EP1413646B2 - Process for electroless plating of metals - Google Patents

Abstract

In an electrolyte for electroless plating with nickel films with residual compressive stress, containing a nickel base salt (I), reducing agent, chelant, accelerator and stabilizer, (I) is Ni acetate in an initial concentration of 12-26 g/l. An Independent claim is also included for the process for electroless plating with this electrolyte, in which uniform Ni films are deposited at a constant high rate of deposition of not less than 7-12 microns/hour, with a throughput of not less than 15-22 MTO (metal turn-over) = 70-110 g Ni/l.

Description

This invention relates to an electrolyte for electroless deposition of residual pressure nickel layers containing a metal base salt, a reducing agent, a complexing agent, an accelerator and a stabilizer.

In addition to electrolytic processes for coating workpieces with a metal layer, so-called external current or electroless plating processes have long been known. Under electroless or even chemical metallization is a chemical surface refinement of almost all metals and many non-conductors to understand. It differs significantly in its chemical, physical and mechanical characteristics of galvanically applied metal coatings. It is advantageous, for example, that the chemical metallization takes place uniformly in the deepest boreholes and fits and, moreover, a virtually constant and contoured layer thickness is produced. These methods are particularly often used for coating non-conductive substrates, for example plastic parts, in order to make them conductive, for example by a metallic surface, and / or to give them an aesthetic appearance. Likewise, by such methods, the material properties of the substrates thus treated can be improved. Thus, depending on the method, for example, the corrosion resistance, hardness and / or wear resistance of the material used can be improved.

The electroless plating with metals is based on an autocatalytic process, so that it is also referred to as autocatalytic coating. In order to reduce the metal ions contained in the deposition bath (electrolyte) to elemental metal in such a coating process, the electrolyte must be added to a corresponding reducing agent, which is oxidized during the reaction itself. In addition, other components, such as phosphorus and / or additional metals, such as copper, etc., are often incorporated into the coating.

Thus, in the case of an electroless metal bath by the use of hypophosphite as a reducing agent metal coatings are produced with a relatively high phosphorus content. The corresponding reaction equation is as follows:

MSO ₄ + 6NaH ₂ PO ₂ → M + 2H ₂ + 2P + 4 NaH ₂ PO ₃ + Na ₂ SO ₄

Since the proportion of phosphorus has a significant influence on layer properties such as hardness and corrosion resistance, this is selectively introduced depending on the intended use of the coated article. For example, in the case of non-magnetic coatings with maximum hardness, a phosphorus content of ≥ 10% by weight is desired. In addition, such electroless deposited metal-phosphorus coatings have a higher hardness and better wear resistance than electrodeposited coatings.

However, hypophosphite baths for electroless deposition of metals tend to become unstable during deposition, as the concentration of metal and hypophosphite ion progressively decreases as the concentration of orthophosphite ion continues to increase and the counterions of the metal and hypophosphite ions increase Form of, for example, sodium sulfate. The electrolyte is thus consumed ".

The lifetime of such electroless baths is thus limited because the electrolyte can only be used for a certain number of coating runs with uniform coating results. The age of a bath is usually given in metal turn-over (MTO), where 1 MTO is equal to the amount of metal deposited from the bath. This corresponds to the originally used concentration of the metal ions, in each case based on the total volume of the bath, in the bath. In the processes currently known in the art, the degradation products in the electrolyte reach such a high concentration after about 5 to 10 MTO that a high deposition rate and a consistently high quality of the deposited metal can no longer be guaranteed. The electrolyte is then either to replace or regenerate using appropriate tools.

However, the required disposal of the spent baths as well as the required new batch of fresh baths disadvantageously leads to high costs and a significant environmental impact.

The regeneration of an electrolyte for nickel deposition means at least withdrawing the resulting orthophosphite as reaction products and optionally an addition of metal and Hypophosphitionen. In previously known processes, interfering components are separated from the bath, for example by adsorption on ion exchange resins or by electrodialytic processes. Although such methods allow a significantly longer life of the baths, but they are usually connected by the complex structure, etc. with very high operating costs.

Another, less expensive form of regenerating baths for the electroless deposition of metals is the in situ precipitation and separation of unwanted ions in the form of sparingly soluble compounds and the subsequent replenishment of required and consumed in the course of bath life time. As precipitant but usually only rare metals in question, the acquisition in turn is very expensive. In addition, the dissolved in the bath dissolved components of these additives can affect the quality of the metal coating.

Furthermore, methods are already known in which interfering precipitations of metal orthophosphite are prevented by the addition of complexing agents, and thus, by the targeted reduction of the concentration of dissolved free nickel ions, the stability of the baths can be significantly improved. Thus, in the past a wide variety of bath additives have been proposed, but all have the disadvantage that a uniform, non-porous and adherent deposition of metal-phosphorus coatings from such baths with an economically acceptable deposition rate of 7-10 microns / h and with compressive stresses at Phosphorus content of the coating of> 10% over a longer period is not possible. Usually, the life or the period of application of such baths is 7 to max. 10 MTO, using no S ^2- containing accelerators.

The invention has for its object to provide an electrolyte for electroless deposition of Nichel, from the over a long period uniform, pore and crack-free metal-phosphorus coatings with constant layer properties and high phosphorus content, at an increased deposition rate, can be deposited. It is also an object of the present invention to provide an electrolyte having high stability and durability, which contains complexing agents and stabilizers which are effective in a wide volume range and greatly contribute to increasing the deposition rate and prolonging the life of the bath. Another object of the present invention is to provide a process for the electroless deposition of nickel with compressive residual stress.

The object is achieved by means of an electrolyte according to claim 1 .

The publication DE 40 05 088 discloses a saccarin-containing nickel plating bath for electroless deposition of uniformly blackened nickel layers.

The patent US 3, 597, 267 discloses a nickel acetate-containing electrolyte for the electroless deposition of nickel at a high deposition rate.

Chen CJ et al describe in "Internal stress and adhesion of amorphous Ni-Cu-P alloy on aluminum", Thin Solid Films, No. 1-2, 1 July 2000, pages 106-113 , the electroless deposition of nickel-copper-phosphorus alloys with compressive residual stress in a nickel sulfate and saccarin-containing electrolyte.

By means of the electrolyte according to the invention, the disadvantages known in the state of the art are eliminated by providing a novel composition of the electrolyte and in this way achieving considerably better deposition conditions, thereby simplifying implementation and making it more economical. This is primarily due to the advantageous composition of the electrolyte. In particular, by the use of metal salts whose anions are volatile, preferably metal acetates as the electrolyte base salt, the life of the electrolyte at high deposition rates and uniformly deposited layers with constant layer properties can be significantly extended.

The electrolyte of the invention is basically composed of one or more metal base salts selected from the group consisting of nickel acetate, nickel formate, nickel oxalate, nickel nitrate, nickel propionate, nickel citrate and nichel ascorbate, preferably metal acetate and a reducing agent, sodium hypophosphite. Furthermore, various additives, such as complexing agents, accelerators and stabilizers, which are advantageously used in acidic electrolytes for the electroless deposition of nickel, are added to the electrolyte. Since the deposition rate is significantly higher in an acidic medium, an acid is preferably added to the electrolyte as a complexing agent. The use of carboxylic acids and / or polycarboxylic acids turns out to be particularly advantageous since, on the one hand, it determines the advantageous solubility of the metal salts and the controlled control of the free metal ions and, on the other hand, prescribes the adjustment of the pH required for the process due to their acid strength . facilitated. The pH of the electrolyte is advantageously in the range of 4.0 to 5.2. In addition, the dissolved metal is particularly advantageously complexed by the use of carboxylic acids and / or polycarboxylic acids whose salts and / or derivatives, preferably hydroxy (poly) carboxylic acids, particularly preferably 2-hydroxypropanoic acid and / or propanedioic acid. At the same time, these compounds serve as activators and as pH buffers and contribute significantly to the stability of the bath by their advantageous properties.

A sulfur-containing heterocycle is added to the electrolyte as accelerator. The sulfur-containing heterocycle used is saccharin, its salts and / or derivatives, particularly preferably sodium saccharin. In contrast to the S ^{2 -based} accelerators known and commonly used in the prior art, the addition of saccharinate, even in higher concentrations, does not adversely affect the corrosion resistance of the deposited metal layers.

Another important prerequisite for rapid and high-quality deposition of metal layers is the use of suitable compounds for stabilizing the electrolyte. For this purpose, a number of different stabilizers are known in the art. However, since the stability of the electrolyte according to the invention is significantly influenced by the use of metal salts whose anions are volatile, the acetates, formates, nitrates, oxalates, propionate, citrate and ascorbate of the metals, particularly preferably metal acetate, advantageously only small amounts of stabilizers used. This is on the one hand more economical, on the other hand precipitates etc. are avoided, which can be caused by the addition of additional substances and thus significantly shorten the life of the electrolyte. Thus, advantageously only small amounts of a stabilizer are added to the electrolyte according to the invention in order to counteract a spontaneous decomposition of the metallizing bath. These may be, for example, metals, halogen compounds and / or sulfur compounds, such as thioureas. In this case, the use of metals as stabilizers has proved to be particularly advantageous. Preference is given here to the use of lead, bismuth, zinc and / or tin, which are particularly preferably in the form of a salt whose anion contains at least one carbon atom. These salts are preferably one or more of the salts from the group consisting of acetates, formates, nitrates, oxalates, propionates, citrates and ascorbinates, more preferably acetates.

Depending on which additional properties the metal layers should have, besides phosphorus further components, such as, for example, additional metals, preferably cobalt, and / or finely dispersed particles are incorporated into the layer. In addition, the electrolyte according to the invention has smaller amounts of additional components, such as, for example, salts, preferably potassium iodide.

With regard to the above object, this is achieved by a method according to claim 10 .

By using the method according to the invention, the quality of the metallizing bath is surprisingly improved and the service life is considerably prolonged. This has the advantage that not only high deposition rates are achieved by the use of the method according to the invention, but also that the nickel layers obtained by the process are uniform and of high quality, have a very good adhesion and are consistently free of pores and cracks. In addition, the metallization of the surface is improved, especially by more complex substrates. In particular, it is advantageous that uniform nickel layers with compressive residual stresses at a consistently high deposition rate in the range of at least 7 to 14 .mu.m / h, preferably 9 to 12 .mu.m / h with a throughput of at least 14 to 22 MTO = 70 to 110 g Ni / l are deposited ,

Surprisingly, a deposition of high-quality metal-phosphorus layers with phosphorus contents greater than 10% is possible under the same process conditions. This results in the advantageous use of the method according to the invention in a wide variety of areas. For example, the corrosion-resistant metal layers deposited according to the invention are suitable for coating keys or locks, valves, pipelines, etc. Due to the high phosphorus content, the layer becomes non-magnetic and is therefore ideal for coating connectors and contacts as well as housings for electronic devices, etc. Due to the very good wear resistance, the layers produced by the method according to the invention are preferably used in the field of mechanical engineering for coating running surfaces, couplings, pump housings, etc.

As already described above, the method proposed by the invention is characterized in particular by the composition of the electrolyte. It is therefore advantageously in an economical and environmentally friendly compared to the conventional methods. The electrolyte according to the invention can be regenerated, for example, by means of electrodialytic method. When using metal salts whose anions are volatile, the separation effect of the electrodialysis plant is significantly increased. With the same salt load of orthophosphite-containing but sulphation-free electrolytes, the number of electrolysis cells for separating Ortophosphitionen can be reduced at the same separation efficiency.

At the beginning of the process, the base electrolyte of the electrolyte according to the invention is applied. This contains essentially the following composition: 4 - 6 g / l nickel ions 25 - 60 g / l reducing agent 25 - 70 g / l complexing 1 - 25 g / l accelerated 0.1-2 mg / l stabilizer 0-3 g / l other ingredients

The pH range of such a base electrolyte is between 4.0 and 5.0. As already described above, metal salts whose anions are volatile are used as metal receivers. As metal salts whose anions are volatile one or more salts from the group consisting of metal acetates, metal formates, metal nitrates, metal oxalates, metal propionates, metal citrates and Metallascorbinaten, more preferably exclusively metal acetate are used. Since during the reaction, the pH falls through the continuous formation of H ⁺ ions and this must be kept consuming by alkaline media, such as hydroxide, carbonate, or as usually preferred by ammonia in target range, a particular advantage in the sole use of metal salts whose anions are volatile and which originate from the group of acetates, formates, nitrates, oxalates, propionates, citrates and ascorbinates. This is due to the fact that during the deposition of the metal-phosphorus layers anions of the acetates, formates, nitrate, oxalates, propionates, citrates and ascorbinates are formed, which react with the sodium carbon ions from the sodium hypophosphite to form basic sodium salts. The electrolyte according to the invention thus operates throughout the deposition process in a pH range of 4.0 to 5.2, preferably 4.3 to 4.8, without having to be additionally added larger amounts of alkaline media. Due to the extremely advantageous pH self-regulation can be dispensed with during the process on a continuous pH control and alkaline additives.

The starting concentration of the metal-base salts is 0.04 to 0.16 mol / l, preferably 0.048 to 0.105 mol / l, based on nickel, the content of metal being between 0.068 and 0.102 mol / l, preferably 0.085 mol / l.

The reducing agent used is preferably sodium hypophosphite having a starting concentration of 25 to 65 g / l.

As already explained above, the complexing agents used are carboxylic acids and / or polycarboxylic acids, their salts and / or derivatives, preferably hydroxy (poly) carboxylic acids, particularly preferably 2-hydroxypropanoic acid and / or propanedioic acid. By using these compounds, the dissolved nickel is particularly advantageously complexed, so that the deposition rate can be maintained in a corresponding interval of 7 to 14 .mu.m / h, preferably 9 to 12 .mu.m / h with continuous addition of such complexing agents. The starting concentration of the complexing agent in the base electrolyte is between 25 and 70 g / l, preferably 30 to 65 g / l.

The starting concentration of the accelerator, wherein saccharin, its salts and / or derivatives, most preferably sodium saccharin is used, is 2.5 to 22 g / l. Stabilizers used are halogen compound and / or sulfur compound, preferably thiourea. Particularly advantageous, however, is the use of metals, preferably lead, bismuth, zinc and / or tin, particularly preferably in the form of salts whose anions are volatile. These salts are preferably selected from the group consisting of acetates, formates, nitrates, oxalates, propionates, citrates and ascorbinates. Very particular preference is given to the nitrates of the metals used as stabilizers. The starting concentrations of the stabilizers are advantageously from 0.1 to 2 mg / l, preferably from 0.3 to 1 mg / l.

Optionally, further constituents, for example potassium iodide, in a starting concentration of 0 to 3 g / l may also be added to the base electrolyte.

In this basic electrolyte a variety of substrates are introduced and galvanized. To support the lifetime and the stability of the electrolyte, it can be regenerated during the deposition process by means of electrodialysis and / or ion exchange resins. Likewise, supplemental solutions (as exemplified below) may be added to the electrolyte during the deposition process. These replenisher solutions are specially designed to control the individual contents of the basic components and added to the electrolyte in different amounts.

A first replenisher solution includes, for example, the following composition: 500 - 580 g / l reducing agent 5 - 15 g / l complexing 50-150 g / l alkaline buffer 11-20 g / l accelerator 0-3 g / l other ingredients

When creating and using the replenisher solution, the same substances as in the base electrolyte are advantageously used. This results in another very important advantage of the method according to the invention. Since the same substances are used continuously and there are almost no impurities and precipitations, even the compounds from the sink can be returned to the electrolyte. The inventive method thus has a decided material cycle, which can be the process thus more economical and environmentally conscious. The complexing agent content and the content of alkaline buffer are chosen so that, taking into account possible carry-over losses of not more than 40%, an increase to a total content of the complexing agents in the electrolyte to 70 to 90 g / l.

At the same time the content of the accelerator in the electrolyte is controlled so that, for example, in the case of a nickel electrolyte with the use of sodium saccharinate as accelerator per gram of deposited nickel between 0.100 and 0.200 g, preferably 0.150 g are added, wherein the proportion of carryover losses is taken into account. This ensures at the same time a continuous increase to 7.5 - 15 g / l.

As a second replenisher, for example, the following composition can be used: 10 - 50 g / l complexing 0.68 - 2.283 mol / l Metallrezipient 1 - 25 g / l accelerator 40-80 mg / l stabilizer

In this case, the complexing agent of the second replenisher solution may be the same as in the first replenisher or, if necessary, another. Thus, for example, in the case of a content of a hydroxycarboxylic acid, for example 2-hydroxypropanoic acid of 60 g / l, a hydroxycarboxylic acid, for example propanedioic acid with a content of 0.5 g / l, can be used as second complexing agent in the base electrolyte. By adding by means of replenisher, the content of propanedioic acid is then increased by 0.005 to 0.015 g / g of deposited nickel, taking into account the carry-over losses. Due to the continuous increase of propanedioic acid from 0.5 g / l to about 1.2 g / l at 16 MTO equal to 80 g Ni / l, the deposition rate is maintained at the specified interval.

With such an approach, as well as the associated replenisher solution, the use of metal sulfate in addition to the metal base salts described so far, a deposition of adherent metal layers with compressive residual stresses is guaranteed up to a throughput of at least 14 MTO. If only metal-base salts are used whose anion has at least one carbon atom and which preferably originate from the group of acetates, formates, oxalates, propionates, citrates and ascorbinates, the lifetime of the electrolyte surprisingly increases to 22 MTO. The already mentioned compressive residual stress is an extremely important and very desirable layer property. It positively influences the bending cycle stress and increases the ductility. So z. For example, in the case of nickel, metal layers with a ductility of> 0.5% are deposited. Likewise, the residual compressive stresses have a positive effect on the corrosion resistance of the metal-phosphorus layers.

In addition, other components such as additional metals, preferably copper, and / or finely disperse particles, such as finely dispersed fluorine-containing thermoset or thermosetting plastic, may be added to the electrolyte as well as the replenisher, which in the deposited layers additional hardness, dry lubricating effects and / or achieve other properties.

For a more detailed illustration of the invention, a preferred embodiment of the electrolyte according to the invention is described below, to which, however, the invention can not be limited.

Example 1:

composition electrolyte Supplementary solution RA Supplementary solution SA Nickel acetate 4-hydrate (g / l) 12.5 - 25.5 / 200 - 212 Sodium hypophosphite (g / l) 30 - 50 515-565 / Hydroxycarboxylic acid (g / l) 32 - 55 / 25 - 35 Hydroxypolycarboxylic acid (g / l) 0,5 - 5 / / Sodium saccharin (g / l) 2.5 - 22 12.5 - 15 / Potassium iodide (g / l) 0.1 - 2 1 - 2 / Lead acetate (mg / l) 0.3-1 / 60-65 Ammonia 25% by weight (ml / l) 100-150

Such an electrolyte has a self-regulating pH range of 4.3 to 4.8 and allows deposition rates of 8 to 12 μm / hr. The internal stress of the layers deposited therefrom is -10 to -40 N / mm ² . When using the above electrolyte composition metal phosphorus layers with consistently good properties, especially residual compressive stresses at a throughput of 22 MTO equal to 110 g Nill deposited.

By raising the pH range to 4.6 - 5.2 layers with residual compressive stresses of 0 to - 15 N / mm ^{2 are} deposited. The determination of a second pH interval leads to a significant increase in the deposition rate to 12-20 μm / h. The phosphorus content of these layers is 8-10% P. By further raising the pH range to 5.5-6.2, layers with residual compressive stresses of -5 to -30 N / mm ^{2 are} deposited. The phosphorus content of these layers is 2-7% P.

Claims (16)

1. Electrolyte for electroless deposition of nickel layers with residual compressive stresses, including a metal-base salt, a reducing agent, a complexing agent, an accelerator and a stabilizer,
   characterized in that
   the electrolyte comprises as a metal salt the anions of which are volatile at least one salt from the group consisting of nickel acetate, nickel formate, nickel oxalate, nickel nitrates, nickel propionate, nickel citrate and nickel ascorbinate at an initial concentration of 0.01 to 0.30 mol/l and as a reducing agent sodium hypophosphite, wherein the electrolyte comprises as a complex-ing agent carboxylic acids and/or polycarboxylic acids, salts and/or derivatives thereof, and wherein the electrolyte comprises as an accelerator a sulphur-containing heterocycle selected from saccharin, salts and/or derivatives thereof at a concentration between 2.5 g/I and 22 g/I and as stabilizer halogen compounds, sulphur compounds and/or a metal of the group consisting of lead, bismuth, zinc and tin.
2. Electrolyte according to claim 1, characterized in that it comprises nickel sulphate as a further metal-base salt.
3. Electrolyte according to claim 1 or 2, characterized in that it comprises metals and/or finely dispersed particles as a further component.
4. Electrolyte according to claim 3, wherein it comprises copper as a further component.
5. Electrolyte according to one or more of claims 1 to 4, characterized in that said complexing agent is present at a total amount of 70 g/l - 90 g/l.
6. Electrolyte according to any one of the preceding claims, characterized in that it comprises as a stabilizer a metal of the group consisting of lead, bismuth, zinc and tin in the form of a salt the anions of which are volatile.
7. Electrolyte according to claim 6, characterized in that it comprises as anions at least one anion of the group consisting of acetates, formates, nitrates, oxalates, propionates, citrates and ascorbinates.
8. Electrolyte according to one or more of claims 1 to 7, characterized in that it comprises potassium iodide as an additional component.
9. Electrolyte according to one or more of claims 1 to 8, characterized by 0.01 - 0.3 mol/l metal acetate, 30 to 50 g/l sodium hypophosphite-mono-hydrate, 90 to 120 g/l hydroxycarboxylic acid alkaline buffered, 0.5 to 10 g/l hydroxypolycarboxylic acid, 2.5 to 22 g/l saccharinate, 0.1 to 2 g/l potassium iodide, and 0.3 to 1.5 mg/l lead acetate.
10. Method of electroless deposition of nickel layers with residual compressive stress from an electrolyte according to any one of claims 1 to 9,
    characterized in that
    in the course of the process a first and a second supplement solution are added to the electrolyte, wherein the first supplement solution comprises a reducing agent, an alkaline buffer, a complexing agent and an accelerator and the second supplement solution comprises a nickel-base salt, a complexing agent, an accelerator and a stabilizer, wherein as a reducing agent and as a complexing agent in the first supplement solution the same materials as in the base electrolyte are used, and wherein the complexing agent in the second supplement solution is different from the complexing agent in the first supplement solution.
11. Method according to claim 10, characterized in that the method is implemented by use of a closed material cycle.
12. Method according to claim 10 or 11, characterized in that further components of the group consisting of phosphor, additional metals and finely dispersed particles are deposited, too.
13. Method according to one or more of claims 10 to 12, characterized in that metal phosphor layers with phosphor amounts > 10% are deposited.
14. Method according to one or more of claims 10 to 12, characterized in that metal phosphor layers with phosphor amounts of 2 - 10% are deposited, wherein the pH-value in the electrolyte is adjusted to a value between pH 4.6 and pH 6.2.
15. Method according to one or more of claims 10 to 14, characterized in that in the course of the deposition process the total amount of the complexing agent is maintained between 70 g/l and 90 g/l.
16. Method according to one or more of claims 10 to 15, characterized in that the electrolyte is regenerated by means of electrodialysis and/or ion exchange resins.