Abstract:
An electrolyte, system and process for depositing tin, lead or tin/lead alloys upon a substrate by high speed electroplating, which includes a basis solution of an alkyl or alkylol sulfonic acid; and at least one of a solution soluble tin compound or a solution soluble lead compound; and an alkylene oxide condensation compound of (1) an aliphatic hydrocarbon having seven, preferably six or less, carbon atoms and at least one hydroxy group, or (2) an organic compound having no more than a total of twenty carbon atoms in one or two independent or joined rings optionally substituted with an alkyl moeity of less than six carbon atoms.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 07/282,851, filed Dec. 9, 1988, now U.S. Pat. No. 4,880,507, which is a continuation-in-part of application Ser. No. 07/130,759 filed Dec. 10, 1987, now abandoned. 
     TECHNICAL FIELD 
     This invention relates to electrolytes based upon lower alkyl or alkylol sulfonic acids or their derivatives for the high speed electroplating of tin, lead, or tin/lead alloys, particularly those for use in high speed electroplating equipment. 
     BACKGROUND OF THE INVENTION 
     Electroplating baths for depositing tin, lead, or their alloys have been used for many years in electroplating equipment. High speed electroplating equipment and processes are well-known in the industry and generally consist of directing the work to be plated into the electroplating cell from one end, allowing the work to proceed through the electroplating cell and exit thereafter the cell at the other end. The electroplating solution is removed or overflows the electroplating cell into a reservoir and the solution is pumped from the reservoir back into the electroplating cell to provide vigorous agitation and solution circulation. Many variations of these electroplating cells can exist, but the general features are as described. 
     There are a number of desirable features that the electroplating solution should possess for improved operation in this type of equipment or processing, as follows: 
     1. The solution must be able to electroplate the desired alloy deposit at the high speeds required. 
     2. The deposit should be lustrous and fine grained, even at the high current densities required for high speed plating. 
     3. The deposit should have good solderability and be capable of meeting the solderability requirements specified for such deposits. 
     4. The solution should be stable and the additives must withstand exposure to the strong acid solution as well as to the introduction of air which would take place as a result of the vigorous solution movement in high speed plating machines. 
     5. The solution should remain clear and free from turbidity, even at elevated temperatures such as 120-130° F. or higher. Due to the high current densities involved and relatively low solution volumes, these baths tend to heat up in high speed electroplating equipment until the solution reaches equilibrium at an elevated temperature. The additives used must be of a type that will not turn the solution turbid at such elevated temperatures. 
     6. Because of vigorous solution movement and solution mixing with air, there is a strong tendency to produce a foam which is detrimental to the electroplating process. Under extreme conditions, this foam can build up in the reservoir tank with resultant overflow onto the floor, thereby losing a large quantity of solution to the waste stream. In some applications of &#34;controlled depth plating,&#34; the parts to be electroplated are only partially immersed in that a portion of the work is below the solution level. It is desirable to have a distinct and uniform line of demarcation separating the unplated portion from the plated portion of the work. If the solution generates foam, such foam will prevent the formation of a good line of demarcation. Foam can also interfere with the operation of the pump that is being used to generate agitation. Arcing between the anode and cathode is also possible due to the presence of foam. Because of these problems, the additives used should not generate foam in the plating equipment. 
     Many electrolytes have been proposed for electroplating tin, lead, and tin/lead alloys and one of these is described in U.S. Pat. No. 4,701,244. This patent discloses the electroplating of tin, lead or tin/lead alloys from lower alkyl sulfonic acid baths which contain brightening additives as well as many wetting agents of various types. Surfactants claimed to be useful are betaines, alkyline oxide polymers, imidazolinium compounds, quaternary ammonium compounds, ethylene oxide derivatives of amines, phosphonates, amides and many others. 
     U.S. Pat. No. 4,662,999 discloses an electroplating bath for electrodeposition of tin, lead, or tin/lead alloys from alkane or alkanol sulfonic acid baths that also contain surfactants plus other additives. In this patent, the surfactant can be non-ionic, cationic, anionic or amphoteric. A great many examples are given for the various types of surfactants and the patent enumerates a large number of the various types of wetting agents which can be used. 
     U.S. Pat. No. 4,673,470 describes a tin, lead, or tin/lead alloy plating bath based upon an aliphatic or aromatic sulfocarboxylic acid. Instead of the alkene or alkanol sulfonic acids disclosed in previous patents, this patent includes a carboxylic acid radical in the organic sulfonic acid compound. The electroplating baths described contain brightening agents plus a surface active agent, with particular emphasis on those surface active agents that are non-ionic. A very broad group of non-ionic surface active agents as described as being useful, and many different wetting agents are recited. 
     In all of the prior art baths that have been proposed, the wetting agents that have been described to be useful for producing either bright or matte deposits are very broadly described and are deemed equivalent to one another. Numerous examples are given in each of these prior art patents directed to a wide variety of agents of many different types, most of which contain some type of oxide or similar condensation compound. 
     The vast majority of such prior art wetting agents are unsuitable for high speed plating in modern day high speed plating equipment. These wetting agents are mainly incapable of satisfying some or all of the requirements for these electrolytes that are listed above. The present invention resolves this problem by providing specifically preferred agents which are highly useful in high speed electroplating equipment and processes. 
     SUMMARY OF THE INVENTION 
     The invention relates to an electrolyte for depositing tin, lead or tin/lead alloys upon a substrate by high speed electroplating, which comprises a basis solution of an alkyl or alkylol sulfonic acid; and at least one of a solution soluble tin compound or a solution soluble lead compound; and a surfactant of an alkylene oxide condensation compound of an aliphatic hydrocarbon having between one and seven, and preferably less than six, carbon atoms and at least one hydroxy group, or solution soluble derivatives thereof. Preferably, the surfactant imparts to the solution a cloud point of above about 110° F., and the electrolyte may include a brightening agent when bright deposits are desired. 
     A preferred hydrocarbon is an alcohol, such as butyl alcohol. Also, to achieve the desired cloud point, the alkylene oxide compound may be ethylene oxide wherein between about four and 40 moles of ethylene oxide, and preferably between six and twenty-eight, are used to form the condensation compound. Some of the moles of ethylene oxide may be replaced with propylene oxide. 
     Another suitable surfactant is an alkylene oxide condensation compound of an aromatic organic compound having 20 carbon atoms or less; or solution soluble derivatives thereof. This aromatic compound may preferably contain one or two rings, preferably containing between 10 and 12 carbon atoms when two rings are utilized. Also, the aromatic organic compound may include an alkyl moeity of six carbon atoms or less, and one or more hydroxyl groups. Preferably, the aromatic organic compound is benzene, naphthalene, phenol, toluene, bisphenol A, styrenated phenol, or an alkylated derivative thereof. 
     Therefore, the desired surfactants include a organic compound having 20 carbon atoms or less condensed with a sufficient amount of an alkylene oxide compound or solution soluble derivatives thereof to impart a cloud point of above 110° F. to the solution. 
     The invention also includes a system and process for the high speed electroplating of tin, lead, or tin/lead alloys. This system utilizes the high speed electroplating equipment of the type described above. Such equipment includes an electroplating cell, an overflow reservoir adjacent the cell, a pump for returning solution from the reservoir to the cell through one or more sparge pipes, and means for directing a substrate to be plated from an entry point at one end of the cell to an exit at a second end of the cell. The electrolytes of the invention are introduced into the equipment in a manner such that the cell is substantially filled with the electrolyte. Also, the electrolyte continuously overflows into the reservoir and is continuously returned into the cell so that vigorous agitation and circulation of the electrolyte within the cell are achieved. Thus, substrates are continuously electroplated as they pass through the cell. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Tin, lead, and tin/lead alloy electroplating compositions are described herein that are specifically designed to deposit acceptable matte or bright deposits from electrolytes that are suitable for operation at high speeds in modern high speed electroplating equipment. Only a limited number of such wetting agents can satisfy all the requirements listed above for successful high speed electroplating. These compounds comprise relatively low molecular weight ethylene oxide derivatives of aliphatic alcohols containing an alkyl group of less than eight carbon atoms or ethylene oxide derivatives of aromatic alcohols containing a maximum of two aromatic rings which may be alkyl substituted providing the alkyl grouping contains less than six carbon atoms and including bis compounds again provided that the alkyl grouping contains less than six carbon atoms. The aromatic compound, whether alkylated or not, should not contain more than 20 carbon atoms prior to condensation with the alkylene oxide compound. 
     The sulfonic acids that are suitable for this invention include any alkyl or alkylol sulfonic acid having up to 5 carbon atoms. The alkane sulfonic acids, and in particular methane sulfonic acid, are preferred. These acids are generally present in an amount of between 10 and 30 percent by volume of the electrolyte, so that free acid is present. As such, the pH of the electrolyte will be 2 or less, usually less than 0.5. 
     The surface active agents that are suitable for this invention are those that satisfy all of the listed above requirements, namely: those which help produce deposits having good solderability, a matte or lustrous finish with satisfactory grain refinement; and wherein the solution is stable in the acid bath, capable of electroplating at high speeds, has a cloud point above about 110° F., and generates little or no foam during the electroplating operation. 
     Foaming is determined in the laboratory by using a basis solution that is typical of those used in high speed electroplating machines. The solution contains the following: 
     Tin metal (as tin methane sulfonate):20 g/l 
     Methane sulfonic acid: 15% by volume 
     Surface active agent under test: 1% by volume 
     Temperature: ambient to 75° F. 
     The relative degree to which the surface active agents form foam in the basis solution is tested by placing 100 ml of the solution into a 250 ml graduated cylinder. 
     Air is supplied by a commercial laboratory or fish tank aerator and fed into the bottom of the solution in the graduated cylinder through a sparger. Two tests are performed: the first one requires pumping air for two minutes to determine if the foam height exceeds 150 ml or goes over the top of the graduated cylinder. If it does, the surface active agent is considered unsuitable and no further work is done. The second test involves bubbling air into a fresh solution for ten seconds. At the end of ten seconds, the maximum foam height is read on the graduated cylinder and a time for foam to completely dissipate down to the original 100 ml mark is noted. In order for a surfactant to pass such a test, the maximum foam height should not exceed 150 ml, and the time for foam to dissipate should not exceed 20 seconds. 
     Cloud point is measured by taking the basis solution containing 1% of the surface active agent and slowly raising the temperature until the solution begins to turn cloudy. A cloud point above approximately 120° F. is highly satisfactory: those 110° F. or below are generally found to be unsatisfactory. 
     The basis solution for use in high speed electroplating equipment and processes of this invention generally contains relatively high concentrations of metals and acid. Such high concentrations also affect the cloud point of the electrolytes. For example, a surfactant which would impart a high cloud point to dilute electrolytes may impart a low cloud point to these concentrated electrolytes. Therefore, it is important to determine the cloud point for the specific overall electrolyte that is contemplated for electroplating the desired deposit. 
     The high speed electroplating characteristics and deposit grain refinement potential of the solution are determined in a Hull cell operated at 5 amps total current for 1 minute at 120° F., with paddle agitation. The solution contains: 
     Tin metal (as tin methane sulfonate):70 g/l 
     Total methane sulfonic acid: 30% by volume 
     Surfactant: 1-10 ml/l, as required. 
     Under these conditions, the Hull cell panel should show a deposit with no more than 1/4&#34; of burn in the high current density area and the deposit on the balance of the panel should be matte or somewhat lustrous, with a pleasing grey, smooth finish. 
     The stability of the electrolyte containing the surfactant is determined by electrolyzing the bath for at least 20 ampere hours per liter. The characteristics of the electroplating solution and its deposit should not have been affected by electrolysis. 
     The solderability of the deposit is determined by following the methods given in Mil-Std 202 F, dated April, 1986, Method 208 F. The deposit must pass the test as given in this military specification. 
     The surface active agents that are included in this invention all include a hydrophobic organic compound which is condensed with a sufficient amount of an alkylene oxide, preferably ethylene oxide, to satisfy the requirements of high cloud point, stability, and high current density grain refinement. Propylene oxide can also be included with the ethylene oxide; however, the amount of propylene oxide used and its ratio to ethylene oxide use must be such that the cloud point is still high enough to pass the above requirements. Propylene oxide can be included to reduce the foaming characteristics of a surfactant; however, only a limited amount can be used since propylene oxide also lowers the cloud point of the resultant electrolyte. One skilled in the art can easily determine the amount of propylene oxide by routine testing. 
     The organic compound can be any aliphatic hydrocarbon (saturated or unsaturated) of seven carbon atoms or less containing at least one hydroxy group. Similarly, the organic compound can also be an aromatic ring compound such as benzene, naphthalene, phenol, toluene, bisphenol A, styrenated phenol, and the like, providing there is not more than two rings and the length of the substituted alkyl chain is limited to six carbon atoms or less. Also, the ring can be substituted with one or more hydroxyl groups. 
     As an illustration of specific compounds, octylphenol ethoxylate with 12 moles of ethylene oxide would not be suitable for this invention because its foaming characteristics are too great due to the alkyl chain length being too great. Beta-naphthol with 13 moles of ethylene oxide, is suitable for this invention and is capable of passing all of the requirements. Styrenated phenol with two or more moles of styrene condensed with 12 moles of ethylene oxide is not suitable since it has three aromatic rings. Ethyloxylated bisphenol A is also suitable for this invention and is capable of passing all of the above requirements. This compound has two aromatic rings and three alkyl carbon atoms. 
     Other suitable surfactants for this invention can include ethyloxylated butyl alcohol, with or without propylene oxide. As the chain length of the aliphatic alcohol is increased, the foaming characteristics will also increase. The foaming characteristics in this group of compounds can be decreased considerably by the inclusion of some propylene oxide into the molecule. However, this must be controlled to prevent the lowering of the cloud point, which would make the compound unsuitable if the resultant cloud point is less than 110° F. The maximum length of the alkyl group should be eight carbon atoms or less in this series. 
     In this invention, the plating bath contains solution soluble tin and/or lead metals, preferably as alkyl sulfonates or alkanol sulfonates, plus some extra or free alkane or alkanol sulfonic acid. The surfactants suitable for this invention have been described in order to produce suitable deposits which are matte or semi-lustrous: however, it is also possible to improve the brightness of the deposit by adding known brightening agents such as those disclosed in any of the prior art patents listed earlier. The resultant plating bath will then have all of the desirable characteristics of a bright or semi-bright deposit. 
     The surface active agents can be rendered more solution soluble by techniques generally known in the art. Such solution soluble derivatives of the desirable surface active agents can be made, e.g., by sulfating, sulfonating, phosphating, phosphonating, carboxylating, etc., provided the derivative does not impair the suitability of the material for purposes of this invention stated previously. 
     There are a wide variety of high speed electroplating equipment commercially available today. One typical apparatus is disclosed in U.S. Pat. No. 3,819,502 to Meuldjik, while others are disclosed in articles entitled &#34;High Speed Electrogalvanizing Line With Insoluble Anode at Kimitsu Works of Nippon Steel Corporation&#34; by M. Morimoto et al., &#34;Swim Plating as a Continuous Process&#34; by J. J. Miles et al., and &#34;Continuous Plating of Copper, Nickel and Chromium on Wide Steel Strip For Decorative and Function Applications&#34; by H. Wettner. A high speed machine for controlled depth electroplating is disclosed in an article entitled &#34;How to Save Gold With Selective Deposits&#34; by C. D. Eidschun. Each of these papers was presented at the American Electroplater&#39;s Society&#39;s Second Continuous Plating Seminar, Chicago, Illinois, Jan. 24-26, 1977. It must be emphasized that these high speed electroplating units are merely illustrative and fall within the general description given in this application. Those skilled in the art are aware of a wide range of similar machines which are useful for high speed electroplating according to this invention. Generally, solution turnover i.e., volume of the cell, in such equipment ranges between 4 and 6 times (or more) per hour. 
     By &#34;high speed electroplating,&#34; applicants mean those processes which operate at a current density above 200 ASF and above a temperature of about 85° F. in the above-described equipment. Preferably, such processes are carried out at current densities of 500 ASF or above and at temperatures of 95° F. or above. Temperatures of between 120 and 140° F. and current densities as high as 1500 to 2000 ASF or more are not uncommon for such processes. 
     EXAMPLES 
     The scope of the invention is further described in connection with the following examples which are set forth for the sole purpose of illustrating the preferred embodiments of the invention and which are not to be construed as limiting the scope of the invention in any manner. 
     Three stock solutions were used in each example to test the ability of each surfactant to electroplate pure tin, a 90/10 tin/lead alloy and a 60/40 tin/lead alloy. These solutions were as follows: 
     
         ______________________________________             Pure Tin                    90/10   60/40______________________________________Tin metal (as tin methane sulfonate)               72       72      40g/lLead metal (as lead methane               --       18      26sulfonate) g/lMethane sulfonic acid (vol. %)               15       15      15______________________________________ 
    
     The surfactants of each example were added in increments until the optimum amount was reached. Tests of the solutions and the electrodeposits were made using all the test methods listed above: 
     (1) foaming 
     (2) cloud point of solution 
     (3) grain refinement (smooth, light grey satin finish) 
     (4) speed of electroplating 
     (5) solderability of deposits 
     (6) stability of solution 
     Each of the solutions of these examples exhibited a pH of less than 0.5, and was tested in a Hull cell at 5 amps. 
    
    
     EXAMPLE 1 
     Bisphenol A with 8 moles ethylene oxide was used in an amount of between 6 and 12 ml/l. The solutions with this surfactant passed all six tests. 
     EXAMPLE 2 
     Bisphenol A with 10 moles ethylene oxide was used in the same amounts as in Example 1. Solutions with this surfactant, also passed all tests. 
     EXAMPLE 3 
     Sulfated Bisphenol A with 30 moles ethylene oxide was used in an amount of between 3 and 6 ml. Solutions with this surfactant also passed all tests. 
     EXAMPLE 4 
     Beta Naphthol with 13 moles ethylene oxide was used in an amount of between 0.5 and 1 ml. Solutions with this surfactant also passed all tests. 
     EXAMPLE 5 
     (Comparative) 
     Polystyrenated phenol with 12 moles ethylene oxide was used in an amount between 3 and 6 ml/l. This surfactant forms too much foam and is not satisfactory despite that it passed the other tests. 
     EXAMPLE 6 
     (Comparative) 
     Octyl alcohol with 12 moles ethylene oxide was used in an amount of between 3 and 8 ml/l. This surfactant forms too much foam and is not satisfactory. 
     EXAMPLE 7 
     (Comparative) 
     Butyl alcohol with 5 moles ethylene oxide was used in an amount of between 2 and 8 ml/l. Although, the grain refinement of the deposit is not satisfactory, the other tests were passed: thus, the number of moles of ethylene oxide must be increased to at least six or more, as shown by Examples 8 and 9. 
     EXAMPLE 8 
     Butyl alcohol With 16 moles ethylene oxide plus 12 moles propylene oxide was used in an amount of between 1 and 4 ml/l. Solutions with this surfactant passed all tests. 
     EXAMPLE 9 
     Butyl alcohol with 8 moles ethylene oxide plus 6 moles propylene oxide was used in an amount of between 0.5 and 2 ml/l. Solutions with this surfactant passed all tests. 
     EXAMPLE 10 
     Bright deposits can be obtained by adding known brighteners such as aromatic aldehydes such as chlorobenzaldehyde or derivatives thereof, such as benzyl acetone, to any of the above solutions that pass all the tests. 
     While it is apparent that the invention herein disclosed is well calculated to fulfill the objects above stated, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.