Method and apparatus for electroless plating

Description

[0001] 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.

[0002] 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.

[0003] 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.

[0004] It is known to apply some electric potential in the apparatus to prevent plating on the apparatus. This potential is applied by including a cathode in the plating solution and by making the apparatus that contains this solution the anode. The current density on the anode is around 0.05 A/dm² and this is sufficient to prevent plating on the walls of the apparatus. However, the cathode that is included in the solution has a much smaller surface area and therefore a much higher current density, for instance around 30 A/dm². The cathode is therefore liable to be electroplated in accordance with Faraday's laws and the chemical content of the solution is such that the plating is liable to fracture and fall off the cathode into the solution. To prevent the solution being contaminated by fractured metal deposits it is normal to include the cathode in a preforated plastic tube that collects and holds any fractured metal plate but which permits electrical contact between the cathode and the plating solution.

[0005] It is known that when a fresh article is immersed in a fresh electroless plating solution the speed of coating is greater than at any other time. At a time corresponding approximately to the formation of a mono-molecular coating the speed of coating decreases sharply. As the solution becomes aged by use, the speed of coating decreases further.

[0006] It is sometimes desirable to include 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.

[0007] An electroless plating process according to the invention 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.

[0008] 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.

[0009] For instance, in electroplating, increasing the potential difference increases the pH in the solution and there is no significant change in the pH at the time plating starts. In electroless plating, as the potential difference between the article to be plated and the solution is increased by altering the chemical content of the bath, the pH increases until the point at which plating starts, whereupon the pH drops. In the process of the invention, the pH drops when plating starts.

[0010] In 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.

[0011] In electroplating 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. In electroless plating the coating is of uniform thickness, irrespective of the radius of curvature of the surface. In the invention the coating is of uniform thickness.

[0012] 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. In particular, 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/dm² and is generally below 0.1 A/dm². If it is too high then electroplating may occur, with its associated disadvantages of non-uniform coating thickness. If the current density is too low, then 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/dm². Best results are generally obtained with current densities on the cathodic articles of from 0.01 to 0.05 A/d_m2.

[0013] 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.

[0014] The plating metal is preferably cobalt or nickel, generally nickel, but other metals that can be plated include tin, molybdenum and tungsten. In referring to the plating metal being, for instance 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.

[0015] The metal is generally present as salt and this and the other components in a plating solution may be conventional for electroless plating solutions. For instance, 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 BH₄. 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.

[0016] 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.

[0017] Various 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.

[0018] If these organic acids alone do not provide an appropriate buffering action then additional buffer, preferably 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.

[0019] "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. In particular it appears very desirable to include propionic acid, often also with acetic and boric acid, and generally in combination with one or more hydroxy acids.

[0020] 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. Thus, 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. As a result of being able to achieve a good coating at temperatures lower than has normally been required it is possible to achieve good results using solutions that would be unstable at the temperatures that are normally required for coating from them. The preferred temperature is 70 to 85°C.

[0021] 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. In particular, 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.

[0022] It is known, for instance from US Patent Specification 2,762,723, that it is desirable to include in certain electroless plating baths, and in particular nickel plating baths containing hypophosphite, a sulphide ion controller in an amount of up to 200 parts per million. Suitable materials are exemplified in that patent. Particularly preferred materials are mercapto compounds, including mercapto succinic acid, mercapto mercuric acid, glycine and mercapto benzothiazole. In the invention we prefer to include such a compound, generally in amounts of 0.01 to 100 preferably 0.05 to 5, parts per million.

[0023] The solution can include metallic or non metallic particles such as polytetrafluoroethylene, diamond, alumina, chromium, or tungsten carbide particles. As a result of applying the potential difference to the article, but not to the particles, it can be possible to cause preferential coating on the articles so that the particles can be substantially uncoated (or, at least, coated less than in a corresponding process without the applied potential difference) at the time when they are carried uncoated, into the metal plate formed by the electroless plating solution. The particles can therefore serve their intended purpose better than in conventional electroless plating processes, since these result in substantial the particles with metal before they are incorporated into the coating. The particles may be present as, for instance, abrasives, or friction reducers.

[0024] 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.

[0025] The following is an example of the invention.

[0026] 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 µm per hour but after 10 minutes the rate of coating falls to 7 µm per hour. It decreases gradually after that. When the stainless steel vessel is made the anode and the article is made a cathode and a potential difference is applied between the cathode and anode of about 0.1 volts generating a current density on the article of about 0.03 A/dm² the coating rate is initially 15 µm per hour and even after 2 hours it is still 12 µm per hour.

Claims

1. An electroless plating process in which an article is plated while immersed in a plating solution containing a plating metal and a reducing agent and an electrical potential difference is applied during the process, characterised in that the potential difference is applied between the article, as cathode, and an anode in contact with the plating solution.

2. A process according to claim 1 in which the current density on the article is below 1, preferably 0.005 to 0.1, A/dm².

3. A process according to claim 1 in which the current density on the article is from 0.01 to 0.05 A/dm².

4. A process according to any preceding claim conducted in a vessel containing the plating solution and in which the vessel serves as the anode.

5. A process according to any preceding claim in which the solution contains nickel in an amount of from 1 to 10, preferably 2 to 8, g/l, hypophosphite in an amount of 5 to 70, preferably 30 to 50, g/1 and in which the solution is buffered to a pH of 3 to 6, preferably 4 to 5.

6. A process according to claim 5 in which the solution includes organic acid selected from aliphatic monocarboxylic acids, hydroxycarboxylic acids and dicarboxylic acids, preferably selected from acetic acid, propionic acid, citric acid, lactic acid, glycolic acid and succinic acid.

7. A process according to claim 6 in which the solution includes propionic acid.

8. A process according to any preceding claim in which the solution includes boric acid as a buffer.

9. A process according to any preceding claim in which the solution contains 0.01 to 200, preferably 0.05 to 5, ppm of a sulphide controller, preferably from mercapto compounds such as mercapto succinic acid, mercapto mercuri acid, glycine and mercapto benzothiazole.

10. A process according to any preceding claim conducted at a temperature of 50 to 90°C, preferably 70 to 85°C.