Abstract:
The present invention relates to an aqueous plating bath composition for depositing a nickel phosphorous alloy having a phosphorous content in the range of 5 to 12 wt. %. The plating bath comprises a sulfur-containing organic stabilizing agent.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an aqueous plating bath composition for the electroless deposition of nickel phosphorous alloys. 
       BACKGROUND OF THE INVENTION 
       [0002]    Electroless plating of nickel-phosphorous alloys is used in various industries. The deposits derived are used e.g. as wear resistant coatings and barrier layers. 
         [0003]    Such plating bath compositions generally comprise a source of nickel ions, a hypophosphite compound as the reducing agent, at least one complexing agent and at least one stabilizing agent. 
         [0004]    The at least one stabilizing agent present is required in order to provide a sufficient bath lifetime, a reasonable deposition rate and to control the phosphorous content in the as deposited nickel phosphorous alloy. Often, plating baths for deposition of nickel-phosphorous alloys known in the art comprise more than one stabilizing agent. 
         [0005]    Common stabilizing agents are selected from heavy metal ions such as cadmium, thallium, bismuth, lead and antimony ions, inorganic ions such as SCN −  and various organic compounds such as thiourea. 
         [0006]    The patent document U.S. Pat. No. 2,830,014 discloses plating bath compositions for electroplating of nickel which comprise thioalkane sulfonic acids or salts thereof such as mercaptopropane-1 -sodium sulfonate as brightening and ductility-improving agents. 
         [0007]    The patent application U.S. 2005/0013928 A1 discloses an electroless plating pre-treatment solution which comprises 3-mercaptopropanesulfonic acid. The pre-treatment solution reduces the incubation time (time from the start of the supply of an electroless plating solution to the start of the plating reaction) of nickel plating from an electroless plating bath on a copper surface. 
         [0008]    The patent application U.S. 2006/024043 A1 discloses an alkaline aqueous plating bath for deposition of nickel phosphorous alloys which optionally comprises further additives such as thiourea and/or 3-mercapto-1-propane sulfonic acid. 
         [0009]    Such plating bath compositions are not acceptable for industrial applications (see Examples 2 to 5). Furthermore, thiourea is a carcinogenic substance. 
         [0010]    The main disadvantages of known stabilizing agents are
   a) the toxic behaviour of heavy metal ions such as cadmium, thallium, lead and antimony ions and   b) in case more than one stabilizing agent is present in an electroless nickel plating bath, the control of the mixture of stabilizing agents during use of such a plating bath is complex.   
 
       OBJECTIVE OF THE INVENTION 
       [0013]    Therefore it is the objective of the present invention to provide a bath composition for electroless deposition of nickel phosphorous alloys which is free of toxic heavy metal ion based stabilizers. 
         [0014]    It is a further objective of the present invention to provide a bath composition for electroless deposition of nickel phosphorous alloys which allows deposition of a nickel phosphorous alloy having a phosphorous concentration in the range of 5 to 12 wt. %. 
       SUMMARY OF THE INVENTION 
       [0015]    These objectives are solved by an aqueous plating bath composition for electroless plating of a nickel phosphorous alloy, the plating bath comprising
       (i) a water soluble source of nickel ions   (ii) a hypophosphite compound   (iii) at least one complexing agent and   (iv) a stabilizing agent selected from the group consisting of compounds according to formulae (1) and (2):       
 
         [0000]      R 1 S—(CH 2 ) n —SO 3 R 2    (1)
 
         [0000]      R 3 SO 3 —(CH 2 ) m —S—S—(CH 2 ) m —SO 3 R 3    (2)
           wherein   R 1  is selected from the group consisting of hydrogen, methyl, ethyl, propyl, butyl, lithium, sodium, potassium and ammonium,   n ranges from 1 to 6,   R 2  is selected from the group consisting of hydrogen, methyl, ethyl, propyl, butyl, lithium, sodium, potassium and ammonium,   R 3  is selected from the group consisting of hydrogen, methyl, ethyl, propyl, butyl, lithium, sodium, potassium and ammonium and   m ranges from 1 to 6       and having a pH value in the range of 3.5 to 6.5.       
 
         [0027]    From such a plating bath composition, a nickel phosphorous alloy having a phosphorous content in the range of 5 to 12 wt. % of phosphorous is derived by electroless plating. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The aqueous plating bath composition according to the present invention comprises a water soluble source of nickel ions such as nickel sulfate, a reducing agent such as sodium hypophosphite, at least one complexing agent and a stabilizing agent selected from compounds according to formulae (1) and (2). 
         [0029]    The concentration of nickel ions ranges from 1 to 18 g/l, more preferably from 3 to 9 g/l. 
         [0030]    The reducing agent is selected from hypophosphite compounds such as hypophosphorous acid or a bath soluble salt thereof such as sodium hypophosphite, potassium hypophosphite and ammonium hypophosphite. The amount of the reducing agent employed in the plating bath ranges from 2 to 60 g/l, more preferably from 12 to 50 g/l and most preferably from 20 to 45 g/l. As a conventional practice the reducing agent is replenished during the reaction. 
         [0031]    The complexing agents are employed in amounts of 1 to 200 g/l, more preferably from 15 to 75 g/l. 
         [0032]    In one embodiment of the present invention, carboxylic acids, polyamines and sulfonic acids or mixtures thereof are selected as complexing agents. Useful carboxylic acids include mono-, di-, tri- and tetra-carboxylic acids. The carboxylic acids may be substituted with various substituent moieties such as hydroxy or amino groups and the acids may be introduced into the plating solutions as their sodium, potassium or ammonium salts. Some complexing agents such as acetic acid, for example, may also act as a buffering agent, and the appropriate concentration of such additive components can be optimised for any plating solution in consideration of their dual functionality. 
         [0033]    Examples of such carboxylic acids which are useful as the complexing agents include: monocarboxylic acids such as acetic acid, hydroxyacetic acid, aminoacetic acid, 2-amino propanoic acid, 2-hydroxy propanoic acid, lactic acid; dicarboxylic acids such as succinic acid, amino succinic acid, hydroxy succinic acid, propanedioic acid, hydroxybutanedioic acid, tartaric acid, malic acid; tricarboxylic acids such as 2-hydroxy-1,2,3 propane tricarboxylic acid; and tetracarboxylic acids such as ethylene diamine tetra acetic acid (EDTA). 
         [0034]    The most preferred complexing agents are selected from the group consisting of mono-carboxylic acids and di-carboxylic acids. 
         [0035]    In one embodiment, mixtures of two or more of the above complexing agents are utilized. 
         [0036]    The stabilizing agent is selected from compounds according to formulae (1) and (2): 
         [0000]      R 1 S—(CH 2 ) n —SO 3 R 2    (1)
 
         [0000]      R 3 SO 3 —(CH 2 ) m —S—S—(CH 2 ) m —SO 3 R 3    (2)
       wherein   R 1  is selected from the group consisting of hydrogen, methyl, ethyl, propyl, butyl, lithium, sodium, potassium and ammonium, n ranges from 1 to 6,   R 2  is selected from the group consisting of hydrogen, methyl, ethyl, propyl, butyl, lithium, sodium, potassium and ammonium,   R 3  is selected from the group consisting of hydrogen, methyl, ethyl, propyl, butyl, lithium, sodium, potassium and ammonium and   m ranges from 1 to 6.       
 
         [0042]    More preferably, R 1  is selected from the group consisting of hydrogen, sodium and potassium. 
         [0043]    More preferably, n ranges from 2 to 4. 
         [0044]    More preferably, R 2  is selected from the group consisting of hydrogen, sodium and potassium. 
         [0045]    More preferably, R 3  is selected from the group consisting of hydrogen, sodium and potassium. 
         [0046]    More preferably, m ranges from 2 to 4. 
         [0047]    The concentration of the stabilizing agent according to formulae (1) and (2) in the plating bath composition preferably ranges from 1 to 100 ppm, more preferably from 2 to 50 ppm and most preferably from 3 to 30 ppm. 
         [0048]    Since the plating solution has a tendency to become more acidic during its operation due to the formation of H 3 O +  ions, the pH may be periodically or continuously adjusted by adding bath-soluble and bath-compatible alkaline substances such as sodium, potassium or ammonium hydroxides, carbonates and bicarbonates. The stability of the operating pH of the plating solutions can be improved by the addition of various buffer compounds such as acetic acid, propionic acid, boric acid, or the like, in amounts of up to 30 g/L, more preferably from 2 to 10 g/l. 
         [0049]    The pH value of the aqueous plating bath according to the present invention preferably ranges from 3.5 to 6.5, more preferably from 4 to 6. 
         [0050]    Other materials may be included in the nickel plating solutions such as buffers and wetting agents. These materials are known in the art. 
         [0051]    The nickel plating solutions optionally may employ one or more wetting agents of any of the various types heretofore known which are soluble and compatible with the other bath constituents. In one embodiment, the use of such wetting agents prevents or hinders pitting of the nickel phosphorous alloy deposit, and the wetting agents can be employed in amounts up to about 1 g/l. 
         [0052]    The substrate to be plated is contacted with the plating bath at a temperature of at least 40° C. up to 95° C. The electroless nickel plating baths according to the present invention are employed, in one embodiment, at a temperature of from 70° C. to 95° C., and more often, at a temperature of from 80° C. to 90° C. 
         [0053]    The duration of contact of the electroless nickel plating bath with the substrate being plated is a function which is dependent on the desired thickness of the nickel phosphorus alloy. Typically, a contact time can range from 1 to 30 min. 
         [0054]    The substrate to be coated with a nickel phosphorous alloy can be contacted with the plating bath according to the present invention by dipping the substrate into the plating bath or by spraying the plating bath onto the substrate. 
         [0055]    During the deposition of the nickel phosphorous alloy, mild agitation may be employed. Agitation may be a mild air agitation, mechanical agitation, bath circulation by pumping, rotation of a barrel plating, etc. The plating solution may also be subjected to a periodic or continuous filtration treatment to reduce the level of contaminants therein. Replenishment of the constituents of the bath may also be performed, in some embodiments, on a periodic or continuous basis to maintain the concentration of constituents, and in particular, the concentration of nickel ions and hypophosphite ions, as well as the pH level within the desired limits. 
       EXAMPLES 
       [0056]    The invention will now be illustrated by reference to the following non-limiting examples. 
       Example 1 (comparative) 
       [0057]    An electroless nickel plating bath comprising 6 g/l nickel ions as nickel sulfate, hypophosphite as reducing agent, a mixture of lactic acid and malic acid as complexing agents (overall concentration: 150 mmol/l) and lead ions as stabilizing additive was tested. The pH value of this plating bath was 4.8. The plating results are summarised in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 plating results obtained for an electroless nickel  
               
               
                 plating bath known from prior art (MTO = metal turn over). 
               
             
          
           
               
                 Sample 
                   
                 Lead concen- 
                 P concentration in  
                 Plating rate/ 
               
               
                 No. 
                 MTO 
                 tration/ppm 
                 deposited layer/wt.-% 
                 μm/h 
               
               
                   
               
               
                 1 
                 1 
                 0.9 
                 7.0 
                 11.8 
               
               
                 2 
                 2 
                 1.2 
                 7.7 
                 11.9 
               
               
                 3 
                 3 
                 1.3 
                 8.3 
                 11.0 
               
               
                   
               
             
          
         
       
     
         [0058]    The phosphorous concentration in the deposited nickel phosphorous alloys increases from 7.0 wt. % at 1 MTO to 8.3 wt. % at 3 MTO. 
       Example 2 (comparative) 
       [0059]    The same plating bath as described for example 1 was used. Instead of lead ions, 0.6 g/l of a stabilizing agent according to formula (1) with R 1 =hydrogen, R 2 =sodium and n=3 (600 ppm, as disclosed in U.S. 2006/0264043 A1) was added to said plating bath. The pH value of this plating bath composition was adjusted with an aqueous solution of ammonia to 9.25. The plating bath temperature was held at 85° C. in presence of the substrate. 
         [0060]    No nickel phosphorous alloy layer was deposited onto the substrate. Accordingly, the plating rate of this plating bath is 0 μm/h. 
       Example 3 (comparative) 
       [0061]    The same plating bath as described for example 1 was used. Instead of lead ions, 0.6 g/l of a stabilizing agent according to formula (1) with R 1 =hydrogen, R 2 =sodium and n=3 (600 ppm, as disclosed in U.S. 2006/0264043 A1) was added to said plating bath. The pH value of this plating bath composition was adjusted with an aqueous solution of ammonia to 4.8. The plating bath temperature was held at 85 ° C. in presence of the substrate. 
         [0062]    No nickel phosphorous alloy layer was deposited onto the substrate. Accordingly, the plating rate of this plating bath is 0 μm/h. 
       Example 4 (comparative) 
       [0063]    The same plating bath as described for example 1 was used. Instead of lead ions, 0.6 g/l of thiourea (600 ppm, as disclosed in U.S. 2006/0264043 A1) were added to the plating bath as stabilizing agent. The pH value of this plating bath composition was adjusted with an aqueous solution of ammonia to 9.25. The plating bath temperature was held at 85° C. in presence of the substrate. 
         [0064]    No nickel phosphorous alloy layer was deposited onto the substrate. Accordingly, the plating rate of this plating bath is 0 μm/h. 
       Example 5 (comparative) 
       [0065]    The same plating bath as described for example 1 was used. Instead of lead ions, 0.6 g/l of thiourea (600 ppm, as disclosed in U.S. 2006/0264043 A1) were added to the plating bath as stabilizing agent. The pH value of this plating bath composition was adjusted with an aqueous solution of ammonia to 4.8. The plating bath temperature was held at 85° C. in presence of the substrate. 
         [0066]    No nickel phosphorous alloy layer was deposited onto the substrate. Accordingly, the plating rate of this plating bath is 0 μm/h. 
       Example 6 (present invention) 
       [0067]    The same plating bath as described for example 1 was used. Instead of lead ions, a stabilizing agent according to formula (1) with R 1 =hydrogen, R 2 =sodium and n=3 was added to said plating bath. The pH value of this plating bath was 4.8. The plating results are summarized in Table 2. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 plating results obtained for an electroless nickel plating  
               
               
                 bath known from prior art (MTO = metal turn over). 
               
             
          
           
               
                   
                   
                   
                 P concentration  
                 Plating 
               
               
                 Sample 
                   
                 Stabilising agent 
                 in deposited  
                 rate/ 
               
               
                 No. 
                 MTO 
                 concentration/ppm 
                 layer/wt.-% 
                 μm/h 
               
               
                   
               
               
                 1 
                 1 
                 5 
                 8.2 
                 12.3 
               
               
                 2 
                 2 
                 5 
                 7.9 
                 12.3 
               
               
                 3 
                 3 
                 5 
                 8.2 
                 11.2 
               
               
                   
               
             
          
         
       
     
         [0068]    The stabilizing agent according to the present invention shows the desired properties in respect to the stable phosphorous concentration in the deposited nickel phosphorous alloy layers when using the plating bath. Furthermore, the plating rate is sufficient for industrial application.