GB2212000A - Screening assembly for an electronic circuit - Google Patents

Abstract

In order to avoid electromagnetic, particularly RF, interference, an insulating plate 1 bearing a circuit on one side and a conducting layer on the other side is mounted adjacent a second insulating plate 2 having a second conducting layer thereon so that the circuit is disposed between the first and second conducting layers. The peripheries of the respective conducting layers are electrically connected together, for example, by means of a copper strip 3, to effectively screen the circuit from external electromagnetic radiation. <IMAGE>

Description

Screening Assembly This invention relates to a screening assembly for use in screening an electronic circuit from electromagnetic, particularly, RF interference and for screening the region outside the electronic circuit from internally generated signals.

Many electronic circuits are extremely intolerant to stray RF signals and interference arising from sources external to the circuit, which can in extreme cases cause false readings or malfunctioning. Accordingly the screening of such circuits is often required.

The most common method of screening is to place the circuit board or boards within a metal box which in effect serves as a kind of Faraday cage to screen the boards or components mounted within. Such boxes tend however to be bulky and thus limit the minimum size for apparatus in which the electronics are mounted. Problems also arise in taking inputs and outputs from the circuit boards to external circuits outside the screening box.

According to the present invention there is provided a screening assembly for an electronic circuit, comprising an insulating plate upon which is mounted on one side the circuit and on its other side a conducting layer, a second insulating plate having a second conducting layer thereon and mounted spaced from the first plate such that the circuit is disposed between the first and second conducting layers and wherein the peripheries of the respective conducting layers are - electrically connected such that there is electronmagnetic radiation screening of the circuit.

In a preferred embodiment an electronic circuit is also mounted upon the second insulating plate so that the respective conducting layers of the circuits are actually used as the screening box, as well as for grounding appropriate points in the circuitry.

The respective conducting layers may be extended to contact each other or, preferably, the conducting layers are flat and a further member is connected to the peripheries of the two conducting layers to connect the two and to achieve RF sealing and screening. The further member may comprise a strip of conducting material, the edges of which contain fixings to hold and to electrically connect to the conducting layers.

Means may be provided for allowing inputs and outputs from the electronic circuit or circuits within the screening assembly, which means are themselves are screened and may include filters.

A screening assembly constructed in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which; Figure 1 is a perspective view of a screening assembly in partially assembled form; Figure 2 is an enlarged view of part of the strip of conducting material.

Figure 3 is a bottom plan view of the screening assembly Figure 4 is a top plan view of the lower printed circuit board with the top printed circuit board removed; Figure 5 is a partial enlarged sectional view of the lower printed circuit board taken on the lines 5-5 in Figure 4; Figure 6 is an enlarged sectional view taken on the lines 6-6 in Figure 4; and Figure 7 is an enlarged sectional view take on the lines 7-7 in Figure 4.

Referring to Figures 1 and 2, the screening assembly consists of two printed circuit boards 1, 2, and a copper beryllium strip 3. The printed circuit boards each consist of an insulating plate, which bears conducting tracks and components on their inwardly facing surfaces and a copper conduicting ground plane screen on their outwardly facing surfaces. The components are suface mounted and hence do not have leads protruding through the boards, apart from any soldered leads to the ground plane itself.

The copper strip 3 has two pairs of rows of integral fixing tags, 4, 5 and 6, 7. Each board 1, 2 is secured between a respective pair of rows of tags, and the tags are bowed slightly towards the board to ensure good electrical contact. The strip is simply pressed onto the edges of the boards to assemble the screening assembly.

The integral tags 4-7 are pressed out of a flat beryllium copper strip, for example, such a strip may be passed between a pair of rollers bearing suitable indentations.

The strip butts at each end against the metal part of a connector indicated generally by the reference numeral 8 having contact pins 9, 10 connecting to appropriate areas of the conducting tracks on the boards. The pins are both screened and incorporate filters, and will now be described with reference to Figures 3 to 7.

Referring to Figures 4 and 5, the lower set of pins 9 is soldered to appropriate tracks on the lower printed circuit board 2. Corrugated copper plates 11, 12 are provided for mechanical fixing and for screening purposes, and three ferrite slabs 13 to 15 interspersed with two ceramic slabs 16, 17, through each of which the pins 9 pass, are provided for filtering purposes.

The printed circuit board 2 has an aperture 18 cut out of it which is rectangular but which has two notches 18a and 18b at each side. The screening plate 12 has two corresponding lugs 12a, 12b on each side in order to locate the plate relative to the printed circuit board 2. The outer margin of the screening plate 12 is soldered to the ground plane, and the lugs 12a, 12b are soldered to the part of the ground plane at the peripheries of the notches 18a, 18b, so that the ground planes screen is continuous across the whole area of the printed circuit board 2 despite the aperture 18.

The pins 9 are soldered to appropriate tracks on the printed circuit board 2 and, between a flange 9a on the pins and the printed circuit board, they pass through apertures 13a to 15a in the ferrite slabs 13 to 15 and apertures 16a, 17a in the ceramic slabs 16, 17.

The ceramic slabs 16, 17, which are of a high dielectric material, are coated with a conducting layer over the interior cylindrical surface of the apertures 16a, 17a, as well as over the exterior of the slab, except for circular regions 16b, 17b, immediately surrounding the apertures, in order to form the electrode of respective capacitors.

The screening plates 11 and 12 each have longitudial corrugations lla, llb, 12a, 12b which contact opposed surfaces of the slabs 16 and 17 and are soldered thereto, as are the interior of the apertures soldered to the pins 9.

The screening plate 11 has an upstanding flange lla, the upper margin which has a multiplicity of cuts extending vertically (as seen in Figure 5), to define a multiplicity of resilient fingers llb, llc, which are alternatively inclined to either side of the flange.

An identical assembly to that shown in Figure 5 is provided for the printed circuit board 1 and, when the two printed circuit boards are brought together, the resilient fingers extending from each board are pressed together to form a conductive join. Since the edges of the copper strip 3 are in contact with the edges of the upstanding flanges lla, the end face of the screening assembly which houses the connector is screened by the copper strip 3, and by the upstanding flanges extending from each assembly. In the region of the pins 9, the screening plates 11, 12 are soldered to the ceramic blocks 16, 17, and the only unscreened region is therefore the very small annular region 16b, 17b, between the conductive coating on the ends of the block and the pins.

Each region over which the ferrite slabs surrounds the pins forms an inductance associated with that pin, and each region over which the ceramic slab surrounds the pin forms a capacitance with the result that an LCLCL low pass filter is formed between the conducting tracks on the printed circuit board and the ends of the associated pins.

The screening assembly is built up as follows: The screening plates 11, 12 which are coated with solder, the ferrite slabs 13 to 15, the ceramic slabs 16, 17, together with the pins 9, are mounted in a jig, (not shown) and the parts are soldered together by heating.

Then the assembly thus built up is inserted into the aperture 18 in the printed circuit board 2 in a direction parallel to the plane of the printed circuit board until the edge of the screening plate 12 overlaps the ground plane and the inner edges of the pins 9 over-lap the conducting tracks by the required amount.

This will be indicated by the lugs 12a, 12b springing into the notches 18a, 18b.

The over-lapping parts are now soldered to the ground plane on the printed circuit board.

An identical assembly is then built up for the printed circuit board 1 and soldered the printed circuit board in the same way. Each assembly is then reflow soldered to solder the surface mounted components to the board. A high temperature solder may be used for the screening plates 11, 12 to ensure it does not melt during this soldering operation. If desired, the soldering of the sub-assembly shown in Figure 5 to the board may be accomplished during the same heating operation for soldering the components to the board, instsead of a separate operation. Finally, spacers are placed between the printed circuit boards and the two boards brought together. Each resilient finger llb, llc meets an identical finger inclined to the same side of the plane of the associated flange, and the fingers deflect apart until the boards touch the spacers.The copper strip 3 is then pressed onto the edges of the boards and butts against the flange lla at each end, and against a corresponding flange extending from the board 1. The strip can of course be removed for servicing the circuit.

Advantages of the resulting assembly are that the ground planes are responsible for a electromagnetic screening as well as for serving as a grounding point for the circuits, and the screening is completed by a copper strip 3 and the flanges lla. The ceramic and ferrite slabs serve to filter the circuit from electromagnetic interference without the necessity of soldering in separate components.

Of course, variations may be made to the screening assembly described without departing from the scope of the invention. ! Thus, instead of providing an LCLCL filter for each pin 9, the blocks may instead be chosen so as to provide å LCL filter, a CLC filter or a CLCLC filter or, indeed, filters of seven or more components.

Equally, only one of the printed circuit boards could bear a circuit instead of both them.

Attention is directed to our co-pending patent application no: I/7386, which claims an electrical connector of the kind described above.

Claims (8)

1. A screening assembly for an electronic circuit, comprising an insulating plate upon which is mounted on one side the circuit and on its other side a conducting layer, a second insulating plate having a second conducting layer thereon and mounted spaced from the first plate such that the circuit is disposed between the first and second conducting layers, and wherein the peripheries of the respective conducting layers are electrically connected such that there is electromagnetic radiation screening of the circuit.

2. A screening assembly as claimed in claim 1, in which an electronic circuit is also mounted on the second insulating plate.

3. A screening assembly as claimed in claim 1 or claim 2, in which a further member is connected to the peripheries of the two conducting layers.

4. A screening assembly as claimed in claim 3, in which the further member comprises a strip of conducting material.

5. A screening assembly as claimed in claim 4, in which the strip of conducting material has two pairs of rows of integral tags, a respective plate being held between each pair of tags.

6. A screening assembly as claimed in any one of claims 1 to 5, in which, there is provided an electrical connector comprising a plurality of longitudinallyextending contacts, each extending through a layer of magnetically permeable material and through a layer of dielectric material arranged at different positions along the length of the respective contact.

7. A screening assembly as claimed in claim 6, in which the layers of magnetically permeable material and dielectric material are defined by respective unitary blocks extending transverse to the contact.

8. A screening assembly substantially as herein described with reference to the accompanying drawings.