FR2705012A1 - Coating circuit board with photostructurisable material by extrusion - Google Patents

Abstract

In coating a circuits board (I) with photostructurisable material (II) contg. a thermoplastic polymeric binder, an addn.-polymerisable, ethylenically unsatd. cpd. and a photoinitiator, (II) is applied by extrusion at 100-1800 C and distributed over the entire surface with calender or laminator rollers under pressure.

Description

PROCESS FOR COATING PRINTED CIRCUIT CARDS
The present invention relates to a method of coating printed circuit boards, in particular cards comprising holes whose walls are coated on their entirety with a cladding, by means of a photopolymerizable thermoplastic material and containing no solvent.

In the present description and in the claims which are attached to it, the expression “holes with plated walls” will be understood to mean those holes the entire walls of which are coated with a veneer.

 "Research Disclosure" 249019, January 1985 and EP 0 080665 describe methods for preparing by mixture a photopolymerizable compound by spinning in the molten state and the coating of a support with the molten product. Such processes have been used on an industrial scale for the production of photopolymerizable products, such as flexographic printing plates, by coating the surface, essentially plane of the support, with the photopolymerizable composition.

 However, in the manufacture of printed circuit cards, the coating of individual cards having a surface already provided with holes or conductive tracks is provided by the user.

 The purpose of such coatings of reserve materials is, on the one hand, to form the conductive tracks and, on the other hand, to protect them during the welding process.

The coatings are necessary during different stages of the process, in order to protect the underlying material from attacks due to treatment baths, such as developers, mordants and agents to promote electrolytic deposits.

 While, when the requirements for precision of the conductive tracks are low, the resist material is applied so as to form an image by applying screen printing methods, the use of photographic methods is necessary when the fineness of the lines requires a high resolution. . In these latter processes, a uniform reserve layer is first deposited over the entire surface. Next, the layer is exposed so as to form an image through a transparency and washing areas are exposed (in the case of reserve materials operating on positives) or unexposed areas (in the case of materials reserve operating on negatives) of the reserve image.

 Different coating methods are known, which have been adapted to the characteristics of the reserve materials necessary for different uses, as explained below.

The liquid photographic reserves, mostly based on diazo compounds, are applied to the support by spinning or by dipping. They can be applied continuously by various coating methods, in particular by coating with a molten curtain (as described in
US 4,230,793) or by spraying methods, in particular electrostatic spraying methods.

 Electrophoresis deposition produces the reserve layer with another group of liquid photographic reserves.

 Such methods are well suited to liquid photographic reserves having a low viscosity due to the addition of solvents. Thus, it is necessary to dry the cards after the coating step.

 Another coating method is roller coating, in which the liquid coating material is applied by rollers to the surface to be coated. The rollers usually have a surface which has been structured in order to accept the appropriate coating material (PCT / WO 92/07679).

 Such liquid photographic reserves are relatively expensive and are particularly suitable for producing thin layers. However, they have the drawback of not being able to coat both sides of the card simultaneously and of requiring, for the production of a uniform layer, a significant technical effort, the uncured surface of the reserve being also very sensitive to dust and mechanical damage. It has proven difficult to coat a card provided with tracks and holes with a masking layer, since the coating of the vertical surfaces of the holes with the liquid reserve is not satisfactory. In particular, it is difficult to protect the plating with which the walls of the holes are coated. Although the liquid reserve easily penetrates inside the hole, because of its low viscosity, it cannot reliably fill the holes completely, because it’s hard to get the air out of the holes. It therefore follows that the plating of the holes is not sufficiently protected and can be destroyed or undergo a chemical attack during the etching.

 Liquid photographic reserves deposited by electrophoresis ensure a particularly uniform coating. However, their use requires electrically conductive substrates and they cannot, for example, be used as welding masks. The thickness of the coating is limited to a maximum of about 15 µm, which is too thin for use as a reserve for electroplating or coatings; they can therefore only be used as reserves for etching processes.

 As in the case of low viscosity liquid photographic reserves, this method also has specific drawbacks for the coating of printed circuit boards having holes whose walls are plated. The thin layer does not fill the holes enough and for many holes only the wall is coated.

To protect the walls from the holes, when using photographic reserves operating on negatives, it is necessary to harden reliably by exposure the reserve layers on the walls. This is by no means guaranteed in the case of holes having a high depth / diameter ratio.

 In the case of the reserves intended for a method operating on positive, on the other hand, it is necessary to reliably prevent the exposure of the holes during the entire treatment sequence. This is only possible thanks to the use of a residual ring resulting from the exposure in correspondence with the image, and by protecting from ultraviolet rays all the successive stages of the process.

 The use of dry reserve coatings eliminates many of the above-mentioned drawbacks, and has therefore been applied on a large scale. They are described for example in US Patents 3,469,982 and 3,547,730.

They have a sandwich structure comprising a photopolymerizable layer between a temporary support and a transparent protective sheet. The polymerizable layer - in most cases photopolymerizable - is transferred to the printed circuit board, then exposed to form an image through a photographic transparency reproducing the model of the circuit, and after removal of the protective sheet, is eliminated by means an adequate solvent.

 Dry reserves are remarkable for their simplicity of handling for a layer thickness adjustable with great precision by the manufacturer, and for the fact that the surface of the reserve is well protected by a protective layer. However, they have a limited ability to conform to structured supports. A particular drawback is that the only way to protect the plating or strapping of the walls of the holes is to set up a protective "tent" by a process consisting in covering the holes with a layer of photographic reserve. Care must be taken, during the exposure, to ensure the exposure of the reserve not only over the hole, but also over a larger area of mooring around the hole. This requires additional space on the map, and can lead to lack of space problems with ever increasing circuit densities. In addition, this process does not always ensure reliable protection, by setting up a "tent", holes of large diameter.

 Consequently, the problem which the present invention aims to solve is that of carrying out a method making it possible to coat printed circuit cards, in particular cards having holes, with a photopolymerizable material in which it is possible to protect from reliably the plating or strapping of the walls of the holes without lateral overflow, without any drying being necessary, the production of which is not complicated to implement and which makes it possible to produce coatings with a thickness of approximately 5 to 100 μm which adapt well to supports possibly provided with tracks and holes.

The present invention relates to a method of coating a printed circuit board by means of photopolymerizable materials containing a binder based on a thermoplastic polymer, an ethylenically unsaturated compound polymerizable by addition, and a promoter activatable by means of radiation, including the steps of
(a) applying the photopolymerizable material by extrusion
at a temperature of 100 "to 1800 on a surface of the
printed circuit board, and
(b) apply the light-curing material under pressure
by means of at least one roller on the surface of the
card, the roller pressure on the card being 0.2
at 5 kg / cm2.

 According to an embodiment of 1 invention, the card comprises at least one hole with plated walls or a circuit diagram.

 According to another embodiment of the invention, the photopolymerizable material does not contain a solvent and has a viscosity of 10 to 100 Pas at a temperature of 100 to 140 C.

 According to yet another embodiment of the invention, the photopolymerizable material has a viscosity of 20 to 400 Pas at a temperature of 100 to 140 "C.

 According to yet another embodiment of the invention, the light-curing material and the printed circuit board are heated to a temperature of 80 to 100 "C, and that the temperature of the roll is 10 to 50" C higher than that from the menu.

 According to yet another embodiment of the invention, the printed circuit board has holes with modified plated walls and the pressure of the rollers and the viscosity are adjusted so that the photopolymerizable material is forced into the holes.

 According to one embodiment of the invention, the pressure exerted by the roller on the card is from 0.2 to 5 kg / cm2.

 According to another embodiment of the present invention, the card is coated simultaneously on its two faces.

 According to yet another embodiment of the present invention, the photopolymerizable material is covered with a transparent protective sheet after coating the card.

 According to yet another embodiment of the present invention, the transparent protective sheet is applied by laminating to the surface of the photopolymerizable material.

 The present invention also relates to a method of manufacturing a card coated with copper and having holes with plated walls, using a photopolymerizable material containing a binder based on a thermoplastic polymer, a polymerizable ethylenically unsaturated compound. by addition and a promoter activatable by radiation, and comprising the exposure of the coated card through a mask having circuit elements, the elimination of the soluble parts by means of a solvent in order to constitute a reserve for the etching, then etching or electrolytic plating of a discovered copper surface.

 Suitable photopolymerizable materials are known and are used are known and are used in systems operating on negative or positive, photocurable or photodegradable. The preferred photopolymerizable compositions essentially contain a binder based on a thermoplastic polymer, an ethylenically unsaturated compound which can be polymerized by addition and a polymerization promoter capable of being activated by radiation and containing no solvent. Their viscosity is high enough at room temperature so that they do not have a creep characteristic. For a treatment temperature of 100 to 140 ° C., the viscosity is from 10 to 1000 Pas, preferably between 40 and 400 Pas.

 Thermoplastic polymer binders are copolymers of olefinically unsaturated carboxylic acids with other monomers. Preferred binders are copolymers of acrylic acid, methacrylic acid, maleic acid, and itaconic acid with styrene and / or alkyl acrylates and / or alkyl methacrylates. Reaction products of copolymers of olefinically unsaturated carboxylic acids and / or dicarboxylic acid anhydrides with the above-mentioned co-monomers and water, alcohols, and / or amines can also be used; anhydrides of maleic acid, itaconic acid and citaconic acid are preferred.

 The ethylenically unsaturated compounds, polymerizable by adequate addition, capable of being used alone or with other monomers include derivatives of acrylic and methacrylic acid, such as acrylamides, methacrylamides, alkyl acrylates, methacrylates of alkyl, and especially esters of acrylic acid or methacrylic acid with diols or polyols such as ethylene glycol, diethylene glycol, trimethylol propane, pentaerythritol and polyethylene glycols.

 The total amount of monomers in the photopolymerizable compounds is from 10 to 80% by weight relative to the weight of the total mixture.

 Photopromotors or photopromotor systems can be any compound known as such. Examples are: 9-phenyl-acridine, 9,10-dimethylbenzo phenene, benzophenone / Michler's ketone, and hexaarylbisimidazole / mercaptobenzoxazole. The photopromotors can be used in proportions of 0.01 to 10% by weight.

 The light-curing materials can also contain other adjuvants such as, for example, dyes, pigments, plasticizers, adhesion-promoting agents, fillers or fillers and above all stabilizers. The latter ensure the behavior of the material during its short-term heating at the coating temperature, so that no chemical change takes place, and in particular no thermal polymerization.

 The printed circuit board is preferably coated with the photopolymerizable material by hot extrusion. It is also advisable to subject the support to preheating. The preheating temperature is between 80 and 1800C, preferably between 100 and 140 "C. The photopolymerizable material is applied, for example, to the edge of the support in an amount sufficient to allow the coating of the entire surface forming the thickness layer sought.

In another embodiment, the photopolymerizable material is continuously coated to form a film which covers the entire width of the card.

 The high viscosity of the photopolymerizable material also allows the coating of the underside of the support, so that the two faces can be coated simultaneously.

 Separate light-curing materials can be used for coating the top and bottom of the card. For example, it is possible to apply, preferably first of all, a material with a high viscosity on the upper face and, on the lower face, a film-forming material.

 The photopolymerizable material is distributed on the support under pressure due to the rollers of a calender or of a rolling device crossed by the card to be coated. The rolls are heated to a temperature which is preferably 10 to 50 "C higher than that of the card. At this temperature, the viscosity of the photopolymerizable material decreases, so that, under the pressure of the rollers, the material expels from the the air of all irregularities and holes in the card while ensuring the complete coating of its surface and filling the holes. The thickness of the coating can be adjusted by varying the temperature, the pressure of the rollers and the feed speed of the rollers. The accuracy of the coating thickness that can be obtained is + 10% of the total thickness.

 The preferred pressure exerted by the calender or the rollers of the rolling device on the printed circuit board is 0.2 to 5, preferably 0.5 to 2 kg / cm2.

 In a preferred embodiment, a transparent sheet is applied by laminating to the two faces of the support provided with the photopolymerizable material. The sheets prevent the photopolymerizable layers from adhering to the calender rolls or to the transparency in the subsequent exposure stage, and it simultaneously protects the photopolymerizable layers from contact with oxygen in the air, which reduces the sensitivity of light-curing materials. The preferred protective sheets are polyester sheets, for example polyethylene terephthalate.

 Suitable support materials are metals such as aluminum, steel, zinc, copper, and sheets of synthetic resins such as polyethylene terephthalate, cellulose acetate and glass. Synthetic resin cards coated with copper are mainly used for the manufacture of printed circuit cards.

 Exposure to actinic radiation causes photopolymerization, which results in a difference in solubility between exposed and unexposed areas. We carry out the exhibition through a transparency representing the circuit diagram. It is not necessary for this transparency to contain the diagram corresponding precisely to the holes, as is required in the other methods. Rather, it suffices to represent the diagram of the conductive tracks.

 Uncured areas are removed using suitable solvents. A washing time is chosen which is sufficient for the unexposed areas of the surface of the card to be dissolved, and that however the unexposed or only partially exposed material present in the holes, be completely or partially dissolved . The material which remains on the surface constitutes the reserve layer capable of protecting the underlying metal during the next step of etching or electrolytic plating. The light-curing material remaining in the holes protects the plating of the hole.

 Finally, all the remaining reserve is removed by means of a strongly alkaline elimination bath.

 Surprisingly, and contrary to the methods according to the prior art, it has been found that it was not necessary to expose the holes to protect them effectively. This eliminates the need for precision alignment, otherwise necessary between the photomask and the position of the holes. In addition, it is easier during the removal step to remove the non-polymerized photopolymerizable material than the polymerized material. This reduces the risk of residue forming and thus makes the following steps more effective.

 The thickness of the layers obtained by the process according to the invention can be adapted within wide limits, in order to make them correspond to the requirements of the subsequent treatment steps. This thickness can vary between 2 and 100 μm and more. For the etching, layers of 15 to 30 μm are preferred and, for the formation of the layers obtained by electrolytic plating, layers of 30 to 50 μm.

 You can also use this for applying a welding mask. A card with conductive tracks is coated as described above, so that the spaces between the conductive tracks and the holes are well filled due to the high degree of conformability presented by the coating material. The card is equipped after exposure and development.

 Since the good conformability ensures excellent contact between the photopolymerizable material and the card, the welding process can be applied without risking, as would otherwise be possible, that a separation of the weld occurs.

 To further illustrate the present invention, the following examples are provided. All proportions and percentages are expressed by weight, unless otherwise indicated.

Example 1
A photopolymerizable material is prepared from:
- 65 g of a methyl methacrylate / acrylate copolymer
ethyl / methacrylic acid (51/29/20);
- 25 g of trimethylopropane trimethacrylate,
- 0.3 g of Victoria blue,
- 4 g of 4- (dimethylamino) ethyl benzoate,
- 6.5 g of benzophenone,
- 2 g of Michler's ketone.

 The components are mixed intimately in an extruder. A 100 x 200 mm and 3 mm thick printed circuit board provided with holes 0.5 to 6 mm in diameter is heated to 100 ° C. 2 g of the product having the above formula are melted at a temperature of 120 "C. and it is applied by means of a hot glue gun in the form of two strips on the front edge of the upper and lower sides of the card A laminating device (Riston HRL de Du Pont) is loaded with polyethylene terephthalate film Mylar 92A, and the rollers are heated to 140 "C. The card is passed through the laminating device at a feed speed of 0.5 m / min. This makes it possible to equalize the photopolymerizable material and to form a layer with a thickness of approximately 25 μm. All holes are completely filled with the light-curing material.

 The map is exposed through a diagram representing the tracks, the sucking holes not concerned. The holes also remain filled, even after development using a 1% sodium carbonate solution which allows the drawing of the tracks to be updated. The card is then etched in a traditional acid solution for etching and the reserve layer is removed using a 1% potassium hydroxide solution. The holes are now also uncovered and a card is obtained provided with conductive tracks and metallized contacts.

Example 2
A photopolymerizable material is prepared as in Example 1, and it is intimately mixed in an extruder.

A copper coated epoxy resin card of dimensions 100 x 200 m and 3 mm thick is heated to 120 ° C., having holes of 0.5 to 6 mm in diameter. 2 g of the preceding mixture are melted, and it is applied by means of a hot glue gun in the form of two strips on the front edge of the upper and lower faces of the card.

 The card is passed through a laminating device, as in Example 1, at a feed speed of 1.5 m / min, and by the action of the rollers on the photopolymerizable material the photopolymerizable material between the polyester sheet and the support is brought to a thickness of about 75 µm.

 The map is exposed through a diagram representing the tracks, and after removal of the polyester layer, development is carried out using a 1% sodium carbonate solution. Copper is deposited on the exposed areas using a traditional electrolytic plating bath, and the stock is removed using a 1% potassium hydroxide solution. We remove the copper base by etching, and we get the representation of the circuit.

Example 3
A photopolymerizable material is extruded as in Example 1, to obtain a film with a thickness of 125 μm, and it is subjected to calendering at 120 ° C. at an advance speed of 1 m / min so as to produce a layer 40 micrometers thick on both sides of a copper-coated epoxy resin card, dimensions 100 x 200 mm and a thickness of 3 mm, provided with holes with a diameter of 0.5 to 6 mm .

Example 4
A card of 100 x 200 mm and a thickness of 3 mm, provided with holes with a diameter of 0.5 to 6 mm, is heated to 100 "C. 2 g of the material of Example 1 are heated to 1200C, and it is applied in the form of a strip on the front edge of the upper face of the card; the assembly is subjected to calendering at 1200C, at a feed speed of 0.5 m / min. A traditional dry resist film 25 μm thick is applied simultaneously to the bottom surface of the card. The card is treated as in Example 1 and after etching, a card is obtained provided with walled holes. metallic.

Example 5
A printed circuit board provided with conductive tracks of dimensions 100 x 200 mm and 3 mm in thickness is heated to 100 C. At 1200C, 2 g of the photosensitive layer of a film of a dry solder mask are melted commercially available (Vacrel 8100 from Du Pont) and applied in the form of strips on the front edge of the upper and lower sides of the card; the assembly is subjected to calendering at 120 ° C., operating at a speed of 0.5 m / min. The card is exposed through an appropriate photomask on all the zones, except the zones which must remain unexposed for the soldering, and development by washing, as described previously.

 A printed circuit board is produced, on which the spaces between the conductive tracks are filled with the material of the solder mask, the conductive tracks being covered with a layer with a thickness of 25 μm.

Claims (11)

 1.- A process for coating a printed circuit board using photopolymerizable materials containing a binder based on a thermoplastic polymer, an ethylenically unsaturated compound polymerizable by addition, and a promoter activatable by means of radiation, including the steps of  (a) applying the photopolymerizable material by extrusion  at a temperature of 100 "to 180 on a surface of the  printed circuit board and  (b) apply the light-curing material under pressure  by means of at least one roller on the surface of the  menu.  2.- The method of coating a printed circuit card according to claim 1, characterized in that the card comprises at least one hole with plated walls or a circuit diagram.  3.- The method of coating a printed circuit board according to claim 1, characterized in that the photopolymerizable material does not contain solvent and has a viscosity of 10 to 100 Pas at a temperature of 100 to 140 C.  4. The method of coating a printed circuit board according to claim 1, characterized in that the photopolymerizable material has a viscosity of 20 to 400 Pas at a temperature of 100 to 140 "C.  5.- The method of coating a printed circuit board according to claim 1, characterized in that the photopolymerizable material and the printed circuit board are heated to a temperature of 80 to 100 "C, and that the temperature of the roll is 10 to 50 "C higher than that of the card.  6. The method of coating a printed circuit board according to claim 2, in which the printed circuit board has holes with modified plated walls and the pressure of the rollers and the viscosity are adjusted so that the photopolymerizable material is forced into the holes.  7. The method of coating a printed circuit card according to claim 1, characterized in that the pressure exerted by the roller on the card is from 0.2 to 5 kg / cm2.  8.- The method of coating a printed circuit card according to claim 1, characterized in that the card is coated simultaneously on its two faces.  9. The method of coating a printed circuit card according to claim 1, characterized in that the photopolymerizable material is covered with a transparent protective sheet after coating the card.  10.- The method of coating a printed circuit board according to claim 9, characterized in that the transparent protective sheet is applied by rolling on the surface of the photopolymerizable material.  11.- A method of manufacturing a printed circuit by coating a card coated with copper and having holes with plated walls, by means of a photopolymerizable material containing a binder based on a thermoplastic polymer, a compound with ethylenic unsaturation, addition polymerizable and a promoter activatable by radiation, and comprising the exposure of the coated card through a mask having circuit elements, the elimination of the soluble parts by means of a solvent in order to constitute a reserve for etching, then etching or electrolytic plating of an exposed copper surface.