GB2208970A

GB2208970A - A screening housing and mounting for electronic circuit boards

Info

Publication number: GB2208970A
Application number: GB8719748A
Authority: GB
Inventors: John Ernest Salter
Original assignee: British Broadcasting Corp
Current assignee: British Broadcasting Corp
Priority date: 1987-08-20
Filing date: 1987-08-20
Publication date: 1989-04-19
Anticipated expiration: 2007-08-20
Also published as: GB9105748D0; GB8719748D0; GB2241833B; GB2241833A; GB2208970B

Abstract

Electronic circuit boards are shielded against radio frequency interference by a screening housing comprising a sleeve 12 having side panels 2 engaging in channels 10 in side walls 4. Projections in the channels 10 grip the side panels 2 and form an electrically conductive union. The ends of the sleeve 12 are closed by end covers e.g. 16. Flexible conductive gaskets 30 are compressed between the end covers and the sleeve 12 and form an electrically conductive union. One end cover 16 is removable and has a mounting 32 to support a circuit board 34 within the sleeve 12 allowing the board 34 to be removed with the end cover 16. Circuit boards are secured within the housing by a mounting comprising a guide 50 and an electrically conductive gasket 52 lying over the guide. The guide 50 and gasket 52 extend the length of one side of the board 34, the board 34 compressing the gasket 52 against the guide 50 to form an electrical contact. <IMAGE>

Description

A SCREENING HOUSING AND MOUNTING FOR ELECTRONIC CIRCUIT BOARDS This invention relates to the mounting and screening of electronic circuit boards and in particular to a screening housing and mounting for such boards.

Alternating currents in an electrical circuit produce electromagnetic radiation and are a potential source of interference to other circuits or users of the electromagnetic spectrum. In this situation the circuit may require shielding in order to prevent the emission of radio frequency interference (RFI).

Similarly, sensitive electronic circuits may be susceptable to RFI eminating either from other circuits or broadcast transmissions, and in these situations the sensitive circuits may require shielding. The principles of Faraday cages and their screening properties are well known.

Various methods of enclosing electronic circuits in screened enclosures have been employed in order to attempt to solve the problems outlined. An 'ideal' screen could only be achieved by using a uniform structure having no joints, seals or discontinuities. Previous examples include four or five sided boxes with one or two detachable covers or lids. Printed circuit boards may be assembled integrally with the enclosure or fastened within, either by soldering to side wall lugs or by screw fasteners. Covers or lids may be attached by screw fasteners or just clipped in position The disadvantage of previous screens for circuit boards is that it has not been possible to construct an effective RFI screen and retain easy access to the boards for assembly and maintenance.

A further problem encountered with using RFI screens is that circuits constructed on printed circuit boards operating at radio frequencies generally employ an earth plane type of construction and it is important that this earth plane is reliably connected to the housing or shield by a low impedance path.

The various aspects of the present invention are defined in the claims attached hereto, to which reference should now be made.

The sleeve may be formed as a single extruded section or may be assembled from a number of panels and wall sections. Either way, the electrical homogeneity of the screen can be maintained by ensuring electrical conductivity along the full length of all joints and unions and the screen behaves as an 'ideal' screen and is an effective shield from RFI. Preferably, circuit boards are secured to the removable portion of the screen, for example an end cover, and are slidable within the sleeve. The use of a flexible conductive gasket to form the union between the removable portion of the screen and the remainder ensures that the electrical homogeneity is restablished each time the portion is replaced.

A mounting for circuit boards embodying the second aspect of this invention provides an effective way of earthing the boards and obviates the need for numerous fastenings and connections within the screen.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view of a screen embodying the first aspect of the present invention for housing printed circuit boards; Fig. 2 is a view of the screen on the line II-II in Fig. 1; Fig. 3 is a view of the screen on the line III-III in Fig. 1; Fig. 4 is a cross-sectional view of the screen on the line IV IV in Fig. 1; Fig. 5 is a cross-sectional view of a screen embodying the first aspect of the present invention; Fig. 6 is a detailed view of an end cover and circuit board for the screen in Fig. 5; Fig. 7 is a detailed view of an end cover for the screen in Fig. 5; Fig. 8 is a detailed view of a portion of a side wall for a screen embodying the first aspect of the invention;; Fig. 9 is a detailed view of a mounting embodying the second aspect of this invention; and Fig. 10 is a cross-sectional view of a screen embodying the second aspect of this invention along the junction between an end cover and the sreen.

The screen shown in Figures 1 to 5 comprises a pair of spaced apart opposing side panels 2, held apart by a pair of side walls 4.

The side walls 4 are each formed as elongate extruded sections, for example from aluminium, having a wall section 6. Channel support members 8 project outwardly and laterally from both longitudinal edge portions of one face of the wall section 6 and support channels 10, as shown in detail in Fig. 8. Each channel has a pair of opposing walls 11, each wall having a gripping projection 13 on its outermost portion projecting into the channel and extending the full length of the channel wall 11. The side walls 4 are arranged with their channels 10 opposing and the edge portions of the side panels 2 engage with the channels 10 to form a sleeve 12, as shown, for example, in Fig. 5. The gripping projections 13 are forced against the edge portions of the side panels 2 by elastic deformation of the channel walls 11.This ensures firstly that the side panels 2 and side walls 4 are held firmly together and secondly forms an electrically conductive joint keeping the screen electrically homogeneous. In addition, the high pressure contact between the gripping projections 13 and the side panels 2 hinders corrosion at the joints and maintains electrical conductivity between the side panels and side walls. This in turn ensures that the sleeve is an effective RFI screen. A sleeve assembled in this manner would not normally be dismantled.

The screen is completed at its opposed ends by means of the end cover 14 and the end cover and board support 16, respectively shown in Figures 6 and 7. The end cover 14 may also conveniently be formed by extrusion and comprises an end plate 18 and end channels 20. A length of flexible electrically conductive gasket material 22 lies within each channel 20. The end cover 14 is secured by screws 24 which thread into tapped channels 3 formed by the channel support members 8 in each of the side walls 4. A high pressure metal to metal contact is formed around the screws 24 between the adjoining surfaces of the end cover 14 and the side walls 4. The free edge portions of the side panels 2 engage with the channels 20. The gasket material 22 is compressed between the edge of panels 2 and the inner surface of the channels 20 and forms an electrically conductive seal against RFI.In addition, the channels 20 hold the side panels 2 in position, preventing movement and keeping the sleeve 12 rigid.

The end cover and board support 16, is formed in a similar fashion to the end cover 18 with an end plate 26 and channels 28. A flexible electrically conductive sealing gasket 30 lies in the channel 28. The end cover and board support 16 is attached by screws to the remaining open end of the sleeve 12 as described above for the end cover 18 and completes the screen. when completed, the screen prevents RFI either entering or leaving the enclosure.

To support circuit boards within the screen, the end cover and board support 16 is provided with one or more lugs 32 to which a circuit board 34 may be bolted. Once bolted to the end cover and board support 16, the boards may be easily slid in or out of the sleeve 12 by moving the end cover 16. A handle 36 is provided on the end cover 16 to ease removal of the boards and manoeuvre the complete screen. The screen may be housed within a sub-rack, for example by means of guide runners 60 projecting from the wall section 6 of the side walls 4 and extending the full length of the side walls 4 as shown in Fig. 8. The screen is secured by a thumbscrew 38 mounted on the end cover 16, as shown in Figure 1.

To guide the board 34 within the sleeve 12 and ensure a reliable earth connection between the board 34 and the screen, it is preferable to use a mounting for a board embodying the second aspect of the present invention, as shown in Fig. 4 and in detail in Fig. 9. The mounting for each lateral edge of a board comprises a pair of spaced apart L shaped guides 50 for guiding and supporting a board 34, preferably extending the complete length of the lateral edge of the board 34 when mounted. A mounting gasket 52 in the form of a continuous loop of a flexible electrically conductive material extends around the guides 50 as shown in Fig. 9.

The guides 50 are so arranged that the section of the mounting gasket 41 extending between the guides 40 at each end of the sleeve 12 contacts the respective end cover 14, 16 and forms an electrically conductive junction between each of the end covers 14, 15 and each of the side walls 4, as shown in Fig. 10.

The board 34 is secured to the mounting so as to compress the mounting gasket 52 between the edge portions of the board and the guide 50. The gasket 52, when compressed, ensures that a reliable electrical contact is made between the whole of the edge portion of the board 34 and the mounting, thus providing a good earth connection.

The board 34 may conveniently be secured to the mounting by means of a bearing shoulder 54 adjacent the mounting gasket 52. The board 34 is mounted by sliding the edge portion of the board between the bearing shoulder 54 and the gasket 52, compressing the gasket against the guide 50 and securing the board. The bearing shoulder 54 is conveniently formed by the channel support members 8 on the side walls 4 of the screen described above, as shown in Fig. 4 and in more detail in Fig. 8.

The sealing gaskets 22, 30 are formed as resilient strips, whilst the mounting gasket 52 is preferably in the form of a resilient ring. All the gaskets 22, 30, 52 are formed from foamed rubber. The outer surface of the rubber is covered in a screen or mesh of a conductive material, such as stainless steel, monel or tinned copper. Alternatively, beryllium/copper mesh can be used to fill the end channels 20, 28 and form the electrically conductive joint between the end cover 14, 16 and the sleeve 12.

Claims

1. A screening housing for electronic circuit boards, comprising a substantially electrically homogeneous sleeve, an end cover mounted at each end of the sleeve so as to form an electrically conductive union along the junction between the sleeve and the end cover, one end cover being so releasably mounted on the sleeve that a circuit board can be removed through one end of the sleeve.

2. A screening housing according to claim 1, wherein the electrically conductive union between the removable end cover and the sleeve comprises a flexible electrically conductive gasket extending along the junction between the removable end cover and the sleeve.

3. A screening housing according to claim 2, wherein the gasket is a continuous loop of flexible electrically conductive material.

4. A screening housing according to any preceding claim, wherein the removable end cover comprises a mounting for a circuit board disposed to support a board within the housing when the removable end cover is mounted on the sleeve.

5. A screening housing according to any preceding claim, wherein the sleeve comprises two opposing side panels and two opposing side walls connected to and extending between respective edge portions of the side panels, the unions between the side panels and side walls being electrically conductive and substantially electrically homogeneous throughout their length.

6. A screening housing according to claim 4, wherein each side wall comprises channels for engaging the edge portions of the side panels, the walls of the channels having projections disposed to grip the edge portions of the side panels to form the electrically conductive and substantially electrically homogeneous union between the side walls and each of the side panels.

7. A screening housing according to any preceding claim, further comprising a mounting for a circuit board comprising a board support, and a flexible conductive gasket located on the support such that when a board is mounted on the support the gasket lies between a portion of the board extending substantially the complete length of a side of the board and an opposed portion of the support and establishes an electrical contact between the said portion of the board and the support.

8. A mounting for an electronic circuit board comprising a board support, and a flexible conductive gasket located on the support such that when a board is mounted on the support the gasket lies between a portion of the board extending substantially the complete length of a side of the board and an opposed portion of the support and establishes an electrical contact between the said portion of the board and the support.

9. A mounting according to claim 7 or 8 wherein the gasket is a continuous loop of flexible conductive material.

10. A mounting according to any of claims 7 to 9, wherein the support comprises first and second opposing surfaces, the gasket extending between the surfaces, whereby when the board is mounted, the said portion of the board extends between the first surface and the gasket, the gasket establishing an electrical contact between the said portion and the second surface.

11. A screening housing for an electronic circuit board substantially as hereinbefore described having reference to accompanying Figures 1 to 8.

12. A mounting for an electronic circuit board substantially as hereinbefore described having reference to accompanying Figures 1 to 10.