Method of Treating Electroless Nickel Plating Bath

This invention relates to processes for regenerating solutions used in electroless chemical nickel plating, and more particularly to processes for precipitating out contaminants in the plating solution.

Electroless chemical nickel plating is known and has been used for a substantial period of time. The chemical nickel plating solution usually includes an aqueous solution of nickel cations and hypophosphite anions, derived from nickel sulfate, nickel chloride, and sodium hypophosphite.

The process of chemical nickel plating involves the reduction of a nickel salt to metallic nickel and the corresponding oxidation of a reducing agent, such as a hypophosphite. The material to be plated, which is a ferrous or non-ferrous material, is prepared and is immersed in an aqueous acid bath containing the nickel cations and hypophosphite anions. The reduction of the nickel cations and corresponding oxidation of the hypophosphite is believed to cause the formation of phosphite anions, which form nickel orthophosphite, a relatively insoluble material. The presence of orthophosphite solid particles in the bath results in rough deposits and slows down the rate of deposition, two undesirable effects. Because of the characteristics of the orthophosphite solid particles, they cannot be filtered or otherwise mechanically removed from the bath in a practical manner. Therefore, the orthophosphite build-up will eventually contaminate the bath so that it must be discarded. Since the cost of a nickel plating bath is considerable, extending the bath life is a desirable and important consideration.

In order to prevent early precipitation of the orthophosphites, complexing or chelating agents may be used to form nickel complexes and tie up the nickel ions in a substantially undissociable molecule. Since these chelating agents tend to slow down the deposition rate, certain anions (exaltants) are also added to the bath to increase the plating rate. One agent which acts both as a complexing agent and exaltant is sodium fluoride.

Until the present, when sodium fluoride was added to a plating bath, only small quantities were employed, in the nature of 0.01 to 0.04 moles/liter. See, e.g., U.S. Pat. Nos. 2,694,017 and 2,694,019.

The present invention involves the addition of large quantities of sodium fluoride to a standard electroless nickel plating bath. For example, the periodic addition of relatively large amounts of sodium fluoride, e.g., a minimum amount of about 150 pounds of fluoride to a 4500 gallon bath, (1) permits consistent maintenance of the control and integrity of the bath and extends the bath life for an indefinite period, (2) allows heavy deposits (e.g., 0.010 inch) to be made without roughness, and (3) reduces the concentrations of additives needed to maintain control of the bath properties. The fluoride salt is added when roughness begins to appear on the plating workpieces, when the plating rate falls below a predetermined level, or when the pH of the plating solution becomes too low. When the fluoride salt is added, a precipitate is formed which settles to the bottom of the bath tank. The bath can then be transferred from the plating tank to a storage facility, the precipitate removed from the plating tank, and a regenerated bath then returned to the plating tank.

A chemical plating solution which can be used in this invention includes:

1. An aqueous solution of nickel ions in concentrations of about 1.5 ounces per gallon, wherein the nickel ions may be obtained from nickel sulfate (NiSo.sub.4 . 6H.sub.2 O) dissolved in water;

2. sodium hypophosphite in concentrations of about 2.7 ounces per gallon or approximately 1.8 times the concentration of nickel ions;

3. sodium acetate or sodium citrate in an amount equal to about 1 to 1.5 ounces per gallon; and

4. lactic acid or glycolic acid (polyhydroxy acetic acid) in amounts sufficient to adjust the pH to about 5, which has been found to be an optimum operating point.

The plating solution will be operated within a pH range of 3.5 to 6, depending upon the amount of single nickel salts used and whether or not boric acid is added to the solution. The addition of boric acid is optional. When used in amounts of about 1 to 1.5 ounces per gallon, the boric acid may afford additional ductility to the nickel plate and may aid in pH control by acting as a buffering agent.

The plating solution is operated at temperatures of about 180.degree. to 185.degree. F. Once the plating solution is used, the pH of the solution decreases rapidly to as low as 3.5 and the rate of deposition to 0.0002 inches of nickel per hour. While in many plating operations the plating bath can operate at this rate for an extended period of time, it may not be practical to operate at this rate when heavy deposits, such as 0.008 to 0.010 inches of nickel are required.

In order to increase the plating rate of the solution to as much as 0.001-0.0015 inches per hour, the pH must be adjusted upward to 4.5 to 4.6 and maintained at that level. This can be done by adding ammonium hydroxide in the presence of lactic acid, or glycolic acid. Where the latter are not present, a precipitate or impurity will be formed and the nickel will plate on to the precipitate causing the bath to deplete rapidly and uncontrollably.

It is believed that the precipitate or impurity is a by-product of the reduction of nickel with hypophosphite called orthophosphite. When the presence of these orthophosphite particles in the bath slows down the plating rate until it becomes less than 0.0003 inches per hour, it not only becomes impractical to keep the plating solution in operation, but also the alloy of nickel and phosphorus deposited becomes too unlike the alloy deposited from a bath which is plating at a rate of 0.0004 inches - 0.0008 inches per hour. The nickel-phosphorous alloy is too hard, exhibits porosity, and is difficult, if not impossible, to polish to a pit-free, low R.M.S. finish.

To maintain the bath integrity and increase the rate of deposition, relatively large quantities of sodium fluoride are added to the plating solution when the rate of deposition falls below about 0.0002 inches of nickel per hour, and when normal adjustment of the nickel to hypophosphite fails to cause an increase in the plating rate. The sodium fluoride must be added in quantities of about 0.53 ounces per gallon. Thus, in a 4500 gallon bath, a minimum of 150 pounds of sodium fluoride must be added, and it should all be added at the same time. If all of the sodium fluoride is not added to the solution at the same time, the reaction which results in the precipitate may not be complete, and precipitation may be incomplete.

The addition of the fluoride will cause a white precipitate to form which can be filtered from the solution. The precipitate will not be plated with nickel. Since the concentration of the precipitate will increase as the temperature of the solution is increased, the temperature is raised to about 190.degree.-195.degree. F. after the fluoride is added. The solution may then be filtered at the elevated temperature and dropped back to the lower operating temperature (180.degree.-185.degree. F.) for a clear plating solution. Since the addition of sodium fluoride does not affect plating, the addition can be made while the plating solution is operating.

After the precipitate is formed and removed from the solution, there is no significant change, either in the hypophosphite concentration or in the nickel concentration, as compared to the concentrations existing prior to formation and removal of the precipitate.

The sodium fluoride may be added to the solution at a pH of 3.5 and the pH may thereafter be raised as high as 6.

It should be noted that the addition of large quantities of sodium fluoride to a new bath will not increase the plating rate.