WO2002099163A2

WO2002099163A2 - Autocatalytic coating method

Info

Publication number: WO2002099163A2
Application number: PCT/GB2002/002470
Authority: WO
Inventors: Daniel Robert Johnson; William Norman Damerell; Stephen George Appleton; Gregory Peter Wade Fixter
Original assignee: Qinetiq Limited
Priority date: 2001-06-04
Filing date: 2002-05-27
Publication date: 2002-12-12
Also published as: GB0113408D0; WO2002099163A3; ZA200309379B; AU2002302775A1

Abstract

Autocatalytic plating is a form of electrode-less plating in which a metal, for example, cobalt, nickel, gold, silver or copper, is deposited onto a substrate via a chemical reduction process. Coatings derived from this process are usually more uniform and adherent than from other processes and can be applied to unusually shaped surfaces. Non-metallic surfaces can also be coated via this process following suitable sensitisation of the substrate. Autocatalytic deposition is used to coat whole surfaces and the formation of metal patterns requires additional and costly additional processing such as photolithography and etching. This invention therefore provides a method of preparing a substrate material for subsequent autocatalytic deposition of a metal coating in a pre-determined pattern.

Description

AUTOCATALYTIC COATING METHOD

This invention relates to the field of autocatalytic or "electroiess" coating methods.

Autocatalytic plating is a form of electrode-less (electroiess) plating in which a metal is deposited onto a substrate via a chemical reduction process. The advantage of this technology is that an electric current is not required to drive the process and so electrical insulators can be coated. Coatings derived by this technique are usually more uniform and adherent than from other processes and can be applied to unusually shaped surfaces (see Deposition of Inorganic Films from Solution, Section III Ch 1 pp 209-229; Thin Film processes (1978); Publishers Academic Press and, Smithells Metals Reference Book, 7* Edition (1992) Chapter 32, ppl2-20; Publishers Butterworih Heinmann

Processes exist for the autocatalytic deposition of a large number of metals, particularly cobalt, nickel, gold, silver and copper from a suitable solution bath. Basically, the solutions contain a salt of the metal to be deposited and a suitable reducing agent, e.g. hypophosphite, hydrazine, borane etc. When a metal substrate, which is catalytic to the reaction, is introduced into the solution bath it becomes covered with a layer of the coating metal which itself is catalytic so that the reaction can continue.

Deposition will only occur if conditions are suitable on the substrate to initiate and then sustain the autocatalytic process. Therefore in cases where the substrate is a plastic or ceramic, for example, additional steps are required to create suitable surface properties. Usually, in such cases the substrate is "sensitised" with a reducing agent, e.g. SnCl₂. Also, the surface may be "activated" with a thin layer of an intermediate catalytic material, e.g. Palladium (itself a candidate metal for autocatalytic deposition), in order to aid the deposition process. Such "deposition promoting materials" are generally referred to in the literature as "sensitisers" and "activators" respectively. Autocatalytic deposition is generally employed to coat whole surfaces. However, in order to form metal patterns, e.g. for electrical circuits or decorative effects, additional processes such as photolithography followed by etching of surplus metal have to be performed. This is complex and costly.

It is therefore an object of the present invention to provide a method of preparing a substrate material for subsequent autocatalytic deposition of a metal coating in a predetermined pattern that alleviates some of the above-mentioned disadvantages.

Accordingly, this invention provides a method of preparing a substrate material for subsequent metal plating by an autocatalytic deposition process comprising coating some or all of the substrate material with a deposition promoting material (as hereinbefore defined) which is capable, once the coated substrate is introduced into an autocatalytic deposition solution, of facilitating the deposition of a metal coating from an autocatalytic solution onto the coated areas of the substrate wherein the deposition promoting material is printed onto the substrate by a pattern transfer mechanism.

By using pattern transfer mechanisms, such as, inkjet printing, screen printing, pen writing or spray printing, the deposition promoting material can be laid down onto the substrate in a pre-determined pattern. When the substrate is subsequently immersed into an autocatalytic deposition solution deposition of metal will occur only on the patterned areas of the substrate covered by the deposition promoting material. Surrounding areas of the substrate will be unaffected.

The metal coating which is deposited by the autocatalytic deposition process may then also subsequently be coated with further metals through electroiess deposition, provided the first metal coating surface can catalyse or ion exchange with the subsequent metals. For example a sensitised substrate may be autocatalytically coated with a layer of nickel which could then be further coated, via a further electroiess process, with a coating of copper. Alternatively, if the first electroiess coating is copper a further coating of tin may be deposited. It is also possible for the autocatalytic deposition solution to contain two different metal salts which are then co-deposited onto a sensitised substrate at the same time, for example nickel and copper.

An autocatalytically deposited metal pattern may also be further coated with a wide range of metals or compounds by electrodeposition, provided there are continuous electrical paths in the pattern to act as the cathode of an electrolytic bath. An example is the electrodeposition of "chromium" plate onto nickel to prevent tarnishing.

The minimum feature sizes that result from the use of a pattern transfer technique are dependent on the particular mechanism used. For an ink jet printing technique features of the order 20 microns are possible. Screen printing and/or pen writing result in much coarser features being produced, e.g. up to 1000 microns. Features in the range 20-1000 microns are therefore possible depending on the mechanism used.

The use of a pattern transfer mechanism removes or at least greatly reduces the need for any processing (such as etching etc.) after autocatalytic plating has taken place. Therefore the amount of wasted material is reduced and the overall process is simplified which leads to cost savings.

Conveniently, the deposition promoting material can be contained in an ink formulation suitable for use with the chosen pattern transfer mechanism.

The deposition promoting material may comprise a reducing agent (a "sensitiser") such as SnCl₂, glucose, hydrazine, amine boranes, borohydride, aldehydes, hypophosphites, tartrates.

As an alternative to, or as well as, a reducing agent, the deposition promoting material could be an activator such as a colloidal dispersion of a catalytic material. For example palladium, cobalt, nickel, steel or copper could be added to an ink formulation to catalyse a particular metal deposition. As a further alternative, the deposition promoting material could be one that is able to ion exchange with the catalytic material contained within the autocatalytic solution bath. For example, Ni or Fe could be added directly to an ink formulation. Once the coated substrate is introduced into the autocatalytic solution bath the deposition promoting material undergoes ion exchange with the metal in the autocatalytic solution, thereby nucleating deposition of the electroiess coating.

Conveniently, the ink formulation can, in addition to the deposition promoting material, contain binders and fillers which variously can enhance the properties of the final metal coating, enhance the adhesion of the electroiess metal to the substrate and which can provide porous and textured surface effects, which can change the mechanical, thermal, electrical, optical, and catalytic properties of depositing metal.

The inclusion of binders in the ink formulation may additionally serve to prevent loss of adhesion from the printed substrate of the deposition promoting agent during electroiess coating. The inclusion of fillers may serve to improve contact between the deposition promoting agent and the autocatalytic solution bath.

Any organic /inorganic material that will solidify or "set" and be adhered to the printable surface of the substrate may be used as a binder. Examples may be ink solutions containing polymers e.g. poly(vinyl acetate), acrylics, poly(vinyl alcohol) and/or inorganic materials that behave as cements or sol-gels coatings , e.g titanium isopropoxide and other alkoxides.

Fillers comprise insoluble particles contained in the ink that are small enough to transfer from the printer mechanism. Typically, 10- 200 nm carbon black particles are added to colour inkjet inks and 1-100 micron graphitic carbon is added to screen- printable inks used in the fabrication of printed electrical conductors. Ceramics, organic dyes or polymer particles may be added to ink to provide colour and/or texture in the printed product e.g. titania, alumina, mica, glass, acrylics. The ink may therefore be formulated with any of these components and include the deposition promoting material to provide a wide range of properties. As an alternative to including binders and fillers within the ink formulation the substrate may incorporate a porous layer which can influence the adhesion, scratch resistance and texture of the subsequent electroiess metal coating.

Where a chemical reducing agent is deposited onto a substrate to become the deposition promoting agent, the method may conveniently comprise a further step of immersing the now "sensitised" substrate into an intermediate solution bath of reducible metal ions (prior to the final autocatalytic solution bath), to provide an "activating" metal overlayer on the deposition promoting agent. This further step has the effect of aiding the deposition promoting material and promoting easier deposition of certain metals (such as copper, nickel and cobalt).

For example, for the case of an ink formulation containing SnCl₂ as the deposition promoting material, once the substrate material has had the SnCl₂ applied to it, it can be immersed into an intermediate solution bath comprising a dilute aqueous solution of PdCl₂. This causes the deposition of Pd metal onto the areas of the substrate coated with the deposition promoting material. If the Pd "activated" substrate is now immersed into an autocatalytic solution then autocatalytic deposition will take place onto the Pd metal. Such an intermediate step is useful in cases where the metal to be deposited from the autocatalytic deposition bath is either copper, nickel or cobalt.

As an alternative to the above the ink formulation could contain PdCl₂ instead of SnCl₂. Following deposition of this onto the substrate, an intermediate step could be to convert the PdCl₂ on the surface of the substrate to Pd metal by immersion in a dilute aqueous solution of SnCl . Immersion in an autocatalytic deposition bath could then take place as before.

In a further alternative, the intermediate step could be omitted by using a "reduced" complex as the deposition promoting material, i.e. the deposition promoting material could be formulated to contain a combination of chemical species comprising both a reducing agent and an activator. For example, both SnCl₂ (sensitiser) and PdCl₂ (activator) could be added to the ink formulation. Following deposition of this onto the substrate material the substrate could be introduced immediately into the autocatalytic deposition solution to deposit the metal of choice.

Embodiments of the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows the three stage process of producing a metallised substrate using an ink jet printing system.

Figure 2 shows the three stage process of producing a metallised substrate using a screen printing process.

Turning to Figure 1, an ink jet printing system 1 coats a substrate 3 with an ink formulation containing a deposition promoting material in a user determined pattern 5. The treated substrate 3, 5 is then immersed in an autocatalytic deposition solution 7 to produce a user determined metalised pattern 9.

The substrate 3 used was a sheet of plastic. The ink had a "reduced" complex formulation of 0.01-0. Ig of PdCl₂ dissolved in HCL (0.6ml were used from a 5M aqueous solution of the acid), water, 0.01-0. Ig SnCl₂ plus standard inkjet solvents and binders. The autocatalytic deposition solution comprised a nickel salt and a sodium hypophosphite reducing agent. Following immersion in the autocatalytic solution bath a coating of nickel was found to have been deposited only on the patterned area 5,

Turning to Figure 2, a screen printing system 11 coats a substrate 3 with an ink formulation containing a deposition promoting material in a user determined pattern 5 (like numerals being used to denote like features between Figures 1 and 2). The treated substrate is once again immersed in an autocatalytic deposition solution 7 to produce a user determined metalised pattern 9.

The substrate used was a plastic sheet once again, The ink had a formulation of 0.01- O.lg of PdCl₂, HC1, water, 0.01-0. Ig SnCl₂ and Acheson TiO₂ screen printing paste. Following screen printing the ink formulation was dried and cured before being placed in an autocatalytic solution bath of a nickel salt and sodium hypophosphite. Following five minutes of autocatalytic deposition an approximately 1 micron thick layer of nickel had been deposited onto the screen printed pattern.

The skilled man will appreciate that the above principles can be applied with different autocatalytic materials and solutions and different pattern transfer mechanisms in order to produce the desired metallised and patterned substrate. For example, the inkjet printing ink formulation relating to Figure 1 could also be delivered onto a substrate by means of a fibre tipped pen in order to create the desired pattern.

Claims

1. A method of preparing a substrate material for subsequent metal plaiing by an autocatalytic deposition process comprising coating some or all of the substrate material with a deposition promoting material (as hereinbefore defined) which is capable, once the coated substrate is introduced into an autocatalytic deposition solution, of facilitating the deposition of a metal coating from an autocatalytic solution onto the coated areas of the substrate wherein the deposition promoting material is printed onto the substrate by a pattern transfer mechanism.

2. A method of preparing a substrate material for subsequent metal plating by an autocatalytic deposition process as claimed in Claim 1 wherein the pattern transfer mechanism is screen printing.

3. A method of preparing a substrate material for subsequent metal plating by an autocatalytic deposition process as claimed in Claim 1 wherein the pattern transfer mechanism is ink-jet printing.

4. A method of preparing a substrate material for subsequent metal plating by an autocatalytic deposition process as claimed in any preceding claim wherein the deposition promoting material is contained within an ink formulation

5. A method of preparing a substrate material for subsequent metal plating by an autocatalytic deposition process as claimed in Claim 4 wherein the ink formulation contains additional binders and/or fillers capable in use of enhancing the autocatalytic deposited coating.

6. A method of preparing a substrate material for subsequent metal plating by an autocatalytic deposition process as claimed in any of Claims 1-5 wherein the deposition promoting material comprises an activator comprising a colloidal dispersion of a catalytic material which is capable, once the substrate is introduced into an autocatalytic solution, of initiating and sustaining an autocatalytic reaction.

7. A method of preparing a substrate material for subsequent metal plating by an autocatalytic deposition process as claimed in any of claims 1-5 wherein the deposition promoting material comprises a material that, once the substrate is introduced into an autocatalytic deposition solution, will undergo ion exchange with the metal salt in the autocatalytic deposition solution.

8. A method of preparing a substrate material for subsequent metal plating by an autocatalytic deposition process as claimed in any preceding claim wherein the deposition promoting material comprises a reducing agent.

9. A method of preparing a substrate material for subsequent metal plating by an autocatalytic deposition process as claimed in Claim 8 wherein the deposition promoting material is SnCl _.

10. A method of preparing a substrate material for subsequent metal plating by an autocatalytic deposition process as claimed in claim 8 or 9 wherein the method additionally comprises the step of introducing the substrate after it has been coated with the deposition promoting material into an aqueous metal salt solution with which the deposition promoting material will react to reduce the metal from the aqueous metal solution onto those parts of the substrate that have been coated with the deposition promoting material, the reduced metal being selected such that it is capable, once the treated substrate is introduced into an autocatalytic solution, of catalysing the deposition of a further metal from an autocatalytic deposition solution

11. A method of preparing a substrate material for subsequent metal plating by an autocatalytic deposition process as claimed in Claim 8 wherein the deposition promoting material comprises a combination of reducing agent and activator.

12. A method of preparing a substrate material for subsequent metal plating by an autocatalytic deposition process as claimed in any preceding claim wherein the substrate material comprises a porous surface layer.

13. A method of metal plating a substrate by an autocatalytic deposition process comprising the steps of: a) preparing the substrate material according to the method of preparing a substrate material as claimed in any preceding claim, and

b) introducing the prepared substrate material from step (a) into an autocatalytic deposition solution, the autocatalytic solution comprising a metal salt and a reducing agent.

14. A method of metal plating a substrate by an autocatalytic deposition process as claimed in Claim 13 comprising the further step of introducing the coated substrate from step (b) of Claim 13 into a further autocatalytic solution comprising a further metal salt and a reducing agent.

15. A method of metal platmg a substrate by an autocatalytic deposition process as claimed in Claim 13 comprising the further step of introducing the coated substrate material from step (b) of Claim 13 into an electrolytic bath in order to electrodeposit a further metal.

16. A method of metal plating a substrate by an autocatalytic deposition process as claimed in Claim 13 wherein the autocatalytic solution contains two or more metals salts in solution.

17. An ink formulation for carrying out the method of claim 4, the ink comprising a deposition promoting material and a solvent.

18. An ink formulation as claimed in Claim 17 wherein the solvent is water, ester, alcohol or ketone based.

19. An ink formulation as claimed in Claims 17 or 18 further comprising binder materials.

20. An ink formulation as claimed in any of Claims 17 to 19 further comprising filler materials.

21. An ink formulation as claimed in Claim 19 wherein the binder material comprises poly(vinyl acetate) polymers.

22. An ink formulation as claimed in Claim 19 wherein the binder material comprises acrylic polymers

23. An ink formulation as claimed in Claim 19 wherein the binder material comprises poly(vinyl alcohol) polymers.

24. An ink formulation as claimed in Claim 20 wherein the filler material comprises insoluble particles which are arranged in use to be capable of transferring from the pattern transfer mechanism to the substrate.