GB2320728A - Depositing a metallic film involving pretreatment - Google Patents

Abstract

In a method of depositing a metallic film on a substrate, a polymer resin layer is applied to the substrate. Metallic particles (or particles of a metal compound) are dispersed in this layer. The layer is then subjected to a plating solution to form the metallic film. The metal is more than 80% by weight of the total of metal plus resin. The polymer resin may be printed on the substrate in a pattern corresponding to an electrical circuit wiring pattern. Examples use as the metal a mixture of tin and copper, tin and brass, or tin and zinc; and as the polymer a phenolic varnish optionally with an epoxy resin, or an epoxy resin.

Description

METHOD OF DEPOSITING A METALLIC FILM The present invention relates to a method for deposition of a metallic film on a substrate. The method is particularly, although not exclusively, for the deposition of wiring tracks on substrates of electronic circuit boards.

Conventionally, electrically conductive paths upon printed circuit boards and the like have generally been manufactured by subtractive methods (i.e. by etching away undesired portions of a layer of electrically conductive material - typically copper extending over the surface of a board) or by an additive method (i.e. by building up sequential layers of conductive and dielectric materials upon a substrate).

A subtractive method normally begins with a substrate to which a conductive layer of copper or some other metal has been applied, e.g. by vacuum deposition. The conductive paths which interconnect the circuit components are produced by masking the areas of copper or metal required to make up the circuit with a protective layer, usually referred to as resist. When the substrate is sprayed with a corrosive etchant solution the unprotected areas are etched away. Those areas beneath the resist however are protected by it and so remain on the substrate. The resist layer is then stripped away using a suitable solvent or caustic stripping solution to reveal the desired circuit pattern.

The resist layer pattern may for example be defined using a silk printing screen. The resist pattern may also be produced by exposing a photosensitive resist layer to LW radiation through a suitable piece of art-work, such that the pattern required to define the circuit is rendered insoluble in dilute alkaline solutions. The areas not required to define the circuit are soluble in the dilute alkaline solution and so may be washed away, leaving or "developing" the resist in the pattern of the circuit required. The resists used in this application are then generally stripped in caustic solutions.

Another subtractive process is described in US-A-4 651 417. This uses a substrate which is non-conductive and has been pressed on its surface to produce recesses in the shape of the desired circuitry. The substrate is then vacuum sputtered with a suitable metal and then the circuit built up by electrodeposition. The finished circuit is then produced by physically removing the copper from the substrate surface by sanding or scraping, to leave the circuitry in the recesses as the conductive tracks on the substrate.

One specific type of additive process is commonly known as conductive polymer thick film (PTF). This utilizes metal filled (PTF) inks to define conductive circuit tracks. It has the benefit of being significantly more environmentally friendly as it does not involve the generation and subsequent processing or disposal of considerable quantities of copper solubilized in etchants. The conductive polymer inks are however very limited in their electrical conductivity and the process is therefore limited to a few specific applications.

Some other additive processes include the use of metal filled inks. The need to ensure that such metallic inks are firmly bonded to the substrates employed means that a fairly high percentage of adhesive resin needs to be included to affect the said bonding. The high bonding resin content limits the electrical conductivity.

One proposal for the combination of additive and subtractive type of process is disclosed in WO 90/12482. In this method, a non-conductive substrate is coated with a film of vacuum sputtered metal. This is then coated with a photosensitive resin which is selectively exposed and developed to produce the desired circuit pattern etched into the surface of the resist. The areas of sputtered metal bared by this process are then plated up with copper by electrodeposition to produce circuit tracks of the desired thickness. The photosensitive resist is then stripped and the sputtered metal layer flash etched away to yield the desired circuit pattern. Subsequently further layers of circuitry may be added by sequentially adding further layers of photosensitive material and sputtered metal and repeating the electrodeposition and etching processes.

Another process which is additive, is disclosed in WO 88/08337. This process involves selective catalysation of the surface of the substrate to allow circuit patterns to be produced using electrolytic copper deposition. The initial circuit shape being defined by a photoimageable resist.

A more recent proposal for an additive process is described in WO 93/06943. This is based on the use of a conductive ink including an adhesive resin, as alluded to above.

Specifically, the ink is a mixture of a relatively high melting point metal powder, solder powder, a cross linking flux and a reactive monomer or polymer. The reactive monomer or polymer system acts as an adhesive for bonding the ink to the substrate after curing/drying. The flux may be selected so as only to be activated at a predetermined elevated temperature.

The present invention provides a method in which a resin polymer film containing metallic particles is formed on a substrate and then plated by electroless (i.e. chemical) plating. In principle, it can be utilized as either a subtractive or an additive process.

It is already known to deposit metal on circuit boards by selective adhesive deposition of a polymer with intrinsic electrical conductivity onto a substrate. The conductive polymer film is then electroplated. This process is described in WO 94/26092.

It is also known to form a metallic film over moulded items by electroless plating techniques. These techniques are described in JP-A-05059586 and JP-A-60162783.

Essentially, they involve application of a plastics coating containing metallic particles, over the surface of the article. The metal film is then formed by electroless plating onto the coating.

According to WO 95/02715, a dispersion of divided cuprous oxide in a binder solution is used to deposit a layer on a non-conducting substrate. The oxide is at least partially converted to metallic copper by treating the coated substrate with an acid. This metalcoating layer is then covered with a thin copper film by electroplating.

Similar techniques are described in US-A-3 146 125 and US-A-3 226 256 in which cuprous oxide dispersed in a resin base is coated onto a substrate, e.g. a circuit board.

The cuprous oxide in the coating is then at least partially reduced by e.g. acid treatment to metallic copper and then a copper film is deposited by electroless plating. The latter document also envisages using powders comprising a metal, or mixture of metals in place of the cuprous oxide.

However, in US-A-3 146 125 and US-A-3 226 256, the amount of metal or metal oxide is said to be from 0.25 and 80% of the combined weight of the metal or metal oxide plus the resin.

Despite the number of proposals for plating onto a binder film having metal or metal oxide particles dispersed therein, none has actually succeeded in being developed into a commercially viable product. It has now been found that, surprisingly, significant improvements are obtained by increasing the metal content.

Thus, the present invention now provides a method of depositing a metallic film on a substrate, the method comprising: (a) applying to said substrate, a layer of a polymer resin in which polymer resin layer are dispersed particles of a metal or metal compound, and (b) subjecting said polymer resin layer to an acid and dissolved metal ions to electrolessly plate a metallic film within and/or over the polymer resin layer; wherein the amount of metal is greater than 80% by weight of the total of metal and polymer resin applied in step (a).

Step (b) refers to formation of a metallic film within and/or over the polymer resin layer because, without wishing to be bound by any particular theory or explanation, the applicants have conjectured that the acid treatment allows metal ions in solution to penetrate the layer of metal/metal compound particles deposited in the resin layer.

However, preferred embodiments certainly result in plating of the metallic film at least over the polymer resin layer.

The method of the present invention allows the metallic film to be deposited with significantly better thickness and integrity. Therefore, when used for electrical applications, the resultant film has much improved electrical conductivity, whilst good adhesion to the substrate is maintained. It should be appreciated that the method of the present invention is applicable to productions of all kinds of article wherein a metallic film is applied over a non-conducting article and the term "substrate" is to be construed accordingly. The method may for example be applied to plating of three-dimensional objects having a non-conductive surface, such as decorative articles, automotive components and the like. However, a preferred application is to electrical circuits. In the latter case, circuit tracks of excellent conductivity, resolution and solderability can be produced.

In another aspect, the present invention provides an article in which a metallic film is disposed over a non-conductive surface of the article, an intermediate film being located between the metallic film and the non-conductive surface, which film comprises particles of metal and/or a metal compound at least part of which has been converted to the base metal, said particles being dispersed in a resin binder, wherein the total amount of metal in the film is more than 80% by weight of that of the metal plus the binder.

Where particles of a metal compound are employed, they must at least in part be converted to the corresponding base metal. For example, when the metal compound is a metal oxide, it must be at least partly converted by application of an appropriate reducing agent to the applied particle/resin layer. In that case, reduction is conveniently effected by the acid employed in step (b).

However, preferred embodiments of the present invention employ particles of a metal or a mixture of metals rather than compounds. Of course, particulate mixtures of metals and metal compounds could also be used. The term "mixture of metals" covers both use of particles which consists solely or mainly of different given metals, as well as particles of metal alloys.

When particles of metals or alloys (as opposed to metal compounds such as oxides) are used, it is still necessary to treat the applied film of particulate dispersion/resin binder with the acid in step (b) Step (b) of the process comprises subjecting the polymer resin layer to at least one solution comprising water-soluble metal ions and at least one solution comprising an acid, or a single solution containing both. In other words, the source of water-soluble metal ions (usually a metal salt) and the acid may be combined in a single plating solution or they may be applied in separate solutions, sequentially applied to the polymer resin layer containing the metallic particles. In that case, the acid solution should be applied first. Moreover, where two solutions (acid and then metal ion) are employed separately, preferably the second solution which contains the metal ions also contains some acid, albeit usually at a lower concentration than that used in the primary acid solution.

The plating solution may be applied by immersion of the coated substrate in the solution(s) or else by means such as roller coating, curtain coating, spray coating etc.

After application of any such plating solution(s), the substrate is washed, although washing is not necessary between application of a separate acid solution and metal ion solution.

The nature of the metal particles in the polymer resin layer and the nature of the metal ions in plating solution are chosen so that the latter causes electroless plating. This is dependent on the relative positions of the respective metals in the electrochemical series and suitable examples will be known to persons skilled in the art. However, a copper sulphate (or other copper salt) plating solution is generally preferred. Preferred metals for dispersion in the polymer resin layer include the following metals and alloys: brass; tin/copper; tin/copper/zinc; tin/bismuth/copper; tin/zinc; and zinc.

Generally speaking, the best results are obtained with use of the smallest metallic particles possible, dispersed in the resin layer. Preferably, such metal particles have a size of lOptm or less, typically from 2 to 5 llm (all expressed as number-average particle diameter). However, it is also preferred for the metal particles not to be all spherical or pseudo-spherical. It is preferred to include flake-shaped particles as well.

The amount of metal in the applied layer, relative to the total of metal plus binder resin is at least 80% by weight. However, the amount of metal relative to the curable liquid composition is preferably from 82% to 95% by weight, more preferably from 85% to 90% by weight.

It should be noted that all references herein to the amount of metal as a weight percentage of metal plus resin binder refers to the weight of metal or metal ions. In other words, where a metal compound is present, it is the total amount of metal in the compound and not the amount of compound per se. Any base metal which has been chemically converted (e.g. by use of a reducing agent) and is no longer in compound form, is of course included.

A mixture of metals may be employed for the purpose described in WO 96/22670, namely to allow the metallic component to melt upon application of a subsequent heating step, so that the metallic particles coalesce (reflow). This is especially applicable to production of circuit board tracks. In this case, it is preferable also to incorporate a flux component in the binder/particle dispersion. For reflow, the metal particles should comprise a first component and a second component having a melting point higher than that of the first component. The relatively low melting point component preferably has a melting point in the range of from 100"C to 250"C and the relatively high melting point component has a melting point in the range of from 300 C to 1455 C.

Preferably, the relatively low melting point component comprises at least one metal selected from tin, lead, silver, bismuth, antimony and indium or an alloy of at least any two or more of such metals. On the other hand, the relatively high melting point component comprises at least one metal selected from tin, silver, copper, zinc, iron, manganese, nickel and cobalt or an alloy of at least any two or more of such metals.

After elevation to a temperature greater than the melting point of the relatively lower melting point component, the resulting wiring line has a higher melting point than that of the aforementioned relatively lower melting point component.

The layer of resin binder/particles is preferably applied to the substrate in the form of a liquid composition which is then hardened by drying and/or curing as will be explained in more detail below.

Thus, a further aspect of the present in provides a liquid composition comprising a polymer resin binder or monomer(s) for curing to form a polymer resin binder, and particles of metal or a metal compound, wherein the weight ratio of metal in the particles to the total of the metal plus resin binder or monomer is more than 80%.

As mentioned above, when used in circuit board applications, the process of the present invention is applicable to both additive and subtractive methods. For the additive approach, screen printing of the liquid composition is appropriate.

For a subtractive method, the liquid composition may comprise a monomer and if necessary, an appropriate initiator so that the composition is curable by application of radiation (e.g. UV) through a mask, the unexposed areas being subsequently etchedaway.

As a hybrid additive/subtractive process a radiation curable layer which tracks are formed (as described in WO 96/22670) may be applied to the substrate and then the liquid composition can be deposited in the tracks, prior to hardening/curing.

In general, the liquid composition rr.ay also be applied by any appropriate means such as screen printing, roller coating, dry film lamination, curtain coating or spray coating, as appropriate, according to the precise trade definition method being employed.

In some instances, depending on the nature of the resin, it may be preferable first to apply an adhesive resin layer to the substrate prior to application of the curable liquid composition, to enhance bonding of the cured polymer resin layer to the substrate.

The binder resin may be of the thermoplastic type so that in the applied liquid composition it is in the form of a solution of the resin in an organic solvent which is then dried, optionally followed by curing by application of heat. However, polymerisable liquid compositions which are monomer-based may also contain such solvents.

Suitable polymer resins include polyvinyl butyral (PVB), polyvinyl pyrolidone (pup), CAB, polyvinyl alcohols (PVA), cresylic resole, polystyrene, phenolic novolacs, styrene alkyl alcohols, polystyrenes and combinations of the foregoing.

The liquid composition may optionally contain other additives, including those for rendering the applied layer permeable to the plating ion solution. Various oxides, carbonates, bicarbonates and hydroxides which react with the acid are suitable for this purpose.

To form plated through-holes (PTH's), the liquid composition can be applied through pre-formed holes in the substrate, after completion of step (b).

The substrate-type may be chosen from any suitable material according to the intended end use. However, for printed circuit board use, the usual substrates such as FR4, polyimide and oxide-coated aluminium are suitable.

The metallic film so prepared by the method of the present invention can be conventionally soldered with tin/lead solder by wave or dip method. Hot air solderino is also possible. The metal film could be enhanced, prior to soldering, by electroplating but this is not really necessary in view of the quality of the film produced by the electroless plating technique. In any event, as a practical matter it is very difficult to arrange electrical connections to a complex circuit wiring pattern to ensure that all metal is in electrical contact with the relevant polarity.

In a typical embodiment, the liquid composition is applied by any method as hereinbefore described and then dried from e.g. 10 minutes to 60 minutes, between temperatures of typically from 75"C to 1 500C. Generally speaking, the higher the temperature, the shorter the drying time.

The substrate with cured/dried resin layer containing the metallic particles is then totally immersed in a suitable aqueous dilute acid. This may for example be a mineral acid such as sulphuric, hydrochloric, nitric or phosphoric. Alternatively, an organic acid such as formic acid or acetic acid may be employed. Typical concentrations of the acid in acid solution are from 10 to 25% by weight. The immersion time for such an acid is typically from 1 minute to 30 minutes, e.g. from 2 minutes to 10 minutes and frequently, about 5 minutes.

The substrate is then removed from the acid solution and placed in a solution of a metallic salt, usually copper sulphate or copper sulphate plus an acid such as described above for the acidic solution, but typically sulphuric acid. Typically, the concentration of the metal salt is from 10% to 30% by weight, most preferably from 15% to 20% by weight. Any acid present will normally be in a concentration of from 5% to 20%, e.g.

from 7.5% to 15% by weight.

In the second (metal ion) solution, a plating forms quite quickly, normally in about 30 to 45 seconds. However, the immersion time in the second solution will typically be from 30 seconds to 15 minutes, e.g. from 2 minutes to 10 minutes.

The substrate is then removed and immersed in water to remove any excess acid and dried rapidly to minimise discoloration of the copper. It is then post-baked for around one hour at 150"C to cure the resin and provide final adhesion.

The present invention will now be further explained by way of the following nonlimiting examples.

Examples A-C are liquid compositions including binder resin in which metal or metal compound particles are to be dispersed so that the total of metal to metal plus binder exceeds 80% by weight.

EXAMPLE A Ethyl glycol mono ether acetate 40.4% Butyl lactose 11.6% Cyclohexanone 5.1% Epoxy resin 0.18% Dicyclohexyl phthalate 3.52% Vinyl chloride ester resin 14.70% Plexigum resin 15.52% Filler (talc) 9.70% EXAMPLE B Glycol ether acetate 45.90% Butyl lactose 10.70% Cyclohexanone 5.80% Epoxy resin 2.00% Dicyclohexyl phthalate 4.00% resin Vinyl VYHM-copolymer 16.70% Acrylic resin 16.70% EXAMPLE C Aromatic solvents (mixed) 41.5% Ethylene glycol monoethyl ether 20.5% Propylene glycol monoethyl ether 18.5% Ethyl Cellulose 7 resin 14.5% Ethyl Cellulose N200 resin 0.5% Examples 1-9 are examples of the present invention.

EXAMPLE 1 600 mesh spherical tin type SC10 62% ex Osprey metal 10p copper flake type QZP0100 23% ex Degussa PVB/Phenolic varnish 15% The PVB/Phenolic varnish comprised Butyl carbitol 74.5% Hydroquinone 0.5% ex Rhone Poulenc Polyvinyl butyral resin 12.5% ex Hoechst Chemicals Phenolic Novolac resin 12.5% ex Hoechst Chemicals The above formulation was compounded on a triple roll mill, and printed via a 61 T.'cm polyester screen to form a defined test pattern on FR4 laminate. The sample was predried for 20' at 850C, allowed to cool and its resistivity recorded.

The test pattern used comprised a lmm wide track convoluted to a total length of 1500mm. The resistance between the limits of this track is used as a measure of the resistivity of the material.

After printing resistance was in excess of 20MQ; in practical terms non-conductive.

The sample was then immersed for 10 minutes in an aqueous solution comprising 10% sulphuric acid 30% copper sulphate, rinsed in water, dried rapidly to prevent water staining and cured at 150"C for 60 minutes.

After cure the resistance was approximately 70-80Q.

EXAMPLE 2 As for Example 1 except: 600 mesh spherical tin SC10 60% ex Osprey 10u copper flake QZP0100 20% ex Degussa 20,u Zinc flake Z2012 5% ex Roland Britten PVB/Phenolic varnish as 1 15% After printing and predry, the sample was immersed for 10 minutes in a 10% aqueous solution of sulphuric acid, and subsequently transferred to a copper salt/acid solution as in Example 1.

After acid treatment the conductivity was 9.6 ohms. After copper salt treatment and final cure this fell to 4 ohms.

EXAMPLE 3 As for Example 1 except: 600 mesh spherical tin SC10 62% 5 micron brass flake ultrafine FT 23% ex Debdale Ltd.

PVB/Phenolic varnish as 1 15% The sample was treated as in Example 2. Resistance after predry was in excess of 20MS2, after final cure this fell to approximately 3-5 ohms. This material was very viscous and suited for the application of conductive ink into preformed photoimaged test patterns.

EXAMPLE 4 As for Example 1 except: 600 mesh spherical tin SCl0 56% 5 micron brass flake ultrafine FT 21% PVB/Phenolic varnish as 1 13% Epoxy resin (1) 10% (1) 70% Epoxy EPPN 201 Ex. Nippon Kayaku t 30% propylene glycol methyl acetate as solvent.

The epoxy resin was included to give enhanced adhesion. The sample was treated as in Example 2. Resistance after predry was in excess of 20M Q after final cure this fell to approximately 20 ohms.

EXAMPLE 5 600 mesh spherical tin SC10 60% 20 micron zinc flake Z2012 25% PVB/Phenolic varnish as 1 15% This sample was treated as Example 2. Resistance after predry was in excess of 20MQ, after final cure this fell to approximately 15 - 20 ohms.

EXAMPLE 6 600 mesh spherical tin SC10 62% 5 micron brass flake ultrafine FT 23% Epoxy resin (as Ex. 4) 15% The sample was treated as for Example 2 except for an acid immersion time of 60 minutes. The resistance after predry was greater than 20MQ after acid treatment alone this fell approximately 150 ohms and after copper plating and cure, to 4 ohms.

EXAMPLE 7 As for Example 3 but with the following process change: The printed sample was predried at 850C for 30 minutes then immersed in the acid solution for 10 minutes. It was then baked for 1 hour at 1500C to cure the resin and bind the ink into place. The conductivity of the sample at this point was approximately 30 ohms. It was then subjected to the copper plating bath, plated rinsed in water and air dried. The conductivity was 1.5 ohms.

EXAMPLE 8 As for Example 7 but with the following process change: The printed sample was baked at 150"C for 30 minutes and the binder resin cured. The formulation used contained only 4% solid binder resin. The process was then carried out as in Example 3. The resistivity after predry was in excess of 20my, after acid pretreatment was in excess of 100Q and after plating and final cure approximately 100 ohms.

EXAMPLE 9 101,1 Bismuth tin alloy MCP137 62% available from MCP 5u brass flake ultrafine FT 23% PVB/Phenolic varnish as 1 15% This material contains a 4% binder resin solids and was processed by printing onto FR4 and reflowing in the manner described in WO 96/22670.

It was then subjected to the process treatment as in Example 2. This material was more conductive after full cure being between 1.5 and 2.3 ohms and had excellent cross-hatch adhesion.

In the light of this disclosure, modifications of the described examples, as well as other examples, all within the scope of the present invention as defined by the appended claims, will now become apparent to persons skilled in the art.

Claims (10)

CLAIMS:

1. A method of depositing a metallic film on a substrate, the method comprising: (a) applying to said substrate, a layer of a polymer resin in which polymer resin layer are dispersed particles of a metal or metal compounds and (b) subjecting said polymer resin layer to an acid and dissolved metal ions to electrolessly plate a metallic film within and/or over the polymer resin layer; characterised in that the amount of metal is greater than 80% by weight of the total of metal and polymer resin applied in step (a).

2. A method according to claim 1, wherein the amount of metal is from 82 to 95% by weight of the total of metal and polymer resin.

3. A method according to claim 1 or claim 2, wherein the amount of metal is from 85 to 90% by weight of the total of metal and polymer resin.

4. A method according to any preceding claim, wherein step (a) comprises applying a liquid composition comprising the polymer resin and particles to the substrate and hardening the composition to form the layer.

5. A method according to claim 4, wherein the liquid composition comprises a thermoplastic resin dissolved in an organic solvent in which the particles are dispersed.

6. A method according to claim 4 or claim 5, wherein the curable liquid composition is printed onto the substrate in a pattern corresponding to an electrical circuit wiring pattern.

7. A method according to any preceding claim, wherein step (b) comprises subjecting said polymer resin layer to at least one solution comprising water-soluble metal ions and at least one solution comprising an acid, or else at least one solution containing both acid and water-soluble metal ions.

8. A method according to claim 7, wherein the water-soluble metal ions and the acid are contained in the same solution.

9. An article in which a metallic film is disposed over a non-conductive surface of the article, an intermediate film being located between the metallic film and the nonconductive surface, which film comprises particles of metal and/or a metal compound, at least part of any compound being converted to the base metal, said particles being dispersed in a polymer resin as binder, characterised in that the total amount of metal in the film is more than 80% by weight of that of the metal plus the binder.

10. A liquid composition comprising a polymer resin or monomer(s) for curing to form a polymer resin, and particles of a metal or metal compound, characterised in that the weight ratio of metal to the total of the metal plus resin and/or monomer is more than 80%.