WO2016193736A1

WO2016193736A1 - Selective electropolishing method, apparatus and electrolyte

Info

Publication number: WO2016193736A1
Application number: PCT/GB2016/051629
Authority: WO
Inventors: Peter Benjamin Neill SCOTT; Chan Kwee SYH
Original assignee: Datum Alloys Pte. Ltd.; Datum Alloys Ltd
Priority date: 2015-06-02
Filing date: 2016-06-02
Publication date: 2016-12-08
Also published as: GB2538996A; GB201509563D0

Abstract

A method, apparatus and electrolyte for selective electropolishing comprising a metallic workpiece that acts as an anode for selective electropolishing of an electropolishing area of a surface of the workpiece, an electrolyte medium applied to a predetermined depth on the electropolishing area and in electrical contact with the anode workpiece, and a cover that acts as a cathode positioned in electrical contact with the electrolyte medium. The electrolyte comprises a silica based colloid. A running current is applied with a power source through the cathode cover, electrolyte medium and anode workpiece to selectively electropolish the surface of the workpiece within the electropolishing area where the electrolyte medium is in contact with the workpiece surface.

Description

SELECTIVE ELECTROPOLISHING METHOD, APPARATUS AND ELECTROLYTE

FIELD OF THE INVENTION

This invention relates generally to electropolishing of a metallic material, and more particularly to a method, an apparatus and an electrolyte for selective electropolishing of a metallic material.

BACKGROUND OF THE INVENTION

Electropolishing is commonly used in various metal finishing industries to remove defective layers of material from a surface of metallic workpiece. For example in the surface mount technology (SMT) industry, electropolishing is used to deburr and polish the surfaces of metallic sheet stencils. For instance, electropolishing is frequently used to remove the burnt marks, "melted" material caused by the laser beam, and other residue dross around apertures and recesses due to the laser beam entry into the metallic sheet. In the electropolishing process, the removal rate of material is greater at such protrusions than with the recesses along metallic surfaces.

Attempts have been made to remove such undesired laser cut rough surface alterations to make the metallic surface smoother. The most widely used method in the surface mount technology (SMT) industry is the use of a higher power (high current, and typically 400 V) rectifier on cathodes and the workpiece anode submerged in a heated bath filled with a liquid polishing chemical electrolyte such as mixtures of phosphoric acid and sulphuric acid. The entire laser cut metallic sheet stencil is submerged into the bath and a voltage current is applied across the electrodes to effect the polishing action.

There is concern with polishing the entire workpiece such as the metallic sheet stencil as there is a camera vision feature fiducial marker that is crucial for alignment of the stencil in surface mount technology (SMT) manufacturing process. When the stencil is submerged, the fiducial marker is also polished which may result in removal or degraded fiducial marker that may cause errors in alignment during the manufacturing process. Conventionally, the fiducial marker may be taped over in an attempt to protect the fiducial marker; however, this adds an additional step in the metal finishing process.

There is also concern of the polish chemical electrolyte bath being reused each time for different metallic workpieces. The metallic material debris removed from each workpiece precipitates and remains in the bath which may alter the effectiveness of the electrolyte action of the bath. The electrolyte chemical polish is conventionally heated during the electropolishing process contributing to evaporation into the atmosphere over time requiring the bath to be topped up. Additionally, toxic vapours are emitted during the conventional electropolishing process requiring adequate ventilation. Running the conventional electropolishing process requires time consuming and costly maintenance and upkeep. There is a need for a selective electropolishing method, apparatus and/or electrolyte that addresses or at least alleviates the problems associated with conventional electropolishing processes used in the metal finishing industries.

SUMMARY OF THE INVENTION

An aspect of the invention is a selective electropolishing method which comprises providing a workpiece for selective electropolishing an electropolishing area of a surface of the workpiece, the workpiece to act as an anode; positioning a spacer on the surface of the workpiece; applying an electrolyte medium on the electropolishing area of the surface of the workpiece, wherein the electrolyte medium comprises an electrolyte acid compound and a gelling compound, and wherein the gelling compound is a silica based colloid; placing a cover on the electrolyte material to cover the electropolishing area on the surface of the workpiece, the cover to act as a cathode; electrically connecting the cathode cover and the anode workpiece to a power source; running an electrical current from the power source through the cathode cover, the electrolyte medium, and the anode workpiece for a predetermined period of time to electropolish the selected electropolishing area of the surface of the workpiece.

An aspect of the invention is a selective electropolishing apparatus comprising a workpiece for selective electropolishing an electropolishing area of a surface of the workpiece, the workpiece to act as an anode; an electrolyte medium in electrical contact with the surface of the workpiece in the electropolishing area of the surface of the workpiece, wherein the electrolyte medium comprises an electrolyte acid compound and a gelling compound, and wherein the gelling compound is a silica based colloid; a cover in electrical contact with the electrolyte medium to cover the electropolishing area of the surface of the workpiece, the cover to act as a cathode; a power source electrically connected to the cathode cover and the anode workpiece, and supply an electrical current from the power source through the cathode cover, the electrolyte medium, and the anode workpiece for a predetermined period of time to electropolish the selected electropolishing area of the surface of the workpiece.

An aspect of the invention is an electropolishing electrolyte for use in selective electropolishing comprising an electrolyte acid compound and a gelling compound, wherein the gelling compound is a silica based colloid. The gelling compound may be a waterborne silica based colloid. The electrolyte acid compound may comprise 55% to 75% acid by weight of the total weight of the electrolyte acid compound. The electrolyte compound may comprise a sulphuric acid and phosphoric acid. The electropolishing electrolyte may further comprise water. The electropolishing electrolyte may comprise 25-75% by weight of acid, 15-25% by weight of gelling compound, and/or 10-25% by weight of water compound.

An aspect of the invention is an electropolishing kit for selective electropolishing of a workpiece, the kit comprising: a spacer for positioning on the surface of a workpiece; an electrolyte medium for bringing into contact with the surface of the workpiece at an electropolishing area of the surface of the workpiece, wherein the electrolyte medium comprises an electrolyte acid compound and a gelling compound, and wherein the gelling compound is a silica based colloid; and a cover for placing in electrical contact with the electrolyte medium, wherein the cover is configured to act as a cathode.

The present invention provides an electrolyte medium which is easy to handle and safe to use. The present invention also provides a method which is safe, simple and easy to perform and does not require specialist knowledge or skill. The present invention also provides an apparatus and a kit with which the method of the invention may be performed. The apparatus and kit are simple and easy to assemble and do not require expensive specialist equipment. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated herein and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. While the invention will be described in connection with certain embodiments, there is no intent to limit the invention to those embodiments described. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the scope of the invention as defined by the appended claims. In the drawings:

FIG. 1 shows a simplified schematic block diagram of an apparatus for selective electropolishing of a surface of a workpiece in accordance with an embodiment of the invention;

Fig. 1A shows a simplified schematic block diagram of an apparatus comprising a sacrificial layer for selective electropolishing of a surface of a workpiece in accordance with an embodiment of the invention;

FIG. 2 shows a simplified schematic block diagram of an apparatus for selective electropolishing of a surface of workpiece in a horizontal orientation in accordance with an embodiment of the invention;

FIG. 3 shows a flow diagram of a selective electropolishing method for selective electropolishing of a surface of a workpiece in accordance with an embodiment of the invention;

FIG. 4 shows a simplified schematic block diagram of an apparatus for selective electropolishing of a surface of a workpiece in accordance with an embodiment of the invention;

FIG. 5 shows a simplified schematic block diagram of an apparatus for selective electropolishing of a surface of workpiece in a horizontal orientation in accordance with an embodiment of the invention;

FIG. 6 shows a simplified schematic block diagram of an apparatus for selective electropolishing of a surface of workpiece in a horizontal orientation showing the arrangement of the cathode, spacer, and workpiece in accordance with an embodiment of the invention;

FIG. 7 shows a simplified schematic block diagram of an arrangement of the cathode, spacer, and workpiece of the area within dashed circle of FIG. 6 in more detail in accordance with an embodiment of the invention;

FIG. 8 shows a top plan view of spacer in accordance with an embodiment of the invention;

FIG. 9 shows a cross-sectional view of the spacer taken along dashed line A-A of FIG. 8 in accordance with an embodiment of the invention;

FIG. 10 shows a perspective view of an apparatus for selective electropolishing of a surface of a workpiece in accordance with an embodiment of the invention; and

FIG. 11-18 show scanning electron microscope (SEM) images of untreated workpiece surfaces in FIG. 11, 13, 15, and 17 before electropolishing, and respective treated surfaces of the workpiece surfaces in FIG. 12, 14, 16, and 18 after electropolishing in an apparatus for selective electropolishing in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

A method, apparatus and electrolyte for selective electropolishing a surface of a metallic workpiece are disclosed comprising a workpiece that acts as an anode for selective electropolishing an electropolishing area of a surface of the workpiece, a spacer positioned on the workpiece with an electrolyte medium applied to a predetermined depth on the electropolishing area and in electrical contact with the anode workpiece, and a cover that acts as a cathode positioned in electrical contact with the electrolyte medium. The spacer may or may not be present in the apparatus of the invention. When the spacer is present, the cover may or may not be in contact with the spacer. A running current is applied with a power source through the cathode cover, electrolyte medium and anode workpiece to selectively electropolish the surface of the workpiece within the

electropolishing area where the electrolyte medium is in contact with the workpiece surface.

As used herein, the term "workpiece" is typically taken to mean a metal object, for example a metal sheet.

As used herein, the term "spacer" is taken to mean an object which is capable of containing the electrolytic medium on the surface of the workpiece. For example, the spacer may comprise a continuous barrier delineating a portion of the surface of the workpiece, for example by forming a loop of continuous physical contact with the workpiece. The spacer may be shaped to contact a portion of the surface of the workpiece. Typically, the spacer comprises a non-conducting material.

As used herein, the term "electropolishing area" is taken to mean the portion of the surface of the workpiece which is to be electropolished. The electropolishing area typically corresponds to the portion of the surface of the workpiece contained within the area of spacer or beneath the cover, preferably within the spacer.

As used herein, the term "cover" is taken to mean an object adapted to make electrical contact with the electrolyte medium between itself. Typically, the cover may be shaped to contact the spacer or to fit within the spacer. Typically, the cover comprises a conducting material e.g. a metal. Further typically, the cover is shaped so as to substantially contain the electrolyte medium between itself, the spacer and the workpiece.

As used herein, the term "power supply" is taken to mean a unit capable of supplying a continuous voltage for a period of time. The power supply is capable of supplying a positive voltage to the anode (that is, the workpiece) and a negative voltage to the cathode (that is, cover). The power supply typically applies a voltage of from 1 to 50 V, for example 5 to 40 or 10 to 30 V, e.g. 12 or 24 V. The power supply typically provides a direct current, typically from 0.1 to 5 Amps, for example from 1 to 2.5 Amps or 1.5 to 2 Amps.

The power supply used in the present invention supplies a low voltage compared to the voltages used in existing electropolishing process. Existing electropolishing processes are high-power processes and typically involve the application of a voltage of around 400 V. By contrast, the present process is a low-power process. The voltage used in the present process is much lower, for example from 10 to 30 V. The method of the present invention is therefore much safer, and much more user- friendly, than known electropolishing processes. Similarly, the apparatus and kit of the invention are much safer and more user-friendly than known electropolishing apparatus, and moreover do not require expensive specialised high-energy power supplies.

Referring to FIG. 1, a simplified schematic block diagram 10 is shown of an apparatus for selective electropolishing of a surface of a workpiece 12 in accordance with an embodiment of the invention. The apparatus comprises an anode workpiece 12, power source 14, cathode cover 16, a spacer 18, and an electrolyte 20. The electrolyte fills the space defined by the walls of the spacer, the surface of the workpiece 22 and a side wall of the cathode cover. A power source 14 is electrically connected to the anode workpiece 12 which is positively charged and a cathode cover 16 which is negatively charged to complete the electro-chemical circuit. During electropolishing, the cathode cover is in physical contact with a spacer 18. The spacer is in physical contact with a surface of the anode workpiece 12. The spacer acts as an insulator, and contains and holds the electrolyte medium within the electropolishing area. The power source 14 provides a direct current (dc) and applies a voltage such as 12V, 24V, or the like, to ensure a running current of approximately, 1.5 Amp, 2.0, 2.5Amp, or the like, to run a current through a cathode 16, the electrolyte 20 within an

electropolishing area 22 on the surface of the workpiece, and the anode workpiece 12. It will be appreciated that the configurations and settings of the power source stated here may vary depending on the required specification, and are not to be considered exhaustive.

The apparatus may comprise a sacrificial layer. Typically, the sacrificial layer sits between the cathode and the electrolyte medium, and may be in electrical contact with both the cathode and the electrolyte medium during the method of the invention. For example, the sacrificial layer may be in physical contact with the electrolyte medium and the cathode in the method of the invention. Typically, therefore, the sacrificial layer is shaped to cover the cathode, for example the sacrificial layer may have the same shape as the portion of the cathode which would be in contact with the electrolyte medium in the absence of the sacrificial layer. Further typically, the apparatus or kit of the invention may include a fastener to attach the sacrificial layer to the cathode, for example a clip, screw, clamp, or adhesive. Alternatively, the apparatus or kit may not include such a fastener and the sacrificial sheet may, for instance, fit within the spacer and thus be held in position by the spacer.

Use of a sacrificial layer during the method of the invention is advantageous as it protects the cathode from damage during electropolishing. Moreover, the sacrificial layer can be replaced if it becomes damaged itself. If a sacrificial layer is included in the kit or the apparatus of the invention, it can therefore extend the life of the cathode in the apparatus or kit of the invention. A sacrificial layer is typically a metal sheet, for example a stainless steel sheet. The sacrificial layer is typically from 0.1 to 2 mm thick, for example from 0.5 to 1 mm thick, for example 0.5 mm thick.

Referring to FIG. 1A, a simplified schematic block diagram is shown of an apparatus for selective electropolishing of a surface of a workpiece 12 in accordance with an embodiment of the invention. The apparatus comprises an anode workpiece 12, power source 14, cathode cover 16, a sacrificial layer 18a, and an electrolyte 20. The electrolyte fills the space between the surface of the workpiece 22 and the sacrificial layer 18a. A power source 14 is electrically connected to the anode workpiece 12 which is positively charged and a cathode cover 16 which is negatively charged to complete the electro-chemical circuit. During electropolishing, the cathode cover is in physical contact and electrical contact with a sacrificial layer 18a. The sacrificial layer is in physical and electrical contact with an electrolyte medium 20. The electrolyte medium 20 is also in physical and electrical contact with an electropolishing area 22 on the surface of the workpiece. The power source 14 provides a direct current (dc) and applies a voltage such as 12V, 24V, or the like, to ensure a running current of approximately, 1.5 Amp, 2.0, 2.5Amp, or the like, to run a current through a cathode 16, the sacrificial layer 18a, the electrolyte 20 within an electropolishing area 22 on the surface of the workpiece, and the anode workpiece 12. It will be appreciated that the configurations and settings of the power source stated here may vary depending on the required specification, and are not to be considered exhaustive.

FIG. 2 shows a simplified schematic block diagram 30 of an apparatus for selective electropolishing of a surface of workpiece in a horizontal orientation in accordance with an embodiment of the invention. The horizontal orientation shows the cathode cover 16 is placed on the spacer 18, with the electrolyte 20 applied on the anode workpiece within the electropolishing area 22 defined by the spacer placement on the surface of the workpiece.

In some embodiments of the method of the invention, the spacer is removed from the surface of the workpiece before the cathode is placed in contact with the electrolyte medium. For example, in these embodiments the method comprises: placing the spacer on a workpiece, the workpiece to act as an anode; placing the electrolyte medium on the anode workpiece within the area defined by the spacer; removing the spacer from the anode workpiece; placing a cover to act as a cathode in contact with the electrolyte medium; electrically connecting the cathode cover and the anode workpiece to a power source; running an electrical current from the power source through the cathode cover, the electrolyte medium, and the anode workpiece for a predetermined period of time to electropolish the selected electropolishing area of the surface of the workpiece. Thus in the apparatus of the invention, the spacer may or may not be present. Optionally, a sacrificial layer may be placed on the electrolyte medium after removing the spacer and prior to placing the cathode cover in contact with the electrolyte medium. In this case, the cathode cover is placed in contact with the sacrificial layer rather than with the electrolyte medium itself. It is found that removal of the spacer prior to addition of the cathode or sacrificial layer results in improved electrical connection between the cathode and the electrolyte medium.

FIG. 3 shows a flow diagram 50 of a selective electropolishing method for selective electropolishing of a surface of a workpiece in accordance with an embodiment of the invention. The metallic workpiece 12 is provided for electropolishing. The spacer 18 is placed in a position 52 on a surface of workpiece to be electropolished. An electrolyte 20 is applied 54 on surface of workpiece within an electropolishing area defined by the spacer 18. The cathode cover is placed 56 on the spacer and electrolyte. A force may be applied onto the cathode cover to discharge electrolyte medium out through an outlet to ensure the electrolyte completely covers the workpiece surfaces within the electropolishing areas. The power source is electrically connected 58 to the cathode cover 16 and anode workpiece 12. The cathode cover 16 is electrically connected to the negative terminal of the power source or ground, and the anode workpiece is connected to the positive terminal of the workpiece. A current is run or applied 60 through cathode cover, electrolyte medium, and anode workpiece to electropolish the surface of the workpiece within the electropolishing area. The current is removed 62 after a predetermined time, such as 25 seconds, 50 seconds, 2 minutes, 4 minutes, or the like. The cover, spacer, and electrolyte are removed 64 from the workpiece. It will be appreciated that the periods of time stated may vary depending on the required specification, and are not to be considered exhaustive.

In an embodiment, the spacer may be fixed to the surface of the workpiece by an adhesive, tape, or the like. The electrolyte may be applied to a predetermined depth, such as to be level with the height of the spacer, such as for example 1.5mm or the like. The spacer may any suitable thickness to allow the electropolishing action, and a dielectric or insulator, and acid resistant material that does not interfere with the electropolishing process. Such materials of the spacer may be glass, PERSPEX, acrylic resins, or the like. PERSPEX is a registered trademark in some countries of Lucite International Ltd of Southampton, United Kingdom. The spacer may have window dimension of, for example, 240mm x 170mm, and the cover may have dimensions smaller to allow an outlet or discharge of any excess electrolyte medium. It will be appreciated that the dimensions stated here may vary depending on the required specification, and are not to be considered exhaustive.

The electrolyte medium, also referred to herein as the electrolyte composition, is suitable for use as an electrolyte in an electropolishing process.

Typically, the electrolyte medium is a gelling agent acid mixture or blend. That is, the electrolyte medium typically comprises a gelling agent and an acid mixture. The electrolyte medium may also comprise one or more further components, for instance a diluent (usually a water compound, e.g. an aqueous solution or suspension, e.g. water). The electrolyte medium may therefore be described as an electrolyte composition comprising a gelling agent and also comprising one or more of an acid mixture and a diluent. Preferably, the electrolyte medium comprises a gelling agent and an acid mixture. More preferably, the electrolyte medium comprises a gelling agent, an acid mixture, and a diluent (usually a water compound, e.g. water).

The acid mixture, which may also be referred to as an electrolyte acid compound, may comprise different concentrations of acids such as phosphoric acid (H3PO4), sulphuric acid (H2SO4), and/or the like. Typically, the acid mixture is an aqueous acid. By "aqueous acid" it is meant that the acid mixture comprises water as a solvent. Typically, the acid mixture contains 40 to 95% by weight of one or more acids, compared to the total weight of the acid mixture (i.e. compared to 100% of the weight of the acid mixture). For example the acid may comprise 50 to 85% or 55 to 75% acid by weight of the total weight of the acid mixture. Typically, the acid mixture comprises 5 to 60% by weight of water (compared to the total weight of the acid mixture), for example 15 to 50% or 25 to 45% water by weight (compared to the total weight of the acid mixture). The acid mixture may optionally also comprise one or more inhibitors, for example aryl sulfonic acids. For example, the acid mixture may comprise from 10 to 40% inhibitor by weight (compared to the total weight of the acid mixture). In this case, the percentage of water by weight (compared to the total weight of the acid mixture) may be lower, for example from 5 to 15% by weight.

An example of an acid component which may be used as an acid mixture is a metal electrolytic treatment compound such as Electrogleam 55 available from MacDermid, Inc. of Waterbury, Connecticut, United States of America. In that case, the composition contains: about 55 to about 75% by weight of a combination of orthophosphoric acid and sulphuric acid (in a 1:1 to 2:1 ratio); about 10 to 20% by weight of hydroxyacetic acid; about 5 to 35% of aryl sulfonic acids (typically benzene sulfonic acid and toluene sulfonic acids); and about 5 to 15% by weight of water. The gelling agent (also referred to herein as a gelling compound) comprises a silica-based colloid. The electrolyte medium therefore comprises a silica-based colloid. By "colloid" is meant a mixture comprising microscopic particles dispersed throughout a medium in which the particles do not dissolve. Typically, the medium is a liquid, e.g. water. By "silica-based colloid" is meant a mixture comprising microscopic particles of silica dispersed in a liquid. Particles other than silica may be present in the silica-based colloid, as long as the silica-based colloid comprises silica particles.

Typically, silica makes up at least 50% by weight, more preferably, at least 75%, more preferably at least 90% by weight of the total weight of dispersed particles in the silica-based colloid. Preferably, the dispersed particles in the gelling agent substantially consist of silica. Thus, the silica-based colloid is typically a dispersion of silica particles in a liquid, typically in water. Typically, the silica particles are amorphous, for example the silica particles may be non-crystalline. Typically, the liquid is water. By silica is meant silicon dioxide, that is, SiC .

The gelling agent may be a waterborne gelling agent containing a blend of, for example water (H2O) and a gelling agent such as waterborne colloidal silica (Si0₂) dispersions or the like. The gelling agent, and hence the electrolyte medium, comprises a silica-based colloid (i.e. colloidal silica). For example, the gelling agent (and hence the electrolyte) may comprise a dispersion of colloidal silica particles in water. A dispersion of colloidal silica particles typically comprises amorphous silica particles. The particle diameter of colloidal silica in the gelling agent is typically between 2 and 100 nm, as may be measured, for example, by laser diffraction. Typically, where the gelling agent comprises colloidal silica, the density of the gelling agent is typically between 0.9 and 1.4 g cm ³, for instance between 1.0 and 1.2 g cm^"3 or between 1.05 and 1.15 g cm^"3, e.g. 1.1 g cm^"3. The percentage of colloidal silica as a percentage of the total weight of the gelling agent is typically from 5 to 50% by weight, for example from 5% to 30% or from 10 to 20% by weight e.g. 15% by weight (compared to the total weight of the gelling agent).

Where the gelling agent comprises colloidal silica, the gelling agent typically has an alkaline pH (for example, a pH of from 8 to 11, e.g. from 10 to 11) before it is mixed to form the electrolyte medium.

Where the gelling agent comprises colloidal silica, the gelling agent is typically stabilised by electrostatic stabilisation. Typically therefore the gelling agent comprises a stabilising agent, for instance sodium ions, for example in the form of Na20. The amount of stabilising agent is typically from 0.1 to 2% by weight compared to the total weight of the gelling agent. For example, the stabilising agent may comprise from 0.5 to 1%, e.g. 0.8% by weight of the gelling agent, compared to the total weight of the gelling agent.

An example of a typical gelling agent is Bindzil GB3000, manufactured by Akzo Nobel. In this case, the gelling agent comprises amorphous silica dispersed in water. The gelling agent comprises 10 to 20% silica by weight (compared to the total weight of the gelling agent). The gelling agent also comprises 0.75 to 0.85% by weight Na₂0 as a stabilising agent (compared to the total weight of the gelling agent).

The electrolyte medium or electrolyte composition may also comprise a diluent. This diluent is typically an aqueous solution (an aqueous solution being a solution containing, for example, at least 70% or at least 80% water by total weight of the diluent) or water. Preferably, the diluent is water. The electrolyte medium (or electrolyte composition) is obtainable by combining the gelling agent with the acid mixture, and optionally with one or more further agents for example a diluent. The electrolyte medium (or electrolyte composition) may contain the compounds (that is, may contain the three components described above) in various proportions.

Typically, a diluent is not used and the electrolyte medium comprises 55% to 95% by weight of electrolyte acid compound and 5% to 45% gelling agent by weight (compared to the total weight of the electrolyte medium). For example, the electrolyte medium may comprise 65% to 90% or 75% to 85% by weight of electrolyte acid compound and 10% to 35% or 15% to 25% by weight of gelling agent (compared to the total weight of the electrolyte medium).

Where a diluent is used, the electrolyte medium comprises for example 25-75% by weight of electrolyte acid compound, 10-25% by weight of water compound; and 15-35% by weight of gelling compound. For example, the electrolyte medium typically comprises 25-75% by weight of electrolyte acid compound, 10-35% by weight of water compound; and 15-40% by weight of gelling compound. These percentages are given as percentages of the total weight of the electrolyte composition.

Typically, therefore, the electrolyte medium comprises from 0.003 to 22.5% by weight of a silica- based colloid, compared to the total weight of the electrolyte medium. For example, the electrolyte medium may comprise 0.5 to 20% by weight silica-based colloid (i.e. colloidal silica) or 1 to 15% by weight silica-based colloid, e.g. 2 to 10% by weight silica-based colloid (compared to the total weight of the electrolyte medium).

Similarly, the electrolyte medium typically comprises from 10% to 90% by weight of acid. For example, the electrolyte medium typically comprises from 25% to 75% by weight of acid or from 40% to 60% by weight of acid (compared to the total weight of the electrolyte medium).

In some embodiments, the electrolyte medium of the invention comprises:

- 55 to 95% of acid electrolyte compound by weight (compared to the total weight of the electrolyte medium), wherein the acid electrolyte compound comprises 55 to 75% by weight of acid (compared to the total weight of the acid electrolyte compound), and wherein the acid electrolyte compound comprises a sulphuric and a phosphoric acid; and

5 to 45% of gelling agent by weight (compared to the total weight of the electrolyte medium), wherein a silica based colloid comprises 10 to 20% by weight of the gelling agent.

In some embodiments, the electrolyte medium of the invention comprises:

65 to 90% of acid electrolyte compound by weight (compared to the total weight of the electrolyte medium), wherein the acid electrolyte compound comprises 55 to 75% by weight of acid (compared to the total weight of the acid electrolyte compound), and wherein the acid electrolyte compound comprises a sulphuric and a phosphoric acid; and

10 to 35% of gelling agent by weight (compared to the total weight of the electrolyte medium), wherein a silica based colloid comprises 10 to 20% by weight of the gelling agent. In some embodiments, the electrolyte medium of the invention comprises:

25 to 75% of acid electrolyte compound by weight (compared to the total weight of the electrolyte medium), wherein the acid electrolyte compound comprises 55 to 75% by weight of acid (compared to the total weight of the acid electrolyte compound), and wherein the acid electrolyte compound comprises a sulphuric and a phosphoric acid;

15 to 35% of gelling agent by weight (compared to the total weight of the electrolyte medium), wherein a silica based colloid comprises 10 to 20% by weight of the gelling agent; and

10 to 25% of diluent, the diluent being water, by weight (compared to the total weight of the electrolyte medium).

In some embodiments, the electrolyte medium of the invention comprises:

- 25 to 75% of acid electrolyte compound by weight (compared to the total weight of the electrolyte medium), wherein the acid electrolyte compound comprises 55 to 75% by weight of acid (compared to the total weight of the acid electrolyte compound), and wherein the acid electrolyte compound comprises a sulphuric and a phosphoric acid;

15 to 40% of gelling agent by weight (compared to the total weight of the electrolyte medium), wherein a silica based colloid comprises 10 to 20% by weight of the gelling agent; and

10 to 35% of diluent, the diluent being water, by weight (compared to the total weight of the electrolyte medium). In some embodiments, the electrolyte medium of the invention comprises:

25 to 75% of acid by weight (compared to the total weight of the electrolyte medium), wherein said acid comprises sulphuric and phosphoric acid; and

1 to 15% of silica-based colloid by weight (compared to the total weight of the electrolyte medium).

In some embodiments, the electrolyte medium of the invention comprises:

40 to 60% of acid by weight (compared to the total weight of the electrolyte medium), wherein said acid comprises sulphuric and phosphoric acid;

2 to 10% of silica based colloid by weight (compared to the total weight of the electrolyte medium).

The viscous gelling agent, comprising silica based colloid, increases the viscosity of the electrolyte medium acid mix into a paste, gel, sol, emulsion, viscous solution, cream, slurry or the like, to be contained within the spacer during the electropolishing process. The gelling agent is typically viscous because the gelling agent is capable of forming a gel. The process of forming a gel involves the formation of links between species in the gelling agent. For example, where the gelling agent comprises colloidal silica, the colloid particles (that is, silica colloids) are capable of forming a gel. Specifically, when mixed with acid the colloidal silica particles can cross-link to one another by the formation of siloxane bridges. The gelling process (that is, the cross-linking process) leads to an increase in the viscosity of the electrolyte medium.

After electropolishing, the electrolyte medium may be washed off the other components such as the spacer, workpiece surface, cover and the like, with water, or the like. The viscous gelling agent makes the electrolyte medium viscous, such that it can be handled in a safer manner than in a liquid form of the electrolyte solution. Additional advantages are also realized in transporting the viscous electrolyte medium with increased viscosity.

There are further advantages to a gel as an electrolyte medium (as opposed to a viscous liquid, for example a viscous liquid comprising a powder such as a silica powder or hydrated silica). The use of a gel improves the electropolishing performance of the electrolyte medium compared to said viscous liquid comprising a powder, particularly in a static system. Specifically, the inventors found that when an electrolyte medium comprising hydrated silica was utilised in the apparatus of the invention, no electropolishing was observed. However, when the gel electrolyte was used, electropolishing took place.

Without wishing to be bound by theory, the inventors note that the formation of a gel (that is, the cross-linking or gelling procedure) creates a three-dimensional structure by the linking of species in the gelling agent (particularly colloidal silica particles). The gelling agent of the invention comprises a silica-based colloid. That is, the gelling agent of the invention comprises colloidal silica particles. The electrolyte of the invention typically comprises colloidal silica particles bonded to one another. Typically, colloidal silica particles are bonded to one another in the electrolyte by siloxane bridges. The bonding process may be initiated by anions present in acid. For example, the bonding process may be initiated by sulphate anions.

The electrolyte typically therefore comprises a network of colloidal silica particles bonded to one another. The network of colloidal silica particles typically has a three-dimensional structure. Said network typically comprises voids. The network of colloidal silica particles, and hence the electrolyte, may therefore be described as having a sponge-like structure, often referred to as a sponge-like nanostructure.

The presence of a three-dimensional structure within the electrolyte medium is believed to allow circulation of electrons through the structure and thus to assist the electropolishing process. In particular, the presence of voids within the network of colloidal silica particles typically allows fluid to flow easily throughout the electrolyte. For example, the acid mixture can move easily throughout the electrolyte, e.g. the acid mixture can move easily from void to void. It is therefore believed that the sponge-like structure allows the acid to be refreshed, for example at the cathode or at the surface of the workpiece, during electropolishing. Moreover, it is thought that the presence of a three-dimensional structure throughout the whole electrolyte medium assists the movement of electrons between the cathode and the anode.

Hydrated silica powder does not form such extensive networks as silica-based colloids do, throughout the electrolyte medium. It is speculated that hydrated silica powder, which has much greater crystallinity than a silica-based colloid, is more stable and therefore less likely to form siloxane bridges between powder grains than form between silica colloids. Accordingly, the inventors suggest that current may flow less easily through an electrolyte comprising hydrated silica powder than through an electrolyte comprising a silica-based colloid. Similarly, fluid (for example acid mixture) can move less easily through an electrolyte comprising hydrated silica powder than through an electrolyte comprising a silica-based colloid. Hence the electrolyte medium of the invention results in more effective, or more efficient, electropolishing than known electrolytes comprising silica powder such as hydrated silica powder. It is noted that electropolishing has been observed using an electrolyte medium comprising hydrated silica powder rather than a gel, for example in US patent application US 2013/0319878 Al. However, as is discussed below, the use of hydrated silica powder in the electrolyte medium does not allow good current flow. Electropolishing is believed to occur in the process described in US

2013/0319878 Al because that process involves the continuous flow of an electrolytic solution into the apparatus, and the movement of said apparatus over the surface to be electropolished. It is suggested that the continuous flow of electrolyte through the apparatus enhances the circulation of electrons and thus permits electropolishing.

FIG. 4 shows a simplified schematic block diagram 80 of an apparatus for selective electropolishing of a surface of a workpiece 82 in accordance with an embodiment of the invention. In this embodiment the workpiece 82 is supported by a support 84, workbench, work top or the like. The work top support 84 is preferably an insulator or dielectric and an acid resistant material, such as for example, glass, PERSPEX, and the like. The workpiece has a bore 86, hole, or the like through the workpiece with sides 88 of the bore. Such a workpiece may be a metallic sheet stencil or the like. Although a bore is shown, there may be recesses instead or in addition to the bore 86. The electrolyte medium 20 fills the spaces of the recesses, bores 86, holes and the like, to the support 84. Accordingly the surfaces that are included in the electropolishing area of the workpiece includes the surface of the sides, side wall surfaces 88, or the like of the bores 86, holes, recesses, or the like. FIG. 5 shows a simplified schematic block diagram 90 of an apparatus for selective electropolishing of a surface of workpiece in a horizontal orientation in accordance with an embodiment of the invention. The horizontal orientation shows the cathode cover 16 is placed on the spacer 18, with the electrolyte 20 applied on the anode workpiece within the electropolishing area 22 including the stencil bores 86 and side walls 88 forming the surface of the electropolishing area defined by the spacer 18 placement on the surface of the workpiece.

The apparatus of the invention is particularly suited to the electropolishing of microscale features. In part, this is due to the typical small size of the apparatus. The apparatus is typically less than 1 000 mm in diameter, for example from 200 to 600 mm or 400 to 500 mm in diameter. Preferably the apparatus is from 400 to 500 mm in diameter. The size of the electropolishing area (that is, the area to be electropolished) is typically smaller than the size of the whole apparatus. The electropolishing area is typically the area contained within the spacer, also called the spacer window. The spacer may have window dimensions smaller than the dimensions of the apparatus, that is, less than 1 000 mm. The dimensions of the spacer window are typically from 100 to 300 mm, e.g. 240 x 170 mm.

The small size of the apparatus and the spacer window has various advantages. For example, where microscale features are to be electropolished, the use of a small apparatus such as that of the present invention reduces the amount of electrolyte medium wasted by electropolishing areas of the surface which are not in need of electropolishing. Electropolishing is often needed to remove burrs or to smooth rough edges created by, for example, micro-cutting or laser cutting, and thus electropolishing is only needed at the rough edges.

Furthermore, the small size of the apparatus enables it to be positioned over areas in a complex workpiece which would be hard or even impossible to reach with a larger apparatus. For instance, the apparatus may be positioned in between layers of a workpiece having a plurality of layers. Additionally, the small size of the apparatus enables the electropolishing to be performed selectively, so as to avoid certain areas of the surface (such as those having a fiduciary marker) while electropolishing a nearby area.

The apparatus of the invention is a static apparatus, meaning that it is not moved over the surface of the workpiece during the electropolishing process. The apparatus of the invention is placed at a specific position on the surface of the material and therefore selectively electropolishes that area of the surface. For instance, the apparatus and in particular the spacer may be fixed to the surface of the workpiece, for example by a clamp, an adhesive, tape and the like. Fixing the apparatus and in particular the spacer to the surface of the workpiece is taken to mean that the apparatus (and in particular the spacer) is fixed in position relative to the workpiece.

Furthermore, the electrolyte medium typically does not flow through the apparatus. The electrolyte medium is typically added to the apparatus prior to electropolishing, for example the entire electrolyte medium may be added to the apparatus prior to electropolishing. The electrolyte medium is typically removed from the apparatus after electropolishing so that it may be replaced. Typically, no electrolyte medium is added to or removed from the apparatus of the invention during the electropolishing process. Removing and replacing the electrolyte medium has the added advantage that any debris incorporated into the electrolyte medium during the electropolishing process is removed from the system and does not affect the electropolishing conditions in subsequent uses of the apparatus.

As a result of the static nature of the apparatus of the invention, it provides highly reproducible electropolishing conditions. It is possible that the absence of electrolyte movement reduces irregularities in the electropolishing effect across the surface. The highly reproducible

electropolishing conditions further suit the apparatus to the electropolishing of microscale features, where small differences in electropolishing conditions could lead to large differences in the electropolishing effect of microscale features on the workpiece. This in turn may affect the properties of the workpiece. For workpieces having microscale features, therefore, it is desirable to electropolish the workpiece under highly reproducible conditions, such as may be attained by the present apparatus.

FIG. 6 shows a simplified schematic block diagram 100 of an apparatus for selective electropolishing of a surface of workpiece in a horizontal orientation showing the arrangement of the cathode 16, spacer 102, and workpiece 12 in accordance with an embodiment of the invention. The spacer 102 in this embodiment has a rebate 104, in which the cover 16 is placed after the electrolyte is applied in the electropolishing area of the surface of the workpiece within the area defined by the spacer. The rebate 104 of the spacer also may define the depth of electrolyte medium. Dashed circle 106 marks the portions of the spacer, cathode, and anode arrangement that is shown in more detail in FIG. 7

FIG. 7 shows a simplified schematic block diagram 120 of an arrangement of the cathode 12, spacer 102, and workpiece 12 of the area within dashed circle 106 of FIG. 6 in more detail in accordance with an embodiment of the invention. Excess electrolyte medium 122 is shown escaping the space defined between the cover 16, surface of the workpiece, and rebate of the spacer. The space formed between the spacer surface and the cover forms an outlet 124 for the excess electrolyte medium to discharge or escape. FIG. 8 shows a top plan view 150 of spacer 152 in accordance with an embodiment of the invention. The spacer 152 comprises a spacer rebate 154 forming a spacer window 156 which defines electropolishing area of workpiece. Dashed line A-A is the line along which cross-sectional view is taken in FIG. 9

FIG. 9 shows a cross-sectional view 170 of the spacer 152 taken along dashed line A-A of FIG. 8 in accordance with an embodiment of the invention. The width or thickness 172 of rebate 154 may assist in determining depth of electrolyte medium. The rebate 154 provides side walls that may hold the electrolyte medium to a predetermined thickness on the electropolishing area on the surface of the workspace. The width or thickness of the rebate may be for example 1.5mm, 2.0mm, or the like. Accordingly, the layer of electrolyte medium may also be 1.5mm, 2.0mm or the like. The width or thickness of spacer 174 may be for example 5mm, 6mm or the like. The depth of spacer rebate 176, shown as the horizontal shelf of the spacer on which the perimeter of the cover rests or is in position during electropolishing may be, for example, 1.0mm, 1.5mm or the like. It will be appreciated that the dimensions stated here may vary depending on the required specification, and are not to be considered exhaustive.

FIG. 10 shows a perspective view 200 of an apparatus for selective electropolishing of a surface of a workpiece in accordance with an embodiment of the invention. An anode workpiece 212 is connected to the positive terminal of the power source 214, and the cathode cover 216 is connected to the negative terminal of the power source 214. A spacer 218 is positioned on the surface of the workpiece, defining the electropolishing area on the surface of the workpiece. An electrolyte medium 220 fills the space defined between the anode workpiece 212, spacer 218 and cathode cover 216 and conducts the electrical current running through the cathode cover, electrolyte medium, and the workpiece from the power source. Any excess electrolyte medium 220 is shown protruding as discharged electrolyte medium from space between perimeter of cover 216 and rebate of spacer 218. An outlet 222 formed by a channel formed of space between perimeter of cover and surface of spacer provides the escape for the excess electrolyte medium. The cover has cover ribs 230 to provide the cover with more rigidity. The cover ribs may be the same metallic material as the metallic material of the cover, or a material different to the metallic material of the cover so long as the cover is electrically connected to the power source.

In this embodiment, the spacer 218 is fitted with a spacer contact 240 for anode workpiece to be electrically connected to the positive terminal of the power source, through the spacer. The spacer is a metallic material such as stainless steel, or the like.

FIG. 11-18 show scanning electron microscope (SEM) images of untreated workpiece surfaces in FIG. 11, 13, 15, and 17 before electropolishing, and respective treated surfaces of the workpiece surfaces in FIG. 12, 14, 16, and 18 after electropolishing in an apparatus for selective electropolishing in accordance with an embodiment of the invention. In the untreated workpieces shown in FIG. 11, 13, 15, and 17, undesired surface projections are shown, for example in FIG. 11 the SEM image 250 shows burrs or projections 252, uneven surfaces or burnt mark 254, and the like. FIG. 12, 14, 16, and 18 show electron micrographs of treated surfaces of the workpiece after electropolishing the untreated workpiece shown in FIG. 11, 13, 15 and 17, respectively.

FIG. 12 shows a SEM image 260 of a surface of a workpiece after electropolishing in an apparatus for selective electropolishing in accordance with an embodiment of the invention with an electrolyte having a first concentration at 12 V, for 25 seconds of electropolishing. The first concentration of the electrolyte medium contains 50 ml of the electrolyte component, 65 ml of gelling agent, and 25 ml of water. The SEM image showing 250 the untreated workpiece before electropolishing is shown in FIG. 11. It can be seen in comparing FIG. 11 and FIG. 12 that the first concentration level of electrolyte medium effectively removes burnt marks 254 around the edge of the aperture and material chip that stuck on laser cut wall.

FIG. 14 shows a SEM image 280 of a surface of a workpiece after electropolishing in an apparatus for selective electropolishing with a second, higher concentration of electrolyte concentration than shown in FIG. 12 in accordance with an embodiment of the invention. The second concentration of the electrolyte medium contains 100 ml of, and 25 ml of. The SEM image 270 showing the untreated workpiece before electropolishing is shown in FIG. 13. It can be seen in comparing FIG. 13 and FIG. 14 that the second, higher concentration does have an impact on the aperture wall compared to the first, lower concentration electrolyte medium.

FIG. 16 shows a SEM image 300 of a surface of a workpiece after electropolishing in an apparatus for selective electropolishing with second, higher concentration of electrolyte, at a higher voltage of 24 V from power source for the same amount of time of 25 seconds as the treated workpiece shown in FIG. 12 in accordance with an embodiment of the invention. The SEM image 290 showing the untreated workpiece before electropolishing is shown in FIG. 15. It can be seen in comparing FIG. 15 and FIG. 16 that the second, higher concentration does have an increased polishing action shown as a whiter surface.

FIG. 18 shows a SEM image 320 of a surface of a workpiece after electropolishing in an apparatus for selective electropolishing with the second, higher concentration for a longer or extended period of time, such as 50 seconds, than shown in the treated workpiece shown in FIG. 12 in accordance with an embodiment of the invention. The SEM image 310 showing the untreated workpiece before electropolishing is shown in FIG. 17. It can be seen in comparing FIG. 17 and FIG. 18 that a better result is achieved with increasing the polishing time and higher gel concentration.

The selective electropolishing method and apparatus in accordance with embodiments of the invention have a footprint that is smaller than the conventional electropolishing heater bath system. The selective electropolishing method and apparatus may also be conducted in different orientations, such as horizontal and the like, in contrast to the vertical orientation of the conventional electropolishing heater bath systems. The selective electropolishing method and apparatus requires minimal maintenance when compared with conventional electropolishing heater bath systems. The electrolyte may be used fresh during each electropolishing in the selective electropolishing method and apparatus unlike the reused electrolyte of the conventional systems. The selective electropolishing method may be configured for different areas, such that the electropolishing area may be set for depending on each particular specification, such that only the surfaces of the workpiece that require electropolishing are actually electropolished.

FORMULATION EXAMPLE

An electrolyte composition according to the invention was made up using an acid mixture and a gelling agent. No diluent was used. The acid mixture comprised:

270 ml (459 g) phosphoric acid 81% v/v

514 ml (771g) sulphuric acid 50% v/v

Acid percentages given relate to the mass of acid by weight compared to 100% of the weight of that acid, so 81% v/v phosphoric acid refers to a phosphoric acid solution comprising 81% phosphoric acid by weight compared to the total weight of the phosphoric acid solution. In total, the acid mixture comprised 62% acid by weight (compared to 100% of the weight of the acid solution).

The gelling agent was Bindzil GB3000.

The formulation of the electrolyte was therefore: Phosphoric Acid 81% 270ml (459g)

Sulphuric Acid 50% 514ml (771g)

Bindzil GB3000 216ml (238g)

Total 1000ml (1468g)

COMPARATIVE EXAMPLE

The inventors performed an experiment to compare the electropolishing effect of a static apparatus according to the present invention using (A) an electrolyte comprising a silica-based colloid, and (B) and an electrolyte comprising hydrated silica powder. The apparatus was placed at a fixed position on the surface of a workpiece. According to the method of the invention, in both (A) and (B) a voltage was applied across the electrolyte medium, via the cover and the anode.

In example (A), using the silica-based colloid, a viable current was attained and the electropolishing area of the surface was effectively electropolished.

However, in example (B), using hydrated silica powder, insufficient current flow was observed and the electropolishing area of the surface was not effectively electropolished.

SUMMARY

Embodiments of the invention have been described herein. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by the applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims

CLAIMS:

An electropolishing electrolyte for use in selective electropolishing comprising:

an electrolyte acid compound and a gelling compound, wherein the gelling compound is a silica-based colloid.

The electropolishing electrolyte of claim 1 wherein the gelling compound is a waterborne silica based colloid.

The electropolishing electrolyte of claim 1 or claim 2 wherein the electrolyte acid compound comprises 55% to 75% acid by weight compared to the total weight of the electrolyte acid compound.

The electropolishing electrolyte of any one of claims 1 to 3 wherein the electrolyte acid compound comprises a sulphuric acid and phosphoric acid.

The electropolishing electrolyte of any one of claims 1 to 4 further comprising water.

The electropolishing electrolyte of any one of claims 1 to 5 comprising 25-75% by weight of acid.

The electropolishing electrolyte of any one of claims 1 to 6 comprising 15-25% by weight of gelling compound.

The electropolishing electrolyte of any one of claims 1 to 7 comprising 10-25% by weight of water compound.

A selective electropolishing method comprising:

providing a workpiece for selective electropolishing an electropolishing area of a surface of the workpiece, the workpiece to act as an anode;

positioning a spacer on the surface of the workpiece;

applying an electrolyte medium on the electropolishing area of the surface of the workpiece, wherein the electrolyte medium is an electropolishing electrolyte as claimed in any one of claims 1 to 8;

placing a cover on the electrolyte material to cover the electropolishing area on the surface of the workpiece, the cover to act as a cathode;

electrically connecting the cathode cover and the anode workpiece to a power source; running an electrical current from the power source through the cathode cover, the electrolyte medium, and the anode workpiece for a predetermined period of time to electropolish the selected electropolishing area of the surface of the workpiece.

The selective electropolishing method of claim 9 wherein the method comprising placing the cover on the spacer and the electrolyte material to cover the electropolishing area on the surface of the workpiece, the cover to act as a cathode.

11. The selective electropolishing method of claim 9 or claim 10 comprising placing a sacrificial layer on the electrolyte medium prior to placing the cover on the electrolyte material.

12. The selective electropolishing method of claim 9 comprising removing the spacer from the anode workpiece after applying an electrolyte medium on the electropolishing area of the surface of the workpiece and prior to placing the cover on the electrolyte material.

13. The selective electropolishing method of any one of claims 9 to 12 further comprises fixing the spacer on the surface of the workpiece.

14. The selective electropolishing method of claim 13 further comprises fixing the spacer on the surface of the workpiece with an adhesive.

15. The selective electropolishing method of any one of claims 9 to 14 wherein electrically connecting the cathode cover to the negative terminal of the power source and electrically connecting the anode workpiece to the positive terminal of the power source.

16. The selective electropolishing method of any one of claims 9 to 15 which further comprises removing the electrolyte medium from the surface of the anode workpiece after applying a current.

17. The selective electropolishing method of any one of claims 9 to 16 wherein applying the electrolyte medium within the area formed by the spacer, the spacer defining the electropolishing area of the surface of the workpiece.

18. The selective electropolishing method of any one of claims 9 to 17 wherein applying the electrolyte medium on the surface of the workpiece to a predetermined depth.

19. The selective electropolishing method of any one of claims 9 to 18 wherein applying the electrolyte medium to a depth of the spacer.

20. The selective electropolishing method of claim 9 or claim 10 wherein placing the cover on the spacer forms an outlet for the excess electrolyte medium to escape from a space formed between the perimeter of the cathode cover and the spacer.

21. The selective electropolishing method of any one of claims 9 to 10 wherein electrically connecting the anode workpiece with the power source via a connector in the spacer, the connector in electrical contact with the anode workpiece. 22. A selective electropolishing apparatus comprising:

a workpiece for selective electropolishing an electropolishing area of a surface of the workpiece, the workpiece to act as an anode;

an electrolyte medium in electrical contact with the surface of the workpiece in the electropolishing area of the surface of the workpiece, wherein the electrolyte medium is an electropolishing electrolyte as claimed in any one of claims 1 to 8; a cover in electrical contact with the electrolyte medium to cover the electropolishing area of the surface of the workpiece, the cover to act as a cathode;

a power source electrically connected to the cathode cover and the anode workpiece, and supply an electrical current from the power source through the cathode cover, the electrolyte medium, and the anode workpiece for a predetermined period of time to electropolish the selected electropolishing area of the surface of the workpiece.

23. The selective electropolishing apparatus of claim 22 further comprising a spacer positioned on the surface of the workpiece.

24. The selective electropolishing apparatus of claim 23 wherein the apparatus comprises a cover on the spacer and in electrical contact with the electrolyte medium to cover the electropolishing area of the surface of the workpiece, the cover to act as a cathode. 25. The selective electropolishing apparatus of claim 23 or claim 24 further comprising a

fastener to fix the spacer onto the surface of the workpiece.

26. The selective electropolishing apparatus of claim 25 wherein the fastener is an adhesive. 27. The selective electropolishing apparatus of any one of claims 22 to 26 wherein the cathode cover is electrically connected to the negative terminal of the power source and the anode workpiece is electrically connected to the positive terminal of the power source.

28. The selective electropolishing apparatus of any one of claims 23 to 27 wherein the

electrolyte medium is within the area formed by the spacer, the spacer defining the electropolishing area of the surface of the workpiece.

29. The selective electropolishing apparatus of any one of claims 22 to 28 wherein the

electrolyte medium is a predetermined depth on the surface of the workpiece in the electropolishing area.

30. The selective electropolishing apparatus of any one of claims 23 to 29 wherein the

electrolyte medium has substantially the same depth as the depth of the spacer. 31. The selective electropolishing apparatus of any one of claims 24 to 30 further comprising an electrolyte outlet formed between the cover and the spacer for allowing excess electrolyte medium to escape.

32. The selective electropolishing apparatus of claim 31 wherein the outlet is formed space formed between the perimeter of the cathode cover and a surface of the spacer.

33. The selective electropolishing apparatus of any one of claims 23 to 32 wherein the spacer further comprises a connector for electrically connecting the anode workpiece with the terminal of the power source.

34. The selective electropolishing apparatus of any of claims 22 to 33 wherein the apparatus comprises a sacrificial layer between the cathode and the electrolyte medium.

35. An electropolishing kit for selective electropolishing of a workpiece, the kit comprising: a spacer for positioning on the surface of a workpiece;

an electrolyte medium bringing into contact with the surface of the workpiece at an electropolishing area of the surface of the workpiece, wherein the electrolyte medium is as claimed in any one of claims 1 to 8; and

a cover for placing in electrical contact with the electrolyte medium, wherein the cover is configured to act as a cathode.

36. The electropolishing kit of claim 35 further comprising a power source for electrical

connection to the cathode cover and to the anode workpiece, wherein the power source is for supplying an electrical current through the cathode cover, the electrolyte medium, and the anode workpiece for a predetermined period of time to electropolish the selected electropolishing area of the surface of the workpiece.

37. The electropolishing kit according to claim 35 or claim 36 further comprising a fastener to fix the spacer onto the surface of the workpiece.

38. The electropolishing kit according to any of claims 35 to 37 further comprising a connector for electrically connecting the anode workpiece with the terminal of the power source.

39. The electropolishing kit according to any of claims 35 to 38 further comprising a sacrificial layer suitable for placing between the cathode and the electrolyte medium.