METHOD FOR PREPARING AN ELECTROLYTE SOLUTION FOR ELECTROPLATING TIN OR TIN-LEAD ALLOYS

Abstract: This invention relates to electrolytes for plating tin, lead and particularly tin-lead alloys comprising a lead and/or tin alkyl or alkylol sulfonate, an alkyl or alkylol sulfonic acid in a sufficient amount to impart a pH below about 3 to the bath, and various additives to improve the brightness of the deposits, the useful current density ranges, and/or the solderability of tin-lead alloy deposits including quaternary nitrogen wetting agents containing a fatty acid radical, alkylene oxides, pyridine compounds, aromatic aldehydes, acetaldehyde and/or a bismuth compound, and methods for the preparation thereof.

Description: The invention relates to a method for preparing an electrolyte solution baths for electroplating tin or tin-lead alloys. The time-honored method for plating tin-lead alloys, commonly called Solder Plate, is to have these two metals present in solution as fluoborates with an excess of fluoboric acid and boric acid. In order to obtain smoother deposits, it is common to add various types of proteins, such as peptones, glue, gelatin or meat proteins. The fluoborate-based bath containing peptone is most commonly used and it is commercially successful in obtaining smooth matte deposits. Brighter tin-lead solder deposits can be obtained from these fluoroborate based baths by adding various brightening agents, such as aromatic aldehydes, pyridine compounds, ethylene oxide wetting agents and fomaldehyde. The main problem with these fluoborate-based baths is the fluoborate ion itself. Fluoborates are not only highly poisonous but are also highly corrosive to the equipment used in the plating environment. Fluoborates are also difficult to remove from the waste water that follows the plating operation and presents a serious problem for the waste-disposal engineer. Alkane or alkyl sulfonic acids containing 1 to 5 carbon atoms in the alkyl group have previously been used in electrolytic plating baths. See, for example, U.S. Patent No. 2,525,942 to Proell. Alkanolsulfonates, phenolsulfonates and cresolsulfonates have also been used in electroplating baths. See, for example, the article entitled ELECTRODEPOSITION OF BRIGHT TIN-LEAD ALLOYS FROM ALKANOLSULFONATE BATH byN. Dohi K. Obata, Proceedings of Interfinish 80, as well as U.S. Patents Nos. 4,132,610 and 3,905,878. U.S. Patent No. 4,459,185 to Obata et al. discloses alkyl or alkanol sulfonic acid baths for plating tin, lead, or tin-lead alloys. These baths contain at least one cationic, amphoteric or nonionic surfactant along with a levelling agent. None of the prior art, however, discloses the applicants' novel methods for preparing tin-lead electrolytes. Furthermore, none of the prior art recognized the addition of various additives for specific purposes, as is disclosed hereinbelow. This invention relates to a method for preparing electrolytes for electroplating tin, or tin-lead alloys The electroplating bath comprises a tin and optionally a lead alkyl sulfonate, an excess of an alkyl sulfonic acid sufficient to bring the pH of the bath below about 3 and advantageously to less than about 2, and various additives to improve the brightness of the deposits, the useful current density ranges, and/or the solderability of the tin-lead alloy deposits. Such additives include certain alkylene oxide compounds, quaternary nitrogen wetting agents containing a fatty acid radical, aromatic aldehydes, acetaldehydes and/or bismuth compounds. In one aspect, the present invention provides a method for preparing an electrolyte solution for electroplating tin or tin-lead alloys which comprises a suitable solvent, a soluble divalent tin compound optionally in combination with a soluble divalent lead compound; a soluble alkyl sulfonic acid in an amount sufficient to cause the solution to have a pH of less than about 3; and one or more wetting agents comprising a soluble alkylene oxide condensate compound having at least 8 moles of ethylene oxide present in an amount sufficient to cause the electrolyte solution to have a cloud point above 32° C (90⁰F), and/or a soluble quaternary ammonium fatty acid compound; characterised by the step of including a reducing agent in the solution in an amount sufficient to prevent or minimize the oxidation of divalent tin to tetravalent tin. The tin and optional lead compounds useable according to the invention are those which are soluble in the alkyl sulfonic acids and which will form an alkyl sulfonic acid salt. Certain alkyl sulfonic acids and their tin and lead salts are known compounds and may be readily prepared by those skilled in the art in the manner disclosed, for example, in U.S. Patent No. 2,525,942, as well as in the prior art cited in that patent. The preferred alkyl sulfonic acids useful according to the invention are those which are water soluble or soluble in the overall electrolyte. This would include the lower alkyl sulfonic acids containing about 1-7 carbon atoms. The tin and optional lead metals can be added to the baths in various forms and do not have to be added as a soluble alkyl sulfonate salt. Lead, for example, can be added as lead acetate. Thus, these baths may contain ions other than sulfonate ions so long as sufficient sulfonate ions are present to produce the advantageous results of the invention. However, the metals should predominate as sulfonates in the electrolytes of this invention. The amount of the total metal salt when using alkyl sulfonic acids is advantageously less than about 8 percent by weight when the electrolyte is used at low current densities and high throwing power is desired. Increased amounts of the metal salts significantly decrease the throwing power of the solution particularly at low current densities. The most advantageous amounts are from about 4.5 to 2 percent of the metal salt or about 3 to 1 percent of the metal or metals as the sulfonic acid salt. The invention includes improved tin and optional lead alkyl sulfonic acid baths containing quaternary nitrogen-fatty acid based wetting agents to improve the high current density range and throwing power of the bath as well as the surface finish of the resulting deposit. The preferred quaternary nitrogen-fatty acid based wetting agents are amphoteric and are the water or solution soluble imidazoline, monoazoline and/or amidobetaine compounds. These types of wetting agents are well known in the art and are commercially available from Lonza Inc. under the tradename AMPHOTERGE, by Miranol Chemical Company, Inc. under the tradenames MIRANOL, and MIRATAINE and by Mona Industries Inc. under the tradename MONATERICS. Other manufacturers of these materials are also available. AMPHOTERGE compounds are alkyl dicarboxy imidazoline surfactants, which can generally be categorized as an alkyl imidazolines. Also, as noted below, salts of these compounds may also be used. The only limitation on the use of these compounds is that they must be soluble in the electrolyte. The MONATERICS wetting agents include ampholytic amino-betaine and imidazoline derivatives. This would include monazoline compounds such as 1-(hydroxyethyl)-2-alkyl imidazolines. The most preferred compounds are the imidazolines having the formula: in which R is a fatty acid radical; R₁ is H, Na or CH₂ COOM; R2 is COOM. CH₂ COOM or CHOHCH₂SO₃M; M is Na, H or an organic base; and R₃ is OH, an acid salt or the salt of an anionic surface active sulfate or sulfonate. The most advantageous compounds are those in which R contains 6 carbon atoms or more, R₁ is CH₂COOM, R₂ is COOM and R₃ is OH. Best results to date have been obtained from the compound where R is C₇H₁₅ (capric), R₁ is CH₂COONa, R₂ is COONa and R₃ is OH (AMPHOTERAGE KJ-2). Carboxylic or sulfonic acid-fatty acid wetting agents containing tertiary nitrogen, such as the compound of formula (5), can also be used and the quaternary formed in situ by the acid contained in the bath. This same compound where R is C₁₂H₂₀ (coconut oil), also gives excellent results. Oleic hydroxyethyl alkyl imidazole and N' cocoyl-N-N' dimethyl glycine are other specific examples of imidazoline wetting agents. Other compounds found to be useful according to the invention include: where R has the same meaning as assigned above with respect to formula (1). The chain lengths of the various alkyl radicals and group can also be varied so long as water or solution solubility is maintained and the compounds retain wetting agents properties. The compounds of formulas (3) and (4) are, at present, of secondary interest. Examples of some specific compounds of formulas (3) and (4) include dimethyl [3 cocoamidopropyl] ammonioethanoate and cocoamidopropyl dimethyl ammonia-2-hydroxy-propane sulfonate. They produce improved finishes and throwing power but not as well as the compounds of formulas (1) and (2). Compounds according to formulas (1) and (2) produce solder deposits of a smooth matte finish equal in every way to those obtained from the peptone containing fluoborate based baths that are commonly used in industry today. R has been designated in the above formulas as being a fatty acid radical. The fatty group can be saturated or unsaturated. The only upper limit on the number of carbon atoms would be the solubility of the compound in the bath solution and this may vary depending on the compound being used. For example, the compound of formula (2) can contain more carbon atoms in the fatty group than that of formula (1) and still retain water solubility. The particular cation in the above formulas is not critical but sodium is preferred. Mixtures of the wetting agents can also be used. The amphoteric wetting agents useful according to this invention are broadly defined as quaternary nitrogen wetting agents containing an acid group derived from a fatty acid and one or more solubilizing groups. The solubilizing group can be a carboxylic or sulfonic acid group, alkanol group, etc. The amount of these wetting agents is not critical and optimum amounts will vary depending on the particular agent selected for use and the particular bath in which it is used. A sufficient amount of the wetting agent should be used to obtain the results desired. Generally, 1 to 2 ml/l of the wetting agents give excellent results with pure tin and 60/40 tin-lead alloy baths. Higher amounts could be used but there is no particular reason to do so. As the lead content of the bath is increased, additional amounts of these wetting agents may have to be employed. The addition of the quaternary nitrogen wetting agents markedly improved the coverage of electrodeposit at low current densities (54-215 Am -2 (5-20 ASF) and the baths are thus particularly advantageous for rack plating, such as circuit boards, where low current densities are employed and high throwing power is required. With the more advantageous wetting agents, coverage can be obtained at current densities even as low as 11 or 22 Am -2 (1 or 2 ASF). The plating baths of this invention can also be advantageously used to produce bright deposits by the addition of materials that are also commonly used for the fluoborate type baths when bright deposits are desired. These known brighteners are advantageously aromatic aldehydes such as chlorobenzaldehyde, benzaldehyde, and benzal acetone. Other brightening agents which have been used with the fluoborate baths include aromatic pyridine compounds, ethylene oxide wetting agents and formaldehyde. These materials can be used in substantially the same proportions as used in the fluoborate baths. Suitable ethylene oxide wetting agents which have been used as brighteners include condensation products of ethylene oxide and alkyl phenols or higher alcohols (as shown in British Patent Specification No. 1,151,460 and U.S. Pat. No. 3,905,878); aliphatic alcohol, sorbitan alkyl ester, or alkyl phenol ethylene oxide condensates (U.S. Pat. No. 3,875,029); alkyl phenoxy poly (ethyleneoxy) ethanols (U.S. Pat. No. 4,242,182); water soluble ethers of polyglycols (U.S. Pat No. 3,361,652); polyethylene glycol-derived alkylphenol, ether, or alkylamide compounds (U.S. Pat. No. 3,661,730); polyoxy alkylated compounds such as polyethoxylated alkyl phenols or polyethoxylated fatty acid monoalkanolamides and related amines (U.S. Pat. No. 4,118,289); and condensation products or polyalkylene oxides with other compounds (Table III of U.S. Pat. No. 4,000,047). The previously described alkylene oxide compounds have been used in fluoborate tin and tin-lead plating baths for many years. The prior art always considered all alkylene oxide compounds as being substantially equivalent to each other. For small scale or test applications, this may be true. It has now been found, however, that many of the alkylene oxide compounds cannot be used commercially in alkyl sulfonic acid based plating baths. Those alkylene oxide compounds having a cloud point below about 32°C (90°F) cause reduced cathode efficiency and the resulting electrodeposits exhibit poor solderability in comparison to baths containing an alkylene oxide compound whose cloud point is above about 32°C (90°F). Some alkylene oxide compounds, when used in the alkyl sulfonic acid based baths, will be clear when the alkylene oxide is initially added, but after about a day's use, will become cloudy. The plating bath can then be expected to lose cathode efficiency and produce a deposit with reduced solderability characteristics. As noted above, it is the cloud point of the alkylene oxide in the particular electrolytic solution being used that is important rather than the cloud point of the alkylene oxide in water alone, since the environmental conditions of the bath change the cloud point. This phenomenon can be either detrimental or beneficial. It is detrimental when the bath environment causes the cloud point to decrease below 32°C (90°F). This can occur either upon formulation of the bath or after use of the bath over time. Other agents and compounds in the bath affect the cloud point of baths containing such alkylene oxide compounds. These agents can change the cloud point which the alkylene oxide compound would have in water by to a higher or lower value. The addition of an imidazoline compound for example, will raise the cloud point of baths containing various alkylene oxide surface active agents. Thus, in such instances, alkylene oxide compounds having a cloud point below about 32°C(90°F) in water alone can be used in such electrolytes provided that the cloud point is not so low that it cannot be raised to the desired level (i.e., at least above 32°C(90°F) by the addition of the imidazolines or other similar agents. Only baths containing alkylene oxide compounds which have or can achieve the appropriate cloud point (i.e., above 32°C(90°F) in the bath environment can be used according to this invention. These baths can be used to produce smooth matte deposits in the low current range 54-215Am -2 (5-20 ASF) and, after fusing, the deposit surfaces are smooth, finer grained, bright and showed good solderability. In the high current density range the deposits are dark with 20 to 50% of the area burnt. However, the use of the proper alkylene oxide compounds increases the plating range over which useful deposits can be obtained. According to this invention, the most preferred alkylene oxide wetting agents are the condensation products of ethylene oxide and/or propylene oxide with styrenated phenols, high alcohols, alkyl phenols, alkyl napthols, fatty acid amines, alkyl amines, esterified sorbitan, phosphate, or fatty acid amides. Only those condensates which utilize at least 8 and preferably 12 or more moles of ethylene oxide, will consistently provide the necessary bath cloud point of greater than 32°C(90°F). Examples of preferred compounds useful for forming such condensates with the alkylene oxide components include the styrenated phenols preferably mono, di-or tri-styrenated phenol, mono-or di-styrenated cresol, and mono- or di-styrenated phenylphenol. Typical of the higher alcohols are octanol, decanol, lauryl alcohol, tetradecanol, hexadecanol, stearyl alcohol, eicosanol, cetyl alcohol, oleyl alcohol and docosanol. Illustrative of the alkyl-phenols are mono-, di-or tri-alkyl substituted phenols such as p-tertiary-butylphenol, p-isooctylphenol, p-nonylphenol, p-hexylphenol, 2,4-dibutylphenol, 2,4, 6-tributylphenol, p-dodecylphenol, p-laurylphenol and p-stearylphenol. The alkylnaphthols include alkylated α- or β-naphthols. The alkyl substituent in the alkylnaphthols includes methyl, ethyl, propyl, butyl, hexyl, octyl, decyl, dodecyl and octadecyl, and may be in any position of the naphthalene nucleus. The fatty acid amide may be amides of propionic, butyric, caprylic, lauric, myristic, palmitic, stearic and behenic acids. The phosphates are esters obtained by esterifying one or two hydroxyl groups of phosphoric acid with a C₁-C₂₀ alcohol. Typical of the sorbitan esterified with a higher fatty acid are mono-, di-or tri-esterified 1,4-, 1,5-, or 3,6-sorbitan, for example, sorbitan monolaurate, sorbitan mono-palmitate, sorbitan monostearate, sorbitan oleate, sorbitan dilaurate, sorbitan dipalmitate, sorbitan distearate, sorbitan dioleate and mixed fatty acid esters of sorbitan. Those skilled in the art are aware of other compounds which may be used to form the alkylene oxide compounds of the invention. As noted above, at least 8 moles of ethylene oxide compounds are required. The use of the quaternary nitrogen-fatty acid compounds, particularly the imidazolines, used in combination with the alkylene oxide compounds, gives further improvements than would be expected, i.e., they act synergistically. The pyridine or quinoline compounds useful according to this invention are those described in U.S. Patent No. 4,000,047. The pyridine compounds are preferred. The amounts of pyridine or quinoline compounds incorporated into the electrolytes of this invention can be the same as disclosed in U.S. Patent No. 4,000,047. The aromatic aldehydes, as well as the amounts, useful according to this invention are also those described in U.S. Patent No. 4,000,047. Thus, U.S. Patent No. 4,000,047 is incorporated herein by reference, particularly with regard to its disclosure of the types and amounts of pyridines, quinolines, and aromatic aldehydes which can be used in accordance with this invention. Bismuth compounds may be used in this invention to improve the low current density of the deposits. The bismuth compounds useful in this invention are those which are water or solution soluble and the anion produced by the bismuth compound should not interfere with the tin or lead salts, such as by causing precipitation thereof. Bismuth nitrate is an example of an acceptable bismuth compound. The amount of bismuth should be sufficient to obtain the desired low current density range. As little as 0.1 g/l of bismuth metal as bismuth nitrate can lower the useful current density range to close to zero Am -2 , thus rendering the baths very useful for barrel plating. Bismuth alone does not give the brightness at low current densities. However, bright deposits may be obtained if an aromatic aldehyde and/or an ethylene oxide wetting agent is used in combination with the bismuth compounds. Acetaldehyde may be used to increase the high current density of the plating bath. The degree of increase is extraordinary, unexpected and highly dramatic. The amount of acetaldehyde should be sufficient to raise the useful current density range to that desired. 1 ml/l has been found sufficient to raise the current density range from a high of 431Am -2 (40 ASF) (without acetaldehyde) to 2150Am -2 (200 ASF). The particular combination of additives employed with the basic electrolyte will depend on the results or types of deposits desired. Matte deposits with good solderability can be obtained using an alkylene oxide of the appropriate cloud point and an imidazoline or quaternary compound. Bright deposits can be obtained by adding ethylene oxide and an aromatic aldehyde. The useful current density can be extended to the high range 215to 2150Am -2 (20 to 200 ASF) by the addition of acetaldehyde and to the low range of slightly above zero to about 431 by the addition of a bismuth compound. The plating bath can be prepared by placing tin and optional lead sulfonic acid salt in an excess of the alkyl sulfonic acid, adjusting the acid content to the required pH, removing any undissolved matter by filtration, and then diluted with water to the final desired volume. The plating bath is generally operated at ambient temperatures. Agitation is desirable for increased plating speed. Various alloys can be produced depending on the tin-lead metal ratios employed in the plating solutions. For plating a 60:40 tin-lead alloy, for example, 20 g/l of tin metal and 10 g/l of lead metal can be used. With such a bath, alkylsulfonic acid is advantageously present at about 150 g/l of 100 percent alkylsulfonic acid. The addition of a small amount of a reducing agent to the baths is an essential feature of the invention. This assists in keeping the tin in the soluble divalent state. This helps avoid conversion of divalent tin to tetravalent tin, which is generally insoluble in the bath enviroment. About 0.5 g/l of resorcinol or a similar agent is generally sufficient to accomplish this purpose. Examples : The scope of the invention is further described in connection with the following examples which are set forth for the purposes of illustration only and are not to be construed as limiting the scope of the invention in any manner. The following Examples employe the following basic electrolyte bath: Hull Cell panels are run for 5 minutes at 1 amp. total current in all of the following Examples: Example 1. 10 g/l polyoxyethylene lauryl ether (23 moles ethylene oxide condensate) (Brij-35-Atlas) was added to the basic electrolyte. The bath produces a Hull Cell panel that is smooth and matte in the low current density range with about 30 to 50% burn in the high current density areas. After fusing, the deposit surface is very smooth, bright and leveled showing good solderability. Example 2. To Example 1, 1 g/l capric dicarboxyimidazoline amphoteric surfactant (AMPHOTERGE KJ-2-Lonza) is added. This bath provides an increased high current density range with no evidence of burning or darkening of the deposit. After fusing, the deposit is very smooth, bright and leveled showing good solderability. Example 3. 10 g/l nonyl phenol/12 ethylene oxide condensate are added to the basic electrolyte. The bath provides a smooth deposit with only 50% of the high current density range dark and burnt. Example 4. To Example 3, 0.18 g/l of a picolinic acid, 0.1 g/l of allylether of orthoxyhydroxybenzaldehyde and 0.16 ml/l orthochlorobenzaldehyde are added. The deposits from this bath exhibit a bright current density range from 160 to 430 Am -2 (15 to 40 ASF). The low current density range is hazy to dull and the high current density range dull burnt. Example 5. To Example 4, 1 ml/l acetaldehyde was added. the deposit is brilliant to current density ranging from 215-2150Am -2 (20 to 200 ASF). This example show the power of acetaldehyde in the alkyl or alkylol sulfonic acid electrolytes containing the brighteners of Example 10 to drastically raise the useful current density range. Example 6. To Example 4, 0.1 g/l bismuth metal as bismuth nitrate is added. The bath provides a deposit with brilliance at current density ranging from near 0 to 430 Am -2 (0 to 40 ASF). This example shows the power of bismuth in the alkyl sulfonic acid electrolyte containing the brighteners of Example 4 to drastically reduce the useful current density range to very low values. Example 7 (Comparative). 0.1 g/l bismuth metal as bismuth nitrate is added to the bath of Example 1 and there is no improvement in the low current density areas. This shows the low current density improvement as synergism provided by the use of bismuth in combination with the aromatic aldehydes and brighteners listed in U.S. Patent No. 4,000,047. Example 8. Example 5 is repeated but in this case the picolinic acid was left out. The deposit is brilliant with the bright current density ranging from 430 to 2150Am -2 (40-200 ASF). This example shows that it is possible to eliminate the pyridine type materials of patent 4,000,047 and still get good results in the higher current density ranges. There is some loss in the low current density range - from a lower limit of 215Am -2 (20 ASF) in Example 5 to 430Am -2 (40 ASF) in this example, but this loss is not significant in those applications where only the high current density range is needed or desired. Example 9. (Comparative) A plating bath was prepared as follows: Hull cell tests were performed at 1 A for 5 minutes and produced a matte deposit with nodules and poor throwing power. In all of the above Examples, the electrolytic solutions were diluted with water to form the desired volume.

Claims: A method for preparing an electrolyte solution for electroplating tin or tin-lead alloys which comprises a suitable solvent, a soluble divalent tin compound optionally in combination with a soluble divalent lead compound; a soluble alkyl sulfonic acid in an amount sufficient to cause the solution to have a pH of less than about 3; and one or more wetting agents comprising a soluble alkylene oxide condensate compound having at least 8 moles of ethylene oxide present in an amount sufficient to cause the electrolyte solution to have a cloud point above 32° C (90⁰F), and/or a soluble guaternary ammonium fatty acid compound; characterised by the step of including a reducing agent in the solution in an amount sufficient to prevent or minimize the oxidation of divalent tin to tetravalent tin. A method according to claim 1 wherein the reducing agent is resorcinol. A method according to claim 1 or claim 2 wherein a combination of an alkylene oxide condensate compound and a guaternary nitrogen fatty acid compound are used as wetting agents. A method according to any preceding claim wherein the alkyl sulfonic acid is methane sulfonic acid and the solvent is water. A method according to any preceding claim wherein the divalent tin compound, and optionally the divalent lead compound, are added to the solvent in the form of an alkyl sulfonate salt. A method according to any preceding claim wherein the alkylene oxide is ethylene oxide. A method according to any preceding claim wherein the quarternary ammonium fatty acid compound is in the form of a soluble imidazoline, monoazoline or amidobetaine compound. A method according to any preceding claim wherein the electrolyte solution further comprises an aromatic aldehyde in an amount sufficient to improve the brightness of the deposits obtainable by electroplating. A method according to any preceding claim wherein the electrolyte solution further comprises a soluble bismuth compound in an amount sufficient to improve the low current density range for electroplating. A method according to any preceding claim wherein the electrolyte solution further comprises acetaldehyde in an amount sufficient to improve the high current density range for electroplating. A method for electroplating tin or tin-lead alloys upon a suitable substrate comprising the steps of: preparing an electrolyte solution in accordance with the method of any preceding claim; immersing at least a portion of the substrate into the electrolyte solution; and electroplating tin or tin-lead alloy upon the immersed portion. A method according to claim 11 further comprising the step of agitating the solution to increase the speed of the electroplating.

Inventors: NOBEL, FRED I., OSTROW, BARNET D., SCHRAM, DAVID NOEL
Applicants: LEARONAL, INC.
Application year: 1985
Main classification: C25D3
IPC codes: C25D3