The invention relates to a process for the preparation of a fibre-reinforced resin- coated sheet.

The formation of layers having a different thickness plays an important role in the production of printed circuit boards. Thus, for example, copper foils are coated with epoxy resin compositions dissolved in solvents and subsequently the solvent is evaporated. The sheets obtained thereby are then laminated under pressure on already structured so-called inner layers under vacuum (US 5,718,039, US 6,187,416, EP 1 108 532 A1). A further example is the application of a solder mask on the final printed circuit board. The corresponding compositions also con¬ tain solvents that need to be removed by heat (EP 0 323 563 A2).

The preparation of prepregs is done by applying resin compositions to glass fabric in specific coating units called treaters. In this process the fabric is transported on rolls and is dipped first in a solution of the resin to be applied in an organic solvent and subsequently passed through a long (vertically) arranged oven so that the sol¬ vent is uniformly evaporated from both sides. Because of the forces which act on the glass fabric, it is not possible to use material of any thickness, rather it is nec¬ essary for this material to have a minimum thickness so that it can be processed. Thus, for example, the thinnest commercially available prepreg has a thickness of 50 μm.

Further, solvent-free processes are known wherein the resin component and the curing agent component are melted separately and only mixed on the glass fabric

(see Kelly Graham, Solventless Prepreg Manufacturing Process, www.cϊ rcuitree.com).

The solvent-based processes are disadvantageous in that they require a large amount of energy to evaporate the solvent, they are harmful to the environment, they require sophisticated industrial hygiene, the solvents are combustible, waste disposal is complicated and entails additional considerable costs.

Known solvent-free systems for the preparation of prepregs are disadvantageous in that they require sophisticated equipment for the processing of molten resin sys¬ tems having a relatively high viscosity. The control of the high reactivity of such a system comprising binding agent and curing agent which are added at the same time in the mixing head may prove difficult as well. In addition, only a limited num¬ ber of formulations can be used in this process without the complicated re¬ adjustment of all process parameters.

It is the object of the invention to provide a solvent-free process for the preparation of a fibre-reinforced resin-coated sheet that does not have the aforementioned disadvantages. The process should result in a uniform layer thickness. Further, the process should allow the use of very thin fibre-reinforced materials without the oc¬ currence of a partial or complete destruction of the fabric. Thus, the component layers in the preparation of printed circuit boards comprising resin/glass fi- b re/copper-composites should become thinner and lighter. Finally, the process should provide a fibre-reinforced resin-coated copper sheet enabling a faster laser drilling in the preparation of printed circuit boards, thus resulting in an increased production efficiency, since more holes can be drilled per time unit with the same capacity of laser drillers.

This object is achieved by a process that allows the solvent-free coating of con¬ ductive materials or discharging materials (metal sheets, polymers that have been made anti-static, conductive polymers, conductive glass or glass having a conduc¬ tive coating, metallized polymers) in combination with woven and non-woven fab-

rics. In particular, the invention relates to a process for the solvent-free preparation of a fibre-reinforced resin-coated sheet comprising the following steps:

(i) applying a coating powder on a substrate selected from a woven or non- woven fabric, wherein the coating powder is charged by friction in the pres- ence of magnetic particles and then transported by means of a fluidized bed and/or optionally one or more mixing rolls and subsequently transferred to and applied onto said substrate by means of an electric field between the brush drum and a substrate roll bearing said substrate,

wherein said substrate is in intimate contact with a conductive or discharging sheet,

(ii) melting and partially curing the layer of coating powder obtained on said sub¬ strate for the preparation of a fibre-reinforced resin-coated sheet.

According to a preferred embodiment of the process according to the present in¬ vention, glass fibres or high-performance fibres are used as fibre-reinforced sub- strate material. Preferred high-performance fibres are aramide fibres, carbon fibres and ceramic fibres.

In the process according to the present invention very thin woven fabric or mats consisting of glass or high-performance fibres can be used. In general their thick¬ ness is 5-200 μm and preferably 15-80 μm.

Using the process according to the present invention an extraordinarily homoge¬ neous or uniform layer of coating powder can be obtained. Thus, for example, lay¬ ers having a thickness of 60 μm can be obtained by the process according to the present invention, the difference in thickness throughout the layer being ± 15%, preferably ± 10% and most preferably ± 5%.

This distinguishes the process according to the present invention from processes known in paint technology such as powder spray-coating. These processes result in a non-uniform thickness of the coating which is not acceptable in the field of printed circuit boards.

-A-

In the process according to the present invention the so-called electromagnetic brush is used and the process is based on a principle which is used, in similar form, with laser printers and copying machines.

Inter alia, the invention is based on the surprising finding that the filler particles of the composition or formulation are distributed within the fabric in an entirely uni¬ form way, i.e., they do not "stick" to the surface.

The powder to be applied is mixed with the carrier particles or so-called carriers in a container. These carriers consist of a magnetic core having a polymeric coating.

The powder can be electrostatically charged by means of several rollers whereby it sticks to the carriers. By means of mixing rolls the powder is continuously trans¬ ported together with the carriers to the so-called brush roll which is provided with magnets in its interior. Alternatively, the transport to the brush drum can be done by a fluidized bed. The magnetic carrier particles together with the powder adher¬ ing to them adhere now to the brush drum. Upon application of a corresponding voltage between the brush drum and the substrate drum, the powder particles are transferred to the substrate (for example, a fabric/metal foil sandwich), whereas the carrier particles remain in the system. This way, uniform thick layers can be applied onto a substrate.

Brief description of the drawings:

Figure 1 schematically describes a coating unit having mixing drums. The carrier and power are mixed in the container (10). Thereby the powder is charged and adheres to the oppositely charged carriers. The transport to the brush drum (1) is made by the mixing rolls (4). The mixing rolls or mixing drums, respectively, may have magnets in their interior. Alternatively, they have wings or paddles on the outside. These serve to mix the carriers and the powder or coating powder, re¬ spectively. The mixing rolls transport the particles to the brush drum (1 ). The brush drum has magnets in its interior. By applying a voltage between the brush drum (1) and the substrate drum (2), the positively charged powder particles are repelled and are transferred to the grounded substrate (6) which is on the substrate drum

(2). The substrate (for example, a copper sheet) is unreeled from a roll (6). The substrate must be a discharging substrate.

Figure 1 further shows a heating device (7) for melting the coating powder and for sintering the powder, respectively. (4) denotes a metal plate or sheet stripping off the carriers so that they are transferred back into the container. Finally, (8) de¬ notes a roll for winding up the coated substrate.

Figure 2 shows a modification comprising a fluid ized bed: The mixing rolls (3) shown in Figure 1 are replaced with a fluidized bed (represented in Figure 2 by dots). The fluidized bed is generated by blowing air into the mixture of carriers and powder or coating powder, respectively, which fluidized bed mixes and thus causes an opposite electrostatic charge of the carriers and the powder. In addition, the transport to the brush drum is made.

The particle size of the coating powder is in general < 150 μm and preferably < 100 μm and most preferably < 50 μm.

The size of the carrier particles is in general 10-150 μm and preferably 20-100 μm.

In the process according to the present invention a curable coating powder is pref¬ erably used which comprises particles obtainab Ie by

(i) mixing

(a) a polymeric binder, an oxazine resin, a cyanate ester or a maleimide,

(b) a hardener or initiator,

(c) a coating additive,

(d) optionally a filler,

(e) optionally a compatibilizing polymer

and optionally further components

(ii) melt extruding the mixture obtained in step (i) and

(iii) milling and sieving the extruded mixture.

According to a preferred embodiment of the invention, the coating powder has a glass transition temperature in the uncured state of at least 2O°C, preferably at least 25°C and more preferably at least 3O ⁰ C and has a glass transition tempera¬ ture in the cured state of at least 150 ⁰ C ₁ preferably at least 160 ⁰ C and more pref¬ erably at least 170°C.

In the process according to the present invention curing is preferably made up to a degree of 1 to 70%, preferably 10 to 50% (as measured by DSC).

Furthermore, the polymeric binder is preferably essentially an epoxy resin which is solid at room temperature. The glass transition temperature of the resin should preferably be at least 25°C.

The coating powder used in the invention can preferably also comprise a mixture of epoxy resins. This mixture preferably has a glass transition temperature of > 25 ⁰ C in the uncured state. Its molecular weight (number average molecular weight) is generally > 600.

Suitable epoxy resins for the preparation of the coating powder used in the inven¬ tion are described, for example, in: Clayton A. May (Ed.) Epoxy Resins: Chemistry and Technology, 2nd ed., Marcel Dekker Inc., New York, 1988.

Preferred mixtures of epoxy resins on the basis of bisphenol A and bisphenol A diglycidyl ether. The epoxy equivalent weight of these resins is > 300 g/equivalent. Such a resin is, for example, D.E.R. 6508 (available from Dow Chemicals).

Epoxy resins on the basis of bisphenol F and bisphenol S can optionally also be added.

Furthermore, the mixture can comprise multifunctional epoxy resins. The function¬ ality of these resins is > 3. Examples for such multifunctional epoxy resins are ere-

sol-novolak epoxy, phenol-novolak epoxy and naphthol-containing multifu notional epoxy resins.

Examples for the aforementioned epoxy resins are bisphenol A epoxy resin such as D.E.R. 667-20, D.E.R. 663UE, D.E.R. 692H, D.E.R. 692, D.E.R. 662E, D.E.R. 6508, D.E.R. 642U-20 (available from Dow Chemicals), cresol-novolak epoxy res¬ ins such as Araldite ECN 1299, Araldite ECN 1280 (Vantico), EOCN-103 S, EOCN-104, NC-3000, EPPN 201 , EPPN-502 H (Nippon Kayaku), naphthol epoxy resins such as NC 7000-L (Nippon Kayaku) and brominated Epoxy resins such as Araldite 8010 (Vantico), BREN-S (Nippon Kayaku), ESB-400 T (Sumitomo) and Epikote 5051 (Resolution). Moreover, modified epoxy resins can also be used. Such modifications are, for example, the use of chain reaction terminating agents to control the molecular weight, so-called ,,high-flow" resins, and the use of multi¬ functional monomers to prepare branched resins.

A particularly preferred coating powder used in the invention comprises, as com- ponent (a), about 50-90 wt.-% of epoxide and about 5-20 wt.-% of cyanate ester, as component (b), about 0.5-5 wt.-% of dicyandiamide and about 0.1-2 \vt.-% of 2- phenylimidazole, for example about 85 wt.-% of epoxide, 10 wt.-% of cyanate es¬ ter, about 2 wt.-% of dicyandiamide as hardener and about 1 wt.-% of 2- phenylimidazole as initiator.

As mentioned above, apart from the epoxy resins, cyanate esters can also be used as polymeric binders. In the preparation of the coating powder used in the invention, these can be used both in monomeric form as well as in the fo rm of oli¬ gomers or prepolymers.

Suitable cyanate esters are bifunctional cyanate esters, such as BADCy, Primaset Fluorocy, Primaset MethylCy, or multifunctional cyanate esters, such as Primaset BA-200, Primaset PT 60, Primaset CT 90, Primaset PT 30. All of the aforemen¬ tioned bifunctional and multifunctional cyanate esters are available from Lonza, Basel, Switzerland.

Especially preferred cyanate esters are BADCy and its prepolymers (e.g. Primaset BA-200).

Apart form the cyanate esters, the component (a) can also comprise 1-oxa-3-aza- tetralin-containing compounds (oxazine resins). In the preparation of the coating powder used in the invention, these are also initially employed in monomeric form.

Preferred oxazine resins are those which are obtained either by reacting bisphenol A with aniline and formaldehyde or by reacting 4,4'-diaminodiphenyl methane with phenol and formaldehyde. Further examples may be found in WO 02/072655 and EP 0 493 310 A1 as well as in WO 02/055603 and the Japanese patent applica- tions JP 2001-48536, JP 2000-358678, JP 2000-255897, JP 2000-231515, JP 2000-123496, JP 1999-373382, JP 1999-310113 and JP 1999-307512. Further examples may be found in Macromolecular Chemistry, Macromolecular Symposia (1993), 74 (4th Meeting on Fire Retardant Polymers, 1992), 165-71 , EP 0 493 310 A1 , EP 0 458 740 A1 , EP 0 458 739 A2, EP 0 356 379 A1 and EP 0 178 414 A1.

The maleimides used in the preparation of the coating powder of the invention are also known per se to the skilled person and are described, for example, in Shiow- Ching Lin, EIi M. Pearce, High-Performance Thermosets, Carl Hanser Verlag, Mu¬ nich 1994, Chapter 2.

The component (b) of the resin composition used in the invention comprises a hardener or initiator. Such hardeners and initiators are known per se to the skilled person and comprise latent hardeners with low activity at room temperature, such as phenolic hardeners, such as D.E.H. 90, D.E.H. 87, D.E.H. 85, D.E.H. 84, D.E.H. 82 (available from Dow Chemicals, US), dicyandiamide or derivatives thereof, such as Dyhard OTB, Dyhard UR 200, Dyhard UR 300, Dyhard UR 500, Dygard 100, Dyhard 100 S, Dyhard 100 SF and Dyhard 100 SH (available from Degussa, Germany), bisphenol A, acid anhydrides, such as phthalic acid anhy¬ dride, tetrahydrophthalic acid anhydride, trimellitic acid anhydride, pyromellitic acid anhydride, hexahydrophthalic acid anhydride, HET-acid anhydride, dodecenyl succinic acid anhydride, bicyclo[2.2.1]hept-5-en-2,3-dicarboxylic acid anhydride,

aromatic and aliphatic amines, such as diaminodiphenylsuifone, diaminodiphenylether, diaminodiphenylmethane or ring-substituted dianilines, such as Lonzacure ^® M-DEA, Lonzacure ^® M-DIPA, Lonzacure ^® M-MIPA, Lonzacure ^® DETDA 80 (all of the aforementioned compounds are available from Lonza, Basel, Switzerland).

Preferably, dicyandiamide or modified dicyandiamide is employed.

In the resin composition used in the invention, the hardeners or initiators are used in an amount of below 10 wt.-%, preferably below 5 wt.-% (lower limit: about 0.1 wt.-%).

Preferred initiators are imidazoles and derivatives thereof, such as 2- methylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole, 2-phenyl-4- methylimidazole, bis(2-ethyl-4-methylimidazole), 2-undecylimidazole, 2,4-diamino- 6(2'-methyl-imidazole(1 '))ethyl-s-triazine and 1-cyanoethyl-2-undecylimidazole. Furthermore salts formed from imidazoles and carboxylic acids can be used. Fur- ther initiators are 1 ,8-diaza-bicyclo(5.4.0)undecene (DBU) and boron-trihalide- amine complexes, such as BF3-amine. Further examples may be found in Clayton A. May (Ed.) Epoxy Resins: Chemistry and Technology, 2nd ed., Marcel Dekker Inc., New York, 1988.

The resin composition used in the invention further comprises coating additives as component (c). These comprise flow-control agents, degassing agents and lubri¬ cants. These are known per se to the skilled person. Typical examples are butyl acrylate polymers as flow-control agents, benzoin as degassing agents and waxes as lubricants. Furthermore, for example stabilizers can be used as coating addi¬ tives.

The resin composition used in the invention contains the coating additives in an amount of generally 0.1-10 wt.-%, preferably 0.2-5 wt.-%.

Coating additives also comprise adhesion promoters. These are useful for provid¬ ing adhesion to the copper substrate.

The coating powder used in the invention may further comprise organic and inor¬ ganic fillers (d).

These fillers are suitably employed in the coating powder used in the invention in an amount of 5 to 300 wt.-%, preferably 10 to 200 wt.-%, more preferably 10 to 100 wt-%. The stated amounts relate to the sum of components (a), (b) and (c) of the coating powder.

Examples for organic fillers are fluorine containing polymers, such as polytetra- fluoroethylene (PTFE), tetrafiuoroethylene/hexafluoropropylene copolymer (FEP), tetrafluoroethylene/ethylene copolymer (E/TFE), tetrafluoroethylene/hexafluoro- propylene/vinylidene fluoride terpolymer (THV), poly(trifluorochloroethylene)

(PCTFE), trifluorochloroethylene/ethylene copolymer (E/CTFE), polyvinyl fluoride) (PVF), poly(vinylidene fluoride) (PVDF), perfluoroalkoxy copolymer (PFA), tetra- fluoroethylene/perfluoromethyfvinylether copolymer (MFA), furthermore poly( vinyl chloride) (PVC), polyphenyl ether (PPO), polysulfone (PSU), polyaryl ether sulfon (PES), polyphenyl ether sulfon (PPSU), polyphenylene sulfide (PPS), polyether ketone (PEK) and polyether imide (PEI).

Especially preferred organic fillers are tetrafluoroethylene/hexafluoropropylene copolymer (FEP), ethylenetetrafluoroethylene copolymer (ETFE) and polyphenyl ether (PPO).

In the coating powder used in the invention, there may preferably be used organic fillers which do not melt upon processing. Alternatively, there can be used fillers which melt and show phase separation upon cooling.

Apart from the organic fillers, inorganic fillers may also be used in the coating powder.

Such fillers are, for example, fused silica, such as Silbond 800 EST, Silbond 800 AST, Silbond 800 TST, Silbond 800 VST ₁ Silbond 600 EST, Silbond 600 AST, Sil¬ bond 600 TST, Silbond 600 VST (available from Quarzwerke Frechen, Germany), fumed silica, such as Aerosil 300 and Aerosil R 972, precipitated silica, such as

Ultrasil 360, Sipemat D 10, Sipemat 320 (available from Degussa, Germany), cal¬ cined kaolin, such as PoleStar (Imerys, St Austell, UK), Santintone (Engelhard Corporation, Iselin, NJ, US), aluminium oxide, magnesium oxide, zirconium oxide, aluminium silicates, calcium carbonate and barium sulfate, silica glass and kaolin being preferred fillers. Furthermore, there may be mentioned ceramics, especially those with low or negative coefficients of expansion.

The advantages of the coating powder used in the invention are that it is possible, in order to optimise the properties of the product, to select from a variety of fillers the one which best satisfies the relevant requirements. For example, a given ep- oxy resin mixture can, thus, be modified as needed. Even fillers which are difficult to process can be incorporated without problems. Thus, electric properties such as the dielectric constant (DR), the dielectric loss factor (tan δ), the breakdown resis¬ tance, the surface resistance, the volume resistance and mechanical properties such as bending strength, impact strength, tensile strength as well as further mate- rial properties such as the coefficient of thermal expansion (CTE), fiammability and others can be adapted as desired. The filler does not have to be solvable or stably dispersable in organic solvents. Consequently, it is possible to use materials as fillers which could previously not or only hardly be used in sequential build-up (SBU), such as the aforementioned organic fillers.

The electrical and mechanical properties of the coating powder and of the coating layer prepared therefrom can be influenced and controlled by the fillers.

Thus, for example fillers with a low dielectric constant, such as PTFE, FEP and kaolin may be employed in order to prepare coating layers with a correspondingly low dielectric constant.

Further electrical properties can be controlled in an analogous way.

The mechanical properties which can be influenced by the fillers comprise, in par¬ ticular, properties such as the coefficient of thermal expansion, impact strength, and tensile strength.

The following fillers are particularly suitable for controlling the coefficient of thermal expansion: silica glass, kaolin, calcium carbonate and ceramics with a negative coefficient of expansion.

Bending strength can be influenced or controlled, for example, by PPO.

According to a preferred embodiment of the invention, the cured coating powder has a coefficient of thermal expansion (CTE) of < 70 ppm/°C and preferably < 60 ppm/°C in the x-, y- and z-direction.

According to a further preferred embodiment, the dielectric constant of the coating in the cured state is < 3.8, preferably < 3.6. Moreover, glass transition tempera- tures of the cured formulation of above 150 ⁰ C, preferably above 16O ⁰ C, are pre¬ ferred.

Furthermore, flame-retard ant materials may be used as fillers. Examples for these are inorganic materials which release water upon heating, such as aluminium hy¬ droxide, which is available, for example, as Martinal OL-104, Martinal OL-111 (Martinswerk GmbH, Bergheim, Germany) or Apyral 60 D (Nabaltec, Schwandorf, Germany), magnesium hydroxide, available, for example, as magnesium hydrox¬ ide 8814 (Martinswek GmbH, Bergheim, Germany) or Mg-hydroxide SIM 2.2 (Scheruhn Industrie-Mineralien, Hof, Germany), phosphorous-containing organic compounds, such as triphenyl phosphate (TPP), tricresyl phosphate (TCP), cresyl diphenyl phosphate (CDP), tertiary phosphin oxides, such as Cyagard ^® and Re- oflam ^® 410, red phosphorous in the form of a dispersion in an epoxy resin, such as Exolit RP 650, or in the form of a powder, such as Exolit OP 930 (both products are available from Clariant GmbH, Frankfurt, Germany) and antimony trioxide.

Furthermore, the flammability of the coating powder of the invention can be influ- enced and controlled by component (c), i.e., the coating additives. In this connec¬ tion, for example, phosphorous-containing and nitrogen-containing flame retar- dants may be mentioned.

The coating powder of the invention can, optionally, further contain compatibilizing polymers. Such compatibilizing polymers are, for example, di- ortriblock copoly¬ mers such as styrene/butadiene/styrene or styrene/butadiene/methyl methacrylate blockcopolymers (Atofina, France).

Furthermore, the coating powder used in the invention can contain conventional additives which are conventionally used in the processing of epoxy resins.

In the preparation of the coating powder used in the invention, the components (a), (b), (c) and, optionally, (d) and (e) are first dry-milled to give a powder.

In doing so, it may be useful to mix and extrude individual components beforehand to prepare a master batch.

This procedure must be used, in particular, when certain components are difficult to incorporate. These are then incorporated into each other beforehand. Such master batches are also commercially available. For example, in the case of the resins, for example, it is possible to mix two resins beforehand. This course of ac- tion is used, in particular, when one of the resins has a low glass transition tem¬ perature. Moreover, this procedure may be used when certain components are used only in small amounts.

The aforementioned components or master batches are premixed and milled in the dry state. Before milling, the mixture may optionally be cooled.

After thorough mixing (and optional cooling), the material is milled in the dry state while maintaining a powder and the powder is subsequently extruded. This extru¬ sion provides complete homogenisation of the components and is a key step in the overall process.

After extrusion, the material is milled in the dry state and the oversize material is separated, wherein a sieve size in the range of less than 10 to 500 μm and pref¬ erably less than 100 μm is suitably used, which guarantees a corresponding parti¬ cle size. Classifying mills such as Hosekawa MicroPul are particularly suitable for milling.

The aforementioned melt extrusion is preferably carried out in such a way that the conversion of the reactive component is less than 20%, preferably less than 10%. This reaction is due to the fact that a melt is formed upon extrusion. The degree of conversion can be determined by the skilled person by thermal analysis. The cor- responding extrusion parameters (for obtaining such a degree of conversion) can be determined by the skilled person by simple experiments. They depend on the type of extruder and the type and amount of the components employed. For ex¬ ample, a Buss co-kneader can be used as extruder, in which the aforementioned components are extruded. As mentioned above, the mass is subsequently cooled and reduced to small pieces. The final coating powder mixtures preferably have an average particle size in the range of 1 to 500 μm, especially of 10 to 100 μm.

The coating powder which are produced in this way are used in the process ac¬ cording to the present invention for the preparation of a fibre-reinforced resin coated copper sheet for use in the production of printed circuit boards.

The following methods can be used in step (ii) of the process according to the pre¬ sent invention for melting:

a) melting in an oven with or without convection,

b) infrared radiation,

c) near infrared (NIR) and

d) induction and optionally

e) excitation by microwaves.

In the process of the invention, the melting is preferably be effected by NIR. This method is described in WO 99/47276, DE 10109847, in Kunststoffe (1999), 89 (6), 62-64 and in Journal fur Oberflachβntechnik (1998), 38 (2), 26-29.

The step of melting is particularly important. Upon heating, a change in viscosity occurs, i.e., the powder first melts. The viscosity of the melt decreases initially. Subsequently, curing and, thus, a rise in viscosity takes place. This operation must

be conducted in the process of the invention in such a way that the viscosity of the melt is initially as low as possible and subsequently good flow is achieved without formation of bubbles, such that a non-porous film is obtained.

Thus, the coating layer is first melted, remains flowable and can, hence, be used for the preparation of a multilayer structure by:

(i) applying the coating powder to a substrate by means of an electromagnetic brush (as described above),

(ii) melting the coating powder followed by cooling,

(iii) laminating the coated substrate to a printed circuit board which may already comprise more than one layer,

(iv) curing,

(v) drilling and through-connecting the individual layers and substrates to pre¬ pare a multilayer structure,

(vi) optionally repeating steps (i) to (v).

In this way non-porous coating layers are obtained.

An essential feature of this process is that curing occurs mainly in step (iv), i.e., after the preparation of the multilayer structure. In this connection, it is important that the films are still flowable during the formation of the structure.

The curing of the melted powder coated layers takes place during pressing or lamination. The pressing or lamination takes place under vacuum and pressure, the corresponding parameters being known to the skilled person. For example, a Lauffer press or an Adara press can be used. The pressing cycles are to be adap¬ ted to the individual material used.

In the last step of this process, the press contacting of the individual layers and substrates takes place in order to prepare the multilayer structure.

Occasionally, the fibre-reinforced resin-coated sheet prepared according to the process described above sho-ws blisters. The occurrence of such blisters can be avoided in that in step (i) the intimate contact between the substrate and the sheet is achieved by depositing the coating powder prior to step (i) on the sheet by methods known per se (powder spraying, powder cloud, electromagnetic brush) to obtain a powder-coated sheet, melting the obtained powder-coating by applying heat to the powder coated sheet, applying the substrate on this coated sheet to obtain a sandwich-like structure, optionally, coat this sandwich-like structure for a second time with coating powder followed by melting and laminating this sand- wich-like structure to form a substrate laminate comprising the substrate and the sheet.

Preferably the coating powder is deposited on the sheet (such as a copper foil) with a thickness in the range of 3-30 μm, preferrably 10-15 μm.

The coated sheet is heated in such a way that the coating powder is only partially cured. In general, the degree of curing is less than 60 %, preferably less than 40%, and most preferably less than 30 %.

For melting the coating powder the methods described hereinbefore such as NIR can be used.

In general, the melting temperature is in the range of 80-250 ⁰ C and preferably in the range of 100-170 ⁰ C.

The melting time is from 0.1 -20 seconds and preferably from 0.1-5 seconds.

As the coating powder is only partially cured, it is possible to laminate the coated sheet (such as a copper foil) and the substrate such as glass fabric.

Lamination is preferably done by roll lamination or belt lamination. Belt lamination is preferred as it increases the intimate contact between the substrate and the sheet.

Usually, lamination is done at a temperature in the range of 100-180 ⁰ C and pref¬ erably in the range of 120-150 ⁰ C.

The lamination pressure ranges from 1-3O N/cm ² and preferably 15-25 N/cm ² .

As already mentioned, the process according to the present invention achieves a more homogeneous coating powder and thus more homogeneous layer thick¬ nesses and edge coating. Further, the process according to the present invention is advantageous in that it does not require any solvents so that costs for the raw materials may be reduced. Further, considerable amounts of energy may be saved. In addition, the costs for the disposal of the solvent (by burning) are not incurred.

A further advantage is that the so-called "skin over effect" can be avoided which occurs upon evaporation of solvents and which makes it difficult to remove the solvent completely from the inner layers. On account of the fact that no solvent needs to be used in the process according to the present invention (only a resin needs to be melted), the whole device for carrying out the process according to the present invention can be constructed in a compact manner and nevertheless be operated at high belt conveyer speed.

Since the device is very short, it does not comprise much freely pending substrate material such as glass fabric.

A further advantage of the process according to the present invention is the con¬ tact-free coating occurring therein. Further, it is possible, as described before, to incorporate various fillers into the coating powder.

Concerning the preparation of fibre-reinforced resin coated metal sheets, it has to be noted further that the stress exerted on the substrate material (the glass fabric, for example) is low because most of the mechanical strain or stress on the metal sheet does not occur, which metal sheet forms part of the composite to be coated. In this way glass fabric can be used which is thinner than the one processed so far.

Finally, there are significant advantages in the processing of the fibre-reinforced resin coated copper sheets (comprising very thin substrate material) obtained by the process according to the present invention in the production of printed circuit boards. In particular, the component layers will get thinner and lighter and the mi- crovia holes can be laser-drilled quicker resulting in an improved production effi¬ ciency because more holes can be drilled per time unit >/vith the same capacity of laser drills.

Particularly structured surfaces can be obtained by the process according to the present invention. The corresponding parameter is the so-called gloss value which can be determined by a glossmeter. The surfaces obtained by the process accord¬ ing to the present invention have in general a gloss value < 60, preferably < 30 and most preferably < 20 (as measured with a glossmeter of Byk Gardner at 60°).

The invention is further illustrated by the following examples.

Example 1 (Preparation of coating powder)

380 parts of a solid epoxy resin (melting point 90-110 ⁰ C, epoxy equivalent weight 400 g/eq), 116 parts of a solid multifunctional epoxy resin (4.3-4.9 eq/kg), 160 parts filler (particle size < 8 μm), 25 parts dicyano diamide (particle size < 6 μm), 16 parts benzoin and 16 parts methyl imidazole and 12 parts of a flow agent (acrylic polymer) and 3 parts adhesion promotion adhesive are mixed in a pre- mixer and then extruded (twin-screw extruder from OMC, Saronno, Italy, type EBVP 20/24; temperature 100 ⁰ C, load 65%). The subsequent milling is done in a mill from Fritsch (pulverisette 14, 15000 rpm). The oversize material is separated by sieving (< 100 μm). 50% of the particles have a particle size < 30 μm.

Example 2 (Preparation of a fibre-reinforced resin-coated copper sheet)

A roll of glass fabric (Hexcel Fabrics, type 106, thickness 40 μm) was layed onto the treated side of a copper sheet having a thickness of 12 μm and the assembly was coated with coating powder (Example 1 ) by an EMB device continuously. The coating powder was melted in a NIR oven so that an optically uniform composite of

coating powder/glass fabric/copper sheet was obtained. No inclusion of air could be detected under the microscope. The powder wetted the fabric and the copper foil well.

This composite was pressed on a FR4 laminate (thickness 0.8 mm). A cross- section of this composite was examined by microscope. An analysis by means of SEM in combination with EDX showed a uniform distribution of the inorganic filler particles in the resin matrix, the glass fabric was uniformly embedded from all sides in the resin. The thickness of the dielectric layer along a sample having a width of 40 cm is listed in the table below. The sheet obtained in this way has a gloss value < 10 (as measured with a glossmeter from Byk Garnder at 60°).

Example 3 (Preparation of a fibre-reinforced resin-coated copper sheet)

Step 1 : Applying a thin layer of coating powder to a copper sheet

A 12 μm copper sheet was coated continuously with a coating powder that was melted immediately after its application in a NIR oven. The EMB device was ad¬ justed in such a way that a 15 μm coating powder layer on the copper sheet was produced (belt speed 5 m/min, brush drum voltage 1200 V). The power of the NIR oven was selected such that the coating powder was not completely cured but had a relatively low degree of curing of 30%.

Step 2: Lamination of glass fabric on the coated copper sheet obtained in step 1

As the applied coating powder was not completely cured, it is possible to laminate the coated copper sheet and glass fabric in the next process step. To achieve this, the coated sheet and glass fabric (Unitika, Type 106) were processed continuously in a roll laminator (Soni R 160). Typical process conditions were a temperature of

14O ⁰ C at a belt speed of 1-2 m/min. A lamination pressure of 2.5 bar was selected. Under these conditions the coating powder melts again and can intimately com¬ bine the copper sheet and the glass fabric without the occurrence of blisters.

Step 3: Applying a second layer of coating powder to the laminate obtained in step 2

In a third step, the composite of copper sheet, coating powder and glass fabric is coated with a further layer of coating powder. Its thickness can be varied by choosing the EMB device parameters accordingly depending on the intended use of the final composite. To achieve this, the roll obtained in step 2 is inserted into an EMB device and continuously coated with a coating powder which is melted again in a NIR oven without being completely cured.

The composite obtained in this way layed up on a core and pressure is applied to to form a laminate. The total thickness of the dielectric layer generated in this manner was 45 μm.

Example 4

Steps 1 and 3 were carried out as described in Example 3. However, in step 2, the roll laminator was replaced with a belt laminator (KFK, from Meyer, Germany). A composite free of blisters could be obtained at a belt speed of 5 m/min, a lamina¬ tion temperature of 160 ⁰ C and a lamination pressure of 20 N/cm ² .

List of reference signs:

(1 ): Brush drum

(2): Substrate drum

(3): Mixing drums

(4): Plate for the retransfer of the carrier particles

(5): Device for adjustment of the height of the brush

(6): Roll for reeling off the substrate

(7): Heating device

(8): Take-up roller for the coated substrate

(9): Container