Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Patent
1993-03-09
1994-08-09
Pianalto, Bernard
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
427 96, 427493, 427512, 427542, 427557, 427561, 427581, B05D 512
Patent
active
053365291
DESCRIPTION:
BRIEF SUMMARY
Underlying the present invention is the task to make available an improved method and an apparatus which makes it possible to coat circuitboards with photopolymerizable lacquers by the curtain pouring process.
The curtain pouring process is described in U.S. Pat. No. 4,230,793. To obtain uniform lacquer layers it is necessary to have as low as possible a lacquer viscosity. This low viscosity, however, causes the lacquer to run off the condutctor edges (edge flight), thus leading to insufficient edge covering.
This problem is described in EP-A 0,075,537 and has been solved by addition of finely divided filler. The required viscosity of 380 mPa.s is reached only at a solids content of 33 to 40 percent by addition of solvents.
The high percentages of solvent require a very slow and costly drying. There is danger of air bubbles and solvent inclusions in particular if the conductors are narrow and high.
It is, therefore, an object of the invention to provide an ecophile possibility of coating circuitboards, preferably by curtain pouring, which can be carried out with a minimum of solvent or even none at all, and in which the inclusion of air bubbles and solvent residues is minimized even for high and narrow conductors.
It is an object of the invention also to achieve as uniform as possible a coatting with minimum edge flight.
The subject of the present invention therefore is a method which proceeds from a photopolymerizable lacquer low in solvents or free of solvents whose viscositty is reduced by brief heating in an apparatus according to the invention and which then impinges in the form of a heated lacquer curtain on a cooled circuitboard which is shot through the curtain.
In the method according to the invention, photopolymerizable lacquer systems with little to no solvent, having a solids content of 80 to 100 percent by weight, preferably 85 to 95%, are scooped with a heatable roll out of a vat temperature-controlled to 60.degree. to 80.degree. C. and are degased with a heatted doctor blade at temperatures of 60.degree. to 90.degree. C., preferably 70.degree. to 80.degree. C., and are briefly heated by infrared lamps and caused to flow.
BRIEF DESCRIPTION OF THE DRAWING
The drawing shows an apparatus for coating circuit boards according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
In continueous process a curtain of liquid is formed by the flowing down of the lacquer system heated to 100.degree. to 160.degree. C., which has a viscosity of 100 to 500 mPa.multidot.s, preferably 100 to 180 mPa.multidot.s, from an inclined doctor blade, the circuitboards being moved through the curtain at a speed of preferably 150 to 300 m/min, being thus coated. Lacquer layers of a thickness of 5 to 30 .mu.m are obtained.
According to the invention, the circuitboards to be coated are not heated, but rather are cooled to a temperature in the range of from -20.degree. to +25.degree. C., preferably from -10.degree. to +10.degree. C.
At 20.degree. C. the lacquers used according to the invention have a viscosity of 10,000 to 150,000 mPa.multidot.s. As they can contain no or only small amounts of solvents, no fillers are needed to adjust the viscosity. Possibly, for economic reasons, the lacquer system may contain up to 50% by weight, preferably up to 10%, of fillers known in themselves.
The preferred binders for the photo-hardening lacquers are those which contain both photo-hardening unsaturated groups as well as thermally crosslinkable ones, as for instance epoxide groups. This makes it possible to pre-crosslink the coated boards by radiation and subsequently to achieve a final hardening by thermal treatment. Such binders are known e.g. from EP-B 0,194,360.
The apparatus for the coating of circuitboards according to the invention is a coating machine where in a coating installation for the curtain pouring process, above the conveyor belt (7) for the circuitboards, a heatable roll (3) is arranged in a gutter-like heatable vat (2), a heatable doctor blade (4) being applied against the roll, over which infrared lamps
Pianalto Bernard
Rutgerswerke Aktiengesellschaft
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