GB2136315A

GB2136315A - Insulating coatings

Info

Publication number: GB2136315A
Application number: GB08306098A
Authority: GB
Inventors: H F Sterling
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1983-02-04
Filing date: 1983-02-04
Publication date: 1984-09-19
Also published as: AU2505884A; GB2136315B; GB8306098D0

Abstract

Electronic components are provided with insulation on their exposed conductive surfaces only. The component is exposed to radio frequency energy which heats only the conductor regions. The component is next treated with a powdered plastics material which adheres only to the heated regions. A further application of heat fuses the plastics into a contiguous coating on the conductive surfaces.

Description

SPECIFICATION Insulating coatings This invention relates to the provision of insulating coatings on electrically conductive surfaces, and in particular to the provision of insulation on electronic components.

In the manufacture of electronic components, e.g. capacitors, it is common practice to provide insulation of the device in the form of an encapsulating coating of a plastics material. Under circuit fault conditions such components may be subjected to excessive heating resulting in carbonisation of the plastics material. Since carbon is conductive a short circuit path may be established which can then result in excessive circuit damage or, in some circumstances, fire.

As well as the high voltage breakdown mechanisms described above another form of short circuit failure has been recognised in ceramic capacitors. Under some service conditions changes in temperature and humidity can result in the condensation of traces of moisture interfacially between the ceramic body and the plastics encapsulant. Under these conditions metal transport of the electrode material can occur in the form of thin filamentary whiskers which establish a short circuit path.

The object of the invention is to minimise or to overcome those disadvantages.

According to one asepct of the invention there is provided a method of insulating an electrically conductive region of an electronic component, the method including exposing the component to radio frequency energy whereby said conductive region is heated, applying a fusible plastics material to the heated region, and fusing the plastics material to form a contiguous insulating coating on the region.

According to another aspect of the invention there is provided an apparatus for providing plastics insulation on conductive regions of electronic components, the apparatus including a radio frequency generator whereby said conductive regions are selectively heated, means for applying a plastics material to said heated regions, and means for fusing said plastics material to form a contiguous insulating coating on said regions.

The term radio frequency as employed herein is understood to include both the radio and microwave frequency regions of the electromagnetic spectrum.

The process provides a plastics coating only on the exposed conductor regions of the component.

Thus, in the case of a ceramic capacitor, there is no continuous plastics coating between the device leads and hence no risk of the establishment of a carbon path. Further, since only the conductive surfaces are coated there is no hermetic sealing thus overcoming the problem of moisture trapping within the device.

An embodiment of the invention will now be described with reference to the accompanying drawing in which the signal figure is a schematic diagram of an apparatus for applying insulating coatings to components.

Referring to the drawing, the apparatus includes an insulating conveyor 11 on which components, e.g. ceramic capacitors 12, are supported and carried through a heating station 1 3. As the capacitors 12 pass through the heating station 1 3 they are exposed to radiofrequency energy of a sufficient intensity to cause heating of the conductive portions, including the exposed leads 1 21 , to a temperature above the softening point of the intended insulating plastics material.

The capacitors 12 are next transported to a coating station 14 where plastics material is applied to each device in the form of a powder.

This powder is partially melted by contact with the heated conductive surfaces and thus adheres only to those surfaces. Typically the coating station 14 includes a fluidised bed of the powdered plastics material through which the capacitors 12 are passed to effect selective coating.

A number of plastics materials may be used for this purpose. We prefer to employ epoxide resins but other materials include polyolefins, polyesters and polyurethane. Other suitable materials will be apparent to those skilled in the art.

The selectively coated capacitors are carried from the coating station 1 4 to a curing station 1 5 where heat is applied to fuse or cure the plastics material into a contiguous coating on the conductive surfaces. This heating may be accomplished by conventional thermal means or by a further application of radiofrequency energy.

In some applications the coating and curing steps may be repeated to build up a relatively thick selective coating orto provide a layered coating of different plastics materials.

Finally the coated capacitors 1 2 are carried to a collection station 1 5 to await further processing, e.g. testing and packing for dispatch.

In a preferred embodiment of the invention means are provided to apply the radio frequency energy via a suitable water cooled inductorapplicator system as is known in the arts of induction, dielectric and microwave heating technology. By attention to the RF matching and cyclic application of energy the processes of heating, transfer and coating can be speedily effected with highly efficient use of energy. In fact, because the energy dissipation is localised, less energy may be expanded in total, for this specialised and selective coating of components, than is normally used in the thermal fluidised bed coating of these items.

As with other radio frequency applications the physical size and shape of the components will influence the choice of operating frequency. This is known in the art as the "form factor aspect in relevant calculations. These equations must also take into account the electrical resistivity of the metallic conductions which form the connections to the components.

For example, an oscillator may be designed to feed energy into a slot-type inductor-concentrator operating at low impedance and at a frequency of about 27 MHz. A multiplicity of components, e.g.

0.5 cm square and with thin lead wires, suitably mounted in a series parallel sequence may be traversed across the slot in either linear, circular or spiral fashion and then immediately transferred to the plastics coating station. The speed of operation, power output of the generator and duty cycle is arranged according to the particular application envisaged. For larger components a frequency of 10 to 15 MHz could be chosen.

With suitable design and care in the materials of construction together with a suitable duty-cycle it is possible to combine the inductor-applicator head with the ftuidized bed of plastics powder or granules.

It will be understood that in this process the only heated items are those parts of the components which are metallic and then only those parts which are subjected to the concentrated radio frequency field. It will also be appreciated that in order to obtain efficient operation it is necessary to have a duty cycle which applies a relatively high burst of energy for a short period of time and to transfer the component quickly to the coating station.

The process is not of course limited to fluidised bed coating. Thus the plastics material may be applied in the form of sheet or tape or as a moulded preform. It will also be appreciated that the process is not limited to the treatment of capacitors but may also be employed for the treatment of other components.

Claims

1. A method of insulating an electrically conductive region of an electronic component, the method including exposing the component to radiofrequency energy whereby said conductive region is heated, applying a fusible plastics material to the heated region, and fusing the plastics material to form a contiguous insulating coating on the region.

2. A method as claimed in claim 1 wherein the component is a ceramic capacitor.

3. A method as claimed in claim 1 or 2, wherein the plastics material is fused by a further application of radio frequency energy.

4. A method as claimed in claim 1, 2 or 3, wherein the plastics material is applied to the component from a fluidised bed suspension of the powdered plastics.

5. A method as claimed in any one of claims 1 to 4, wherein the plastics material comprises an epoxide resin, a polyolefin, a polyester or a polyurethane.

6. A method of selectively insulating an electronic component substantially as described herein with reference to the accompanying drawings.

7. An electronic component provided with insulation by a method as claimed in any one of claims 1 to 6.

8. An apparatus for providing plastics insulation on conductive regions of electronic components, the apparatus including a radio frequency generator whereby said conductive regions are selectively heated, means for applying a plastics material to said heated regions, and means for fusing said plastics material to form a contiguous insulating coating on said regions.

9. An apparatus for selective insulation of electronic components substantially as described herein with reference to the accompanying drawing.

New claims or amendments to claims filed on June 3, 1983.

Superseded claim

10.

New or amended claims: 1 0. A ceramic capacitor having exposed conductive portions and provided with insulation comprising a fused plastics material disposed only on the exposed conductive portions.