GB2145290A - Cooling avionics circuit modules - Google Patents

Abstract

An avionics unit has an outer housing (1) within which are mounted several circuit modules (10). The housing (1) has opposite walls each formed by inner and outer spaced plates (4 and 7, 5 and 8) having ventilating apertures (6 and 9) arranged such that cooling gas can flow through the housing over the modules (10). Each module (10) is an hermetically sealed unit having a substrate (40) on which electronic circuit elements (41) are mounted and which is backed by a layer (42) of heat-conducting material which contacts a base plate (43). A moulded plastics cover (48) having a peripheral wall (52) seals with the base plate (43) and protects the circuit elements (41) from the cooling gas. One surface of each module has heat-dissipating fins (46) that extend parallel to the flow of cooling gas and which contact the surface of an adjacent module. The modules (10) are resiliently mounted at their edges in the housing. <IMAGE>

Description

SPECIFICATION Avionics units This invention relates to avionics units.

Avionics units usually take one of two different constructions.

In civil aircraft the units have an outer housing provided with ventilation apertures that align with a ventilation duct provided on the airframe. Within the housing a number of circuit boards or modules are slidably received in rails or channels so that they can readily be removed and replaced. On each board there are mounted several circuit elements which may be integrated circuits or discrete components.

Air or other cooling gas is free to flow from the ventilating apertures in the housing over the surface of the boards, and over the circuit elements which are exposed to the gas flow. This arrangement is possible in civil aircraft because there is adequate supply of clean, dry air from the aircraft's airconditioning plant. The housing of such units can be relatively compact and light in weight.

Military aircraft are not usually equipped with sufficient air-conditioning capacity to provide clean, dry cooling airforthe avionics units. The air that is used to cool the avionics units is therefore not purified to any great extent and can be high in moisture, dust and contaminants. For this reason, the circuit board cannot be exposed directly to the cooling air since it could lead to damage to the circuit elements. Avionics units for military aircraft therefore generally differ from those used in civil aircraft in that the circuit board modules are mounted in an inner chamber within the housing that is sealed from the flow of cooling air. The circuit board modules are provided with heat conducting flanges that are clamped firmly against metal rails on the sides of the inner chamber.Heat dissipated by the circuit elements is conducted through the substrate on which they are mounted, to the flanges and hence to the metal rails. A heat exchanger is located outside the inner chamber in thermal contact with the rails, the cooling gas being supplied through the heat exchanger. This arrangement avoids any contact of the cooling gas with the circuit elements themselves. The arrangement does, however, have significant disadvantages.

Because of the large distance between the heat dissipating components and the flow of cooling gas, the removal of heat is not very efficient, this is especially the case for components mounted furthest from the cooling flanges which may thereby be prone to overheating. The need to use heat exchangers makes the military unit heavier than the units used in civil aircraft and can reduce the space available within the housing for circuit boards. The heat exchangers are also prone to clogging by contaminants thereby reducing the cooling efficiency. A further disadvantage of military units arises from the requirement that the flanges of the board be firmly clamped to the casing to ensure good thermal contact. This makes it more difficult to insert and remove the boards.Firmly clamping the relatively flexible boards at their edges also makes them prone to flexing when vibrated; this can lead to damage to the circuit elements, or to the mounting and connection of the circuit elements with the boards.

Both the civil and military avionics units suffer from an additional disadvantage because the circuit elements on the modules are exposed. This makes them prone to damage and contamination during handling and also makes servicing more difficult because the boards can only be removed from the avionics unit if special precautions are taken to avoid contamination. Obviously, it would be desirable if boards could be removed from the avionics unit whilst it was installed in the aircraft on a runway rather than having to remove the entire unit and replace the board in a workshop.

It is an object of the present invention to provide an avionics unit that may be used to alleviate the above-mentioned difficulties and disadvantages.

According to one aspect of the present invention there is provided an avionics unit having an outer casing defining an inner chamber having a plurality of circuit modules removably mounted at their edges in said inner chamber, each module including electronic circuit elements, said housing being arranged to direct a flow of cooling gas through said inner chamber over the surface of said circuit boards, wherein each said module is an hermetically sealed unit having a pair of opposite plate surfaces between which said circuit elements are contained and which protect said circuit elements from said cooling gas.

In this way, an avionics housing similar to that used for civil aircraft may be used with the poor quality cooling air available in military aircraft, but without the risk of contaminating the electronic circuit elements.

The housing may have opposite walls each formed by an inner and outer plate, said inner and outer plates being provided with ventilating apertures and being spaced from one another such that said cooling gas can flow into said inner chamber via the space between said inner and outer plate. At least one of the outer plates of each module may be provided with an extended surface arranged to increase heat transfer from the module to the cooling gas, and the extended surface may be provided by a plurality of parallel fins arranged such that the cooling gas flows between the fins along the length of the fins.The fins of one module may project towards an adjacent module which they may contact One of the plate surfaces of each module may be provided by a cover member having an integral peripheral wall arranged to seal with the other of said plate surfaces of the module, and the cover member is preferably of a plastics material.

The circuit elements may be mounted on one face of a printed circuit board, the other face of the printed circuit board being in contact with a layer of heat-conducting material which is preferably plated on the other face. One of the opposite plate surfaces of each module may contact the layer of heatconducting material, and the one surface being provided with an extended surface arranged to increase heat transfer from the module to the cooling gas. Each module is preferably resiliently mounted within the unit, aiid at least one of the modules may be provided with RF screening.

A military avionics unit 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 view of the avionics unit; Figure 2 is a plan view of a part of the unit of Figure 1; and Figure 3 is an exploded view of a module for the avionics unit With reference particularly to Figure 1,the avionics unit has a housing 1 the construction of which is similar to the conventional housing used in civil aircraft. The unit contains a number of circuit board modules 10 the construction of which will be described later.

The housing 1 is of rectangular shape having a front chamber 2 within which the modules 10 are mounted, and which is separated from a rear chamber (not shown) by a vertical back wall 3. The upper and lower walls of the housing 1 are formed by pairs of plates 4 and 7, and 5 and 8 respectively.

The outer plates 4 and 5 are both provided with ventilating apertures 6, that are located such that they align with ventilating ducts (not shown) provided in the aircraft for the purpose of supplying and removing cooling air to the avionics unit. The outer plates 4 and 5 are separated by a short space from respective inner plates 7 and 8 that extend parallel to the outer plates. Each of the inner plates 7 and 8 also has ventilating apertures 9 so that cooling air can flow through the front chamber 2 over the modules 10 via the spaces between the plates. The ventilating apertures 9 are arranged in rows down either side of parallel mounting rails 20 and 21 which are secured to the upper and lower inner plates 7 and 8 respectively.

The mounting rails 20 and 21, which are shown in greater detail in Figure 2, comprise two short parallel walls 23 and 24 that extend the depth of the front chamber 2 and which contain two serpentine springs 25 and 26 between which the modules 10 are located. The springs 25 and 26 provide a degree of resilience to the mounting of the modules 10 and thereby help damp any vibration.

The back wall 3 of the front chamber 2 carries a number of electrical connectors 31 positioned to mate with cooperating connectors 32 on the rear edge of each module 10.

The front of the housing 1 is normally closed by a removable cover plate (not shown). When open, the modules 10 can be inserted in the unit by sliding along the rails 20 and 21 until the connector 32 on the board mates with the connector 31 on the back wall 3.

The construction of the modules 10 will now be described in greater detail with reference to Figure 3.

Each module 10 is an hermetically sealed unit which contains a rectangular substrate 40 on which a number of electronic circuit elements 41 are mounted. The substrate 40 may be a standard multi-layer printed circuit board having conductive tracks by which electrical interconnection of the various circuit elements 41 are made. At its rear edge the substrate 40 also carries the connector 32 by which electrical connection is made to the module 10. A thick heat-conducting layer 42 of copper is preferably plated on the underside of the substrate 40. Heat dissipation of the circuit elements 41 may be improved by pillars of copper that extend through the thickness of the substrate 40 between the copper layer 42 and the circuit elements 41. The copper layer 42 on the bottom of the substrate 40 lies in thermal contact with a base plate 43.

The base plate 43 may be an extrusion of alumi nium or other metal and has a smooth, flat upper surface 44, and an extended lower surface 45 formed with parallel vertical fins 46. The purpose of the extended surface provided by the fins 46 is to improve the dissipation of heat from the module 10.

A cover 48 seals the module 10, being secured to the base plate 43 and the substrate 40, such as by means of screws 49. A gasket 50 or other sealing element may be sandwiched between the substrate 40 and the cover 48 to improve the seal of the module 10.

The cover 48 comprises a rectangular plate 51 around which extends a shallow peripheral wall 52 the lower edge of which engages the substrate 40 (or gasket 50) and the height of which is just sufficient to keep the plate 51 clear of the circuit elements 41. A portion of the wall 52 at the rear edge of the module 10 is cut away to receive the connector 32. The cover 48 is preferably moulded from a rigid plastics material, for economy.

When located in the avionics unit, the modules 10 are arranged vertically, side-by-side in the manner shown in Figure 2. The fins 46 of one module 10 project towards, or contact, the top surface of the cover 48 of an adjacent module. The ventilating apertures 9 on the inner plates 7 and 8 of the housing 1 are positioned intermediate the fins 46 so that cooling air is thereby channelled to flow along the gaps between the fins, between adjacent modules. If greater heat dissipation is produced from any particular region of the substrate 40, the size or location of the ventilating apertures 9 may be arranged such that a greater amount of cooling air flows over the corresponding region of the module 10.

Because the electronics components are sealed from the cooling air, the purity of the air used for cooling is not important. By avoiding the need to use heat exchangers, the housing can be much lighter than in previous military avionic units. The resistance to flow of cooling air through the avionics unit is also reduced, thereby reducing the pressure loss across the unit. Cleaning of the unit is facilitated since any dirt or contamination that collects on the surfaces of the modules can be removed readily.

Whereas, in previous military units, it is preferable that components dissipating the greatest amount of heat are located towards the edge of the module, closest to the heat exchanger, this need not be done in the present invention, thereby leaving greater freedom in the design of the circuits. Since, in the present invention, there is no need to ensure good thermal contact between the modules 10 and the housing 1, the accuracy with which the modules have to fit in the housing is consequently reduced.

Because the modules 10 do not have to be clamped rigidly against the housing 1 for thermal dissipation, they may be mounted with a degree of resilience at their edges so that, on vibration, flexure of the modules is reduced. The box-like structure of the individual modules 10 is also more rigid than some previous modules which further reduces flexing; the rigidity of the modules also improves the overall rigidity and strength of the avionics unit.

Whilst the avionics unit would normally be closed by a cover plate, the seal provided by the cover plate is not important since the electronic components are protected within their modules. The cover plate and the front face of the avionics unit need not therefore be made with great precision since an hermetic seal is not required. A consequence of allowing cooling air to flow through the interior of the housing, instead of sealing the interior of the housing, as with previous military avionics units, is that the overall sealed volume within the unit is substantially reduced. This reduces the effect on the unit of rapid changes in external atmospheric pressure, such as are experienced during change in altitude. In this way, for example, condensation within the unit, and forces caused by pressure differences are reduced.

Because each module 10 is a sealed unit, it can be removed from the housing 1 without risk of damage to the circuit elements it contains. This enables individual modules to be removed and replaced when the avionics unit is installed in an aircraft which is, for example, waiting on a runway exposed to rain or other foul weather.

A further advantage is achieved by the present invention since it enables RF screening to be incorporated in each module. If the cover plate 48 of the module is of plastics material it must include an electrically conductive layer if screening on both sides is desired. Because each module can be screened from each other, the same housing can be used to accommodate modules made by several different manufacturers, since there is no risk that RF signals produced by one module will interfere with an adjacent module. In this way, it may be possible to reduce the overall space occupied in an aircraft.

Claims (15)

1. An avionics unit having an outer housing defining an inner chamber having a plurality of circuit modules removably mounted at their edges in said inner chamber, each module including electronic circuit elements, said housing being arranged to direct a flow of cooling gas through said inner chamber over the surface of said circuit modules, wherein each said module is an hermetically sealed unit having a pair of opposite plate surfaces between which said circuit elements are contained and which protect said circuit elements from said cooling gas.

2. An avionics unit according to Claim 1,wherein said housing has opposite walls each formed by an inner and outer plate, and wherein said inner and outer plates are provided with ventilating apertures and are spaced from one another such that said cooling gas can flow into said inner chamber via the space between said inner and outer plate.

3. An avionics unit according to Claim 1 or 2, wherein at least one of the outer plates of each module is provided with an extended surface arranged to increase heat transfer from the module to the cooling gas.

4. An avionics unit according to Claim 3, wherein said extended surface is provided by a plurality of parallel fins arranged such that the cooling gas flows between the fins along the length of the fins.

5. An avionics unit according to Claim 4, wherein the fins of one module are arranged to project towards an adjacent module.

6. An avionics unit according to Claim 5, wherein the fins of one module contact an adjacent module.

7. An avionics unit according to any one of the preceding claims, wherein one of said plate surfaces of each module is provided by a cover member having an integral peripheral wall arranged to seal with the other of said plate surfaces of the module.

8. An avionics unit according to Claim 7, wherein said cover member is of a plastics material.

9. An avionics unit according to any one of the preceding claims, wherein the said circuit elements are mounted on one face of a printed circuit board, and wherein the other face of said printed circuit board is in contact with a layer of heat-conducting material.

10. An avionics unit according to Claim 9, wherein the said layer of heat-conducting material is plated on the other face of the printed circuit board.

11. An avionics unit according to Claim 9 or 10, wherein one of said opposite plate surfaces of each module contacts said layer of heat-conducting material, and wherein the said one surface is provided with an extended surface arranged to increase heat transfer from the module to the cooling gas.

12. An avionics unit according to any one of the preceding claims, wherein at last one of said modules is provided with RF screening.

13. An avionics unit according to any one of the preceding claims, wherein each said module is resiliently mounted within said unit.

14. An avionics unit substantially as hereinbefore described with reference to the accompanying drawings.

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