This invention relates to electroless cobalt plating.

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Electroless deposition in general is a known technique. It is a process whereby metal ions in solution are reduced to metal atoms on a surface which is to be plated. Electroless plating is distinguished from electroplating, in which the reducing power is supplied by electricity, by the provision of a chemical reducing agent.

Electroless- cobalt deposition is itself known. Pearlstein and Weightman (J. Electrochem. Soc. (1974) 1023-1028) have described the electroless deposition of cobalt from acid baths. They reported that the coercivity of certain electroless cobalt-boron deposits was low, and electroless cobalt has been used in the preparation of magnetic memory surfaces.

Although nickel is perhaps the most common metal deposited by electroless techniques, cobalt does have certain advantages: for example, electroless cobalt depositions have been able to withstand higher temperatures than corresponding nickel depositions, which is advantageous in some applications.

Because, in an electroless plating bath, the reducing agent is present at the same time as the metal ion to be reduced, the bath is thermodynamically unstable. As a practical matter, therefore, the metal ions which are to be deposited are usually complexed so as to prevent their reduction to metal in the bulk of the solution;

but the metal ions should not be so strongly complexed that the deposition of the metal onto the substrate to be plated will be prevented. The surface of the substrate will usually be to some degree catalytic (either inherently or by being rendered catalytic) for the reduction of the metal ion to the metal.

Thallium has previously been reported as a stabiliser for certain electroless nickel deposition systems. Unfortunately, thallium does not seem to be particularly effective for cobalt, and in any case it is extremely toxic. There is therefore a need for a means of stabilising baths for producing electroless cobalt deposits.

It has now been discovered that, in appropriate amounts, cadmium can act as a stabiliser for such baths. It also appears that cadmium can act as a brightener and increases bath life. Cadmium can also act as a grain refiner.

According to a first aspect of the present invention, there is provided an electroless cobalt plating composition comprising a source of cobalt ions, a reducing agent and cadmium ions in an amount effective to stabilise the composition but insufficient substantially to prevent cobalt deposition on a substrate.

For the source of cobalt, any convenient' and sufficiently soluble salt of cobalt may be used. Cobalt sulphate or cobalt chloride is generally found to be convenient. The source of cobalt may be present

in an amount of from 1 to 100 g/1, for example 10 to 45 g/1, typically about 30 g/1, calculated as cobalt sulphate hexahydrate.

The cobalt deposit may contain a minor amount, for example up to 15, 20, 25 or 30%, of an alloying metal such as nickel. In such a case, the amount of cobalt in the plating composition may be (but is not necessarily) slightly less than the middle of the broad and preferred ranges given above. A source of alloying metal such as nickel may be present to provide metal ions in an amount of from 0.1 to 3 g/1, preferably 0.5 to 1.5 g/1; these amounts could be realised by providing, for example, 0.45 to 13.5 g/1, and preferably 2.25 to 6.75 g/1, nickel sulphate hexahydrate. <

As a practical matter, a complexor will u-sually be present to maintain the cobalt and/or alloying metal in solution under the appropriate operating conditions. Any suitable complexor may be used, but carboxylic acids (for example C-_-Cg carboxylic acids) , particularly dicarboxylic acids (for example C ₂ -C ₁₀ carboxylic acids) may be advantageous. Succirtic acid has been found to be the most appropriate, but other suitable acids include acetic, glycollic, oxalic, glutaric and adipic acids. The complexor may be present in an amount of from 1 to 100 g/1, for 'example 5 to 4.0 g/1, typically about 25 g/1. It will be appreciated that both the source of cobalt and the complexor may be provided as a single source, for example cobalt succinate.

(References in this specification to weak (eg organic) acids are to be taken to include their salts, and vice versa, as the exact nature of the species present will depend on the pH.)

Any appropriate reducing agent may be used. Hitherto, dimethylamino borane (DMAB) has been found to be the most appropriate. It appears that the use of such a reducing actually yields electroless deposits of cobalt-boron, much in the way as the use of hypophosphite in electroless nickel plating produces nickel-phosphorus. It is to be understood that in this specification the word "cobalt", when referring to the deposited metal includes such entities as cobalt-boron alloys, cobalt phosphorus alloys and compositions of cobalt, boron and phosphorus.

Boron hydrides and their derivatives in general and alkyl amino boranes (including DMAB) in particular are suitable reducing agents. Mixtures of more than one reducing agent (such as DMAB and hypophosphite) can be used.

The reducing agent may be present in an amount of from 0.5 to 20 g/1, for example from 2 to 6 g/1, typically about 4 g/1.

The amount of cadmium that is to be present is to be chosen.so as to be sufficient to stabilise the bath, but insufficient substantially to prevent the cobalt being deposited on the substrate. Too high a concentration of cadmium is believed to poison the catalytic qualities of the substrate. The amount of

cadmium that is suitable for use in the invention may vary depending on the bath conditions, but will generally be in the order of less than 15 ppm, for example from 0.1 to 8 ppm, typically from 0.5 to 7 ppm, and preferably about 6 ppm. Appropriate amounts can be determined by simple experimentation within the general teaching of this specification.

The cadmium may be added in ionic form or covalently bound in a compound or as the free metal, in which case it will generally dissolve to form a salt. It is preferably supplied as a soluble salt, such as cadmium acetate. The amount of cadmium present is calculated by reference to the metal.

The plating composition can contain a further stabiliser and/or grain refiner. An example is lead. The use of lead in electroless cobalt plating baths is the subject of another application filed today „by the same applicants as are named in this application and claiming the same two priorities; where the law allows, the other application is herein incorporated by reference. The amount of lead that is to be present may be chosen so as to be sufficient, in combination with other bath components, to refine the structure of the deposit produced and/or to stabilise the composition (particularly when depositing composites) , but insufficient substantially to prevent the cobalt being deposited on the substrate. Too high a concentration of lead is believed to poison the catalytic qualities of the substrate. The amount of lead that is suitable for use in the invention may vary depending on the composition conditions, but will generally be in the

order of from 0.5 to 40 ppm, for example about 1 to 20 ppm. Appropriate amounts can be determined by simple experimentation within the general teaching of this specification. The lead may be provided simply by adding a source of lead ions to the composition (for example in an amount of from 1 to 10 ppm, typically 3 to 5 ppm) and/or it may be provided, in the case of composite plating compositions, by pre-treating additive particles (when present) with lead and adding the so-treated particles to the plating composition. Additive particles are discussed further below. This pre-treatment technique is preferred when composites are to be deposited, as it tends to ensure that there is a higher concentration of lead on the surfaces of the additive particles rather than on the surface which is intended to bear the deposited plate, thereby effectively directing plating onto the desired surface. It is not, however, essential, as the lead in the plating composition may preferentially adsorb onto the additive particles, particularly if the additive particles are angular rather than smooth. As an alternative to lead, additive particles may be pre-treated with cadmium, tin, zinc, mercury, thallium, bismuth and/or antimony.

The plating composition may contain an accelerator. Accelerators act to improve the plating rate. Sulphur-containing compounds in general are good accelerators, although their use in excess can be marked by a deterioration in surface quality. Some sulphur-containing compounds such as thiourea are initially effective but prone to hydrolysis. Preferred accelerators are therefore sulphur-containing compounds

which are generally stable to hydrolysis. An example of such a compound is mercaptobenzothiazole (MBT) which may be used in an amount of from 0.1 to 20 ppm, for example 1 to 5 ppm and typically about 2 ppm.

The composition may contain a moderator. Lactic acid has been found to have an effect on the overall quality of the plating operation, for example by enhancing stability and improving the structure of the deposit. Lactic acid is known for use in nickel-hypophosphite electroless plating compositions, where it has the effect of removing an unwanted by-product. Here it is having a different effect. The lactic acid may be present in the composition in an amount of from 0.1 to 30 ml/1, for example 1 to 10 ml/1, typically about 5 ml/1.

The composition may contain a wetting agent, for example to reduce surface tension as an aid to the release of gas bubbles. A wide range of surfactants are suitable, particularly nonionic and anionic surfactants. Ethoxylated alkyl phenols, exemplified by those sold under the trade mark TRITON XT 100, are a

_• suitable class of nonionic surfactants, and sulphosuccinates, such as those sold under the trade mark LUTENSIT ABP are a suitable class of anionic surfactants. The wetting agent may be present in an amount of from 1 to 75 ppm for anionic surfactants or from 1 to 40 ppm for nonionic surfactants. Amounts in the range of from 1 to 5 ppm are typically used.

As mentioned above, the composition may contain additive particles for forming a composite coating. The additive particles may be of any suitable material. Additive particles which may be used include graphite, polyfluorocarbons such as polytetrafluoroethylene (ptfe) , carbides, oxides, borides and nitrides; even non-catalytic metallic particles such as molybdenum can be co-deposited to provide another family of composites. Silicon carbide is a particularly preferred additive particle because it is available in purified form and a wide range of particle sizes, is reasonable in cost and has been found to be excellent in performance. Chromium carbide and chromium oxide particles are the additive particles of choice in the aeronautical industry. The particles may range from 0.5 to 50 microns in size, for example 1 to 5 microns and typically about 2 microns. The additive particles may be present in the electroless deposition composition in an amount of from 0.1 to 50 g/1, typically 0.5 to 10 g/1, for example about 2 g/1.

The pH of the plating composition will generally be on the acid side, but if the pH is too low, the plating rate is reduced. It is therefore preferred for the bath pH to be in the range of from 4.5 to 6.5, for example about 4.8 to 5.5.

According to a second aspect of the invention, there is provided a process for the electroless deposition of cobalt onto a substrate, the process comprising contacting the substrate with a composition comprising a source of cobalt ions, a reducing agent and cadmium

ions in an amount effective to stabilise the composition but insufficient substantially to prevent cobalt deposition on the substrate.

The temperature of the composition in use may be adjusted as desired. A higher temperature results in a faster plating rate, but there are corresponding sacrifices in the energy consumption of the system. It is generally preferred for the temperature to be in the range of from 60 to 90°C, for example from 70 to 85°C.

The method of contacting the surface of the substrate to be plated with the composition will usually be immersion in a bath of the composition. However, it should be understood that immersion is not necessarily the only suitable method. It may, for example, be possible to develop a suitable spray-plating method.

The time of contact between the substrate and the plating composition will naturally depend on the thickness of the deposit required and the plating rate. Contact times in the order of 5 minutes to 5 hours may be appropriate, with contact times of from 30 minutes to 2 hours being typical.

The plating rate itself will generally be optimised to a degree that is compatible with the quality required of the finished product. Plating rates in the range of 20 to 30 microns per hour are generally achievable by means of the invention, but higher quality deposits may be achievable by more modest plating rates. Plating

rates in the range of from 10 or 15 to 20 microns per hour, for example about 17 microns per hour, have been found to be optimal.

In order to enhance the evenness of the deposit, it is usual to agitate the composition during plating. Agitation may be achieved by any suitable means such as mechanical stirring, pump agitation, magnetic stirring and air agitation.

The plating may take place on any suitable substrate. Steel and other metals are preferred substrates. It is not essential that any pre-treatment of the substrate take place, although for non-conductive substrates an activator such as palladium may be deposited. Cleaning and desmutting operations may be appropriate, as will be known to those skilled in the art. When working with steel, a thin electroless nickel deposit may be used with advantage as an undercoat. Any suitable electroless nickel plating composition may be used, such as that supplied by Lea Manufacturing Co Ltd of Buxton, England under the trade mark ALLIED KELITE 794. Plating may take place for as little as 5 minutes to produce a very thin substantially pure nickel layer.

Preferred features of all the aspects of the invention are as for the first aspect, mutatis mutandis.

The invention will now be illustrated by the following examples.

11

Example 1

A cobalt electroless plating bath (3 litres) of the following composition was prepared:

The solution remained stable during heat up. A small test coupon of steel (38 cm ² ) was plated in 200 ml for 90 minutes, during which time the weight gain was 3.9482 g and the plating rate was 12.97 microns/hour.

The bath was replenished by the addition of CoS0 ₄ (20 g) and DMAB (4 g) and plating continued for a further 90 minutes. The replenishment and plating procedures were repeated a further four times, so that the coupon had been plated for a total of 9 hours: the bath was still stable. New test coupons were then electrocleaned at 2 V for 10 minutes, after which they were subjected to two further 90 minute plating periods in the same bath as previously, with prior replenishment of cobalt and reducing- agent as before. Again, bath stability was maintained throughout.

Example 2

A cobalt electroless plating bath of the following composition was prepared:

The solution remained stable during heat up. A small test coupon of steel (38 cm ) was plated in 200 ml for 30 minutes, during which time the weight gain was 0.3798 g and the plating rate was 22.79 microns/hour. The bath remained stable throughout.

Example 3

The procedure of Example 2 was repeated except that the cadmium was present at 4 ppm. The weight gain was 0.3695 g and the plating rate was 22.17 microns/hour. The bath remained stable throughout.

Example 4

The pr.ocedure of Example 2 was repeated except that the cadmium was present at 6 ppm. The weight gain was 0.3543 g and the plating rate was 21.26 microns/hour. The bath remained stable throughout.

Example 5

This example, like the others, was carried out in a 3 litre tall form beaker at 70°C on a stirrer hotplate with digital thermocouple temperature control.

Example 1 was repeated except that the bath contained 6g/l cobalt (as 26.7 g/1 CoS0 ₄ .6H ₂ 0) and lg/1 nickel (as 4.46 g/1 NiS0 ₄ .6H ₂ 0) . This bath produces cobalt coatings containing approximately 10% by weight of nickel. The presence of cadmium to the plating bath causes the surface of the deposit to appear laminar, when viewed under a metallurgical microscope (magnification 200X) . This contrasts with a nodular appearance observed if cadmium is not present.

Example 6

This is a comparison example, carried out in 3 litre tall form beaker at 70°C on a stirrer hotplate with digital thermocouple temperature control.

An electroless cobalt solution was prepared as in Example 1 but containing 6g/l cobalt (as cobalt sulphate hexahydrate) no added cadmium and 2ppm lead. Test coupons were plated with additions made of cobalt sulphate and dimethylamine borane only to maintain the bath chemistry. When 18g of cobalt metal had been deposited and the additions made, plating out occurred on the bottom of the beaker. The solution was filtered and reheated to operating temperature whereupon decomposition began to occur. Filtration was again carried out and the solution regenerated by the

addition of cobalt sulphate and dimethylamine borane. The lead in the bath was replenished by the addition of 6mg lead to give a working concentration of 2ppm. The bath was reheated to 70°C and plating resumed. No sign of solution decomposition was evident until a further 18g metal had been deposited.

The deposits produced from the above experiments, although smooth, had a matt appearance. When viewed under a metallurgical microscope (magnification 200X) the surface of the coating exhibited a nodular structure.

Example 7

The procedure of Example 6 was repeated, except that in order to refine the deposit structure, 2ppm cadmium (as cadmium acetate) was added to the plating bath. No deleterious affect on plating rate was noticed. The deposit produced from this solution had a high degree of reflectivity. When viewed under the microscope (magnification 200X) , the surface structure appeared to be laminar as opposed to nodular when no cadmium is present in the solution.