Coating processes – Measuring – testing – or indicating
Reexamination Certificate
2001-08-31
2002-12-31
Barr, Michael (Department: 1762)
Coating processes
Measuring, testing, or indicating
C427S438000, C427S443100
Reexamination Certificate
active
06500482
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a novel composition and process for its use in electrolessly plating nickel deposits. The composition and process all for the electroless plating of nickel without the use of ammonium hydroxide, which has objectionable odor, complicates water waste treatment and is considered an air pollutant.
BACKGROUND OF THE INVENTION
Methods for the electroless deposition of metals are now widely known and used in industry to deposit a variety of metals, including nickel onto various substrates. In general, electroless deposition compositions contain a salt of the metal to be deposited, a reducing agent capable of reducing metal ions to the metal in the presence of a catalytic surface, a chelating agent to maintain the metal in solution, and a pH-adjusting agent. Other substances such as stabilizers, brighteners, surfactants and other similar additives may also be present.
Electroless nickel plating solutions are probably the most widely used electroless plating solutions. These plating solutions are a delicate blend of several ingredients, each performing specific functions. They generally contain nickel salt such as nickel chloride, nickel carbonate and/or nickel sulfate. In addition they can be chelated with a variety of organic acids and chelators. The most widely used electroless nickel baths in industry today utilize the hypophosphate ion as a reducing agent and use aqueous ammonia to adjust the pH of the solution. These plating solutions can also employ various stabilizers, buffers, and surfactants. Electroless nickel plating commercially occurs at temperatures ranging from about 175° to 195° F.
Electroless nickel compositions of this type are generally replenishable in that while they are used to plate nickel, the nickel, chelator, reducing agent and other components may be added back to the bath in concentrated form to replace the constituents used in plating. In this manner, the bath is maintained in peak condition for continuous or repeated use for many metal turnovers. One metal turnover is reached when metal is plated out of the bath in an amount equal to the initial, starting metal content in the bath.
As plating continues however, the pH of the solution will drop and this will need to be monitored and adjusted higher to keep the bath in optimum condition for plating. The pH of the solution naturally drops during plating because the plating reaction produces hydrogen in both gaseous and ionic form. This production of hydrogen obviously continues to acidify the solution as plating proceeds. Generally, pH maintenance is accomplished with the addition of aqueous ammonia and pH is controlled within the range of about 4-7. In addition to the use of ammonia, pH is also controlled to some extent by buffers in the plating solution.
Historically, alkali stronger than ammonium hydroxide, such as alkali metal hydroxides, have not been useful in controlling the pH of electroless nickel solutions of this type, since they have generally been found to adversely affect the stability of the plating solution and/or the quality of the deposit. In some cases they have been found to cause catastrophic solution failure. In part, these difficulties are believed to be due in part to the fact that alkali metal hydroxides lack the ability to chelate nickel ions and that they may cause sharp changes in pH on a localized basis. However, despite these difficulties, efforts continue to develop an electroless nickel hypophosphite plating process which can effectively use alkali metal hydroxides to adjust and maintain pH. It is believed that use of alkali metal hydroxides in this manner would be advantageous since the plating solution would be easier to waste treat than those containing ammonia, and since the concentrated nature of the alkali metal hydroxides can also provide advantages.
Thus, it is an object of this invention to provide a process for electrolessly plating nickel which process utilizes, at least in part, alkali metal hydroxides to adjust and maintain the pH of the plating solution.
SUMMARY OF THE INVENTION
The inventors herein propose a process for plating electroless nickel from a nickel hypophosphite plating solution wherein a portion of the plating solution is continuously or regularly removed from the plating tank (i.e. the tank where plating occurs), cooled to below about 140° F. and placed in a container separate from the plating tank. While in the separate container, the removed portion of the plating solution is mixed, the pH is measured, and alkali metal hydroxide is added with mixing in order to adjust the pH of the removed portion of the plating solution to within the optimum range. The removed portion of the plating solution is preferably then filtered and then it is returned to the plating tank. Replenishment of other materials such as nickel salts, chelators, reducing agent and other additives may occur in the separate container or in the plating tank, however, alkali metal hydroxides are preferably only added in the separate container, with mixing, and after the removed portion of the plating solution is cooled to less than about 140° F.
REFERENCES:
patent: 4692346 (1987-09-01), McBride et al.
patent: 4780342 (1988-10-01), LeBlanc, Jr.
patent: 5112392 (1992-05-01), Anderson et al.
patent: 5609767 (1997-03-01), Eisenmann
patent: 5635253 (1997-06-01), Canaperi et al.
Barr Michael
Carmody & Torrance LLP
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