CH710579A1 - A method of electroless plating of a precious metal. - Google Patents

Abstract

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

Description

Technical area

The present invention relates to the field of metal plating of substrates. More specifically, it relates to autocatalytic plating of precious metals on substrates.

State of the art

[0002] Electroless plating of non-conductive or semiconductor substrates, in particular of plastics such as ABS, polycarbonate, polypropylene, etc., is well known as an alternative to physical vapor deposition. . These plated substrates are used for example for aesthetic elements in automobiles and consumer electronics, and also as a basis for printed circuit boards in electronic articles.

EP 2 559 786 discloses a tin-free variant of such a method. This process comprises steps of pretreatment of the substrate (degreasing, solvent swelling), application of a promoter such as sulfuric acid, chromic acid, alkaline permanganate or plasma etching, followed by application a neutralizer to neutralize any residues left by the promoter, and the application of a conditioner. This can be followed by a microgravure. The surface of the substrate is then activated by immersion in an aqueous solution of a precious metal catalyst, comprising nanoparticles of silver, gold, platinum, palladium, iridium, copper, aluminum , cobalt, nickel and iron, together with a stabilizing compound such as gallic acid, its derivatives and its salts, followed by conventional electroplating of copper, copper alloy, nickel or nickel alloy.

However, these methods comprise many steps and take time, since the surface must be prepared so 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 rinses.

In addition, to obtain a finish with a precious metal, this finish must be applied electrolytically on conventional electroless plating, since the latter is based on copper or nickel.

An object of the invention is therefore to reduce the number of steps required for the autocatalytic deposition of a precious metal layer on a non-conductive substrate.

Disclosure of the invention

More specifically, the invention relates to a method for forming a precious metal layer on a surface of a substrate, comprising the steps of providing a substrate, and activating a surface of said substrate (which may be either the entire surface of the substrate or a surface thereof selectively wetted) by wetting it with a suspension of nanoparticles of precious metal in a solvent, said solvent comprising at least one organic solvent, so as to deposit therein nanoparticles of precious metal. This wetting adheres the nanoparticles, which act as catalysts for autocatalytic precious metal deposition on the substrate surface by Van der Waals forces. The use of such a suspension of precious metal nanoparticles in an organic solvent, also called colloidal suspension, works for this purpose and is simpler, since a polarized attraction between the nanoparticles and the surface does not need to be generated as in the case of an aqueous diet.

The thus activated surface of the substrate is then immersed in an aqueous solution, often called a plating solution, comprising at least one precious metal salt so that a layer of precious metal is deposited on said surface. This deposition is catalyzed by previously deposited nanoparticles.

It should be noted that the precious metal of the layer may be identical to or different from the precious metal of the nanoparticles. If it is the same metal, the catalyst effect can be improved and the speed and quality of the deposited precious metal layer can be improved.

Accordingly, a layer of precious metal can be formed directly by autocatalytic plating on the surface of the substrate without an intervening adhesion layer, and without requiring any subsequent electrolytic deposition, in a very limited number of steps and by use. simple, low toxicity chemistry with low disposal costs of reagents.

Advantageously, the suspension comprises precious metal nanoparticles having diameters in the range from 0.1 to 900 nm, more specifically from 1 to 100 nm, more specifically from 2 nm to 20 nm.

The wetting of the surface can be carried out by at least one of the following methods: dipping the substrate in said suspension; immersing the substrate in a bath of said suspension; spraying said suspension on said surface of the substrate; printing said suspension on said surface of the substrate, for example by ink jet printing, pad printing, coil-to-coil printing, screen printing, aerosol jet printing, or any convenient printing method.

Soaking, immersion and spraying are very simple processes, which can be combined with masks deposited beforehand, while the printing of the colloidal nanoparticle suspension on the surface of the substrate allows a very precise determination. the location of the deposited precious metal layer, with a high resolution and using standard technology, particularly as an inkjet printing.

These masks, or the ink jet printing of the suspension, allow the wetting of the surface selectively to form precious metal patterns deposited.

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

The solvent used for the suspension of colloidal nanoparticles advantageously comprises undecane (CH3 (CH2) 9CH3), which has a viscosity suitable for suspending nanoparticles of precious metal.

[0017] The precious metal comprises at least one of: silver, gold, platinum, rhodium, iridium, osmium, palladium, rhenium, ruthenium, gallium, indium, tellurium. In a particularly economical variant, the precious metal is silver or an alloy thereof, the latter being preferably a precious alloy thereof, that is to say consisting predominantly of silver.

As is usually 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.

The precious metal layer is advantageously deposited in a thickness such that the conductivity of said precious metal layer is at least 60000 S / m. This conductivity is adequate to allow electrolytic deposition of another metal layer over it.

The invention further relates to a substrate comprising a layer of precious metal deposited thereon by means of a method as defined above.

Finally, the invention further relates to a method for forming a metal component in a mold formed by a structured substrate, wherein a layer of precious metal is deposited in the mold by the aforementioned method, the metal component being subsequently removed of the mold.

Brief description of the drawings

Other details of the invention will be described in more detail in the description which follows, with reference to the appended figures, which show: <Tb> Fig. 1: <SEP> a schematic flow diagram of the main steps of the process according to the invention; <Tb> Fig. 2: <SEP> a photograph of an activated substrate immediately after immersion in the plating solution; <Tb> Fig. 3: <SEP> a photograph of an activated substrate after several minutes of immersion in the plating solution; <Tb> Fig. 4: <SEP> a photograph of an activated substrate after 30-40 minutes of immersion in the plating solution; <Tb> Fig. 5: <SEP> a photograph of a selectively activated substrate after it has been removed from the plating solution in which it has been immersed for 30-40 minutes; <Tb> Fig. 6: <SEP> a schematic flow diagram of a process for preparing a metal component, incorporating the process of the invention.

Embodiment of the invention

FIG. 1 very generically illustrates the main steps of the method of the invention.

In their most generic form, the three main stages are: Step 101: providing a substrate 1; Step 102: activation of a surface of the substrate 1 by wetting it with a suspension 2 of nanoparticles of precious metal in a solvent, the solvent comprising at least one organic solvent; Step 103: depositing a layer of precious metal on the activated surface by immersing the surface thus activated in an aqueous solution comprising at least one precious metal salt.

The intermediate steps and the prior art, degreasing, rinsing, drying, etc., have not been shown in FIG. 1.

First, in step 101, a substrate 1 is obtained. This can be in virtually any non-conductive material and / or dielectric and / or semiconductor. In practice, it would usually be a resin or a thermoplastic or thermosetting polymer. A non-limiting 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, polyethers, nylon, polyethylene, polystyrene, styrenes blends, polycarbonates, polychlorotrifluoroethylene, and vinyl polymers and copolymers, such as vinyl acetate, vinyl chloride, chloride copolymers vinyl acetate, vinylidene chloride; vinyl alcohol, vinyl butyral, vinyiformal, ABS (acrylonitrile-butadiene-styrene), allyl phthalate, furan, copolymers of melamine-formaldehyde, phenol-formaldehyde and phenol-furfural, esters polyacrylates, silicones, urea-formaldehyde, epoxy resins, allyl resins, glyceryl phthalates, and polyesters. Other examples are known to those skilled in the art and need not be repeated. Other suitable classes of materials are ceramics, glasses, glass ceramics, textiles and semiconductors such as silicon.

These substrates may ultimately form decorative elements in automobiles, consumer electronics, etc., molds for electromechanical micromechanical components, printed circuit boards, or for any other conceivable use.

Unlike the autocatalytic deposition processes of the prior art, only a basic preparation of the substrate is required, namely a degreasing. It is in no way required to provide the surface with roughness and / or a structure, for example by etching (deep reactive ion etching, plasma etching, chemical etching or the like) or by mechanical means (sandblasting, projection bead, etc.), although the process is compatible with such pretreatments that can be performed if desired. Similarly, the application of various promoters, neutralisers, conditioners, etc., and the use of stabilizers, are not required.

If necessary, a mask may be applied to the surface of the substrate to hide areas on which it is not desired to deposit the precious metal layer 5, for example by lithography, inkjet printing or screen printing of a suitable masking substance, application of adhesive films, or by any other suitable known method.

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 ensure that a certain amount of the precious metal nanoparticles 3 adheres to the surface of the substrate 1. The diameter of the precious metal nanoparticles 3 is in the range from 0.1 to 900 nm, ideally from 1 to 100 nm, even more ideally from 2 to 20 nm. The precious metal of the nanoparticles 3 may be identical or different from the precious metal of the final deposited layer 5, and is applied in the form of a colloidal suspension. The solvent forming the liquid component of the colloidal suspension comprises at least one organic solvent, and may moreover exclusively comprise one or more organic solvents practically without water; however, it may also contain water in the case where the selected solvent is miscible with water, such as an alcohol. In practice, undecane (CH3 (CH2) 9CH3) alone has proven to work well.

This wetting can be carried out simply by immersion in a bath of the colloidal nanoparticle suspension 2, as shown in FIG. 1. However, it is also possible that it can be applied in the form of a spray, or by means of ink jet printing. The latter is particularly advantageous if the substrate is intended to be a printed circuit board (PCB), since ink jet printing can selectively wet the surface so as to define the desired conductive paths of the PCB with high accuracy and an extremely high definition. In the deposition step 103, the precious metal layer 5 will only be formed on the regions of the substrate surface which have been activated by the nanoparticle catalyst, that is to say which comprise nanoparticles of metal valuable 3 deposited on these, leaving uncoated other regions that do not include nanoparticles of precious metal 3.

The precious metal nanoparticles 3 adhere to the surface of the substrate 1 by Van der Waals forces rather than by ionic attraction; the Applicants have surprisingly found that these Van der Waals forces are sufficient to adhere the nanoparticles 3 to the surface of the substrate 1, thereby eliminating the use of various promoters, neutralizers, conditioners, etc., necessary to obtain the adhesion of the nanoparticles in the aqueous regimes used in the prior art. This adhesion occurs in a single step that can be performed at room temperature using simple and safe chemistry.

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 thereon act as a catalyst for the autocatalytic deposition of the precious metal layer 5. in other words, each individual nanoparticle acts as a catalyst seed to grow the precious metal layer 5, which is deposited by autocatalytic reduction from a solution. The nanoparticles of precious metal 3 are visible to the naked eye, making appear with a light brown color areas of the surface of the substrate 1 on which they were deposited.

Once the surface of the substrate 1 has been activated, the substrate 1 is rinsed to remove the excess solvent and the nanoparticles 3 not adhered, and if necessary can be removed any mask applied. 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 that used for the nanoparticle catalyst; however this is not strictly necessary, and these precious metals may be different. In the case where 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.

The aqueous solution 4 further comprises, as is known, a reducing agent such as citric acid, ascorbic acid, hydrazine, sodium hypophosphite, sodium borohydride, boranes. amine or formaldehyde. As a result, an autocatalytic reaction seeded by the 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 substrate surface. 1.

Finally, rinsing the substrate 1 to remove traces of the aqueous solution.

As a result, a layer of precious metal 5 can be deposited directly on the surface of a non-conductive or semiconductor substrate 1 without deposition of an intermediate metal layer, for example nickel or copper, and without requiring electrolytic deposition of the precious metal, in only two basic steps: a single step of activation of the surface 102, and a single deposition step 103.

Once the precious metal layer has been formed on the surface of the substrate, any other desired metal layers may be formed thereon by electrolytic deposition. In the case where the substrate is structured so as to form a mold for a (micro) mechanical part 9, the precious metal layer is formed on the inner surface of the mold, this layer of precious metal forming a conductor to allow the mold to to be then filled with metal by galvanic formation in a manner similar to the well-known LIGA process (German lithography, Galvanoformung, Abformung), followed by the separation of the piece thus formed out of the mold. Fig. 6 illustrates in more detail such a method for forming a metal part 9.

In step 201, there is a structured substrate 1, the substrate comprising a cavity forming a mold. This step corresponds to step 101 of the basic method as described above. The substrate 1 can 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. Then, an electrical contact is made with the precious metal layer 5, and the cavity is filled by electrolytic deposition of a metal 7 such as copper, nickel, silver, or any other suitable metal, thus forming a metal part 9.

The metal part 9 is then released from the substrate 1, either mechanically or by removing the substrate 1 by dissolution in a suitable solvent.

Practical example

The following example describes a method concretized and tested in the laboratory for depositing a silver layer 5 on an ABS substrate 1. However, it should not be considered as limiting the invention. In particular, the lengths of time mentioned can easily vary according to the needs of the person skilled in the art, it is possible to use other precious metals, other reducing agents, other pH reducing agents and other solvents. and steps can be implemented such as degreasing in any known manner with any known chemicals or deposited products.

1. Degreasing ABS substrate

There is an ABS substrate 1 which is first cleaned with ultrasound in an ethanol bath for 5 minutes, then air dried for 30 seconds. Then, it is cleaned for 3 minutes with a cleaner such as, but not limited to, MP-900 (a deposited alkaline degreasing agent comprising a mixture of sodium hydroxide and ethanolamine, available from Dow Chemical), after which is rinsed thoroughly with water and ethanol. The substrate is then air dried with compressed air.

2. Surface activation

The defatted substrate 1 is immersed for 5 minutes in a bath containing a suspension 2 of 0.4% by weight of Ag-D0 2 in undecane (CH 3 (CH 2) 9 CH 3). Ag-D02 is a silver nanoparticle powder comprising particle sizes ranging from 2 nm to 20 nm, typically provided in the form of a paste comprising 5-20% undecane, which can be easily mixed with more undecane to obtain the desired concentration.

After immersion, the substrate 1 is removed from the bath and rinsed twice with undecane to remove the excess nanoparticles, and then rinsed twice with ethanol to remove the undecane. The substrate 1 is then dried.

3. Autocatalytic deposition

The substrate 1 thus activated 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. <tb> Silver nitrate (AgNO3) <SEP> 0.004% <SEP> Precious metal salt <tb> Nitric acid (HNO3) <SEP> 0.3% <SEP> pH reduction <tb> CTAN (cetyltrimethylammonium nitrate) <SEP> 0.01% <SEP> Surfactant <tb> Ascorbic acid <SEP> 0.02% <SEP> Reducing agent <tb> Water <SEP> The rest <SEP> Solvent

The solution is stirred well with a magnetic stir bar, and the deposition is carried out until the desired thickness of the silver layer is reached. A mirror finish is formed in 0.5 to 2 minutes, after which the layer 5 gradually whitens. A maximum practical layer thickness is reached after about 40 minutes. The deposition can be stopped at any time when the desired thickness or finish of the silver layer thus deposited has been obtained.

Subsequently, the substrate 1, naturally comprising the silver layer 5 deposited on it, is thoroughly rinsed with deionized water and then in ethanol, after which it is dried with compressed air.

As can be seen from this concrete example, very few process steps are necessary in comparison with the prior art, which has the result that it requires less raw materials, fewer different chemicals. less rinsing and therefore less waste both in time and materials. The process of the invention is therefore extremely economical, it should also be noted that all the chemicals used are commonly available, are low in toxicity and have a low impact on the environment, which reduces the cost of acquisition and disposal of used chemicals.

Figs. 2 - 5 illustrate various different ABS 1 substrates at different stages of the aforementioned process.

FIG. 2 illustrates an activated substrate 1 just after immersion in the aqueous solution 4, showing the discoloration of the ABS substrate 1 due to silver nanoparticles 3 deposited thereon.

FIG. 3 illustrates an activated substrate 1 after 1-3 minutes of immersion in the aqueous solution 4, with a thin layer similar to a metallic silver mirror 5 which has been deposited thereon.

FIG. 4 illustrates an activated substrate 1 after 30-40 minutes of immersion in the aqueous solution 4, with a significantly thicker layer (0.2 to 10 μm) of metallic silver deposited on the substrate 1. This thicker layer appears white to the naked eye, because of the texture of metallic silver as the layer grows.

FIG. 5 illustrates a substrate selectively coated with metallic silver 5. Prior to the application of the silver nanoparticle catalyst to the surface of the substrate 1, the periphery of the substrate 1 has been masked so as to prevent adhesion thereto of the nanoparticles of the substrate. As a result, only the rectangular central portion of the surface of the substrate 1 has received the silver coating 5.

Although the invention has been described with reference to specific embodiments, it is not intended that the scope of the invention, as defined in the claims, be limited by them.

Claims (15)

A method for forming a precious metal layer (5) on a surface of a substrate (1), comprising the steps of: - to have a substrate (1); Activating a surface of said substrate (1) by wetting said surface with a suspension (2) of precious metal nanoparticles (3) in a solvent, so as to deposit nanoparticles of precious metal (3), said solvent comprising at least an organic solvent; - Immerse the surface thus activated in an aqueous solution (4) comprising at least one precious metal salt so as to deposit a layer of precious metal (5) on said surface. The method of claim 1, wherein said precious metal layer (5) is formed directly on said surface. 3. Method according to one of claims 1 and 2, wherein said layer of precious metal (5) is formed of the same precious metal as said nanoparticles of precious metal (3). The process according to one of claims 1 to 3, wherein said suspension comprises precious metal nanoparticles (3) having diameters in the range of 2 nm to 20 nm. 5. Method according to one of claims 1 to 4, wherein said wetting of the surface is carried out by at least one of the following methods: - Soaking the substrate (1) in said suspension (2); - Immersing the substrate (1) in a bath of said suspension (2); Spraying said suspension (2) on said surface of the substrate (1); Printing said suspension (2) on said surface of the substrate (1). The method of claim 5, wherein said wetting of said surface of the substrate (1) is performed selectively. The method of any of the preceding claims, wherein said substrate (1) comprises at least one of: plastic or resin; a ceramic; a glass; a ceramic hob; a textile; silicon. The process of any one of the preceding claims, wherein said solvent comprises undecane. 9. Process according to any one of the preceding claims, wherein said precious metal comprises at least one of: silver, gold, platinum, rhodium, iridium, osmium, palladium, rhenium, ruthenium, gallium, indium, tellurium. The method of claim 9, wherein said precious metal is silver or an alloy thereof. The method of any of the preceding claims, wherein said aqueous solution (4) further comprises at least one reducing agent such as ascorbic acid. The method of claim 11, wherein said aqueous solution (4) further comprises a surfactant and a pH reducing agent. 13. A method according to any one of the preceding claims, wherein said layer of precious metal (5) is deposited in a thickness such that the conductivity of said precious metal layer (5) is at least 60,000 S / m. 14. Substrate (1) comprising a layer of precious metal (5) deposited thereon by means of a process according to any one of the preceding claims. A method for producing a metal part (9), comprising the steps of: A precious metal layer (5) is provided on a surface of a substrate (1) by means of the method of any one of claims 1 to 12, the substrate (1) being structured so as to form a mold for said metal part (9); Forming said metal part (9) by electroforming metal (7) on said precious metal layer (5); Separating said metal part (9) from said substrate (1).