GB2270207A

GB2270207A - Cooling of electronics equipment

Info

Publication number: GB2270207A
Application number: GB9317902A
Authority: GB
Inventors: David Murray
Original assignee: DSK TECHNOLOGY INTERNATIONAL L
Current assignee: DSK TECHNOLOGY INTERNATIONAL L
Priority date: 1992-08-27
Filing date: 1993-08-27
Publication date: 1994-03-02
Also published as: GB9218233D0; GB9317902D0

Abstract

A circuit board 2 is a snug fit within a cavity 1a in a two-part enclosure 1b, 1c made from an aluminium alloy or other good heat conducting material. At least the spaces between the heat generating components 2a and adjacent internal faces of the enclosure cavity are filled and bridged by a material or compound possessing enhanced thermal conductivity, so that heat generated by the components 2a is transferred directly to the surrounding enclosure, in a direction away from, and generally perpendicular to, the plane of the circuit board. The thermally conductive material may be an aluminium oxide filled silicone elastomeric sheet or a potting compound such as epoxy resin. Alternatively the inner face or faces of the enclosure members may be configured to engage or be close to the heat generating components. <IMAGE>

Description

COOLING OF ELECTRONICS EQUIPMENT The present invention relates to the cooling of electronics equipment incorporating circuit boards or the like.

Electronics equipment, for example as used in industrial or other harsh environmental conditions, must withstand mechanical shock and vibration as well as hostile temperature environments. The most difficult to accommodate is often the temperature, particularly if the environment is such that the equipment must be located in a sealed, fluid tight enclosure, for example to isolate the equipment from explosive or inflammable liquids, gases or mixtures, or to prevent the ingress of fluids. Electronics equipments are generally more reliable if cool, but an enclosure in the form of a sealed case or housing providing mechanical protection possesses the disadvantage that it also provides thermal lagging, causing the internal temperature to rise considerably above that of the surroundings.

Conventional methods of reducing the temperature inside enclosures include: 1. Maximising the surface area of the enclosure (by corrugating the surface); 2. Placing electronic components with a high power consumption on heat sinks; and 3. Placing high power circuit board-mounted components near the edge of the board, and using metal heat conducting planes and ladders to remove the heat to the edge of the board in a direction along the plane of the board, and then to the enclosure walls.

The circuit boards are usually mounted in an array within the enclosure, with their faces generally parallel and spaced apart, and spaced from the surfaces of the enclosure. Heat is therefore transferred from the circuit boards to the enclosure by convection of air within the enclosure, which is relatively inefficient. Additionally or alternatively , heat transfer occurs by conduction from the components across the circuit board to the edges of the latter, and then to the enclosure walls. Since the circuit board is formed from an electrically insulating material, such as a glass fibre-filled resin, which has a relatively poor thermal conductivity, it is necessary to provide additional means to provide heat conduction to the edges of the board.This may take the form of a heat conducting plane, such as an aluminium layer, extending over the board beneath and in thermal contact with the components, the layer being etched so that it does not contact and short circuit the connections to the components. Alternatively, a heat conducting ladder is provided, for example composed of a row of strips of etched copper or aluminium which extend across the board to metal bars mounted at the edges of the board. These methods possess the disadvantage that they rely on heat conduction to the edges of the circuit board where there is a relatively localised, small contact area for heat conduction to the enclosure. In addition, each element of the heat conducting path, from the components, across the board, to the enclosure, has a thermal resistance.Furthermore, the provision of a metal heat conducting plane or ladder, and the need to position the components which generate significant heat near the edges of the board, increase the complexity and cost of the board.

It is an object of the present invention to provide an improved method of cooling enclosed circuit boards or the like, which reduces the number of heat conducting elements, reduces the heat conducting path length, and increases the conducting area, thereby reducing thermal resistance.

According to one aspect of the invention, there is provided a circuit board or the like in combination with an enclosure therefore, the enclosure being configured to accommodate the circuit board with at least one face thereof, or at least one heat generating component associated therewith, in close proximity to a corresponding face of the enclosure, whereby heat transfer from the circuit board or component to the enclosure will primarily occur by conduction in a direction generally perpendicular to the plane of the circuit board.

From another aspect , the invention provides an enclosure for a circuit board or the like, for use in the combination just defined.

In a preferred embodiment, the or each circuit board is accommodated within, and is a snug fit in, a cavity in its own associated enclosure. The opposite faces of the circuit board are located in contact with, or in close proximity to, associated opposite internal faces of the closely conforming enclosure cavity. Any spaces between the or each heat generating component on the circuit board and the associated adjacent internal faces of the enclosure cavity, or between the faces of the circuit board (or the components or connections, etc., mounted on and standing proud of the circuit board) and the faces of the enclosure cavity, are preferably filled with an electrically insulating material possessing enhanced thermal conductivity, i.e. a thermal conductivity better than that of air and of the material of the circuit board.

This approach ensures that the walls of the enclosure cavity are very close to, and effectively in intimate thermal contact with, the components, etc. on the circuit boards. This allows the main thermal transfer to be conduction from the heat generating components in a direction generally perpendicular to the plane of the circuit board in a direction away from the circuit board, not towards or through the circuit board, for example, not along the plane of the latter as is conventional. The less efficient thermal transfer by convection or radiation is also greatly reduced in favour of conduction.

In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which: Figure 1 is a perspective view of a unit comprising or forming part of a piece of electronics equipment embodiment the invention; and Figure 2 is an exploded view, on an enlarged scale, of one of the circuit boards and an associated enclosure shown in Figure 1.

Figure 1 depicts a unit comprising a number of enclosures 1, each containing an associated printed circuit board (2 in Figure 2). The enclosures 1 are interconnected by a back-plane 3 and are retained assembled to the latter by a front panel 4. The back-plane 3, and/or the front panel 4, may, for example, be relatively simple printed circuit boards, provided with electrical connections, such as connectors comprising plugs or sockets, for carrying signals, etc. to and between the various circuit boards and/or other components, etc. (not shown) of the equipment.

In a conventional unit, the elements 1 in Figure 1 would correspond to exposed printed circuit boards, and the unit, or at least the top, bottom and ends thereof between the back-plane 3 and front panel 4, would be closed by walls to form a sealed enclosure or housing. The cooling flow of external air would be around the sides of the housing as indicated by the broken lines 5.

In the unit embodying the invention as shown in Figure 1, the individual circuit boards are sealed within their own separate individual enclosures 1, and the top, bottom and ends of the unit are open. This allows external cooling air to additionally flow around and between the individual enclosures as indicated by the arrows 6, very much closer to the components on the individual circuit boards which require to be cooled.

One embodiment of circuit board enclosure is shown in Figure 2. The circuit board 2 is a close or snug fit in a cavity la in a rear part lb of the enclosure 1, and is sandwiched between the rear part lb and a front part lc which is a close fit in the cavity la. The heat generating components 2a of the electrical circuit on the board are close to the internal faces of the walls of the front and rear enclosure parts, and may be in direct thermal contact therewith.

However, although the components 2a are diagrammatically shown as being planar and flush with the face of the circuit board 2, and the components and their interconnections may be formed as planar circuits, in practice the components and their interconnections, or at least some of them, will usually stand proud of one or both faces of the circuit board to varying degrees. In addition, the walls of the enclosure cavity may be spaced from the components, etc., for example if the enclosure is formed from an electrically conductive material. Thus, there will usually be spaces between the components, etc., and the adjacent face or faces of the enclosure cavity, and/or spaces between the components, etc. themselves.At least the former spaces are preferably filled with a heat conducting material or compound, so that at least the components are effectively in thermal contact with the adjacent face or faces of the enclosure cavity. This may be achieved by sandwiching a sheet or mat of enhanced thermally conductive material between one or both faces of the circuit board and the adjacent face or faces of the enclosure cavity. The material may, for example be a highly conformable thermally conductive elastomer, such as an aluminium oxide-filled silicone elastomer, of the type marketed under the trade name "CHOTHERM" by Chomerics (UK) Ltd. of Marlow, Bucks.

The sheet material may be 1 to 5 mm thick and preferably 2 to 3 mm thick. This sheet material possesses the advantage that it enables the circuit board to be readily removed from its enclosure and repaired/replaced if required. This material possesses the added advantage that it serves to cushion the circuit board and its components against vibrations or other mechanical shocks to which the enclosure is subjected in use, and which might otherwise cause damage.

Alternatively, the assembled circuit board and enclosure may be impregnated with a heat conducting potting compound, such as a two component epoxy resin which, upon mixing, cures to form a non-removable seal with good thermal properties. As a further alternative, a heat sink compound, which preferably is also an electrical insulator, such as a metal oxide paste containing silicone, may be applied to the faces of the individual heat generating components, to the faces of the circuit board, and/or to the faces of the enclosure parts or zones thereof, before assembly. The latter alternative also enables the circuit board to be removed from its enclosure and repaired or replaced if required.

The enclosure parts, or at least the part adjacent the heat generating components, are also formed from a material possessing superior thermal conductivity, for example a metal such as an aluminium alloy. The front panel and/or back plane may also incorporate, or be formed from, a material possessing an efficient thermal conductivity, such as an aluminium alloy.

One or both enclosure parts lb, lc may have seals along their mating edges or surfaces to seal the circuit board within the cavity la. The or each seal may comprise a gasket of the gas and/or liquid sealing type, and may be formed from a natural or synthetic rubber or plastics material or compound. The gasket may be preformed, or may be applied as a liquid or fluid compound, and cured if necessary. The gasket material may also serve to bond the two enclosure parts together, although separate or additional permanent or releasable fixing means may be provided. One or more radio frequency interference (RFI) gaskets may be provided, intended to prevent radio frequency (RF) energy from being emitted from the enclosure, or to prevent the internal circuits from being disturbed in their operation by the presence of powerful external RF fields.The or each RFI gasket may be formed from wire mesh, and may be in addition to, or incorporated in, a gas/liquid sealing gasket.

A connector 8, by which electrical contact is made via the back-plane 3 to other circuit boards, or to external connectors, is shown mounted at the right of the board 2.

It will be understood that various modifications may be made without departing from the scope of the present invention as defined in the appended claims.

For example each enclosure may accommodate more than one circuit board or the like, disposed side-by-side in a common plane. Additionally or alternatively, at least one circuit board could be disposed with one of its faces, i.e. the face carrying the heat generating components, adjacent and in heat conducting relationship with one internal face of one enclosure part, and at least one other circuit board could be disposed with one of its faces, i,e.

its face carrying the heat generating components, adjacent and in heat conducting relationship with the internal face of the other enclosure part, with a space, or a filling of a heat conductive material or compound, between the two boards.

The heat generating components could be provided with their own discrete elements of heat conducting material or compound interposed between the components and opposed internal face of the enclosure so as to form discrete thermally conducting bridges therebetween.

Instead of or in addition to providing a heat conducting material or compound to fill any gaps between the heat generating components and the opposed internal faces of the enclosure, one or both of the opposed surfaces of the enclosure could be contoured to engage or be closer to the individual components. Additionally or alternatively, spacers, such as metal or plastics pillars or shims could be interposed between the components and opposed internal faces of the enclosure, and preferably fixed to one or the other, with or without the use of a heat conducting material or compound.

The unit shown in Figure 1 incorporates a plurality of mutually parallel circuit board/enclosure assemblies 1 disposed between a back plane 3 and front panel 4. However, numerous other configurations could be employed, using one or more assemblies.

For example, the one-piece front panel may be omitted, and the back plane may form the back wall of an open-fronted housing having top, bottom and side walls.

The circuit board/enclosure assemblies may be slidably mounted between the top and bottom walls, with openings being provided in the latter walls to permit the passage of cooling air. The assemblies may be generally "L" or shaped in plan, with the base limb of the "L", or crosslimb of the "T" being formed by an end face, plate or compartment, at the front of the enclosure, which carries external connectors etc. for making external connections to or from the associated circuit board. The circuit boards may be connected by their associated edge connectors, or by soldering etc., to the external connectors or associated circuitry. The said limbs of adjacent enclosures thus, in effect, build up adjacent segments of a composite front panel, for example a front panel 4 similar to that as viewed in Figure 1, but which is split vertically between adjacent enclosures 1. The enclosures may be secured in the housing by screws or other fasteners passing through the laterally projecting portions of said limbs, externally of one or both sides lb, lc of the enclosure, and anchored in pillars in the housing.

The rear connector 8 shown in Figure 2 may be located on a portion of the circuit board 2 which projects rearwardly of the rear wall of the enclosure 1, for example through a slot or cut out in the rear wall of one enclosure part. The connector 8 will then cooperate with a recessed complementary connector in the back plane 3, with a sealing gasket being located between the back plane and rear wall of the enclosure, around the connector 8 and/or rearwardly projecting portion of the circuit board.

The connector 8 or equivalent may be releasably connected to edge contacts at the rear of the circuit board, or may be soldered or wired thereto.

One or both of the enclosure parts may be corrugated, externally finned, or otherwise configured to increase surface area and therefore enhance heat dissipation.

The invention is applicable to circuit boards, such as printed circuit boards, in which the circuitry is located on one or both sides of the board. The invention is particularly applicable to boards in which discrete circuits, each incorporating heat generating components, are provided on opposite sides of the board, for example by known surface mounting techniques.

Claims

1. A circuit board or the like in combination with an enclosure therefore, the enclosure being configured to accommodate the circuit board with at least one face thereof, or at least one heat generating component associated therewith, in close proximity to a corresponding face of the enclosure, whereby heat transfer from the circuit board or component to the enclosure will primarily occur by conduction in a direction generally perpendicular to the plane of the circuit board.

2. The combination claimed in claim 1, wherein the or each circuit board is accommodated within, and is a snug fit in, a cavity in its own associated enclosure.

3. The combination claimed in claim 2, wherein the opposite faces of the circuit board are located in close proximity to associated opposite internal faces of the closely conforming enclosure cavity.

4. The combination claimed in claim 1, 2 or 3, wherein the at least one heat generating component is in heat transfer contact with the adjacent internal face of the enclosure.

5. The combination claimed in claim 1, 2 or 3, wherein the at least one heat generating component is spaced from the adjacent internal face of the enclosure, the space therebetween being bridged by a material possessing an enhanced conductivity which is better than that of air, and of the material of circuit board.

6. The combination claimed in claim 5, wherein the enhanced thermal conductivity material is an electrically insulating material.

7. The combination claimed in claim 5 or 6, wherein the enhanced thermal conductivity material is in the form of a sheet or mat.

8. The combination claimed in claim 7, wherein the enhanced thermal conductivity material is an elastomer.

9. The combination claimed in claim 8, wherein the elastomer is a highly conformable aluminium oxide-filled silicone elastomer.

10. The combination claimed in claim 5 or 6, wherein the enhanced thermal conductivity material is a potting compound, such as a curable two-component epoxy resin.

11. The combination claimed in claim 5 or 6, wherein the enhanced thermal conductivity material is a siliconecontaining metal oxide paste.

12. The combination claimed in any of claims 4 to 11, wherein any spaces between one or both internal faces of the enclosure and one or both faces of the circuit board and/or the components or connections, etc., mounted on and standing proud of the circuit board, also contain, and are preferably filled with, an enhanced thermally conductive material.

13. The combination claimed in claim 7 and 12, wherein the sheet material is substantially coextensive with a major proportion of one or both pairs of opposed faces of the circuit board and enclosure.

14. A circuit board and enclosure combination substantially as hereinbefore described with reference to the accompanying drawings.

15. An enclosure for a circuit board or the like, for use in the combination claimed in any preceding claim.