Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Patent
1997-06-24
1999-04-27
Talbot, Brian K.
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
427421, 427426, B05D 512
Patent
active
058979131
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method for applying a protective coating to the components of a printed circuit board, said coating being comprised of a polymerizable material such as a resin.
Protection of components of printed circuit boards used in a harsh environment (temperature, moisture, impacts, vibrations, etc.) by applying to them a coating comprised of a polymer resin is known. Such an operation is often called "varnishing" or "tropicalization" of boards.
In general, a protective coating is made of a single layer of resin applied by sprinkling, or by spraying, or by dipping the board in a liquid resin bath.
However, the protection provided by this resin layer often proves to be inadequate. The complexity of printed circuit boards, due to the presence of active and/or passive components of highly variable size and fragility such as connectors, the various housings of electronic or integrated-circuit components, etc. requires protective coatings of different types depending on the type of component in question. To accomplish this, different molds and different types of resins have to be made for selectively applying an appropriate coating to each type of board component. Specifically, connectors require a coating based on a viscous resin for impact strength in particular, while components of SMD (surface mounted device) housings require protection against moisture for example by means of a highly fluid resin for small housings or a semi-viscous resin for larger housings. As a result, protection of the various components on a single printed circuit board requires production of a resin-holding mold for each component type so that the resin is applied selectively without flowing beyond the component to be protected.
This being the case, protection of the components on a printed circuit board requires prior fabrication of several molds and a coating operation for each mold used. Implementation is lengthy and complex, resulting in very high manufacturing costs.
SUMMARY OF THE INVENTION
The goal of the invention is to remedy these drawbacks and, for this purpose, the invention provides a method for applying a protective coating to the components on a printed circuit board, characterized in that it consists in preparing the material of which the coating is composed from an initial mixture of at least two ingredients and adding to this initial mixture at least one third ingredient in a variable quantity in order to vary the viscosity of the initial mixture according to the type of component to be protected during the coating of the printed circuit board components.
According to another characteristic of the method according to the invention, the two ingredients of the initial mixture are an isocyanate and a polyol, with these two ingredients reacting with one another to form a resin.
According to another characteristic of the invention, the third ingredient for varying the viscosity of the initial mixture is a thixotropic agent or a catalyst.
In general, the method according to the invention consists in producing the initial mixture in a mixing unit equipped with an ejection nozzle and, according to another go characteristic of the method, the third ingredient is advantageously added to the initial mixture in an output chamber of the unit that communicates with the ejection nozzle, the protective coating then being ejected from the nozzle and being applied selectively to the various components, depending on the viscosity required for each type of board component, by displacing the nozzle relative to the board or vice versa.
According to one important advantage of the invention, it is possible to coat the components on a printed circuit board with a protective coating made from one and the same initial mixture whose viscosity can be varied according to the type of component to be protected, using a single apparatus which, in a single operation or pass, is capable of coating all the components to be protected that protrude on at least one side of the printed circuit board.
REFERENCES:
patent: 3440086 (1969-04-01), Kerns
patent: 4317158 (1982-02-01), Dequasie
patent: 4533598 (1985-08-01), Downey et al.
patent: 4970575 (1990-11-01), Soga et al.
patent: 5043221 (1991-08-01), Koleske
patent: 5330101 (1994-07-01), Turner et al.
Electronic Packaging Production, vol. 30, No. 4, Apr. 1990, Massachusetts, US, pp. 39-40, XP000117358 Engineer's Fact File: "Conformal coating materials".
Dessaux Christophe
Moreau Emmanuel
Giat Industries
Talbot Brian K.
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