Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/52—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating using reducing agents for coating with metallic material not provided for in a single one of groups C23C18/32 - C23C18/50
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/31—Coating with metals
  • C23C18/32—Coating with nickel, cobalt or mixtures thereof with phosphorus or boron
  • C23C18/34—Coating with nickel, cobalt or mixtures thereof with phosphorus or boron using reducing agents
  • C23C18/36—Coating with nickel, cobalt or mixtures thereof with phosphorus or boron using reducing agents using hypophosphites

Definitions

• Metals especially nickel, cobalt and copper, can be plated onto an article either by electroplating or by electroless plating.
• This invention relates to electroless plating.
• the article In electroplating the article is immersed in a solution containing a dissolved compound of the metal and an electric potential is applied sufficient to cause electrolysis and deposition, in accordance with Faraday's laws, of the metal onto the article, the article serving as the cathode.
• the thickness of the eating tends to vary according to the radius of curvature of the surface. Increasing the potential difference tends to increase the pH value of the solution and the rate of plating.
• the article In electroless plating the article is immersed in a solution of a salt of the plating metal and a reducing agent and coating occurs primarily as a result of chemical reduction of the metal salt.
• particulate material in the metal coating that is deposited on the immersed article. It is normally desirable that this particulate material is uncoated at the time that it is incorporated in the metal coating on the immersed articles. However, if the chemicals in the solution provide a coating on the immersed articles, they are liable to coat the particulate material as well. Because of its particulate nature it is impossible to protect it by making it an anode in the way that the apparatus can be protected, as described above.
• An electroless plating process is one in which an article is plated while immersed in a plating solution containing a plating metal and a reducing agent and in which the rate of plating is increased or maintained by electrically applying a potential different between the article, as cathode, and an anode in electrical contact with the plating solution.
• the process is an electroless plating process, even though a potential difference is applied electrically with the article as a cathode, because the coating is not deposited in accordance with Faraday's laws and the mechanism, process conditions and results of the coating are all similar to those that are characteristic of electroless plating and are very different from those that are characteristic of electroplating.
• the amount of coating deposited is in accordance with Faraday's laws and it is possible to calculate the theoretical maximum weight of coating that can be obtained, having regard to the content of the solution and the potential difference. In the invention it is easily possible to achieve a coating weight very much greater than the maximum weight obtainable in accordance with Faraday's laws.
• the thickness of the coating varies according to the radius of curvature of the surface with the most highly curved parts carrying the thickest coating.
• the coating is of uniform thickness, irrespective of the radius of curvature of the surface.
• the coating is of uniform thickness.
• the potential difference that has to be applied electrically in the invention between the article, as the cathode, and an anode in electrical contact with the plating solution is generally very much lower than the potential difference required for causing electroplating.
• the current density on the cathodic article is generally very much lower than the amount required for electroplating with a similar solution.
• the current density on the cathodic article that is being plated is preferably below 1 A/dm 2 and is generally below 0.1 A/dm 2 . If it is too high then electroplating may occur, with its associated disadvantages of non-uniform coating thickness.
• the process may not provide any significant advantage compared to conventional electroless plating process without applied potential difference, and so generally the current density should be at least 0.005 A/dm 2 . Best results are generally obtained with current densities on the cathodic articles of from 0.01 to 0.05 A/d m2.
• the anode must be in electrical contact with the plating solution. It can be provided by one or more electrodes immersed in the solution, for instance in porous tubes, as is conventional with cathodes in prior electroless plating solutions, but it is generally preferred that the apparatus containing the solution should serve as the anode.
• the current density on the anode is then generally between 0.1 and 10 times, preferably 0.3 to 3 times the current density at the cathode.
• the plating metal is preferably cobalt or nickel, generally nickel, but other metals that can be plated include tin, molybdenum and tungsten.
• nickel we mean that it is either nickel alone or is a mixture of nickel with a metal that forms an alloy in the plating with it, for instance, chromium or tungsten.
• the metal is generally present as salt and this and the other components in a plating solution may be conventional for electroless plating solutions.
• the reducing agent may be a reducing agent that is conventional for the particular plating metal, for instance, formaldehyde for copper, or hydrazine or a boron based reducing agent or hypophosphite for nickel or cobalt. Suitable boron based reducing agents include dimethyl or diethyl amine borane and BH 4 .
• the preferred plating solution is a nickel plating solution containing a nickel salt and hypophosphite as the reducing agent.
• the pH of the solution is generally acidic and in the range 3 to 6, preferably 4 to 5, but alkaline solutions having a pH of 7 to 10, especially around 8 or 9, are usable.
• the solution typically contains nickel or other plating metal in an amount of 1 to 10, preferably 2 to 8 g/1 and hypophosphite or other reducing agent in an amount of 5 to 70, preferably 30 to 50, g/l, the reducing agent typically being present in an amount of from 4 to 8 times the amount of plating metal and 10 to 90, preferably 40 to 70, g/1 of one or more organic acids.
• organic acids are known for use in electroless plating solutions for the purpose of stabilising the solution and can be used in the invention. They are generally aliphatic carboxylic acids. Suitable monocarboxylic acids are acetic and propionic acids and typically are present in amounts up to 30, preferably 5 to 20, g/l in total. The use of propionic acid, for instance in amounts of 2 to 20, preferably 2 to 10, g/1 is particularly preferred, especially in combination with acetic acid which typically is present in amounts of 5 to 20, preferably 5 to 15 g/l. Suitable hydroxy substituted aliphatic acids include lactic acid, glycolic acid and citric acid, typically in amounts of from 20 to 60, most preferably 25 to 40, g/l. If citric acid is present the amount is generally from 2 to 15 g/l. Suitable dicarboxylic acids include succinic acid, typically in an amount of 2 to 10 g/1.
• additional buffer preferably boric acid
• boric acid may be introduced in an amount to buffer the pH to the chosen value, typically around 4.5.
• Suitable amounts of boric acid for this purpose are generally from 2 to 15 g/l.
• the choice of the organic acids and buffering agent can affect the results obtainable when applying a potential difference in accordance with the invention in that better results are obtainable with some combinations of acids than with others.
• the application of the potential difference can have the advantage, under unchanged temperature and other process conditions, of causing a faster coating rate.
• the process is generally conducted at an elevated temperature in the range 50 to 90°C.
• a particular advantage of the process is that it can be possible to achieve at a lower temperature with the applied potential difference the same coating rate as is obtainable at a higher temperature without the applied potential difference.
• a plating solution that requires an optimum coating temperature of around 90°C may, in the invention, give equivalent results at a temperature of around 80 or 82°C.
• the preferred temperature is 70 to 85°C.
• the solution will normally contain stabilisers and other conventional additives and it is a further advantage of the invention that it can be possible to obtain good coating from a solution that has been stabilised to such an extent that it would not normally be capable of electroless plating.
• iT can be possible to obtain good coating from a solution that has deteriorated, for instance as a result of prolonged use and reuse with replenishment, to such an extent that it would be incapable of giving adequate coating in the absence of the applied potential difference.
• the solution can include metallic or non metallic particles such as polytetrafluoroethylene, diamond, alumina, chromium, or tungsten carbide particles.
• metallic or non metallic particles such as polytetrafluoroethylene, diamond, alumina, chromium, or tungsten carbide particles.
• the solution may be replenished (for instance for nickel) during use in conventional manner or it may be replenished by the method described in our European application 83304214.6.
• a conventional electroless nickel plating solution is formulated from 5 g/1 nickel (introduced as sulphate), 40 g/1 hypophosphite, 3 ppm mercapto benzothiazole, 30 g/1 lactic acid, 4 g/1 succinic acid, 10 g/1 acetic acid, 4 g/1 propionic acid, 6 g/1 citric acid and 6 g/1 boric acid.
• the solution is held at about 80°C in a stainless steel tank. When fresh articles of steel are immersed in the solution coating is formed on them at a rate of about 10 ⁇ m per hour but after 10 minutes the rate of coating falls to 7 ⁇ m per hour. It decreases gradually after that.
• the coating rate is initially 15 ⁇ m per hour and even after 2 hours it is still 12 ⁇ m per hour.

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• Chemical & Material Sciences (AREA)
• General Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemically Coating (AREA)

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• 1983
  • 1983-09-08 GB GB838324060A patent/GB8324060D0/en active Pending
• 1984
  • 1984-09-06 EP EP84306103A patent/EP0141507A3/de not_active Withdrawn

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