Coating processes – With post-treatment of coating or coating material – Heating or drying
Patent
1998-06-03
1999-11-02
Cameron, Erma
Coating processes
With post-treatment of coating or coating material
Heating or drying
427 58, 427 96, B05D 302
Patent
active
059766298
DESCRIPTION:
BRIEF SUMMARY
This invention relates to coating compositions and in particular coating compositions which are used are to prevent access of potentially problem-causing materials (such as solvents including water) to the surfaces which have been coated.
By way of example, electronics assemblies such as printed circuit boards, especially those intended for use in high-humidity environments, can be affected by the condensation of atmospheric moisture on their surfaces. This moisture may form a thin water-film between areas of opposite polarity and allow "crosstalk" between conductive areas.
In order to reduce this effect, assemblies have, in the past been coated with a variety of resinous materials including acrylics, epoxies, polyurethanes and silicones. These materials also protect the assembly from rough handling in service and from process chemicals. However, apart from when silicone resins are used, the final surface of the coating can still become coated with a film of moisture if the conditions are right.
Some silicone resins can be expensive and are affected by certain common chemicals and also by water under certain circumstances.
In addition, all of the materials mentioned above need to be applied in coatings of substantial thickness, for instance 0.025 mm (0.001 in) to 0.075 mm (0.003 in), in order to be effective. Accordingly, "contact areas" on the board, such as gold-plated edge-fingers (and also some components themselves, e.g. trim-potentiometers and coils) must be prevented from becoming coated with resin. This is normally a very tedious, time-consuming, labour-intensive and therefore costly process, which needs to be repeated as any "masking materials" applied to the assemblies also need to be removed.
Furthermore, some of the resins mentioned above need to be heat-cured. This requires equipment and is also energy-consumptive. The applied heat may also damage the assembly which has been coated by the resin. Even the shrinkage of the resin during cure can affect delicate components.
Finally, some of the solvents used to dissolve and "thin" the resins may be flammable, toxic and/or may chemically affect the assemblies or may be "ozone-depleting".
Some of the above-mentioned disadvantages have been overcome in the past by the use of very low solids-content solutions of fluoro-acrylate resin polymers in perfluorocarbon liquids (such as "FLUORAD" FC-722 marketed by the 3M Company). Unfortunately, when dry, these resins tend to crack when flexed. They are also quite expensive.
Fluoroacrylate-polymer resins are also chemically attacked by a number of common chemical agents such as isopropanol, dilute acetic acid and some other common solvents. They also have limited solubilities in perfluorocarbon liquids so that around 6-8% by weight is the maximum concentration that can be achieved in this type of solvent.
SUMMARY OF INVENTION
According to the present invention there is provided a method of protecting an assembly, or part thereof, comprising applying a composition to said assembly, or part thereof, and drying, or allowing to dry, the applied composition to leave a thin coating of a resin on said assembly, or part thereof, characterised in that the applied composition comprises a multicyclic perfluorinated cycloalkane resin and a fluorocarbon liquid solvent therefor.
The coating provided by the method of the present invention provides an improved material, compared with the prior art coatings discussed above. It is not only resistant to "attack" by solvents, including water and common organic solvents, it is also more resistant to chemical attack. Furthermore, such a coating is inherently flexible and will not crack when flexed at low temperatures.
Preferably, the resin has the formula C.sub.n F.sub.m where n is from 25 to 60 and n<m<2n+2. The molecular weight of such a material lies between 1200 and 2400.
The perfluorinated cycloalkane may be made by fluorination of a cycloalkane by fluorine gas and CoF.sub.3 catalyst. A preferred cycloalkane is phenanthrene.
The solvent may be any suitable fluorocarb
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patent: 5026929 (1991-06-01), Marsella et al.
Moore et al, ACS Symp. Ser. (1995), 614 (Microelectronics Technology), pp. 449-470, 1995.
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