Replenishment of electroless nickel solutions
United States Patent Dutkewych et al.  
 Apr. 8, 1975 REPLENISHMENT OF ELECTROLESS NICKEL SOLUTIONS Primary Examiner-Martin P. Schwadron [75] Inventors: Oleh Borys Dutkewych. Medfield. Ellammer lx obert Mlller Muss; Lebert Arthur Hoffman, Attorney, Agent. or F rm-Dike. Bronstem. Roberts. Westminster. Calif. Cubhman &amp; Pfund [73] Assignee: Shipley Company, Inc., Newton.  
  Mass. [57] ABSTRACT [22] Filed: 1974 This invention concerns replenishment of aqueous [21] Appl. No.: 435,690 electroless nickel plating solutions and to apparatus useful therefor. The process of the invention comlRelated. U.S. Appllcatlon Data prises addition of replenishers in essentially dry form g f -t -p f 9- in a manner that avoids triggeringof the bath. :fypical whlch a commuanon&#39;mpan of replenishers that may be added in dry form include 197] PatNo&#39; 3770464 one or more of a soluble source of nickel ions such as nickel chloride. a reducing agent for nickel ions. bath [52] US. Cl. 106/1; l 7/130 E stabilizers brightencrs Surfactants and flu3 likc TheSC it. i i g i y be added singularly or admixed with [58] Fleld 0 card 106/1 117/130 each other. Essentially dry materials are used for re- Il7/l60 l37/68 plenishment rather than a solution as in the prior art. [5 R f Ct d to prevent volume growth of the plating solution.  
 e erences e UNITED STATES PATENTS 10 Claims, 1 Drawing Figure 2.929.742 3/1960 Minjer et al. 106/1 X 2o| HQ I I&#39;flk I V H3 J REPLENISHMENT OF ELECTROLESS NICKEL SOLUTIONS CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of copending US. patent application Ser. No. 313,178, filed Dec. 7, 1972 which is a continuation-in-part of US. patent application Ser. No. 188,243, filed Oct. 12, 1971, 3,770,464.  
 BACKGROUND OF THE INVENTION This invention relates to electroless nickel plating and more particularly, to replenishment of depleted electroless nickel plating solutions.  
  As is known in the art, electroless nickel deposition refers to the chemical deposition on a receptive surface of an adherant nickel coating in the absense of an external electric source. Such deposition is useful, for example, in the provision of corrosion resistant surfaces, for decorative purposes, and the like.  
  A number of electroless nickel deposition processes and solutions have heretofore been known and are disclosed in numerous patents including US. Pat. Nos. 3,719,508; 3,717,482; 3,424,597; 3,420,680; 3,338,726; 2,929,742; 2,819,188; and 2,762,723, all incorporated herein by reference.  
  Known electroless nickel deposition solutions generally comprise at least four ingredients dissolved in a solvent, typically water. They are, (l) a source of the nickel ion, (2) a reducing agent such as hypophosphite or amine borane, (3) an acid or hydroxide pH adjuster to provide required pH and, (4) a complexing or chelating agent for metal ions sufficient to prevent their precipitation in solution.  
  A large number of suitable complexing agents for electroless nickel solutions are described in the above noted publications. For example, malic acid, lactic acid, glycine and other complexing agents are disclosed in the aforesaid US. Pat. No. 2,819,188, to Metheny et al. In some formulations, the complexing agents are helpful but not a necessity.  
  Deposition from said solutions occurs by the reduction of the nickel ions to nickel by reducing agents initiated by the presence of a suitable catalytic surface, for example, surfaces of iron, cobalt, nickel or palladium, or various surfaces of catalized plastic as disclosed in U.S.&#39;Pat. No. 3,011,920. It is known in the art that in use, an electroless nickel deposition solution becomes depleted in solution constituents. For example, nickel ion concentration becomes depleted through deposition on the substrate. Hypophosphite becomes depleted as it is used to reduce the nickel ion to a metallic nickel. The concentration of the comlexing agent is somewhat lowered by drag-out on the parts passing through the plating bath. Consequently, with use, as the solution becomes depleted, the deposition rate decreases to a point where the solution is no longer useable.  
  In order to increase the useful life of an electroless nickel deposition solution, it has been the practice in the art to replenish the solution with solution constituents at frequent intervals during the plating cycle. This has been accomplished by adding one or more of the solution constituents to the plating solution in the form of a concentrated aqueous solution.  
  A major difficulty with the prior art procedures for replenishment of an electroless nickel deposition solution is that there is an increase in volume of the solution each time it is replenished with the liquid replenisher. This is due to the fact that as the solution becomes depleted of its solution constituents, there is no substan- 5 tial corresponding decrease in volume, as the only losses in volume occur as a result of evaporation and drag-out. For example, in large volume continuous plating operations, there may be as many as five plating cycles per 24 hour period. The plating solution is replenished with an aqueous solution of the constituents after each cycle. Each replenishment results in about 10% increase in volume of the plating solution which means that there is about a 50% volume increase every 24 hours. As a result, the cost of the plating process is substantially increased and a greater volume of solution must alternately be dumped. With code regulations preventing dumping of industrial waste containing nickel values, this becomes an. ever increasing problem. Volume build up is particularly acute with room temperature or low temperature electroless nickel plating processes which are being increasingly used, especially in connection with the electroless nickel plating of catalyzed plastic substrates. With high temperature electroless nickel plating processes, which are normally used on materials whose surfaces are themselves catalytic and thus do not need to be specially catalyzed, volume build up is not as significant a problem. This is because these processes are normally run at temperatures of about F or above, and evaporation of the solution effectively decreases the volume of liquid contained in the bath, which includes the amount of liquid added by normal replenishers.  
  Attempts have been made in the prior art to add replenishers in dry form to avoid volume growth of the plating solution. However, these attempts have been unsuccessful for a number of reasons. The most obvious method of adding dry ingredients to a plating solution would be to simply drop the dry powder into the top of the plating tank. However, this method is not operative because the powder in solution acts as localized areas of high concentration and as nucleating sites for plating, resulting in triggering of the bath and loss thereof. Also, particles in the plating solution hitting the work-piece to be plated results in defects in the coherency of the metal plate.  
  In US. Pat. No. 2,955,944, issued Oct. 11, 1960, there is described a method of maintaining the nickel content of an electroless nickel plating bath by addition of a water insoluble solid nickel compound, such as nickel oxide, nickel carbonate or nickel hydroxide, which is solubilized by the action of acid included in, or generated by, the bath. This process did not, however, solve all of the problems associated with attempts at dry replenishment. For example, the difficulty obtained by contact between the surface to be plated and the solid replenisher remained. Further, problems arise concerning compatibility between the anion of the insoluble replenisher and the rest of the bath. Moreover, the process of that patent, whereby the solid replenisher is rendered soluble by acid attack, is obviously totally unuseable with the low temperature electroless nickel plating baths, which are normally alkaline. Thus, a process which would avoid volume build up in these baths without causing the other problems mentioned above remained to be found.  
 STATEMENT OF THE INVENTION The subject invention provides a method of replenishing a stable plating bath with replenishers that are essentially in dry form without triggering the bath thereby avoiding loss of solution by volume growth. The method is based upon the discovery that the replenishers may be added to a stable plating bath if they are kept out of contact with air and are rapidly dispersed and dissolved in solution. This can be accomplished by adding the replenisher below the surface of the solution with agitation to avoid localized areas of high concentration and to facilitate dissolution. Typical replenishers that may beused are one or more of a soluble source of nickel ions such as nickel chloride, a reducing agent for nickel ions, such as hypophosphite, a chelating or complexing agent for nickel ions, and known bath stabilizers, brighteners, surfactants and the like. These ingredients may be added singularly or admixed with each other. Preferably, at least the nickel salt, complexing agent,&#39;and most preferably, the stabilizer are added together to facilitate the solution of the nickel salt in the plating solution.  
  Essentially dry materials are used for replenishment rather than concentrated solutions as in the prior art to prevent volume growth of the plating solution. Consequently, with the avoidance of volume growth, there is less of a problem of dumping waste and accordingly, this invention is a contribution to pollution control.  
 DESCRIPTION OF THE DRAWINGS In the drawings:  
  FIG. 1 represents a preferred plating apparatus in combination with apparatus for adding dry powders; and  
  FIG. 2 represents a cross-sectional view of the apparatus of FIG. 1 taken along the section line 22.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS For purposes of definition, essentially dry form, as usedto discribe the replenisher herein, is intended to mean a composition having a solids content of at least 75% by weight. Thus, the term contemplates highly concentrated solutions substantially more concentrated than prior art replenishers, as well as dry powders. In some instances, moisture content is desirable as it facilitates molding to tablet or other desired shapes. In this respect, it should be noted that those substances added in smaller concentrations such as the replenishers for the stabilizer, surfactant, brightener, complexing agent, and the like may be in liquid form admixed with dry powders of the nickel salt and the hypophosphite, as the minor concentration of these constituents will still keep the total solids content within 75%.  
  As noted above the replenisher composition for the subject invention is in essentially dry form and comprises one or more of a soluble nickel salt that is a source of nickel ions in an aqueous electroless nickel solution, a reducing agent such as a hypophosphite, a small amount of a complexing agent for nickel ions, stabilizers, brighteners, surfactants and the like. The materials contemplated for use in the subject invention are old, exemplified in the above referenced patents and conventionally used in the formulation of electroless nllcel depbsition solutions. Typical examples of soluble nickel salts for purposes of the present invention include nickel chloride, nickel acetate, nickel formate, nickel nitrate, nickel sulfate, nickel ammonium chloride, nickel ammonium sulfate, etc. Typical examples of suitable complexing agents for the nickel ions include hydroxyacetic acid, malic acid, lactic acid, succinic acid, glycine, Rochelle salts, the sodium salts of ethylenediaminetetraacetic acid, modified ethylenediaminetetraaceticacids such as N-hydroxyethylenediaminetriacetate, hydroxylalkyl substituted dialkylenetriamines, sodium salicylate and sodium tartrate. Other complexing agents for nickel ions are well known in the art.  
  Materials known to the art as catalytic poisons to the deposition of electroless metals are frequently used in controlled amounts as stabilizers for the plating solutions. The most widely used group of compounds of this nature are heavy metals, such as lead, selenium, tellurium, bismuth, arsenic, antimony, tin, cadmium, and mercury, such as disclosed in US. Pat. No. 2,762,723, and sulfer compounds such as hydroxypropane sulfonic acids disclosed in US. Pat. No. 3,420,680, or thio compounds such as thiourea. Other suitable stabilizers are well known in the art.  
  The relative proportions of the ingredients comprising the dry replenisher mixture may vary within broad limits dependent upon the requirements and use of the electroless copper deposition solutions to which the replenisher is added. In general, the nickel salts and hypophosphites are added in about the same relative proportions as found in fresh electroless nickel deposition solutions, e.g. a range of nickel to hypophosphite concentration of from about I to 10 to 10 to I. If a complexing agent is used as a constituent of the dry replenisher mixture, its relative proportion in the mixture is substantially less than in the original copper deposition solution as it is not consumed Tn the plating operation but only depleted in that solution by means of drag-put. Typically, the chelating agent may constitute from about 0.01 to 0.50 moles per mole of nickel salt. The dry replenisher may contain substituents in addition to those noted above. For example, additives may be used to facilitate production such as molding agents, of which glycerol is a good example as it is also a chelating agent for the nickel ions. Other additives may include agents to protect against decomposition, sublimation, handling and the like. As noted above, any of the chelating agents, stabilizer, surfactant or brightener, may be in liquid form if blended with the nickel salt, reducing agent or both, as the small amount of such additive will still result in a replenisher in essentially a dry form having a solids content in excess of by weight. In addition to the above ingredients, acid, such as sulfuric acid, or base, such as sodium hydroxide, may require replenishment to maintain the necessary pH of the electroless plating solution.  
  The replenisher should not be added to the electroless copper deposition solution by simply dropping the same onto the surface of the solution as this would cause triggering and interfere with the deposit on a work piece. It is a discovery of this invention that the replenisher may be successfully added to a stabilized bath out of contact with air and in a manner to rapidly disperse powders and dissolve the same. This may be accomplished by adding the replenisher directly to the solution below its surface with solution agitation to facilitate dispersing the dry replenisher in solution and dissolve the same as rapidly as possible.  
  As noted above, the bath to which the replenisher is added should be substantially stable. Upon depletion of a plating bath there may also be depletion of stabilizer through drag-out or otherwise. Thus, the bath should be stabilized prior to dry replenishment. This may be accomplished by one of several methods. The stabilizer may be blended with the other replenisher ingredients or added alone. Alternatively, the situation can be avoided by an initial bath formulation having excess stabilizers, provided it does not poison the bath. What cannot be done is the addition of the stabilizer to an unstable bath subsequent to replenishment of other ingreclients.  
  A preferred method and apparatus for adding replenisher is illustrated in the drawings, wherein FIG. 1, there is illustrated in the apparatus comprising a plating section on the right and a replenishment section, greatly enlarged for purposes of illustration, on the left. The plating section consists of plating apparatus 100 of a catalytically inactive material such as polyethylene or polypropylene. This section is divided into plating tank 101 and overflow tank 102. Catalytically active parts are plated in plating tank 101 by racking the same and immersing them in the plating tank (parts and racks not shown). Contact of the Catalytically active part with the solution in the plating tank will result in plating.  
  Plating tank 101 is separated from overflow tank 102 by barrier 103 having overflow lip 104. As the plating solution passes over barrier 103 from plating tank 101 into overflow tank 102, it may pass through filtering means 105 such as a cloth bag. Within overflow tank 102, there may be placed a heat exchanging device 106 to maintain a desired solution temperature. Thus, in normal use without replenishment, the flow of plating solution is in a closed loop from plating tank 101 over the overflow lip 104 into overflow tank 102, out of overflow tanks 102 through outlet 107 having valve 108 in the open position and through pump 109 back into plating tank 101 through a return sparger line 110 and solution sparger 111. During this operation, valves 112 and 113 are in the closed position.  
  When replenishment is desired, valves 108 and 112 are in the open position and valve 113 is in the closed position. Solution then flows out of overflow tank 102 through outlet 107 and pump 109 into the replenishment apparatus 200 through line 201, valve 112 and inlet 202. Line 201 is sized to allow approximately 75-90% of the output from pump 109 to continue recirculating through plating apparatus 100. lnlet 202 discharges into a cylindrical chamber 203 designed to affect a swirling motion to the solution. The relationship of inlet 202 to cylindrical chamber 203 can be seen in more detail in FIG. 2 which is a cross-sectional view taken along section line 2-2. From FIG. 2, it can be seen that the inlet 202 discharges into the side of chamber 203 to cause a swirling motion. Within chamber 203 there are contained dry replenisher powders 204 which powders are wetted and passed through disperser plate 205 containing openings 206. The powders 204A wetted with solution pass into a porous container 207, such as a cloth bag polypropylene felt, held in chamber 208. The raining action of the solution as it passes through disperser plate 205 causes further dissolution of the powder in porous container 207 at a relatively constant rate. The replenished relatively concentrated solution then passes from chamber 208 through outlet 209, through pipe 210 and into the filter bag which may be of the same material as porous container 207, but with decreased porosity. The filter bag 105 insures that no undissolved constituents enter the solution to form localized areas of high concentration and- /or nucleating sites which would result in the aforesaid problems.  
  The replenisher powders 204 are added to the replenisher apparatus 200 by any convenient means. For example, with reference to the drawing, there is depicted an assembly comprising a hand turn wheel 211 secured with yoke 212. When turning the wheel, cap 213 is raised and the entire assembly moves upwards, and pivots sideways on pivot point 214A or 2148. With the removal of cap 213 there is ready access to upper chamber 203. If it is desired to change or replace porous container 207, this is readily accomplished by removing disperser plate 205 and withdrawing bag connector 216 and container 207 readily slips off bag connector 216.  
  The size of chamber 208 and the length of porous container 207 may vary depending upon the combined volumes of tanks 101 and 102 and the rate of solution flow through the apparatus. In this respect, it can be seen that container 203 is connected to container 208 mechanically and can be readily separated therefrom. Bag connector 216 is provided with a shoulder which sits on flange 217. To separate container 203 from container 208, scerws 218 are removed and flange 217 is separated from flange 219. Pressure is maintained between chambers 203 and 208 with O ring 221.  
  Should it be desired to drain chamber 208 after the dry replenisher is dissolved and/or prior to a subsequent dry replenisher addition, this may be accomplished by closing valve 112, removing cap 213 and opening valve 113. This will permit pump 109 to drain chamber 208 and pipe 210 returning solution to tank 101 through sparger line and sparger outlet 111.  
  Should it be desired to drain tank 101, this can be accomplished by removing cap 115 and permitting the fluid to flow out of the tank. Pump 109 is used to drain solution.  
  It should be understood that while the replenisher has been referred to as essentially dry powders, it may be in the form of a a molded shape, such as that of a tablet or alternatively packaged in a solution soluble plastic bag such as a bag formed from carboxyl methyl cellulose.  
  A particular advantage of the dry replenisher apparatus and process of the invention is the ability to maintain the primary components of the plating solution at a predetermined and desired level. As previously described when the operating parameters of the bath are known, such as a constituent concentrations, bath temperature, surface area in relation to bath volume and plating time, the amount of solution depletion can be predetermined and anticipated. Similarly, when this apparatus is used as described and the solution flow is controlled while maintaining an operating temperature, the rate of dissolution of the dry replenisher into the plating solution can be predicted and controlled. Under the conditions previously described, the rate of dissolution of the replenishers will be approximately 1% per minute. As a net result, the use of a dry replenisher can permit a product user to maintain constituent levels at a desirable point within a relatively small tolerance.  
  The invention will be further clarified with reference to the following examples.  
 EXAMPLE 1 With reference to the apparatus depicted in the drawings, a tank having a capacity of gallons canibe filled with an electroless nickel depositing solution having a composition of 20 g/l nickel sulfate hexahydrate, g/l sodium hypophosphite monohydrate, g/l hydroxyacetic acid, lead as a stabilizer in the form of-2 mg/l lead chloride, and sufficient ammonium chloride to obtain a pH of between about 9 and 10. This solution is maintained at about 90F by recirculating the same from the plating chamber to the overflow tank containing a heating coil and a filter bag at-a rate of about 200 gallons per hour. The solution is used to deposit nickel on a polyacrylonitrile-butadiene-styrene substrate which has been prepared and catalyzed in the manner disclosed in U.S. Pat. No. 3,668,130, with a loading of about one-half square foot of substrate per gallon of plating solution. The nickel, hypophosphite and hydroxide contents of the bath begin depleting immediately, and after about half an hour a noticeable decrease in plating rate takes place. A dry replenisher is prepared by mixing 227 grams of nickel sulfate hexahydrate, 151 grams of sodium hypophosphite monohydrate and 8 milligrams of lead chloride until the mixture is homogenious. The replenisher is placed in a polypropylene bag 207 in the replenishment apparatus depicted in the drawing in the manner described above. The solution is then recirculated from the overflow tank through the replenishment apparatus at a rate of about 200 gallons per hour and back to the filter bag 105 of the overflow tank where it is filtered and recirculated into the plating tank. Within about twenty minutes the replenisher constituents are substantially dissolved and the plating rate will return to normal.  
  The following are examples of suitable replenisher compositions, with percentages being by weight:  
 EXAMPLE 2 Nickel chloride hexahydrate 50%. Sodium hypophosphite monohydrate 50%.  
 EXAMPLE 3 Nickel acetate Sodium hypophosphite monohydrate 40%.  
 Hydroxyacetic acid 20%.  
  Each of the above formulations may be molded into tablets if desired, tablets about 2 inches in diameter by one-fourth inch in thickness being convenient. Other formulation will bereadily apparent to those skilled in the art.  
  The specific embodiments described herein are meant to be exemplary only, and various modifications will be apparent to those in the art. The claims below are intended to cover all such modifications as fall within the true spirit and scope of the invention.  
 I claim:  
  1. In a method for replenishing an electroless nickel plating solution comprising nickel ions, a source of hypophosphite reducing agent and a complexing agent to maintain said nickel ions in solution, with at least one or more replenisher constituents in essentially dry form, said replenisher constituents being selected from a group of soluble nickel salts, a source of hypophosphite, stabilizers and mixtures thereof, the improvements comprising maintaining said solution stable, adding and substantially dissolving said replenisher constituents beneath the level of said plating solution while maintaining said essentially dry replenisher constituents substantially out of contact with air and the surface of the plating solution and avoiding localized areas of high concentration of the replenisher constituents by rapidly dispersing the same in said plating solution.  
  2. The method of claim 1 where the solution is maintained stable by replenishment with a stabilizer prior to or during replenishment with at least the nickel salt.  
  3. The method of claim 1 where the solution is maintained stable by use of an excess ofa non-poisoning stabilizing agent in the original solution make-up.  
  4. The method of claim 1 where the localized areas of high concentration of replenisher constitutents are avoided and rapid dissolution of constituents is accomplished with solution agitation.  
  5. The method of claim 1 where the replenisher constituents are added directly to the plating solution beneath the level of the plating solution.  
  6. The method of claim 1 where said replenisher constituents are added and substantially dissolved beneath the level of the plating solution by circulating a minor portion of the plating solution external to the major portion of said solution, through a mass of said replenisher constituents to substantially dissolve the same whereby the replenisher constituents are maintained substantially out of contact with the surface of the plating solution, and recirculating said minor portion of solution enriched in replenisher constituents back to the major portion of said solution.  
  7. The method &#39;of claim 6 where said minor portion of solution is passed through said replenisher constituents.  
  8. The method of claim 6 where said minor portion of solution has its speed increased through said replenisher constituents by imparting a swirling motion to&#39; said minor portion of solution.  
 9. The method of claim 1 where said replenisher agent for the plating solution.