GB2383337A

GB2383337A - Electroplating plant and method

Info

Publication number: GB2383337A
Application number: GB0130649A
Authority: GB
Inventors: Andrew Derek Turner
Original assignee: Accentus Medical PLC
Current assignee: Accentus Medical PLC
Priority date: 2001-12-21
Filing date: 2001-12-21
Publication date: 2003-06-25
Also published as: GB0130649D0

Abstract

In an electroplating process, the electrical efficiency of dissolution of the metal at the anode 20 may differ from that of deposition of the metal at the cathode 18, so the composition of the electroplating solution 16 will gradually change. Part of the electroplating solution 16 is therefore used to perform deposition using a counter-electrode 30 at which no dissolution can occur, or used to perform dissolution using a counter-electrode at which no deposition can occur. This may be in a treatment cell 24, and the counter-electrode 30 may be surrounded by a bipolar or an ion permselective membrane 32 to prevent destruction of other ions within the solution 16. The optimum composition of the electroplating solution 16 can hence be maintained. Alternatively an anode assembly (36, Fig 3) may be used in the cell 14 comprising an electrode (37) coated with a cation selective membrane (38) and having a backing plate 39. In a further embodiment (40, Fig 4) a cathode assembly may be used in the cell 14 comprising a cathode (42) surrounded by a membrane 44. When electrolyte is circulated to a treatment cell this other cell may take the form of a electrodialysis stack (50, Fig 5).

Description

Electroplating Plant and Method The present invention relates to a method of electroplating, and to a plant for performing this method.

Electroplating is a well known procedure. The electroplating of objects is carried out using an electroplating solution, an anode, and a cathode, one of which is connected to the objects to be plated; typically the reverse reaction is arranged to occur at the other electrode so that the concentration of the solution remains steady. Some known electroplating solutions are copper cyanide, silver cyanide, and nickel sulphamate.

For example copper electroplating may be carried out with the objects connected to a cathode, with an aqueous solution of copper sulphate as the electroplating solution, and with a copper anode; at the anode copper dissolves, while at the cathode copper is deposited on the objects. This is a somewhat simplified picture, as in reality there will be competing electrochemical reactions such as hydrogen evolution. Consequently the electrical efficiency with which copper dissolves (in terms of quantity of copper per unit charge passed) may differ from the efficiency with which copper is deposited, with the consequence that the concentration of copper in the electroplating solution will gradually change.

According to the present invention there is provided a method for electroplating objects, in which the objects are electroplated in an electroplating bath using an electroplating solution containing ions, the electrical efficiency of dissolution differing from the electrical efficiency of deposition, in which part of the electroplating solution is treated to maintain the ionic

concentration of the electroplating solution.

For example, if the efficiency of dissolution is greater than that of deposition, the treatment might be that part of the electroplating solution is used to perform deposition using a counter-electrode at which no dissolution can occur. If the efficiency of deposition is greater than that of dissolution, the treatment might be that the part of the electroplating solution is used to perform dissolution while preventing deposition of the metal. For example where deposition occurs at a cathode, metal ions may be prevented from reaching the cathode by a bipolar membrane, a monovalent cation-selective membrane (where the depositing cation has an ionic charge of 2+ or higher), or by an anion-selective membrane in contact with a gas permeable cathode (so that hydrogen is evolved instead). Alternatively, the part of the electroplating solution might be treated by an electrodialysis process where excess acid is recovered to dissolve a basic salt of the metal, which can be added subsequently to the plating solution.

This method enables the concentration of the electroplating solution to be maintained at its optimum value.

The part of the electroplating solution that is subjected to the treatment (for example being used to perform deposition with the said counter-electrode) may be within the electroplating bath, or alternatively it may be in a separate electrochemical cell through which part of the electroplating solution is circulated. The latter arrangement is preferable where the two processes are operated at different temperatures.

If the solution contains a component which can be

oxidised and destroyed or deposited at an anode, the counter-electrode may be an anode separated from the said part of the electroplating solution by a bipolar membrane. This would apply, for example, with anions such as citrate, sulphamate or cyanide, or cations such as cobalt (II) or lead (II) as these may be oxidised to insoluble oxides Co304 or PbO2.

A bipolar membrane is permeable to neither anions nor cations. It may be considered as comprising a layer of anion-selective material on one side, and a layer of cation-selective material on the other side. In the presence of an electric field, if the potential difference across the membrane is at least 0.84 V, then water is split into hydrogen and hydroxyl ions within the membrane, the hydrogen ions emerging from the cationic side and the hydroxyl ions emerging from the anionic side. Thus with such a counter-electrode, hydrogen ions are introduced into the said part of the electroplating solution, but no anions from the electroplating solution can be deleteriously affected by the anode.

In addition, an oxidation-resistant cation exchange membrane combined with a porous gas-evolving anode can be used to prevent anion access to the anode surface by Donnan exclusion.

In a second aspect, the invention provides an electroplating plant for performing such a method.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying figures in which: Figure 1 shows a diagram of an electroplating plant;

Figure 2 shows a diagrammatic cross-sectional view of part of the plant of figure 1, indicating the chemical processes; Figure 3 shows a diagrammatic side view of an anode assembly for use in an electroplating plant; Figure 4 shows a diagrammatic side view of a cathode assembly for use in an electroplating plant; and Figure 5 shows a diagrammatic view of an electrodialysis stack for use in an electroplating plant.

Referring to figure 1, an electroplating plant 10 is shown for plating a multiplicity of small metal objects 12 with copper. The plant 10 includes an electroplating bath 14 containing an electroplating solution 16. The objects 12 are enclosed within a plastic barrel 18 that rotates during operation and which incorporates electrical contacts 19 (shown only diagrammatically) to the objects 12, so that the objects 12 form a cathode, and at least the lower half of the barrel 18 is immersed in the solution 16. A copper electrode 20, for example comprising shot or discs contained in a metal current feeder basket (fabricated eg from an expanded titanium mesh) arranged as an anode, is also immersed in the solution 16. Optionally the bath 14 may contain several such cathode barrels 18 and several such copper anodes 20.

Part of the electroplating solution 16 is transferred by a pump 22 to a treatment cell 24, circulating through the cell 24 and returning to the bath 14 through a return pipe 25. The treatment cell 24 contains two electrodes that interact with the solution

16: a copper cathode 28, and an anode 30 (for example made from nickel) separated from the solution 16 by a bipolar membrane 32 with its cationic layer at the outside, with aqueous sodium hydroxide solution 34 in the space between the membrane 32 and the anode 30. In this example the electroplating bath 14 is maintained at 65 C, whereas the treatment cell 24 is maintained at 40 C, and a heat exchanger or heat pump 26 is therefore provided which transfers heat from the solution leaving the plating bath 14 to the solution returning through the return pipe 25; this is because bipolar membranes are typically stable only at temperatures below about 45 C.

In this example the plating solution 16 is an alkaline cyanide solution; it contains 100 g/litre of copper in the form of sodium cupro-cyanide, NaCu (CN) , and also 10 g/litre of hydroxide in the form of sodium hydroxide and potassium hydroxide, and 18 g/litre of free cyanide provided in the form of sodium cyanide and potassium cyanide. As a consequence of oxidation of the cyanide ions during operation, there may also be carbonate ions present. This solution 16 gives good quality copper deposition without excessive power consumption, but the electrical efficiency of deposition is only about 85 to 93% (deposition of one equivalent of copper per Faraday would represent 100% efficiency); this is due to competing electrochemical processes at the cathode 18 such as hydrogen evolution. In contrast, the electrical efficiency of dissolution at the anode 20 is 100%, and as a consequence, during operation, the concentration of copper in solution tends to gradually rise for example to 120 g/litre.

Considering now that portion of the solution 16 that is transferred to the treatment cell 24, as shown

diagrammatically in figure 2, copper is deposited on the cathode 28, releasing cyanide ions into the solution 16.

The bipolar membrane 32 prevents any ions, in particular cyanide ions, from reaching the nickel anode 30; instead water molecules are split into hydroxyl and hydrogen ions within the membrane 32, the hydrogen ions emerging into the electrolyte solution 16, and the hydroxyl ions emerging into the sodium hydroxide solution 34. At the anode 30 oxygen is evolved and hydrogen ions generated.

There is consequently no overall change in the composition of the sodium hydroxide solution : the overall effect is merely generation of oxygen gas at the anode 30 and generation of hydrogen ions emerging from the bipolar membrane 32. The electrolyte solution 34 may be recirculated through the electrode assembly to assist in the disengagement of oxygen gas prior to release to the atmosphere and to prevent local overheating.

Hence the treatment cell 24 reduces the concentration of copper in solution, enabling it to be held at the optimum value. It also has the desirable effects of increasing the concentration of cyanide ions, and reducing the excess alkalinity. It will be appreciated that the cathode 28 onto which the excess copper is deposited can be subsequently used as an anode 20 in the plating bath 14, or cut into pieces for use in an anode barrel in the plating bath 14; indeed the deposition cathode 28 may be a plastic barrel provided with electrical contacts and containing pieces of copper, and these pieces can subsequently be transferred to an anode barrel in the plating bath 14. It will be appreciated that only a small portion of the solution 16 need be treated in this way, for example less than 20%, and that the electric current passed through the treatment cell 24 would be less than 20% of that passed through the plating bath 14. The treatment cell 24 would

typically be of much smaller volume than the plating bath 14.

It will be appreciated that, with this electrolyte composition, there is a gradual increase in the concentration of bicarbonate and carbonate ions during operation (due to absorption of carbon dioxide from the atmosphere, and to decomposition of the cyanide). During passage through the heat exchanger 26, sodium bicarbonate may be crystallised out (because of its reduced solubility at lower temperatures), and may be separated using a filter, hydrocyclone or gravity separator (eg lamellar) (not shown). This crystallisation step may be assisted by subjecting the electrolyte 16 to intense ultrasound as it emerges from the heat exchanger 26 to minimise the degree of supersaturation required, and therefore grow larger crystals.

An alternative anode assembly 36 which would be suitable in this situation, as cyanide ions are prevented from contacting it, is shown in figure 3, to which reference is now made. It consists of an electrode 37 for example of nickel (or of precious metal oxide coated titanium, or lead dioxide coated titanium) which is porous and which has a cation selective membrane 38 covering the surface contacting the electrolyte. The rear surface of the porous electrode contacts a porous plastic backing plate 39 whose opposite surface is sealed. Water diffusing through the cation selective membrane 38 undergoes electrolysis, the oxygen gas escaping through the pores of the electrode 37 and the plastic backing plate 39. The cation-selective membrane 38 may be of a perfluorinated sulphonated ethylene (such as Nafion : trade mark). Such an anode assembly 36 might be used in the treatment cell 24, or alternatively it might be immersed in the electroplating bath 14 as the

membrane 38 is stable at elevated temperatures as long as it remains hydrated.

It will be appreciated that some of the parameters are specific to a particular plating solution and electroplating system. For example, if the electroplating solution 16 was an aqueous solution of copper sulphate, then the treatment cell 24 may be omitted, and an electrolyte treatment anode (for example of platinised or iridium oxide coated titanium) included in the plating bath 14. In this case most of the electric current would be passed through the copper anode (or anodes) 20, but a sufficient proportion passed through the treatment anode to prevent changes in copper concentration in the plating solution.

In an alternative, where the electrolyte solution is neutral or acidic, then the treatment cell 24 may be provided as described but with the anode 30 being of lead, platinum-coated titanium, or a conductive ceramic, which may be immersed in aqueous sulphuric acid electrolyte between the bipolar membrane 32 and the anode 30.

There are also some electrolysis situations in which the dissolution efficiency may be less than the deposition efficiency. For example this can arise with nickel plating if the nickel alloy anode at which dissolution occurs becomes partly passivated. This would lead to a gradual decrease in the concentration of the metal ions. This may be overcome electrolytically in analogous ways to those described above, for example using a treatment cell 24 containing an anode at which dissolution occurs, and a cathode surrounded by a bipolar membrane 32 (but with its cationic layer on the surface closest to the cathode). (This is substantially as

described above in relation to figure 2, but with opposite polarity.) In operation the bipolar membrane 32 prevents metal ions from being deposited on the cathode, while dissolution takes place at the anode. This is preferably carried out within a treatment cell 24 because bipolar membranes are preferably not exposed to such elevated temperatures as may be used in the plating bath 14.

An alternative is to use a cathode assembly 40 as shown in figure 3, to which reference is now made, in which a cathode 42 (for example of stainless-steel) is surrounded by a membrane 44 selective to monovalent cations. Dilute sulphuric acid might for example be used between the cathode 42 and the membrane 44. Such a cathode assembly 40 may be installed within an electrolysis bath 14 for example used for nickel plating.

Since the nickel ions are divalent, the membrane 44 prevents them contacting the cathode 42 ; hydrogen ions generated at the anode 20 (as a consequence of the water electrolysis that is the cause of the reduced anode dissolution efficiency) can however pass through the membrane 44, and are electrolysed to form hydrogen gas.

The release of the hydrogen may be assisted by recirculation of the electrolyte to a disengagement vessel, through which an excess of air might be passed to dilute it to less than the lower explosive limit prior to passing into the ventilation system.

Alternatively a cathode assembly may be used analogous to that of figure 3 (but of opposite polarity), in which a porous cathode (for example of stainlesssteel) has an anion selective membrane covering the surface contacting the electrolyte. The rear surface of the porous electrode contacts a porous plastic backing plate whose opposite surface is sealed. Water diffusing

through the anion selective membrane undergoes electrolysis, the hydrogen gas escaping through the pores of the electrode and the plastic backing plate. The metal cations from the electrolyte 16 are prevented from reaching the cathode by the membrane. Hydroxyl ions generated by electrolysis emerge through the membrane into the electrolyte 16 to react with excess hydrogen ions generated at the anode 20.

Rather than treating the electrolyte by electrolysis, it may instead be treated by electrodialysis. For example with an electrolyte 16 of copper sulphate solution, if the dissolution efficiency is less than deposition efficiency the proportion of copper ions becomes less and so the proportion of sulphuric acid becomes higher. Part of the electrolyte 16 may be circulated through an electrodialysis stack 50 as shown diagrammatically in figure 5. The electrodialysis stack 50 comprises several parallel flow channels 52 (only two are shown) for the electrolyte 16 undergoing treatment, alternating with several concentrate channels 54 (only three are shown), adjacent channels 52 and 54 being separated by ion-selective membranes. The flow channels 52 and the concentrate channels 54 form a stack, and are arranged between an anode 56 and a cathode 58 immersed in suitable electrolytes. Each flow channel 52 is defined between a monovalent cation-selective membrane C (on the side nearest the cathode 58) and an anionpermeable membrane A (on the side nearest the anode 56).

The membranes at the end of the stack adjacent to the electrode chambers may be bipolar membranes B.

In operation, that part of the electrolyte 16 that flows through the flow channels 52 has hydrogen ions removed through the monovalent cation-selective membrane C and sulphate ions removed through the anion-selective

membrane A. The electrolyte 16 from which the excess sulphuric acid has hence been removed can be returned to the plating bath 14. If desired, the sulphuric acid from the concentrate channels 54 may be reacted with, for example copper carbonate, to generate copper sulphate solution for adding to the electrolyte 16 in the plating bath 14.

Claims

Claims 1. A method for electroplating objects, in which the objects are electroplated in an electroplating bath using an electroplating solution containing ions, the electrical efficiency of dissolution differing from the electrical efficiency of deposition, in which part of the electroplating solution is treated to maintain the ionic concentration of the electroplating solution.
2. A method as claimed in claim 1 wherein'the electrical efficiency of dissolution is greater than the electrical efficiency of deposition, and in which part of the electroplating solution is used to perform deposition using a counter-electrode at which no dissolution can occur.
3. A method as claimed in claim 1 wherein the electrical efficiency of deposition is greater than the electrical efficiency of dissolution, and in which part of the electroplating solution is used to perform dissolution using a counter-electrode at which no deposition can occur.
4. A method as claimed in claim 2 or claim 3 wherein the said counter-electrode is within the electroplating bath.
5. A method as claimed in claim 2 or claim 3 wherein the part of the electroplating solution treated with the said counter-electrode is in a separate electrochemical cell through which part of the electroplating solution is circulated.
6. A method as claimed in any one of the preceding claims wherein the counter-electrode is separated from

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the said part of the electroplating solution by a bipolar membrane.
7. A method for electroplating objects substantially as hereinbefore described with reference to, and as shown in, figure 1 and figure 2 or figure 3 or figure 4 or figure 5 of the accompanying figures.
8. An electroplating plant for performing a method as claimed in any one of the preceding claims.