WO2016102473A1

WO2016102473A1 - Method for electroless plating of a precious metal

Info

Publication number: WO2016102473A1
Application number: PCT/EP2015/080802
Authority: WO
Inventors: Cyrille GAUTIER; Lynda Si-Ahmed
Original assignee: Metalor Technologies International Sa
Priority date: 2014-12-23
Filing date: 2015-12-21
Publication date: 2016-06-30
Also published as: CH710579A1

Abstract

Process for forming a precious metal layer (5) on a surface of a substrate (1) comprising steps of: providing a substrate (1); activation of a surface of said substrate (1) by wetting the said surface with a suspension (2) of precious metal nanoparticles (3) in a solvent so as to deposit precious metal nanoparticles (3) thereupon, said solvent comprising at least one organic solvent; immersion of the thus-activated surface in an aqueous solution (4) comprising at least one precious metal salt so as to deposit a precious metal layer (5) on the said surface.

Description

METHOD FOR ELECTROLESS PLATING OF A PRECIOUS METAL Technical Field

[0001] The present invention relates to the field of metallic plating of substrates.

More specifically, it relates to electroless plating of precious metals on substrates. State of the art

[0002] Electroless plating of non-conducting or semi-conductive substrates, particularly of plastics such as ABS, polycarbonate, polypropylene and so on, is well known per se as an alternative to physical vapour deposition. Such plated substrates are used for e.g. aesthetic elements in automobiles and consumer electronics, and also as the basis for printed circuit boards in electronics.

[0003] The document EP 2 559 786 describes a tin-free variant of such a process.

This process comprises steps of pre-treatment of the substrate (degreasing, solvent swelling), application of a promoter such as sulfuric acid, chromic acid, alkaline permanganate or plasma etching, followed by applying a neutralizer to neutralize any residues left by the promoter, and application of a conditioner. This may be followed by micro-etching. Then, the surface of the substrate is activated by immersion in an aqueous solution of precious metal catalyst, comprising nanoparticles of silver, gold, platinum, palladium, iridium, copper, aluminum, cobalt, nickel and iron together with a stablilising compound such as gallic acid, derivatives and salts thereof, followed by conventional electroless copper, copper alloy, nickel or nickel alloy plating.

[0004] However, such processes comprise many steps and are time consuming, since the surface has to be prepared such that the nanoparticles in the aqueous catalyst solution can adhere to the surface of the substrate by ionic bonding. This requires many process steps and many rinsings. [0005] Furthermore, to achieve a precious metal finish, this finish must be applied electrolytically to the conventional electroless plating, since this latter is copper or nickel based.

[0006] An object of the invention is thus to reduce the number of steps required to electroless-deposit a precious metal layer on a non-conductive substrate.

Disclosure of the invention

[0007] More specifically, the invention relates to a process for forming a precious metal layer on a surface of a substrate comprising steps of providing a substrate, activation of a surface of said substrate (which may be either the entirety of the surface of the substrate or a selectively-wetted area thereof), by wetting with a suspension of precious metal nanoparticles in a solvent, said solvent comprising at least one organic solvent, so as to deposit precious metal nanoparticles thereupon. Such wetting causes the nanoparticles, which act as catalysts for electroless deposition of precious metal, to adhere to the surface of the substrate by Van der Waal's forces. Using such a suspension of precious metal nanoparticles in an organic solvent, also known as a colloidal suspension, works for this purpose and is simpler, since polarized attraction between the nanoparticles and the surface does not need to be generated as it is in the case of an aqueous regime.

[0008] Subsequently, the thus-activated surface of the substrate is immersed in an aqueous solution, often known as a plating solution, comprising at least one precious metal salt so as to deposit a precious metal layer on the said surface. This deposition is catalyzed by the nanoparticles deposited previously.

[0009] It should be noted that the precious metal of the layer may be the same or different to the precious metal of the nanoparticles. If the metal is the same, the catalyzing effect may be improved and the speed and quality of the deposited precious metal layer may be improved.

[0010] In consequence, a precious metal layer can be formed directly by electroless plating on the surface of the substrate without an intervening adhesion layer, and without requiring any subsequent electodeposition, in a very limited number of steps and using simple chemistry with low toxicity and low reagent disposal costs.

[001 1] Advantageously, the suspension comprises precious metal nanoparticles with diameters in the range of 0.1 -900nm, more specifically 1 -100nm, further more specifically 2nm-20nm.

[0012] The wetting of the surface may be carried out by at least one of the following:

- dipping the substrate in said suspension;

- immersion of the substrate in a bath of said suspension;

- spraying said suspension onto said surface of the substrate;

- printing said suspension onto said surface of the substrate, e.g. by inkjet printing, pad printing, reel-to-reel printing, screen printing, aerosol jet printing, or any convenient printing method.

[0013] Dipping, immersion and spraying are very simple methods, which can be combined with previously-deposited masks, whereas printing of the colloidal nanoparticle suspension onto the surface of the substrate allows very precise determination of the location of the deposited precious metal layer, with high resolution and using standard technology particularly such as inkjet printing.

[0014] Such masks or inkjet printing of the suspension allow the wetting of the surface to be carried out selectively so as to form patterns of deposited precious metal.

[0015] The substrate may comprise at least one of: a plastic or resin; a ceramic; a glass; a glass-ceramic; a textile; silicon. ABS plastic is commonly used.

[0016] Advantageously, the solvent used for the colloidal nanoparticle suspention comprises undecane (CH₃(CH2)9CH₃), which has a suitable viscosity for suspending precious metal nanoparticles.

[0017] The precious metal comprises at least one of: silver, gold, platinum, rhodium, iridium, osmium, palladium, rhenium, ruthenium, gallium, indium, tellurium. In a particularly economic variant, the precious metal is silver or an alloy thereof, this latter preferably being a precious alloy thereof, i.e. one which is majority silver. [0018] As is commonly known, the aqueous solution further comprises at least one reducing agent such as ascorbic acid, and may also comprise a surfactant such as CTAN and a pH-reducing agent such as nitric acid.

[0019] Advantageously, the precious metal layer is deposited to a thickness such that the conductivity of said precious metal layer is at least 60'000S/m. This conductivity is adequate to permit electrodeposition of a further metallic layer thereupon.

[0020] The invention further relates to a substrate comprising a precious metal layer deposited thereupon by means of a process as defined above.

[0021] Finally, the invention further relates to a process of forming a metallic component in a mould formed by a structured substrate, in which a precious metal layer is deposited by the above-mentioned method in the mould, the metallic component being subsequently removed from the mould. Brief description of the drawings

[0022] Further details of the invention will be described in more detail in the following description, in reference to the appended figures which show :

- Fig. 1 : a schematic block diagram of the principle steps of the method according to the invention;

- Fig. 2: a photograph of an activated substrate immediately after immersion in the plating solution;

- Fig. 3: a photograph of an activated substrate after several minutes of immersion in the plating solution;

- Fig. 4: a photograph of an activated substrate after 30-40 minutes of immersion in the plating solution;

- Figure 5: a photograph of a selectively-activated substrate after removal from the plating solution after having been immersed therein for 30-40 minutes;

- Figure 6: a schematic flow diagram of a process of making a metallic component, incorporating the method of the invention.

Embodiment of the invention [0023] Figure 1 illustrates most generically the principal steps of the method of the invention.

[0024] In its most generic form, the three principle steps are:

[0025] - step 101 : providing a substrate 1 ;

[0026] - step 102: activating a surface of the substrate 1 by wetting it with a suspension 2 of precious metal nanoparticles in a solvent, wherein the solvent comprises at least one organic solvent;

[0027] - step 103: depositing a precious metal layer 5 on the activated surface by immersing the thus-activated surface in an aqueous solution 4 comprising at least one precious metal salt.

[0028] The known prior and intermediate steps of degreasing, rinsing, drying etc. have not been represented in figure 1.

[0029] Firstly, in step 101 a substrate 1 is provided. This may be of any substantially non-conducting and/or dielectric and/or semiconducting material. In practice, this would usually be a thermoplastic or thermosetting polymer or resin. A nonlimiting list of such substances includes cellulosic resins such as ethyl acetate, cellulose propionate, cellulose acetate butyrate and cellulose nitrate, acetal resins, acrylics such as methyl acrylate, polyethers, nylon, polyethylene, polystyrene, styrene blends, polycarbonates, polychlorotrifluoroethylene, and vinylpolymers and copolymers, such as vinyl acetate, vinyl chloride, vinyl chloride-acetate copolymers, vinylidene chloride vinyl alcohol, vinyl butyral, vinyl formal, ABS (acrylonitrile-butadiene-styrene), allyl phthalate, furane, melamine-formaldehyde, phenol-formaldehyde and phenol-furfural copolymers, polyacrylic esters, silicones, urea formaldehydes, epoxy resins, allyl resins, glyceryl phthalates and polyesters. Other examples are known to the skilled person and need not be repeated. Other suitable classes of material are ceramics, glasses, glass-ceramics, textiles and semiconductors such as silicon.

[0030] Such substrates may ultimately form decorative elements in automobiles, consumer electronics and so on, moulds for electroformed micromechanical components, printed circuit boards, or any other conceivable use. [0031] Contrary to prior art electroless deposition methods, only a basic preparation of the substrate is required, namely degreasing. Roughening and/or structuring of the surface e.g. by etching (deep reactive ion etching, plasma etching, chemical etching or similar) or by mechanical means (sanding, bead blasting etc.) is by no means required, although the process is compatible with such pre-treatments which may be carried out if desired. Likewise, the application of various promoters, neutralisers, conditioners etc. and the use of stabilisers is not required.

[0032] If required, a mask may be applied to the surface of the substrate to mask off areas on which it is undesired to deposit the precious metal layer 5, such as by lithography, inkjet or screen printing of a suitable masking substance, application of adhesive films, or any other suitable known process.

[0033] After degreasing, in step 102 the surface of the substrate 1 is wetted with a suspension 2 of precious metal nanoparticles 3 in a solvent, which serve as a catalyst for the subsequent deposition, so as to cause a quantity of the precious metal nanoparticles 3 to adhere to the surface of the substrate 1 . The diameter of the precious metal nanoparticles 3 is in the range 0.1 - 900nm, ideally 1 -100nm, further ideally 2-20nm. The precious metal of the nanoparticles 3 may be the same as, or different to, the precious metal of the final deposited layer 5, and is applied as a colloidal suspension. The solvent forming the liquid component of the colloidal suspension comprises at least one organic solvent, and may indeed comprise exclusively one or more organic solvents substantially without water content, however it may also contain water in the case in which the solvent chosen is miscible therewith, such as alcohol. In practice, undecane (CH₃(CH2)9CH₃) used alone has proven to work well.

[0034] This wetting can be carried out simply by immersion in a bath of the colloidal nanoparticle suspension 2, as illustrated in figure 1. However, it is also possible that it can be applied as a spray, or by means of inkjet printing. This latter is particularly advantageous if the substrate is destined to be a printed circuit board (PCB), since the inkjet printing can selectively wet the surface so as to define the desired conductor paths of the PCB with a high precision, and an extremely high resolution. In the deposition step 103, the precious metal layer 5 will only be formed on the regions of the surface of the substrate which have been activated by the nanoparticle catalyst, i.e. which comprise precious metal nanoparticles 3 deposited thereupon, leaving other regions which do not comprise precious metal nanoparticles 3 uncoated.

[0035] The precious metal nanoparticles 3 adhere to the surface of the substrate 1 by Van der Waal's forces rather than by ionic attraction: it is the surprising discovery of the applicants that such Van der Waal's forces are sufficient to cause the nanoparticles 3 to adhere to the surface of the substrate 1 that permits the elimination of the use of the various promoters, neutralisers, conditioners etc. required to obtain adhesion of the nanoparticles in the aqueous regimes used in the prior art. This adhesion occurs in a single step which can be carried out at room temperature, using simple, safe chemistry.

[0036] Once the precious metal nanoparticles 3 have adhered to the surface of the substrate, the surface is activated since the precious metal nanoparticles 3 deposited thereupon act as a catalyst for the electroless deposition of the precious metal layer 5. In other words, each individual nanoparticle acts as a catalyst seed for growth of the precious metal layer 5, which is deposited by autocatalytic reduction from solution. The precious metal nanoparticles 3 are visible to the naked eye, causing the areas of the surface of the substrate 1 upon which they have been deposited to appear lightly brown in colour.

[0037] Once the surface of the substrate 1 has been activated, the substrate 1 is rinsed to remove excess solvent and non-adhered nanoparticles 3, and any applied mask may be removed if required. The substrate 1 is then immersed in an aqueous solution 4 comprising at least one precious metal salt. This precious metal salt may comprise the same precious metal as was used for the nanoparticle catalyst, however this is not strictly necessary and these precious metals may be different. In the case in which the solvent used is miscible with water, it may not be necessary to rinse the substrate, since the excess solvent will dissolve in the aqueous solution and diffuse away from the surface. [0038] The aqueous solution 4 furthermore comprises, as is known, a reducing agent such as citric acid, ascorbic acid, hydrazine, sodium hypophosphite, sodium borohydride, amine boranes or formaldehyde. As a result, an autocatalytic reaction seeded by the deposited precious metal nanoparticles 3 occurs, which reduces the precious metal ions of the precious metal salt so as to deposit a layer 5 of this precious metal on the activated areas of the surface of the substrate 1.

[0039] Finally, the substrate 1 is rinsed to remove traces of the aqueous solution.

[0040] As a result, a precious metal layer 5 can be deposited directly upon the surface of a non-conducting or semi-conducting substrate 1 without deposition of an intermediate metallic layer such as nickel or copper, and without requiring electrodeposition of the precious metal, in only two fundamental steps: a single-step activation of the surface 102 and a single deposition step 103.

[0041] Once the precious metal layer has been formed on the surface of the substrate, any further desired metallic layers may be formed thereupon by electrolytic deposition. In the case in which the substrate is structured so as to form a mould for a (micro)mechanical part 9, the precious metal layer is formed on the interior surface of the mould, this precious metal layer forming a conducter to permit the mould to be subsequently filled with metal by galvanic forming in a similar manner to in the well-known LIGA process (from the German Llthographie, Galvanoformung, Abformung), followed by separating the thus-formed piece from the mould. Figure 6 illustrates in more detail such a process for forming a metallic part 9.

[0042] In step 201 , a structured substrate 1 is provided, the substrate comprising a cavity forming a mould. This step corresponds to step 101 of the basic method as described above. The substrate 1 may be structured by any known means. In step 202, a precious metal layer 5 is formed at least on the interior surfaces of the cavity by means of steps 102 and 103 of the basic method. Subsequently, electrical contact is made with the precious metal layer 5, and the cavity is filled by electrodeposition of metal 7, such as copper, nickel, silver, or any other convenient metal, thereby forming metallic part 9.

[0043] Metallic part 9 is then liberated from the substrate 1 , either mechanically, or by dissolving away the substrate 1 in an appropriate solvent.

[0044] Practical example

[0045] The following example describes a concretised and laboratory-proven method for depositing a silver layer 5 on an ABS substrate 1. However, it is not to be construed as limiting of the invention. In particular, lengths of time mentioned can easily be varied according to the needs of the skilled person, other precious metals, other reducing agents, other pH reducing agents and other solvents can be used, and steps such as degreasing can be carried out in any known manner with any suitable chemicals or proprietary products.

1. Degreasing of the ABS substrate

The ABS substrate 1 is provided and is first ultrasonically cleaned in a bath of ethanol for 5 minutes, followed by 30 seconds of air drying. Subsequently, cleaning with a cleaner such as, but not limited to, MP-900 (a proprietary alkaline degreasing agent comprising a mixture of sodium hydroxide and ethanolamine, available from Dow Chemical), for 3 minutes is carried out, followed by a thorough rinsing in water then in ethanol. Subsequently, the substrate is air dried with compressed air.

2. Surface activation

The degreased substrate 1 is immersed for 5 minutes in a bath containing a suspension 2 of 0.4wt% Ag-D02 in undecane

Ag-D02 is a silver nanoparticle powder comprising particle sizes ranging from 2nm to 20nm, typically supplied as a paste comprising 5-20% undecane, which can easily be mixed with more undecane to achieve the desired concentration. [0050] After immersion, the substrate 1 is removed from the bath and is rinsed twice with undecane to remove excess nanoparticles, and subsequently rinsed twice with ethanol to remove the undecane. The substrate 1 is then dried.

[0051] 3. Electroless deposition

[0052] The thus-activated substrate 1 is immersed at room temperature in an aqueous solution 4 of silver salt together with a reducing agent, namely ascorbic acid. This solution comprises approximately the following components

[0053] The solution is well agitated with a magnetic stirring bar, and the deposition is carried out until the desired silver layer thickness is attained. A mirror finish is formed within 0.5 to 2 minutes, after which the layer 5 whitens progressively. Maximum practical layer thickness is attained after about 40 minutes. The deposition can be stopped at any moment when the desired thickness or finish of the thus-deposited silver layer 5 is attained.

[0054] Subsequently, the substrate 1 , naturally including the silver layer 5 deposited thereupon, is well-rinsed with demineralised water, and then in ethanol, after which it is dried with compressed air.

[0055] As can be seen from this concrete example, very few process steps are required in comparison to the prior art, resulting in requiring fewer raw materials, fewer different chemicals, fewer rinsings and thus less wastage of both time and materials. The method of the invention is thus extremely economic. It should be further noted that all the chemicals used are commonly-available, with low toxicity and low environmental impact, reducing the cost of acquisition and disposal of the chemicals used.

[0056] Figures 2-5 illustrate various different ABS substrates 1 at different stages of the above-mentioned method.

[0057] Figure 2 illustrates an activated substrate 1 just after immersion in the aqueous solution 4, showing the discoloration of the ABS substrate 1 due to the silver nanoparticles 3 deposited thereupon.

[0058] Figure 3 illustrates an activated substrate 1 after 1 -3 minutes of immersion in the aqueous solution 4, with a thin mirror-like layer of silver metal 5 having been deposited thereupon.

[0059] Figure 4 illustrates an activated substrate 1 after 30-40 minutes of immersion in the aqueous solution 4, with a significantly thicker (0.2 to 10 μηη) layer of silver metal 5 deposited on the substrate 1. This thicker layer appears white to the naked eye, due to the texture of the silver metal as the layer grows.

[0060] Figure 5 illustrates a substrate selectively coated with silver metal 5. Before application of the silver nanoparticle catalyst to the surface of the substrate 1 , the periphery of the substrate 1 was masked so as to prevent adhesion of the silver nanoparticles 3 thereto. In consequence, only the rectangular central portion of the surface of the substrate 1 has received the silver coating 5. [0061] Although the invention has been described in reference to specific embodiments, the scope of the invention as defined in the claims it is not to be construed as being limited thereby.

Claims

1. Process for forming a precious metal layer (5) on a surface of a substrate (1 ) comprising steps of:

- providing a substrate (1 );

- activation of a surface of said substrate (1 ) by wetting the said surface with a suspension (2) of precious metal nanoparticles (3) in a solvent so as to deposit precious metal nanoparticles (3) thereupon, said solvent comprising at least one organic solvent;

- immersion of the thus-activated surface in an aqueous solution (4) comprising at least one precious metal salt so as to deposit a precious metal layer (5) on the said surface.

2. Process according to claim 1 , wherein said precious metal layer (5) is formed directly on the said surface.

3. Process according to one of claims 1 or 2, wherein said precious metal layer (5) is formed of the same precious metal as said precious metal nanoparticles (3).

4. Process according to one of claims 1 to 3, wherein said suspension comprises precious metal nanoparticles (3) with diameters in the range of 2nm-20nm.

5. Process according to one of claims 1 to 4, wherein said wetting of the surface is carried out by at least one of the following:

- dipping the substrate (1 ) in said suspension (2);

- immersion of the substrate (1 ) in a bath of said suspension (2);

- spraying said suspension (2) onto said surface of the substrate (1 );

- printing said suspension (2) onto said surface of the substrate (1 ).

6. Process according to claim 5, wherein said wetting of said surface of the substrate (1 ) is carried out selectively.

7. Process according to any preceding clainn, wherein said substrate (1 ) comprises at least one of: a plastic or resin; a ceramic; a glass; a glass-ceramic; a textile; silicon.

8. Process according to any preceding claim, wherein said solvent comprises undecane.

9. Process according to any preceding claim, wherein said precious metal comprises at least one of: silver, gold, platinum, rhodium, iridium, osmium, palladium, rhenium, ruthenium, gallium, indium, tellurium.

10. Process according to claim 9, wherein said precious metal is silver or an alloy thereof.

1 1. Process according to any preceding claim, wherein said aqueous solution (4) further comprises at least one reducing agent such as ascorbic acid.

12. Process according to claim 1 1 , wherein said aqueous solution (4) further comprises a surfactant and a pH-reducing agent.

13. Process according to any preceding claim, wherein said precious metal layer (5) is deposited to a thickness such that the conductivity of said precious metal layer (5) is at least 60'000S/m.

14. Substrate (1) comprising a precious metal layer (5) deposited thereupon by means of a process according to any preceding claim.

15. Process for producing a metallic part (9), comprising the steps of:

- providing a precious metal layer (5) on a surface of a substrate (1 ) by means of the process of any of claims 1-12, wherein the substrate (1) is structured so as to form a mould for said metallic part (9); - forming said metallic part (9) by electroforming metal (7) onto said precious metal layer (5);

- separating said metallic part (9) from said substrate (1 ).