GB2244601A

GB2244601A - Electrical circuit board assemblies

Info

Publication number: GB2244601A
Application number: GB9105072A
Authority: GB
Inventors: Geoffrey Arnold Trussler; Alastair Billing
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1990-03-14
Filing date: 1991-03-11
Publication date: 1991-12-04
Anticipated expiration: 2011-03-11
Also published as: GB2244601B; GB9005742D0; GB9105072D0

Abstract

An electrical circuit board 1 supporting surface-mounted components 2 is mounted on a heat conducting plate 20 of aluminium. The board 1 is mounted on the plate 20 by male and female threaded bolts 8 and 9 which extend through apertures 3 and 13 in the board 1 and plate 20. The diameter of the bolts is less than that of the apertures 3 in the board 1 to allow for limited movement of the plate relative to the board. A layer 30 of a perfluorenated polyether and PTFE grease, or a gel, is placed between the board 1 and plate 20 to improve heat conduction to the plate. A thin film of plastics may be interposed between the board and the layer 30 of grease. <IMAGE>

Description

ELECTRICAL CIRCUIT BOARD ASSEMBLIES This invention relates to electrical circuit board assemblies.

The use of surface-mounted electrical components has brought with it problems, especially where the components and the board on which they are mounted, are subject to large changes in temperature.

Because there is little compliance in the soldered joint of a surface-mounted component on a board, compared with that of leaded components, it is important to reduce the effects of differences in thermal expansion between the components and the board. This can be done by making the board from a sandwich of different materials which results in a substrate having an overall coefficient of expansion closely matched to that of the components.

The boards generally have a heat-conducting layer which helps to conduct heat away from the components. The heat dissipation of a typical board may be of the order of 50 watts, so it is important that this heat is conducted away from the board as efficiently as possible, in order to minimize both the risk of overheating of the components and the thermal expansion effects. This is generally done by mounting the rear surface of the board on a heat conducting plate, such as of aluminium. For maximum efficiency of heat conduction to the plate, a very close thermal contact with the board is necessary, over the entire surface of the board. One way of achieving this, is by means of an adhesive, such as an epoxy resin, which can produce a secure bond and fill any air gaps between the plate and the board to ensure good thermal conduction.

This arrangement, however, has two disadvantages.

Firstly, because the thermal coefficient of the board has been selected to be close to that of the electrical components, it is very different from that of the plate.

This leads to differential thermal expansion effects between the board and plate which can result in bowing of the assembly and hence damage to the joints of the components with the board. Secondly, it is almost impossible to remove the board from the plate. Even if an adhesive is used which can be dissolved by solvents, the area of adhesive exposed to attack by the solvent, at the edge of the board, is so small that removal of the adhesive becomes very difficult. This disadvantage is aggravated by the fact that it is desirable for each side of the heat conducting plate to support a circuit board, so that a fault in one board can make it necessary to dispose of the entire assembly.

The differential thermal effects between the board and the plate can be reduced by using a compliant bonding layer, such as a heat conducting elastomer.

However, in order to produce the necessary compliance in the plane of the assembly, the bonding layer must be relatively thick. This increases the overall weight of the assembly, which can be a severe disadvantage in, for example, aircraft applications. The added weight also makes the assembly more prone to damage. Such an assembly, furthermore, still suffers from the disadvantage that the board cannot readily be removed from the heat-conducting plate for maintenance.

It is an object of the present invention to provide an electrical circuit board assembly that can be used to alleviate these disadvantages.

According to the present invention there is provided an electrical circuit board assembly comprising an electrical circuit board supporting a plurality of electrical circuit components on a first surface thereof, a heat conducting plate arranged on an opposite surface of said board, mechanical means for retaining the board on the plate without preventing limited movement of the plate relative to the board in the plane of the plate, and a layer of a low-volatile grease or gel intermediate the plate and the board which provides a path for thermal transfer between the board and the plate.

The grease may be a perfluorenated polyether and PTFE grease. The assembly may include a thin film of plastics material interposed between the board and the layer of grease or gel.

The board may have a multi-layer construction with a thermal coefficient of expansion in the plane of the board substantially equal to that of the components. The electrical circuit components may be surface-mounted components. The mechanical retaining means preferably includes a bolt extending through an aperture in the board and an aperture in the plate, the diameter of the bolt being less than the diameter of the aperture in the board.

The mechanical retaining means may include a male threaded bolt and a female threaded bolt that are screwed into one another from opposite sides of the assembly. The plate may be of aluminium.

Electrical circuit board assemblies according to the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective, exploded view of one form of assembly; Figure 2 is a sectional side elevation view of a part of the assembly of Figure 1; and Figures 3 are sectional side elevation and 4 views of alternative assemblies.

With reference first to Figures 1 and 2, the circuit board assembly comprises a conventional printed circuit board 1 which supports surface-mounted electrical components 2. The board 1 has a low thermal coefficient of expansion in the plane of the board which is matched to that of the components. This may be achieved in the usual way, such as by making the board of a multilayer or construction, including layers of invar and copper.

Towards each corner, the board has a circular aperture 3 which each receives a respective retaining device 7. The retaining devices 7 each take the form of male threaded bolt 8 screwed into a female threaded bolt 9. The male bolt 8 has a stem 10, the diameter of which is less than that of the apertures 3, and an enlarged, slotted head 11 which is larger in diameter than the apertures. The male bolt 8 extends through the aperture 3 with its head 11 engaging the upper surface of the board 1. The female bolt 9 is received in apertures 13 in a plate 20 of aluminium.

The aluminium plate 20 is rectangular in shape and slightly larger than the board 1, so that it extends beyond the board at its edges. The apertures 13 in the plate 20 align with the apertures 3 in the board 1 and are substantially the same diameter as the stem 7 of the female bolt 9. The head 18 of the female bolt 9 is slotted and engages the lower surface of the plate 20.

When the two bolts 8 and 9 are tightly screwed together, the separation between their heads defines the desired distance between the upper surface of the board 1 and the lower surface of the plate 20. The plate 20 is firmly clamped to a heat dissipating member (not shown) by which heat in the plate is dissipated. The heat dissipating member may either be actively cooled by a gas or liquid or be sufficiently massive to act as a heat sink.

Between the board 1 and the plate 20, there is a layer 30 of a non-volatile grease such as a perfluorenated polyether and PTFE grease of the kind sold by Montedison under the designation Fomblin ZLHT. The layer 30 is applied to the upper surface of the plate 20 such as by painting or screen printing and covers the entire area underlying the board 1. The thickness of the layer 30 is typically about 0.5mm, being thick enough to fill any interstices between the board 1 and the plate 20, and is defined by the height of the retaining devices 7. The grease has a relatively high thermal conductivity so that the layer 30 provides a path for efficient thermal transfer over a large area between the board 1 and the plate 20. The viscosity of the grease ensures that it is retained between the board 1 and plate 20 by its surface tension and does not flow away from the assembly, regardless of the attitude of the assembly. The non-volatile nature of the grease ensures that its performance is not degraded during the normal operational life of the assembly. Because the grease layer 30 is only exposed to atmosphere at the edge of board 1, this further reduces evaporation and protects the grease from attack by solvents or the like.

When the temperature of the assembly is raised, the plate 20 will expand by a greater amount than the board 1. The layer 30 allows for relative movement between the plate 20 and board 1 in the plane of the assembly. The retaining devices 7 allow for limited movement in this plane because of the loose fitting in the apertures 3 in the board. It will be appreciated that the apertures 13 through the plate 20 could alternatively, or additionally, provide a loose fitting with the retaining devices 7.

It will be appreciated that a different number of retaining devices could be used which could be distributed in other ways over the assembly. Alternative types of retaining device could be used which allow for movement of the plate relative to the board, in the plane of the plate, whilst restricting movement away from the board.

The assembly could be modified, as shown in Figure 3, by interposing a thin film 40 of a plastics sheet material between the grease layer 30 and the underside of the board 1. The plastics material may be a plasticized PVC or polyolefin of the kind sold under the name cling film' and has a thickness in the range 8 to 60 micron. The plastics film 40 is sufficiently deformable to ensure intimate thermal contact with the underside of the board 1 and protects the board from actual contact with the grease, making handling easier.

The assembly could include two boards 1 and 1' mounted on opposite faces of the plate 20, as shown in Figure 4. In this arrangement, the retaining devices 7' extend through apertures in both boards 1 and 1' and the plate 20.

The use of the grease layer 30 brings with it several advantages. It produces good thermal contact between the board and the plate without significantly increasing the weight of the assembly. It enables the board and plate to have different thermal expansion properties and to undergo thermal cycling without damage to the mounting of the components on the board. It also enables the board to be removed readily from the plate simply by releasing the retaining devices. This allows the board to be repaired and serviced more easily. This is an especial advantage when the plate supports two boards, as in Figure 4, since in some previous assemblies it has been necessary to dispose of both boards when one was faulty. Because the board is loosely fitted on the on the plate it is free to move relative to the plate on vibration. The layer of grease provides a damping effect on movement of the board so that it is, in effect, cushioned from the effects of vibraction. Instead of grease, a low volatile gel could be used.

Claims

1. An electrical circuit board assembly comprising an electrical circuit board supporting a plurality of electrical circuit components on a first surface thereof, a heat conducting plate arranged on an opposite surface of said board, mechanical means for retaining the board on the plate without preventing limited movement of the plate relative to the board in the plane of the plate, and a layer of a low-volatile grease or gel intermediate the plate and the board which provides a path for thermal transfer between the board and the plate.

2. An assembly according to Claim 1, wherein the grease is a perfluorenated polyether and PTFE grease.

3. An assembly according to Claim 1 or 2, wherein the assembly includes a thin film of plastics material interposed between the board and the layer of grease or gel.

4. An assembly according to any one of the preceding claims, wherein the board has a multi-layer construction with a thermal coefficient of expansion in the plane of the board substantially equal to that of the components.

5. An assembly according to any one of the preceding claims, wherein the electrical circuit components are surface-mounted components.

6. An assembly according to any one of the preceding claims, wherein the mechanical retaining means includes a bolt extending through an aperture in the board and an aperture in the plate, and wherein the diameter of the bolt is less than the diameter of the aperture in the board.

7. An assembly according to Claim 6, wherein the mechanical retaining means includes a male threaded bolt and a female threaded bolt that are screwed into one another from opposite sides of the assembly.

8. An assembly according to any one of the preceding claims, wherein the plate is of aluminium.

9. An assembly substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

10. An assembly substantially as hereinbefore described with reference to Figures 1 and 2 as modified by Figure 3 of the accompanying drawings.

11. An assembly substantially as hereinbefore described with reference to Figures 1 and 2 as modified by Figure 4 of the accompanying drawings.

12. Any novel feature or combination of features as hereinbefore described.