United States Patent (19) 11) 4,162,205 Wilson et al. 45) Jul, 24, 1979 54 METHOD OF ELECTROPLATING TIN AND Primary Examiner-G. L. Kaplan ALKALINE ELECTROPLATING BATH Attorney, Agent, or Firm-Burns, Doane, Swecker & THEREFOR Mathis 75 Inventors: Harold P. Wilson, Huron, Ohio; 57) ABSTRACT Walter C. Bradbury, Townsend, Ga. An improved electrotinplating process and electroplat (73) Assignee: Vulcan Materials Company, ing bath therefor is disclosed. The electroplating bath Birmingham, Ala. contains tin ions, an alkali metal hydroxide, and a bis muth compound selected from the group consisting of (21) Appl. No.: 952,855 at least one alkali metal bismuth salt of a linear polyhy 22). Filed: Oct. 19, 1978 droxymonocarboxylic acid having at least six carbon 51) Int. C.’................................................ C25D3/60 atoms, at least one alkali metal bismuthyl compound of 52 U.S. C. .................................. 204/43 S; 260/447; glucono-delta-lactone, and mixtures thereof. The alkali 260/343.5 metal bismuth compounds of this invention exhibit in 58 Field of Search ............... 204/43 S, 123; 260/447 creased solubility and long-term stability in hot alkaline alkali metal stannate electroplating baths when com (56) References Cited pared to conventional alkali metal bismuthates. A pre U.S. PATENT DOCUMENTS ferred alkalimetal bismuth compound is an alkali metal 3,360,446 12/1967 Jongkind ............................ 204/43 S bismuthyl gluconate. An improved electrotinplating 3,522,155 7/1970 Dow ................................... 204/43 S process is therefore provided which produces a plated 3,663,384 5/1972. Lescure ............................. 204/43 S tin-bismuth alloy which is resistant to the effects of tin pest. FOREIGN PATENT DOCUMENTS 467145 4/1975 U.S.S.R. ................................. 204/43 S 22 Claims, No Drawings 4,162,205 1. 2 quired to place the required amount of bismuth into METHOD OF ELECTROPLATING TN AND solution. In addition, the quality of the electrotinplate ALKALINE ELECTROPLATING BATH may be affected by suspended alkali metal bismuthate THEREFOR and hydrolyzed bismuth stannate precipitates. Such precipitates would promote sludging in the plating bath BACKGROUND OF THE INVENTION since they are autocatalytic to the formation of hydro This invention relates to an improvement in the lyzed stannic hydroxide precipitates. Thus the bath method of producing electrodeposits of tin, and is par would require purification or replacement more often. ticularly concerned with a plating bath from which tin 10 OBJECTS AND SUMMARY OF THE deposits can be obtained which are substantially im NVENTION mune to the effects of "tin pest". It has been found that when materials bearing an It is an object of this invention to provide an im electrodeposit of tin are maintained under conditions proved process for electrodepositing a tin containing wherein the ambient temperature is less than 18 C., the alloy. transformation of white (beta) tin of tetragonal form to 15 It is another object of this invention to provide an the gray (alpha) tin of cubic centered form occurs. This improved tin electroplating bath containing an alkaline transformation is commonly knonw as tin "disease' or compound of bismuth which is more soluble than alkali "tin pest". The result of the transformation is that the metal bismuthates. specific gravity of the tin or tin electroplate is lowered It is yet another object of this invention to provide a from about 7.3 to about 5.75 with its attendant destruc 20 tin electroplating bath containing an alkaline compound tion of the tin's metallic properties. As the tin pest de of bismuth which is stable in hot alkaline alkali metal velops, a loose tin powder is formed which easily sepa stannate solutions. rates from the base metal. The tin then loses its ability to It is still yet another object of this invention to pro protect the base metal. Accordingly, the base metal is vide a tin electroplating bath containing an alkaline more susceptible to the effects of corrosion. 25 compound of bismuth which more effectively releases For many years it has been known that a small bismuth ions within the electroplating bath. amount of bismuth in tin or tin electroplate inhibits the In accordance with one aspect of the present inven formation of the tin pest. It has been reported that a tion, there is provided an aqueous solution for plating concentration of bismuth above 0.12 percent in tin or tin tin-containing alloy on a conductive substrate which alloy inhibits and may even prevent the formation of tin 30 solution comprises tin ions, alkali metal hydroxide, and pest at temperatures as low as -73 C. for indefinite abismuth compound selected from the group consisting periods of time. MacIntosh, R. M. "The Properties of of at least one alkali metal bismuth salt of a linear Tin at Low Temperatures,” Tin Research Institute, polyhydroxymonocarboxylic acid having at least six Columbus, Ohio (Oct. 15, 1953). In the case of tin elec troplate formed by an alkaline process, it is desirable to 35 carbon atoms, at least one alkali metal bismuthyl com limit the concentration of bismuth to below 2 percent to pound of glucono-delta-lactone, and mixtures thereof. avoid the formation of dendrites as well as grainy and In accordance with another aspect of the present very gray tin plates. invention, there is provided in a method for electroplat In U.S. Pat. No. 3,360,446 bismuth esters (i.e., hy ing a tin-containing alloy onto a conductive substrate droxyalkyl bismuthates, preferably polyhydroxy alkyl comprising immersing a conductive substrate to be bismuthates) are disclosed for incorporation in alkaline plated into an aqueous alkaline plating bath containing potassium or sodium stannate tin plating baths in a con tin ions, alkali metal hydroxide, and a bismuth com centration of from 0.05 to 1.5 grams/liter (expressed in pound, and electroplating a tin-containing alloy onto terms of the bismuth metal content) to achieve a bis the substrate as a cathode in the bath, the improvement muth content of from 0.1-0.6% in the electrotinplate. 45 comprising using as the bismuth compound a compound Since the alkaline primary reaction mixture containing selected from the group consisting of at least one alkali the bismuthate ester must be neutralized with a weak metal bismuth salt of a linear polyhydroxymonocar organic acid immediately to prevent decomposition and boxylic acid having at least six carbon atoms, at least the neutralized product must be precipitated with an one alkali metal bismuthyl compound of glucono-delta alcohol several times to be purified, the long termstabil 50 lacton, and mixtures thereof. ity of the bismuthate ester in hot alkaline electrotinplat DETALED DESCRIPTION OF TH ing baths may not be very high. U.S. Pat. No. 3,522,155 discloses the use of alkali INVENTION - metal bismuthates, particularly sodium bismuthates, in It has now been found that an alkali metal bismuth alkaline potassium or sodium stannate tin plating baths 55 salt of a linear polyhydroxymonocarboxylic acid having in a concentration of from 0.05 to 1.0 grams/liter (ex at least six carbon atoms and an alkali metal bismuthyl pressed in terms of the alkali metal bismuthate) for a compound of glucono-delta-lactone are each more solu range of 0.1-0.6% bismuth in the electrotinplate. Labo ble than previously used alkali metal bismuthates and ratory investigations have shown that sodium bismu are also more stable in hot alkaline alkali metal stannate thate has very low solubility in hot alkaline potassium electrotinplating baths. The compounds are also com stannate tin plating baths and tends to hydrolyze and patible with stannic oxide sol. Useful compounds in precipitate. clude sodium bismuthyl gluconate and potassium bis The low solubility of alkali metal bismuthates in alka muthyl gluconate. Particularly useful compounds are line electrotinplating baths normally necessitates the use sodium bismuthyl trigluconate and potassium bismuthyl of a supersaturated solution of the alkali metal bismu 65 trigluconate and mixtures thereof, with the latter com thate. Several disadvantages exist with respect to the pound being preferred. use of such supersaturated solutions if electrotinplating. The presence of at least one of the alkali metal bis Large amounts of the alkali metal bismuthate are re muth compounds of this invention in the electrotinplat 4,162,205 3 4. ing bath supply enough chelating power to increase the Alkali metal bismuthyl gluconates containing more solubility of the bismuth in solution. than three moles of gluconic acid may be formed as For example, it has been found that the addition of long as one mole of the alkali metal hydroxide is added 0.75 grams/liter of bismuth (in the form of sodium bis for each additional mole of acid. For example, an alka muthate) to 500 milliliters of a hot, agitated alkaline tin 5 line solution of potassium bismuthyl tetragluconate may electroplating bath containing 60 grams/liter of tin and be prepared and is illustrated by the formula: 156 grams/liter of potassium provided only 0.0582 grams/liter of bismuth in solution. Additionally, a large amount of suspended insoluble sodium bismuthate was observed. 10 After completion of the reaction the solution may be In comparison, alkaline solutions of the alkali metal dark or black in color and some gray or black residue bismuth compounds of this invention dissolve quite may be present. This residue is thought to be due to readily in alkaline electrotinplating baths and do not reduction of the bismuth trioxide by the gluconic acid, form precipitates. In addition, the bismuth compounds and the residue may therefore contain bismuth metal as of this invention enable substantially greater amounts of 15 well as suboxides such as bismuthite. To prevent any bismuth ions to be placed into solution in alkaline elec possible deleterious effect on the quality of tin plate trotinplating baths than has previously been possible.