a useful gulde for both composltlon even at the lowest and hlghest ranges of blsmuth content.
The method for calculatlng the bath formulatlon of the present invention is based on the desired percentage blsmuth 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 blsmuth 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 blsmuth to tln to provlde the blsmuth concentratlon needed for the bath. The volumes of blsmuth methane sulfonate concentrate and stannous methane sulfonate concentrate for one llter of the bath may then be calculated accordlng to the respectlve blsmuth and tln analyses of the concentrates. Ordlnarlly, the free methane sul-fonlc acld contrlbuted by the concentrates ls subtracted from 250 grams per llter free methane sulfonlc acld and the balance ls used to calculate the volume of 70% methane sulfonlc acld (say at 938 grams per llter 100% MSA) to add before the concentrates.
The blsmuth content of the electroplate, of course, ls determlned by the welght ratlo of blsmuth to tln ln the platlng bath. If a broad range of tln concentratlon of 1-6 oz/gal or 7.5-45 grams per llter ls selected then the blsmuth ln the bath should range from 0.8-44 oz/gal or 6-330 grams per llter for 5-58% bls-muth ln the co-plate. It ls preferable to determlne the best dlstrlbutlon of tln and blsmuth concentratlons ln the platlng bath for a glven blsmuth content ln the electroplate accordlng to the correlatlon curve.
For optlmum bath stablllty and lnhlbltlon of hydrolytlc preclpltatlon of blsmuth, when blsmuth anodes are used and tln ln the co-plate ls replaced by addlng acld stannous methane sulfonate concentrate to the bath at frequent lntervals, the mlnlmum free methane sulfonlc acld concentratlon ls from about 200-250 grams per llter and the preferred concentratlon ls near 250 grams per llter. The broad range of free methane sulfonlc acld concentra-tlon however ls about 100-400 grams per llter.
Electroplatlng baths of the present lnventlon may con-taln conventlonal amounts of addltlves such as surfactants, graln reflners, prlmary and/or secondary brlghteners. Modlfled aromatlc aldehydes (or ketones) and/or modlfled alkylene oxldes or thelr analogs. These addltlves may be components of a brlghtenlng and levellng system such as ~RI-TIN~ and ULTRA STAN-100O produced by .~,.

M & T Chemlcals, Inc., formerly the Vulcan Materials Company.
The BRI-TIN~ addltlve system lmparts a mlrror brlght finlsh to the tln-blsmuth co-plate. ULTRA STAN-100~ ls a system for promotlng satln whlte tln plates havlng excellent reflowlng and solderabillty characterlstlcs ln acld platlng baths. It conslsts of two solutlons, a Prlmary Addltlon Solutlon whlch ls used malnly to make up the bath, and an Actlvator Solutlon whlch ls added malnly to satlsfy platlng demand. These solutlons con-taln the surfactants, brlghtness, levelers and enhancers necessary for promotlng the deslred satln whlte flnlsh to the electro co-plate of tln-blsmuth. The partlcular levellng and brlghtenlng system ls an not essentlal feature, however, of the lnventlon.
Dlfferent levellng and brlghtenlng systems may result ln some alteratlon of the correlatlon of percentage blsmuth ln the co-plate as a functlon of the welght ratlo of blsmuth to total tln ln the bath or cell. Thus, ULTRA STAN-100~ and BRI-TIN~ may result ln correlatlon curves slmllar ln form but dlfferent ln curvature and havlng dlfferent correlatlon equatlon constants.
Such alteratlons, however, may readlly be antlclpated by and accounted for by a person skllled ln the art.
The conductlve substrate or cathode of electroplatlng cells of the present lnventlon may be any ob~ect whlch ls conduc-tlve of electrlclty. Frequently, such ob~ects are composed of metals such as lron, nlckel, stalnless steel, zlnc, copper, or comblnatlons of metals. The foregolng metals are examples of conventlonal conductlve substrates but the spectrum of conductlve substrates whlch may be plated ln accordance wlth the present lnventlon ls not llmlted to the llsted metals.

The anode of electroplatlng cells of the present lnven-tion ls preferably a soluble blsmuth metal anode that functlons as a source of soluble blsmuth. However, other anodes useful ln the present lnventlon lnclude tln metal anodes. Insoluble anodes, such as zlrcalloy, pyrolltlc graphlte and platlnum, could be used ln the present lnventlon but are not preferred slnce they often provlde poor quallty ln tln-blsmuth electroplates and excesslve oxldatlon of stannous tln.
The ratlo of anode area to cathode area needs to be regulated for the proper anode current denslty to control the rate of anode solublllzatlon to meet the platlng requlrement and to prevent bulld-up of the soluble blsmuth concentratlon ln the bath.
Soluble blsmuth bulld-up upsets the welght ratlo of blsmuth to tln to change the blsmuth content of the co-plate. For 5% blsmuth ln the co-plate the ratlo of blsmuth anode area to cathode area prob-ably would be about 1/7-8. For 58% blsmuth ln the co-plate the ratlo of blsmuth anode area to cathode area probably would be in the order of about 0.5/1. For hlgher blsmuth content the ratlo probably would be on the order of 2/1. Blsmuth ln the range of 1-2% ln the tln-blsmuth co-plate probably would requlre a ratlo of blsmuth anode area to cathode area ln the order of 1/10. In that case platlng performance mlght be better lf hlgh grade tln anodes are substltuted for blsmuth anodes, wlth blsmuth added ln concen-trates accordlng to platlng demand and the concentratlon of free acld ln the bath lowered to subtend anode actlvlty.
In a typlcal process of the present lnventlon, an aque-ous acldlc electroplatlng bath ls prepared ln an electroplatlng vessel known to the art and ls clrculated vlgorously at room temperature (15C to 25C). An anode, preferably soluble blsmuth metal anode, which can be wrapped or bagged ln polypropylene, ls lmmersed or placed lnto the bath and the current ls turned on. A
cathode current denslty from about 2 to about 40 amp/ft2 should ordlnarlly be malntalned. The conductlve substrate wlth an anode area/cathode area ratlo ad~usted accordlng to deslred blsmuth content of the tln-blsmuth co-plate ls then lmmersed lnto the aqueous acldlc electroplatlng bath and reclprocated moderately.
The conductlve substrate ls lmmersed ln the bath and remalns lmmersed for a tlme sufflclent to deposlt a varlable alloy coatlng of tln-blsmuth of the deslred thlckness upon the conduc-tive substrate. The conductlve substrate ls subsequently wlth-drawn from the aqueous acldlc electroplatlng bath.
It ls beneflclal to malntaln the current ln the bath untll the conductlve substrate has been completely wlthdrawn.
Thls mlnlmlzes smuttlng of the plate caused by dlsplacement of blsmuth from the solutlon at hlgh concentratlon by the substrate.
The plated conductlve substrate should be washed thoroughly as qulckly as posslble to mlnlmlze stalnlng.
Thls lnventlon ls further lllustrated ln the followlng Examples. It should be understood, however, that the lnventlon ls not llmlted to the speclflc detalls of the Examples.

PREPARATION OF THE CONDUCTIVE SUBSTRATE
Conductlve substrates used for the electrodeposltlon of blsmuth ln Examples 1-4 were steel panels (25 cm2 platlng area) from Hull cell panels strlpped of zlnc electrocoat ln 1:1 HCl and actlvated ln 10% methanesulfonlc acld at room temperature, wlth thorough washlng wlth demlnerallzed water after each treatment.
The strlpped panels then were electroplated wlth 0.15 - 0.25 ml copper in an acld cuprlc methane sulfonate bath as descrlbed ln Table 1 before belng electroplated wlth 0.1 to 1.0 ml blsmuth. It was found that the adheslon of the electro copper plate to the steel panel was very much lmproved by a very short dlp (e.g. 5-10 seconds) of the strlpped panel ln 20-50 grams per llter HN03 at room temperature and by very thorough washlng before actlvatlon ln 10% methanesulfonlc acld.
Table 1 contalns a llstlng of the bath composltlon for the electroplatlng of the Hull cell panels wlth copper and Table 2 contalns a llstlng of the platlng condltlons and solutlon charac-terlstlcs for the bath used to plate the panels wlth copper.
Table 3 llsts the platlng condltlons and solutlon characterlstlcs that were common throughout Examples 1-4.

1 3~3377 Component Concentratlon (qtl) Copper 25 Free Methanesulfonlc Acld 40 Cuprlc Methane Sulfonate Concentrate (129 gtl Cu, 11 gtl Free MSA) 193.8 ml/l 69.5% Methanesulfonlc Acld (938 g/l 100% MSA) 40.4 ml/l Platlng Condltions Temperature 20C-25C (Room Temp.) Agltatlon None Anode Rolled Electrolytlc Cu Ratlo Anode Area to Cathode Area 2:1 Cathode Current Denslty Amp/ft2 2-25 Cathode Current Efflclency 100%
Solutlon Characterlstlcs:
Clarlty Water clear Color Sllght yellow tlnt Resldue Practlcally none Platlnq Conditlons:
Temperature, C Room (20C-25C) Agitatlon solutlon clrculatlon Vlgorous cathode reclprocatlon Moderate Anodes Cast hlgh purlty blsmuth bagged wlth polypropylene Ratlo anode area to cathode area ad~usted accordlng to the deslred percentage of blsmuth ln the tln-blsmuth co-plate.
Solutlon Characterlstlcs:
Clarlty Water clear Color Sllght yellow tlnt Resldue Practlcally none The panels electroplated ln the electroplatlng bath of Table 4 resulted ln a conductlve substrate wlth an electro-deposlted alloy coatlng comprlsed of 95% Tln/5% Blsmuth.

.

ml/l q/l Total Tln 15.0 Stannous Methane Sulfonate Concentrate48.7 Blsmuth 12.0 Blsmuth Methane Sulfonate Concentrate54.4 Free Methane Sulfonlc Acld 250.0 69.5% Methane Sulfonlc Acld 253.0 ULTRA STAN-1000 Prlmary Additlon Solutlon (3-1/2% v/v) 35 ULTRA STAN-100~ Actlvator Solutlon (2-1/2% v/v) 25 Worklng Range of Cathode Current Denslty 2-40 amp/ft2 Cathode Current Efflclency 95+%

The panels electroplated ln the electroplatlng bath of Table 5 resulted ln a conductlve substrate wlth an electro-deposlted alloy coatlng comprlsed of 90% tln/10% blsmuth.

ml/l q/l Total Tln 15.0 Stannous Methane Sulfonate Concentrate48.7 Blsmuth 24.0 Blsmuth Methane Sulfonate Concentrate 109.0 Free Methane sulfonlc Acld 250.0 69.5% Methane Sulfonlc Acld 241 ULTRA STAN-100~ Prlmary Addltlon Solutlon t3-1/2% v/v) 35 ULTRA STAN-100~ Actlvator Solutlon (2-1/2% v/v) 25 Worklng Range of Cathode Current Denslty 2-40 amp/ft2 Cathode Current Efflclency 95+%

The panels electroplated ln the electroplatlng bath of Table 6 resulted ln a conductlve substrate wlth an electro-deposlted alloy coatlng comprised of 42% tln/58% blsmuth. Thls proportlon of blsmuth to tln comprlses a eutectlc coatlng.

, .
~, ml/l q/l Total Tln 7.5 Stannous Methane Sulfonate Concentrate24.4 Bismuth 55.0 Blsmuth Methane Sulfonate Concentrate275.0 Free Methane Sulfonlc Acld 250.0 69.5% Methane Sulfonlc Acld 200.0 ULTRA STAN-100~ Prlmary Addltlon Solutlon (3-1/2% v/v) 35.0 ULTRA STAN-100~ Actlvator Solutlon (4% v/v) 40 0 Worklng Range of Cathode Current Denslty 2-20 amp/ft Cathode Current Efflclency 80%

The panels electroplated ln the electroplatlng bath of Table 7 resulted ln a conductlve substrate wlth an electro-deposlted alloy coatlng comprlsed of 14.5% tln/85.5% blsmuth.

~ 3J~377 ml/l q/l Total Tln 6.4 Stannous Methane Sulfonate Concentrate 20.8 Blsmuth 49.6 Blsmuth Methane Sulfonate Concentrate 228.0 Free Methane Sulfonlc Acld 250 69.5% Methane Sulfonlc Acld 200 ULTRA STAN-100~ Primary Addition Solution 35.0 (3-1/2% v/v) ULTRA STAN-100~ Actlvator Solution 40.0 (4% v/v) Worklng Range of Cathode Current Density 2-20 amp/ft2 Cathode Current Efficiency 80%
Ratlo of Anode Area to Cathode Area 2:1 Tln-bismuth alloys that may be made in accordance wlth the present lnventlon lnclude: 1) 42% tln/58% blsmuth whlch forms a eutectic material having a melting point of about 138C, approx-imately 50C lower than tin-lead eutectic compositlon, and 2) 25/75 or 16/84 tin-bismuth alloys sandwiched in plastic sheets to make formable metalllzed plastic. Other tln-blsmuth alloys may be expected to flnd utility in many applications prevlously filled by tin/lead alloys.
The princlples, preferred embodlments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected, however, ls not limlted to the partlcular embodlments dlsclosed, slnce these are to be regarded as lllustratlve rather than restrlctlve. Varlatlons and changes may be made by those skilled ln the art wlthout departlng from the splrlt of the lnventlon.

Claims (6)

1. An article of manufacture comprising at least one printed wireboard (PWB) with at least one conductive surface, having a tin-bismuth alloy deposited on at least a portion of the surface, the alloy being deposited from an electroplating bath comprising bismuth and tin ion in aqueous solution, wherein the bismuth and the tin ion are present in the bath in amounts sufficient to deposit the tin-bismuth alloy onto the conductive surface of the PWB in a weight ratio relative to each other selected to provide a controlled bismuth content of the tin-bismuth alloy on the conductive surface of the PWB in an amount of greater than about 10 wt.% bismuth, the bath having an electrolyte comprising at least one alkyl sulfonic acid or salt thereof in an amount sufficient to inhibit hydrolytic precipitation of the bismuth ion.

2. The article of claim 1 wherein the bismuth content is greater than about 20 wt.%.

3. The article of claim 1 wherein the bismuth content is between about 20 and about 80 wt.%.

4. The article of claim 1 wherein the alloy comprises the tin-bismuth eutectic.

5. The article of claim 1 comprising a plurality of PWBs.

6. The article of claim 1 wherein the tin-bismuth alloy is deposited on a plurality of surfaces.