GB2252677A - RFI screened housing for electronic circuitry - Google Patents

Abstract

A low cost, two-piece housing for a mobile telephone comprises an open-topped box (1) and a separate lid (2). In one embodiment the box (1) is made of conductive material, e.g. from a folded sheet of aluminium, whereas the lid (2) is made of a pliant insulating material (6), e.g. ABS plastics, having a thin conductive coating (7), e.g. of copper or nickel provided thereon. The lid has at least one integral flange (8, 9) dependant therefrom for engaging the side walls (4) of the box 1. The pliability of the lid enables the lid to fit tightly on the box and the conductive coating ensures effective screening against radio frequency interference. In an alternative embodiment the box (1) is also made of insulating material having a thin conductive coating thereon. <IMAGE>

Description

HOUSING FOR ELECTRONIC CIRCUITRY This invention relates to a housing for electronic circuitry and, more particularly, to a housing which prevents the escape of radio frequency (RF) radiation.

It is well known that radiation generated by circuitry operating at high frequencies can cause interference with radio and television broadcasts, and RF transmissions generally, a phenomenon which is called radio frequency interference (RFI). To prevent RFI the circuitry may be enclosed in an electrically conductive housing. For example, in a mobile radio telephone the circuitry of the transceiver is commonly contained within a conductive housing comprising an open-topped box and a separate lid for closing the box. To prevent RF radiation escaping from the housing it is important for there to be an effective and continuous RFI seal between the lid and the box.

Figure 1 shows in cross-section a part of a prior art transceiver housing comprising an open-topped box 1 and a lid 2. The sealing means comprises a compressible conductive O-ring gasket 3 made, for example, of a metal braid with a sponge rubber core. The gasket 3 is seated in a channel 4 located in the top face of side wall 5 of the box 1. In use, the lid 2 is fastened down to the box 1, e.g. using screws, thus compressing the gasket 3 and forming continuous electrical contact (and hence an effective RF seal) between the lid and the box.

This prior art transceiver housing does, however, have drawbacks. Firstly, the side wall of the box has to be sufficiently wide to accommodate the sealing gasket 3.

Consequently the side wall has to be made thicker than would otherwise be necessary. This necessitates the use of more material for manufacturing the box, which therefore increases the cost, and also results in the box occupying more space. Secondly, the gasket adds further to the expense of the overall housing and, being a separate component, requires an additional step in the assembly of the housing.

Another prior art transceiver housing employing a different RFI sealing means is illustrated in Figure 2.

In this case a series of electrically conductive leaf spring contacts 6 attached to the lid 2, e.g. by rivets 7, bear against the internal face of the side walls 5 of the box 1 (see Figure 2A). The leaf springs 6 are suitably formed as a unitary comb-like member as shown in plan view in Figure 2B. In the case of a rectangular housing four such comb-like members 8 would be used, one along each of the four sides of the lid 2.

This housing arrangement has the advantage that it occupies less space, since the side walls of the box can be made thinner, but manufacture is more costly because of the extra material for the springs and assembly is more difficult and time consuming because of the requirement to attach the individual comb-like members 8 to the lid 2.

WO 83/01174 discloses as prior art a two-part housing for radio frequency circuitry comprising an open-topped box made of, for example cast aluminium and a cover plate made of a resilient material such as cold rolled steel. The cover plate includes integral rectangular-shaped flange projections bent at approximately 90 with respect to the plane of the cover plate. The flange projections are shaped with an elbow or ridge which makes electrical contact against the inside walls of the box when the cover plate is press fitted into the opening in the box, thus providing effective RFI sealing. This arrangement relies on the resilience of the flange projections.

Our co-pending British patent application No. 8921929.9 and corresponding European patent application No.

90310310.9 (our ref PAT 89013) discloses a two-piece transceiver housing for a mobile telephone, which comprises an open-topped box and a separate lid both made of conductive material, specifically diecast aluminium. A plurality of flat projections are formed integrally on the underside of the lid. These projections are arranged to bear internally against the side wall of the box when the lid is in place to prevent radio frequency radiation escaping from the housing without the need for a separate sealing component. The projections have a tapering curvilinear or rectilinear profile. This helps to distribute the stress and enables the projections to suffer slight deflection without breaking even when they are made of cast aluminium which is relatively brittle.Although this reduces the number of parts to two, thus simplifying manufacture and assembly, the housing remains relatively expensive because of the diecasting process for which tooling costs tend to be very high.

Apart from the expense, prior art housings for RF circuitry have had the disadvantage of being relatively cumbersome due to the usually substantial wall thickness.

According to the present invention there is provided a housing for electronic circuitry, comprising an open-topped box having bottom and side walls comprising electrically conductive material, and a lid for closing the box to prevent radio frequency radiation escaping from the housing, said lid being formed from a pliant insulating material having an electrically conductive coating thereon.

A housing in accordance with the invention has the advantage that in particular the lid, but also the box, may have substantially thinner walls and can be manufactured at low cost. The thinner walls mean that the housing may be less cumbersome than its prior art counterparts. Moreover, the configuration of conductive coating on a pliant substrate means that the overall lid is not rigid, but has a degree of pliability or flexibility which is especially conducive for effective RFI sealing.

Suitably the lid substrate is made of plastics, for example ABS or polycarbonate. The conductive coating, which may be a layer of copper and/or nickel for example less than 50 micrometres thick.

The lid may comprise a flange for engaging the side walls of the box for optimum sealing. The flange may comprise a continuous lip or a plurality of discrete finger-like projections in adjoining relationship. In a preferred embodiment, the lid comprises a pair of spaced apart continuous flanges forming a channel therebetween for receiving the top edge of the side walls.

The box may be formed as a one-piece casting, but in a particularly inexpensive embodiment is formed simply from a folded sheet of metal or from an insulating material having a conductive coating thereon. This has the advantage that the housing may use substantially less material than prior art housings and consequently may be less heavy and cumbersome.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a cross-section of part of a prior art transceiver housing, Figure 2A is a cross-section of part of a different prior art transceiver housing, Figure 2B is a plan view of part of the sealing means of the housing in Figure 2A, Figure 3 is a schematic cross-section of a transceiver housing in accordance with the present invention, and Figure 4 is a schematic diagram illustrating how the box of the transceiver housing may be made from a folded sheet of metal.

The housing shown in Figure 3 is intended for containing the transceiver circuitry of a mobile radio telephone, and comprises a generally rectangular box and a lid 2.

The box, which comprises a bottom 3 and side walls 4, may be formed from a folded sheet of aluminium. The pattern of the aluminium sheet before folding is shown in Figure 4. The side walls are folded at right angles to the base to form the box 1, as shown in Figure 3.

The thickness of the aluminium sheet is chosen to provide adequate heat sinking for the particular transceiver circuitry to be enclosed therein and also to provide sufficient mechanical rigidity physically to protect the circuitry.

The lid 2 comprises a substantially flat, plate-like substrate 6 which may be 1 to 2mm thick made of ABS or polycarbonate or any other suitably pliant plastics material. The underside (i.e. the internal face) of the lid substrate 6 is provided with a 25 micrometre thick coating 7 of copper or nickel. The metallization may be deposited by electroplating. It is noted that the lid may thus be thinner than was conventional in the prior art.

The lid 2 comprises adjacent its perimeter a pair of integral flanges 8,9 depending from the underside thereof. The flanges 8,9 are disposed in parallel relationship to form a channel 10 of substantially uniform width therebetween. The width of the channel 10 is dictated by the thickness of the side walls 4 of the box 6 and is selected to provide a tight push fit over the top edge of the side walls 4. Hence the lid provides an effective RFI seal since the metallization 7 forms a continuous screen not only over the underside of the lid 2, but also at the area where the lid 2 adjoins the side walls 4 of the box 1. In a slightly modified embodiment the flanges 8,9 may be disposed in mutually converging relationship in order to promote more intimate engagement with the side walls 4 and so ensure optimum RFI sealing.Additionally, the lid 2 may be secured on the box 1 using clips or screws or indeed any conventional fixing means.

In an alternative embodiment the inner flange 9 may be extended, as shown by the broken line in Figure 3, to reach further down the side walls 4 and may even extend as far as the bottom face 3 of the box at least at the corners thereof. This will help to provide optimum sealing at the corners of the box when the box itself is made of a folded sheet of aluminium as described herein.

It will be understood that the box and lid configuration described herein may be inverted so that the so-called "lid" effectively becomes the base of the housing and the "open-topped box" is therefore disposed on top of the lid.

In view of the foregoing description, it will be evident to a person skilled in the art that various modifications may be made within the scope of the present invention. For example, the metallization on the internal face need not be restricted to the underside of the lid, but may also extend over some or all of the top surface of the lid substrate.

Alternatively, the metallization may be restricted to the top surface of the lid substrate and sufficient of the underside to provide continuous electrical contact with the side walls of the box. Also, the box may be formed other than from a folded sheet of metal, for example it may be cast in one piece from aluminium or other suitable conductive material. Moreover, the box itself need not be formed entirely of conductive material, but like the lid may comprise a layer of metallization on an insulating substrate.

Finally, it is noted that the invention is not restricted to a transceiver housing, nor indeed to radiotelephony applications, but may be used for housing any circuitry where there is a risk of generating or a requirement for shielding from radio frequency interference.

Claims (12)

1. A housing for electronic circuitry, comprising an open-topped box having bottom and side walls comprising electrically conductive material, and a lid for closing the box to prevent radio frequency radiation escaping from the housing, said lid comprising a pliant insulating substrate material having an electrically conductive coating thereon.

2. A housing as claimed in claim 1, wherein the lid substrate comprises a plastics material.

3. A housing as claimed in any of the preceding claims, wherein the conductive coating is provided selectively on the lid substrate.

4. A housing as claimed in claim 3 wherein the conductive coating is restricted to the underside of the lid substrate.

5. A housing as claimed in any of the previous claims, wherein the conductive coating comprises copper and/or nickel.

6. A housing as claimed in any of the preceding claims, wherein the coating is less than 50 micrometres thick.

7. A housing as claimed in any of the preceding claims, wherein the lid comprises a flange for engaging the side walls of the box.

8. A housing as claimed in claim 7, wherein the lid comprises a pair of spaced apart flanges forming a channel therebetween for receiving the top edge of the side walls.

9. A housing as claimed in any of the preceding claims wherein the box comprises an insulating substrate having an electrically conductive coating thereon.

10. A housing as claimed in any of the preceding claims, wherein the box is formed from a folded sheet of material.

11. A housing as claimed in claim 10 when dependant on claim 7, wherein the flange, at least at the corners of the box, extends to the bottom wall of said box.

12. A housing for electronic circuitry substantially as herein described with reference to Figures 3 and 4 of the accompanying drawing.