A METHOD OF COATING RIDGED PLATES, PARTICULARLY PRINTED CIRCUIT BOARDS

Abstract: Ridged plates, particularly printed circuit boards, are coated with varnish, particularly a solder resist, which preferably consists of a light-sensitive varnish, for example a photopolymerizable varnish. The plate is passed through a roll nip in a roll press for single-sided or double-sided coating of the plate. The varnish is applied at a temperature of at least 5 DEG C and with a viscosity of at least 300 mPa.s, wherein the thickness of the varnish film on respective applicator rolls is adjusted to 5-100 mu m in a known manner.

Description: A METHOD OF COATING RIDGED PLATES, PARTICULARLY PRINTED CIRCUIT BOARDS The present invention relates to a method of applying to ridged plates, particularly printed circuit boards, a varnish coating, particularly a solder resist, which preferably consists of a light-sensitive, e.g. photo- polymerizeable varnish or lacquer. In order to protect the conductors on printed circuit boards from contact with solder when, for example, flow-soldering those components which must not come into contact with the solder, it is normal in the manufacture of such cards to coat these components with varnish while leaving those parts of the circuit which shall be soldered free from varnish. Hitherto, this varnish (solder resist or solder mask) has been applied in patterns with the aid of conven- tional screen printing, although because the conductors on the circuit board have become finer and finer with progressively higher degrees of precision and accuracy with respect to holes and hole-collars, the circuit boards are now produced photographically, wherein the pattern is applied subsequent to coating with a light- sensitive film. The film as such can be dissolved in a developing liquid, although it hardens when exposed to light and the light-irradiated parts cannot be dis¬ solved. It is possible, of course, to reason in the opposite direction, namely that a film which has been hardened by irradiation with light becomes soluble. The film can be applied as a foil or as liquid varnish. This film, antisolder varnish film or solder resist, shall be applied so that no air is present between the substrate, the printed circuit board, and the film. When the film is applied in the form of a self-adhesive foil, the foil is normally applied with the aid of a press and under vacuum. It is, however, almost impos¬ sible to apply the film totally on the surfaces of the circuit board between conductors which lie closely together in a dense conductor pattern. If interspaces are present and air thus remains, the film lifts during the soldering process and ruptures. Another possibility of applying a solder resist invol¬ ves screen-printing the whole of the surface of the printed circuit board with a liquid varnish, such as to press the varnish down between the conductors. One drawback with this method is that the varnish is also pressed down into the component holes. Another problem resides in the difficulty of obtaining a sufficiently thick layer of varnish over the conductors, and also that different varnish thicknesses are obtained in the printing direction on conductors which extend perpen¬ dicularly to the printing direction. Another known method of applying solder resists is to curtain-varnish the circuit boards with a very coating of varnish. The drawback with this method is that not all of the air is pressed away in the case of dense patterns, and that varnish enters the component holes. Another drawback is that the varnish can only be ap¬ plied on one side at a time, and that the circuit board must be held completely flat until the varnish has set. Also known to the art is a process in which varnish coating are applied with the aid of a roller coating technique when preparing solder masks (solder resists) , in much the same manner as when preparing etching resists or masks. This process employs the use of embossed rolls, in order to ensure that sufficient varnish is taken up. However, it has only been possible to use this known process with conductors of small height extensions. Furthermore, only small coating thicknesses can be applied when practicing this known process and the process can therefore only be used effectively in some few cases. The process also re¬ quires the use of low-viscosity varnishes, which are associated with small varnish thicknesses and poor covering of the etched areas. The solder resist shall fulfill two functions. Firstly, it shall protect from contact with solder those free copper surfaces that are not to be soldered. Secondly, the solder resist shall also act as an insulator. Con¬ sequently, it is important that the varnish layer is applied to a sufficient thickness. The most serious problem is that of covering the edges of the conductors that have been etched out. When applied to an edge surface, the varnish will contract from the sharp corner(s) of this edge surface as a result of surface tension. An object of the present invention is to eliminate the drawbacks of this known technique. Another object of the invention is to provide a method which will enable a plate to be varnished readily and economically. These objects are achieved with the present inventive method, in which a varnish coating is applied to a plate by passing the plate through a roll nip in a roll press for coating the plate on one or both sides there- of, and by supplying the varnish at a temperature of at least 5°C and a viscosity of at least 300 mPa-s, and adjusting the thickness of the varnish film on respec¬ tive applicator rolls to 5-100 μm, by adjustment in a known manner, e.g. by a pressure roll or doctor blade, for example. A suitable thickness of the varnish film on the ap¬ plicator roll is preferably at least 20 or 30 μm, for example 40 μm, and at most 80 or 90 μm, for example 70 μ . The temperature of the varnish upon application is preferably at highest 60°C, preferably at least 15 to 20°C. It is primarily suitable to work at room temperature, e.g. 18-24°C. The viscosity should not exceed 5000 mPa-s and preferably not 4000 mPa-s. A suitable viscosity lies in the range of 1000-3000 mPa•s. The viscosity is thus the decisive parameter. The viscosity, however, is temperature dependent, although, of course, it is the viscosity of the varnish applied which is essentially decisive. The best results, however, are obtained with a "short" varnish. In other words, the varnish shall not form filaments. This property of varnishes is known, although no method of measuring this property (unit of measurement) has been proposed or established. A double-sided coating machine may, for instance, in- elude two bottom rolls and two top rolls arranged vertically above one another. The rolls axles are mounted horizontally and the distances between the rolls of each roll pair determines the thickness to which the varnish shall be applied to the plate (the panel). The size of the roll nip is determined by the material thickness of the plate. In operation, the varnish is pumped continuously onto the applicator rolls through a perforated pipe for the upper roll pair and through a perforated pipe for the lower roll pair. A copious flow of varnish is neces¬ sary, since there should be maintained a continuous flow of varnish which runs along the rolls and, when reaching the end of the rolls, runs down into a con- tainer and is continuously pumped back onto the rolls. The amount of varnish delivered to the roll should correspond at least to 1.5 times the amount consumed by the coating, preferably at least 5, most preferably at least 10 times this amount. The amount of varnish supplied may be as high as from 25 to 30 times the amount consumed by the coating, and even higher, the higher limits of the amount delivered being limited by practical circumstances. The plates can be placed in a rack, subsequent to having passed through the roll nip. The varnish is pressed down between the conductors already on the first pass through the nip. The viscosity of the varnish is so high that the varnish will not run, even though the plates are placed on edge. In order to apply a varnish layer of sufficient thickness, it may be necessary to apply a further coating of varnish to the plate, for example after 4 minutes. The plates may also be allowed to pass automatically through a further varnish-applying machine, i.e. the plates are fed automatically into a first varnishing machine, passed over a conveyor belt, for example a so- called "hedgehog belt", dried for a couple of minutes so that the varnish or lacquer solidifies and are then fed automatically into a further varnishing machine and then dried to a non-tacky state for subsequent expo¬ sure. The present invention affords a large number of ad¬ vantages, namely a very uniform varnish layer, the absence of varnish in mounting holes, enables the plates to be varnished on both sides thereof, a high production rate, no contaminants collect in the varnish with continuous filtration, a thick varnish layer even over single conductors, the absence of air between closely packed conductors, no setting-up costs when varnishing different plates (printed circuit boards), no material waste. Thus, the invention enables both sides of a plate to be coated simultaneously. It has earlier been possible to coat two sides of a plate by screen printing or curtain varnishing, in which first one side of the plate is coated and then the other side thereof. When practic¬ ing these methods, however, the sensitivity to light and the developing times of the varnish coatings vary on the two sides of the plate, since the sensitivity of the varnish and its hardening when irradiated with light are dependent on the length of time over which the varnish has been dry on the plate. A dry varnish hardens constantly at room temperature and even more so the higher the drying temperature. These difficulties have now been completely overcome. It must also be mentioned that screen-printing coating machines are known in which double-sided printing can be achieved, and also electrostatic spraying machines, although these machines do not yet function satisfactorily. The present invention also overcomes the drawback of oxidation of that side to which no varnish is applied during a single-side coating process (especially in the case of copper) or contamination of said side in some other way such as to make it necessary to clean the other side of the plate when both sides are to be varnished. The task of cleaning the plate is difficult to carry out without negatively influencing the varnish applied to the other side of the plate. Such cleaning is normally necessary, even when both sides of the plate are initially carefully cleaned. It should also be mentioned that plates that have been coated in accordance with the present invention can be placed on edge immediately after applying the coating, due to the viscosity of the varnish; this has not been possible with curtain-varnished circuit boards. It should also be mentioned that the holes in a plate will not become blocked, since the varnish is applied to a thickness of 5-100 μm. The varnish remains at the edge of the hole, meaning that development after exposure will be highly positive. There are no varnish residues which prevent soldering of the holes. By applying varnish to both sides of the plate simul¬ taneously, there is no risk that the highly sensitive first side of the plate will be damaged while hardening in the drying state as varnish is applied to the other side of said plate. The most suitable embodiment of the present invention is therefore the embodiment in which both sides are varnished simultaneously. The invention will now be described in more detail with reference to a working example. Example A printed circuit board provided with an etched conduc¬ tor pattern and drilled, through-plated holes, was varnished with a photo-sensitive varnish on both sides thereof at 24°C, by passing the circuit board through two rubber rolls having a diameter of 70 mm and a hardness of 60° IRH. Varnish was applied continuously to the rolls by means of two applicator rolls. The varnish was applied to the rolls to a thickness of about 30 μm and the varnish had a viscosity of 2000 ±100 mPa-s. All rolls were driven at the same surface speed, about 15 m/min. The viscosity was measured with a Brookfield viscosimeter LVT, spindle 3. The circuit board was passed once more through the roll nip, while the varnish was still wet, and was then placed in a rack to dry. The varnish now had a thick¬ ness of 2-10 μm over large pattern surfaces. If a thicker layer of varnish is desired, the circuit board is dried in air for about 5 minutes and then passed again through the roll nip, as before described. This results in a layer thickness over individual conductors of about 15 μm. The circuit board was finally dried in an oven with good air exchange (for 15 minutes at 80°C), after which it was judged to be sufficiently dry for irradiation in a vacuum frame, without sticking to the film.

Claims: Claims 1. A method of applying to ridged plates, particularly printed circuit boards, a varnish coating, particularly a solder resist, which preferably consists of a light- sensitive, for example a photopolymerizeable varnish, c h a r a c t e r i z e d by passing the plate through a roll nip in a roll press for single-side or double- sided coating of the plate, wherein said varnish is applied at a temperature of at least 5°C and a vis¬ cosity of at least 300 mPa-s; and by adapting the thickness of the varnish film on respective applicator rolls to 5-100 μm in a known manner. 2. A method according to Claim 1, c h a r a c ¬ t e r i z e d in that the viscosity is at most 5000 mPa-s. 3. A method according to Claim 1 or Claim 2, c h a r a c t e r i z e d in that the viscosity is in the range of 1000-4000 mPa-s.

Inventors: THAMS JOHAN-PETTER BRYNJULF, THAMS, JOHAN-PETTER, BRYNJULF
Applicants: THAMS JOHAN PETTER B, THAMS, JOHAN-PETTER, BRYNJULF
Application year: 1991
Main classification: H05K3
IPC codes: B05D7, B23K1, B23K35, H05K3