Abstract:
Improved electroplating bath for depositing bright, metallic tin wherein divalent tin, in the form of stannous sulfate or fluoroborate, is present in conjunction with sulfuric or fluoroboric acid, brighteners including an aromatic amine and an aliphatic aldehyde, a polyalkylene ether surfactant, and an aromatic sulfonic acid to ensure bath stability as well as the requisite brightness. The divalent tin-containing electroplating bath may also be provided with copper or rhodium salts to achieve codeposition of tin with at least one of these alloying metals. The method of utilizing such divalent tin electroplating baths to plate substrates with bright metallic tin is also described and claimed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to depositing tin as well as copper or rhodium alloys thereof on various substrates; more particularly the invention pertains to depositing of bright, metallic tin from stable baths wherein the tin is in the form of divalent tin sulfate or fluoroborate, i.e. stannous sulfate or fluoroborate. 
     BACKGROUND OF THE INVENTION 
     There is a substantial body of prior art patents concerned with tin or tin alloy electroplating baths and processes for utilizing the same. Some of the more relevant patents for the present purposes include U.S. Pat. Nos. 3,730,853 (Sedlacek et al.); 3,749,649 (Valayil); 3,769,182 (Beckwith et al.); 3,785,939 (Hsu); 3,850,765 (Karustis, Jr. et al.); 3,875,029 (Rosenberg et al.); 3,905,878 (Dohi et al.); 3,926,749 (Passal); 3,954,573 (Dahlgren et al.); 3,956,123 (Rosenberg et al.); 3,977,949 (Rosenberg); 4,000,047 (Ostrow et al.); 4,135,991 (Canaris et al.); 4,118,289 (Hsu); and British Pat. Nos. 1,351,875 and 1,408,148. 
     Despite the existence of this extensive literature and the various formulations which have been suggested for commercial applications, there is still a need for electroplating baths which will effectively deposit bright metallic tin on various substrates. Another important characteristic is bath stability, especially premature tin compound precipitation in the bath. The variety of bath formulations proposed heretofore reveal, moreover, that all of the ingredients employed in the bath formulation must be taken into consideration not only with respect to the type of deposit obtained but also with respect to questions of bath stability, by-product formation, etc. 
     OBJECTS OF THE INVENTION 
     One object of the present invention is to provide a tin electroplating bath which ensures the deposition of bright metallic tin on various substrates. 
     Another object of the present invention is to provide a divalent tin electroplating bath of improved stability. 
     A further object of the present invention is to provide an improved electroplating bath for the deposition of alloys of tin with copper and rhodium. 
     These and other objects will become readily apparent from the following description and illustrative embodiments of the present invention. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention it has now been found that by utilizing certain aromatic sulfonic acid additives in conjunction with certain other additives an improved tin electroplating bath, formulated with bath soluble divalent tin compounds, can be achieved. The resulting bath will not only lead to the deposition of bright metallic tin but will be further characterized by outstanding stability. 
     The other bath ingredients will comprise an inorganic acid, an aromatic amine brightener, and a nonionic surface active agent. Preferably, the bath will also contain an aliphatic aldehyde brightener. 
     In accordance with another aspect of the present invention copper or rhodium metals may be effectively co-deposited with the tin from the electroplating baths. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The electroplating baths of this invention are formulated with divalent tin in the form of a bath soluble compound. Typical of such compounds are stannous sulfate, stannous fluoroborate and stannous chloride. Free inorganic acid is also present in amounts sufficient to provide conductivity, maintain bath pH below 2.0 and maintain the solubility of metal salts. It will be understood that the particular acid used will correspond to the anion of divalent tin compound, e.g. sulfuric acid, fluoroboric acid, hydrochloric acid or the like. 
     The brightener system will comprise one or more aromatic amines and, most preferably will comprise a combination of one or more aromatic amines and aliphatic aldehydes. The aromatic or aryl amines useful for the present purposes include o-toluidine; p-toluidine; m-toluidine; aniline; and o-chloroaniline. For most purposes the use of o-chloroaniline is especially preferred. 
     Suitable aliphatic aldehydes are those containing from 1 to 4 carbon atoms and include, for example, formaldehyde, acetaldehyde, propionaldehyde, butyraaldehyde, crotonaldehyde, etc. In this invention the preferred aldehyde is formaldehyde or formalin, a 37% solution of formaldehyde. 
     Nonionic surfactants are employed in the bath to provide grain refinement of the electrodeposit. These can be commercially available materials such as nonyl phenoxy polyethlene oxide ethanol (Igelpal C0630 and Triton Q515); ethoxylated alkylolamide (Amidex L5 and C3); alkyl phenyl polyglycoletherethylene oxide (Newtronyx 675) and the like. 
     The nonionic surface active agents which have been found to be particularly effective for the present purposes are the polyoxyalkylene ethers, where the alkylene group contains from 2 to 20 carbon atoms. Polyoxyethylene ethers having from 10 to 20 moles of ethylene oxide per mole of lipophilic groups are preferred, and include such surfactants as polyoxyethylene lauryl ether (sold under the tradename Brij 25-SP). 
     As previously described, the essential feature of the present invention is to utilize an aromatic sulfonic acid compound in conjunction with the bath ingredients set forth above. These sulfonic acid compounds maintain stability of the plating bath and provide supplemental brightening and grain refinement to the electrodeposit. Preferred aromatic sulfonic acids for these purposes are: 
     o-cresol sulfonic acid 
     m-cresol sulfonic acid 
     phenol sulfonic acid 
     Other phenol sulfonic acid derivatives of phenol and cresol which could be employed are, for example: 
     2,6-dimethyl phenol sulfonic acid 
     2-chloro, 6-methyl phenol sulfonic acid 
     2,4-dimethyl phenol sulfonic acid 
     2,4,6-trimethyl phenol sulfonic acid 
     m-cresol sulfonic acid 
     p-cresol sulfonic acid 
     Sulfonic acid derivatives of alpha- and beta-naphthols are also possible candidates for the aromatic sulphonic acid ingredient. Additionally, the bath soluble salts of the above acids, such as the alkali metal salts, may be used instead of or in addition to the acid. 
     In formulating the plating baths of the present invention, the divalent tin compound will be used in an amount at least sufficient to deposit tin on the substrate to be plated, up to its maximum solubility in the bath. The inorganic acid will be present in an amount sufficient to maintain the pH of the plating bath not in excess of about 2.0. The aromatic amine or the combination of the aromatic amine and the aliphatic aldehyde are present in amounts at least sufficient to impart brightness to the tin electrodeposit, while the nonionic surfactant is present in the bath in a grain refining amount. The aromatic sulfonic acid derivative is present in an amount sufficient to maintain the stability of the plating bath and enhance the brightness of the electrodeposit. 
     More specifically, the ingredients of the aqueous electroplating baths of this invention will be present in amounts within the following ranges: 
     
         ______________________________________             Amounts (grams/liter)Ingredients         Typical    Preferred______________________________________(1)   Tin (II), as stannous sulfate, fluoroborate or chloride          5-50       15-30(2)   Sulfuric, fluoroboric or hydrochloric acid 100-250    160-190(3)   Aromatic Amine    0.3-15     0.5-1.5(4)   Aliphatic Aldehyde                   0.5-11     0.9-5.4(5)   Nonionic surfactant                   0.1-20     0.5-2.5(6)   Aromatic sulfonic acid derivative        0.5-3.0    3-9______________________________________ 
    
     The pH of the bath will not be in excess of about 2.0 and will usually be less than about 1, with ranges from about 0 to 0.5 being typical and ranges from about 0 to 0.3 being preferred. Electroplating temperatures and current densities used will be those at which there are no adverse effects on either the plating bath or the electrodeposit produced. Typically, the temperatures will be from about 10 degrees to 40 degrees C., with temperatures of about 15 degrees to 25 degrees C. being preferred. Typical current densities will be about 10 to 400 Amps/square foot (ASF) and preferably about 25 to 200 ASF. 
     The substrates which may be satisfactorily plated utilizing the electroplating baths of this invention include most metallic substrates, except zinc, such as copper, copper alloys, iron, steel, nickel, nickel alloys and the like. Additionally, non-metallic substrates that have been treated to provide sufficient conductivity may also be plated with the bath and process of the present invention. 
     Another aspect of this invention involves the discovery that copper and rhodium metals can be deposited with tin on the substrates when utilizing the electroplating baths described above without additional additives or complexing agents. In contrast, metals such as nickel, iron and indium did not codeposit under the same conditions. 
     Typically, the copper or rhodium is added to the bath as bath soluble compounds, preferably having the same anions as the divalent tin compounds. The amounts of such compounds added with be sufficient to provide up to about 5% by weight of copper or rhodium, alloyed with tin, in the electrodeposit. Typical amounts of copper and rhodium in the electroplating baths to provide such quantities of the metal in the electrodeposit are about 0.2 to 4 grams/liter and 0.2 to 2 grams/liter, respectively. 
    
    
     The invention will be more fully understood by reference to the following specific embodiments: 
     EXAMPLE I 
     An electroplating bath was prepared from the ingredients set forth below: 
     
         ______________________________________Ingredients            Amount g/l______________________________________Tin (II), as stannous sulfate                  22.5Sulfuric Acid          175o-chloroaniline        1.0, cc/lFormalin               10, cc/lPolyoxyethylene lauryl ether                  1.0o-Cresol sulfonic acid 5.0Water                  Remainder______________________________________ 
    
     This resulting stable bath was operated at 20 degrees C., 30 ASF, with rapid agitation to plate a copper panel. The tin deposit thus formed had a very bright appearance. 
     EXAMPLE II 
     It has been found that there is a side reaction between formaldehyde and the sulfonic acid which causes a precipitate to form and settle out of the bath solution. However, it was further found that if the ortho position, and to a lesser extent the meta position, of the phenol sulfonic acid are blocked by methyl groups, as in o-cresol sulfonic acid, this undesirable side reaction, and hence the precipitation, slows down. The other ingredients of Example I may also be further optimized (e.g., work load, agitation, etc.) to minimize, if not eliminate this precipitate. Utilizing the other ingredients of Example I, a number of the aromatic sulfonic acids were tested to determine bath stability. The results were as follows: 
     
         ______________________________________Additive         Amount (ml/l)                        Stability (hrs)______________________________________o-Cresol sulfonic acid (65%)            8           24m-Cresol sulfonic acid (33%)            6           16Phenol sulfonic acid (65%)            10          12______________________________________ 
    
     EXAMPLE III 
     An electroplating bath was prepared from the following ingredients: 
     
         ______________________________________Ingredients            Amount (g/l)______________________________________Tin II, as stannous sulfate                   30Sulfuric acid          175Copper, as copper sulfate                  0.4Formalin               10, cc/lo-Chloroaniline        0.4 cc/lPolyoxyethylene lauryl ether                  0.4o-Cresol sulfonic acid 0.8______________________________________ 
    
     The resulting bath was operated at 60 asf produced a tin/copper alloy deposit containing 1.0% copper, the deposit was semi-bright. 
     EXAMPLE IV 
     In the formulation of Example III the copper was replaced with rhodium at a concentration of 0.5 g/l from rhodium sulfate. The bath was operated at 60 asf and produced a very bright tin/rhodium alloy deposit containing 0.07% rhodium. 
     When nickel, iron or indium metal were employed in the divalent tin baths of this invention, they failed to codeposit with the metallic tin. 
     EXAMPLE V 
     To demonstrate the stability enhancing effects achieved by the use of an aromatic sulfonic acid in the tin electroplating baths of this invention the following baths were prepared. 
     
         ______________________________________               g/l______________________________________BATH AStannous sulfate       60Sulfuric acid         180o-Cresol sulfonic acid                 5.6Water                 RemainderBATH BStannous sulfate       60Sulfuric acid         180Water                 Remainder______________________________________ 
    
     An electric air compressor with spargers was employed to pump air at a flow rate of approximately 15 cubic feet per minute through the bath in a 1 liter beaker. 
     
         ______________________________________Time           Stannic Tin Conc. (g/l)Period         BATH A   BATH B______________________________________Start          0.3      0.55 days         2.2      9.110 days        3.5      13.6______________________________________ 
    
     In commercial operations air is normally present as a result of agitation, and becomes a serious problem because high rates of agitation will entrap substantial amounts of air which, in the absence of the aromatic sulfonic acid, will cause formation of stannic tin in the bath which is a measure of bath degradation. 
     It will be further understood that the foregoing examples are illustrative only, and that variations and modifications may be made without departing from the scope of this invention.