GB2237147A

GB2237147A - Printed circuit board arrangement.

Info

Publication number: GB2237147A
Application number: GB8923174A
Authority: GB
Inventors: John Reeves
Original assignee: Electricity Council
Current assignee: Electricity Council
Priority date: 1989-10-13
Filing date: 1989-10-13
Publication date: 1991-04-24
Also published as: WO1991006202A1; GB8923174D0

Abstract

A printed circuit board arrangement comprises a four layer printed circuit board (8), a screening cap (6) and a compliant conducting means. An earth plane (10) of an upper conductive layer (1) is maintained in electrical contact with the rim of the screening cap (6) by a fixing means and a resilient gasket (7). An intermediate conductive layer (2) connects the screened components mounted on circuit areas (5). This layer is screened on the upper side by the upper conductive layer (1) and on the lower side by a lower screening layer (3), which is connected at several points to the earth plane (10) of the upper conductive layer (1), in use, and electrically earthed. Power is communicated to and from the circuit areas (5) by tracks on a fourth conductive layer (4). This arrangement provides for simpler construction and easy access for maintenance to screened components mounted on the circuit areas (5).

Description

PRINTED CIRCUIT BOARD ARRANGEMENT The present invention relates to the screening of circuit components in an electronic circuit and particularly to a printed circuit board arrangement for this purpose.

There is a common requirement to provide electromagnetic screening of groups of components in electronic equipment. The group of components is typically mounted on a printed circuit board and screening is normally achieved by mounting the entire board in a screening enclosure which is then located within the equipment. This has the disadvantage that external connections to the board within the screening enclosure must be made via feed through capacitors or bushings. An extra component is therefore required for each connection. In addition two extra electrical joints must be made (one to each end of the feed through) e.g. by soldering.

According to the present invention a printed circuit board arrangement comprises a printed circuit board with a circuit area for interconnecting circuit components mounted on a face of said board and a conductive screening track on said face substantially surrounding said circuit area, a screening cap for covering circuit components on said circuit area and having a peripheral edge registering with said screening track, and connecting means arranged to retain the screening cap in position on said board and provide electrical connection between said screening track and said peripheral edge of the cap over the length of the track around the circuit area.

With this arrangement a continuous screen can be formed incorporating both the screening cap and the screening track surrounding the circuit area.

Conveniently said connecting means comprises fixing means urging the cap towards the board and compliant conducting means around said peripheral edge adapted to provide good electrical contact between said peripheral edge and said screening track. Said compliant conducting means may be integrally connected to said peripheral edge of the screening cap or may comprise a resilient conductive gasket shaped to fit around said peripheral edge of the screening cap.

Preferably said printed circuit board is a multilayer board having an upper surface conductive layer providing said screening track, a lower conductive layer providing a lower screening layer under said circuit area, and at least one intermediate conductive layer between said upper and lower layers and providing interconnection tracks for power and/or signals to and/or from said circuit area, said screening track being electrically connected to said lower screening layer and extending with the lower screening layer to screen said interconnection tracks.

It can be seen that with this arrangement, signal and power interconnections can be made with the screened circuit area without the need of additional components. Furthermore, these interconnections themselves can be fully screened.

Said printed circuit board may further include a fourth conductive layer beneath said lower screening layer, providing power supply tracks which are connected through said lower screening layer to said circuit area.

In a preferred embodiment, the circuit board arrangement includes a plurality of said circuit areas each with a surrounding said screening track and a respective said screening cap, said screening tracks being formed about respective apertures in an earth plane of said upper surface conductive layer, said earth plane and said lower screening layer screening tracks of said intermediate conductive layer interconnecting the respective said circuit areas.

Examples of the present invention will now be described with reference to the accompanying drawing in which: Figure 1 is a schematic cross-section of a multi-layer printed circuit board arrangement embodying the invention; Referring now to Figure 1, this shows diagramatically a cross-section of an area of a four layer printed circuit board 8, which has four parallel metallised layers. The four layers comprise an upper surface conductive layer 1, an intermediate conductive layer 2, a lower screening layer 3, and a fourth conductive layer 4 beneath the lower screening layer.

The upper surface conductive layer 1 forms an earth plane 10 and this earth plane 10 and the lower screening layer 3 are electrically connected together at several points 9 about their area and, in use, are earthed so as to screen the enclosed intermediate conductive layer 2. The earth plane 10 of the upper surface conductive layer 1 and the lower screening layer 3 need only extend over the area of the printed circuit board for which screening is required. To protect the metallised surface conductive layer 1, there is usually a laquer layer 14 deposited on its surface.

Apertures 11 are provided in the earth plane 10 of the upper surface conductive layer 1 in which there are areas designated as circuit areas 5.

Mounted on these circuit areas 5 are electronic components which require screening from the local electromagnetic environment. Each circuit area 5 can be of any shape or size although a circular shape is conveniently used in the current example. Around the periphery of the circuit area, the earth plane 10 has no protective layer 14 and is exposed to form a conductive screening track 13 inside which, providing isolation from the earth plane, is an isolation channel 12. The circuit area 5 may have metallised tracks to allow interconnection of electronic components mounted thereon. Alternatively or in addition, interconnection of components may be made by tracks in the intermediate conductive layer 2.

The components on the circuit areas 5 can be connected to the tracks of the intermediate layer 2 by plated through-holes through the top layer of the printed circuit board. The intermediate conductive layer 2 also provides metallised tracks for interconnection of several circuit areas 5, where more than one circuit area 5 is used on a board. The tracks are provided for communication of signals or power or a combination of both. This method of interconnection of the circuit areas 5 vastly reduces the noise introduced along the interconnection tracks, since the tracks of the intermediate conductive layer 2 are screened by the earth plane of the upper surface conductive layer 1 and the lower screening layer 3.

Although in the above described example the intermediate conductive layer 2 is described as one layer, more than one intermediate conductive layer may be provided for communication of signals between circuit areas and components on a circuit area.

The fourth conductive layer 4 beneath the lower screening layer 3 comprises a layer of metallised tracks for the provision of power to or from the electronic components on the various circuit areas. The power supply tracks need not be so carefully screened since electronic components are generally much less sensitive to noise, generated by electromagnetic pick up, on the power supplies, than on the signal lines. Connections between the power supply tracks and electronic components on the circuit areas 5 are provided by plated through-holes through apertures in the lower screen layer 3.

Although this undesirably introduces breaks in the integrity of the lower screening layer 3, the reduction in the efficiency of the screening is generally small.

The printed circuit board arrangement shown in Figure 1 also comprises a screening cap 6 attached to the four layer printed circuit board 8, such that the peripheral edge of the screening cap approaches the surface of the printed circuit board around the circuit area 5 and registers with the conductive screening track 13. Hence, the screening cap 6 covers the circuit area 5 and the isolation channel 12 without contacting either. Electrical connection between the screening cap 6 and the screening track 13 of the upper surface conductive layer 1 is made by a compliant conducting means comprising a resilient conductive gasket 7. The gasket 7, provides good electrical contact between the screening cap 6 and the screening track along the entire length of the rim and accommodates irregularities between surfaces of the two.The gasket 7 is shaped to fit over the rim of the screening cap 6 such that it overlaps and projects partly up the sides of the screening cap 6 and is sufficiently elastic to retain such a position. A material that has suitable properties for use as such a gasket is a graphite loaded rubber compound. The cap 6 is fixed, e.g. by a bolt, so as to be urged towards the circuit board. Thus, the use of a flexible material such as a rubber compound for the gasket will ensure that pressure is maintained between the faces of the screening track and the rim of the screening cap 6 even when some relaxation of fixing has occured.

The screening cap 6 may be formed from a metal or other conducting material or a suitable metal plated insulating material.

The screening cap 6 may be attached to the printed circuit board by any convenient method, such as centrally located screws or bolts or clips locating with projections around the rim of the screening cap 6.

Thus, when the screening caps 6 is attached to the four layer printed circuit board 8, the electronic components mounted on the circuit area 5 are shielded from electromagnetic radiation from the environment and from neighbouring circuit areas.

Also, signals to and from these circuit areas 5 are shielded by the earth plane 10 of the upper surface conductive layer 1 and the lower screening layer 3.

In another example of the present invention, the screening cap 6 may be integral with the compliant conducting means. This can be achieved by, for example, manufacturing the screening cap from a material such as a suitably plated injection moulded plastic to a shape such that the rim of the screening cap 6 forms a compliant bellows structure. In addition in this embodiment, if the screening cap is formed from an injection moulded plastic with a suitably plated layer, the fixing means may also be integrally formed such that the screening cap 6 has a central fixing peg with a bulbous end which will "pop" fit in to a hole in the printed circuit board.

This arrangement has the advantage that it simplifies the fitting, reduces costs, and reduces the number of components.

Many screening caps can be fitted to a printed circuit board and the design as hereinbefore described allows for the mixing of screened areas, with screening caps containing sensitive areas, and unscreened areas, on which less sensitive components may be mounted. This facilitates a greater flexibility in printed circuit board design.

The printed circuit board arrangement described in the above examples is a much simpler arrangement than prior art screening arrangements.

The number of holes required on the printed circuit board is reduced, no soldering or bending of fixing tags is required to fit the screening cap, no specialised tools are required to fit the screening caps and the need for expensive contact fingers is avoided. These features mean that construction time is reduced and manufacturing is simpler and cheaper.

Also reliability can be improved, since the connections to screened area are simplified and hence chances of poor or wrong connections are reduced.

The above described arrangement also provides easy access to screened components for ervicing due to the ease with which the screening cap can be removed and replaced.

The above described arrangement is suitable for both surface mounted and conventionally mounted components and has a wide range of applications.

Claims

1. A printed circuit board arrangement comprising a printed circuit board with a circuit area for interconnecting circuit components mounted on a face of said board and a conductive screening track on said face substantially surrounding said circuit area, a screening cap for covering circuit components on said circuit area and having a peripheral edge registering with said screening track, and connecting means arranged to retain the screening cap in position on said board and provide electrical connection between said screening track and said peripheral edge of the cap over the length of the track around the circuit area.

2. A printed circuit board arrangement as claimed in claim 1 wherein said connecting means comprises electrically conductive cement securing the peripheral edge of the screening cap to the screening track.

3. A printed circuit board arrangement as claimed in claim 1, wherein said connecting means comprises fixing means urging the cap towards the board and compliant conducting means around said peripheral edge adapted to provide good electrical contact between said peripheral edge and said screening track.

4. A printed circuit board arrangement as claimed in claim 3, wherein said compliant conducting means is integrally connected to said peripheral edge of the screening cap.

5. A printed circuit board arrangement as claimed in claim 3, wherein said compliant conducting means comprises a resilient conductive gasket shaped to fit around said peripheral edge of the screening cap.

6. A printed circuit board arrangement as claimed in any previous claim, wherein said printed circuit board is a multilayer board having an upper surface conductive screening layer providing said screening track, a lower conductive layer providing a lower screening layer, and at least one intermediate conductive layer between said upper and lower layers and providing interconnection tracks for power and/or signals to and/or from said circuit area, said screening track being electrically connected to said lower screening layer and extending with the lower screening layer to screen said interconnection tracks

7. A printed circuit board arrangement as claimed in claim 6, wherein said printed circuit board further includes a fourth conductive layer beneath said lower screening layer, providing power tracks which are connected through said lower screening layer to said circuit area.

8. A printed circuit board arrangement as claimed in either of claims 6 or 7, and including a plurality of said circuit areas each with a surrounding said screening track and a respective said screening cap, said screening tracks about respective apertures in an earth plane of said upper surface conductive layer, said earth plane and said lower screening layer screening tracks of said intermediate conductive layer interconnecting the said circuit areas.

9. A printed circuit board arrangement as hereinbefore described with reference to and illustrated by the accompanying drawings.