WO2004025728A2

WO2004025728A2 - Improvements in or relating to cooling of electronic components

Info

Publication number: WO2004025728A2
Application number: PCT/GB2003/003859
Authority: WO
Inventors: William Eric Ferguson; Peter Dalglish
Original assignee: Bae Systems Plc
Priority date: 2002-09-12
Filing date: 2003-09-05
Publication date: 2004-03-25
Also published as: WO2004025728A3; GB0221123D0; AU2003263316A8; AU2003263316A1

Abstract

Described herein is a method of cooling electronic components which does not rely on conventional convection cooling techniques. A ball grid array (BGA) (24) is mounted on a printed circuit board (PCB) (22). A heat spreading plate (42) covers the BGA (24) and has a hole (46) formed therein for accommodating a metal disk (48). The disk (48) is preferably located in the hole (46) by means of an interference fit. An interface layer (50) is provided between the BGA (24) and the disk (48) to enhance conduction of heat away from the BGA (24) and through the disk (48). The disk (48) and interface layer (50) form a heat path to dissipate heat from the BGA (24) to the plate (42) and then to a finning arrangement (52) which is cooled by a gas or liquid flow passing over and through the arrangement (52).

Description

IMPROVEMENTS IN OR RELATING TO COOLING OF

ELECTRONIC COMPONENTS

The present invention relates to improvements in or relating to cooling of electronic components, and is more particularly, although not exclusively, concerned with the cooling of high power ball grid arrays.

The cooling of commercial off-the-shelf (COTS) components, such as ball grid arrays (BGAs), presents a major challenge to mechanical and packaging engineers, particularly where established techniques on conduction cooling need to prevail. This is the case with military standard hardware where, in order to minimise contamination, primary cooling is achieved by utilising conduction from the component case.

However, recent changes in component packaging have encouraged the use of high power ball grid arrays (BGAs) on printed circuit boards (PCBs). One example of a BGA is the MPC7400 made by Motorola Inc. BGAs are of particular use on PCBs in computers and other applications where high speed processing is required. The BGAs require cooling so that they remain reliable during operation. In a normal computer-related application where BGAs are used as high-speed processors, a sizeable external finned heat sink is fixed to the component lid and cooling is achieved by blowing air from a fan over the finned heat sink. However, this method of cooling is only possible when there is sufficient space available.

In other applications, BGAs are provided with heat sinks mounted on their lids so that cooling is effective. However, the problems associated with conduction cooling are exacerbated by the need to extract heat from the lid of the BGA because the package dimensions vary according to a physical tolerance and the manufacturer/supplier.

In some military applications, there is no space for an external heat sink and there is no direct access to cooling air via a fan. This means that another method of cooling is required. Moreover, if a heat sink is mounted on the lid of a BGA, it may be difficult to dispose of the heat generated through the top surface of the BGA. This is due to the gap between the top of the BGA and the underside of a heat distributor, for example, a finning arrangement, is not constant. Furthermore, the manufacturing tolerances of the BGA and the heat distributor allow unknown gaps to be produced between the BGA and the heat distributor. Additionally, further tolerance problems may be introduced by the process of soldering the BGA to its PCB. It has been noted that the tolerance in the difference between the minimum and maximum heights of a BGA may be around 0.48mm.

It is therefore an object of the present invention to provide an improved cooling system for a BGA which overcomes the problems associated with space limitation and physical tolerances.

In accordance with one aspect of the present invention, there is provided a method of cooling an electronic component mounted on a printed circuit board, the method comprising the steps of:- a) placing a heat distributor plate adjacent to the printed circuit board; b) creating a thermal path between the electronic component and the heat distributor plate; and c) cooling the heat distributor plate.

Preferably, step a) comprises shaping the heat distributor plate to accommodate the electronic component on the printed circuit board and to allow the plate to make contact with the printed circuit board.

Advantageously, step c) comprises blowing a cooling gas or liquid flow through a finning arrangement in thermal contact with the heat distributor plate.

In accordance with another aspect of the present invention, there is provided a cooling arrangement for an electronic component mounted on a printed circuit board, the arrangement comprising:- a heat distributor plate mounted on the printed circuit board over the electronic component, the plate being shaped to accommodate the electronic component; interface means in thermal contact with the electronic component and the heat distributor plate; and cooling means for dissipating heat from the heat distributor plate. Advantageously, the interface means comprises biasing means for forming thermal contact between the electronic component and the heat distributor plate.

The biasing means may comprise a screw mounted in a boss formed in the heat distributor plate.

Alternatively, the biasing means may comprise a disk mounted in a hole in the heat distributor plate as an interference fit.

Furthermore, the biasing means may comprise an insert mounted in a cut-out in the heat distributor plate, the insert being sealed to the plate using a thermally conductive adhesive.

Additionally, the interface means may comprise an interface material and the biasing means biases the interface material into thermal contact with the electronic component.

For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings, in which:-

Figure 1 illustrates a schematic section through a conventional cooling arrangement for a BGA and other components on a PCB;

Figure 2 illustrates a first embodiment of cooling arrangement in accordance with the present invention; Figure 3 illustrates a second embodiment of a cooling arrangement in accordance with the present invention; and

Figure 4 illustrates a third embodiment of a cooling arrangement in accordance with the present invention.

Figure 1 illustrates a conventional cooling arrangement 10 for a PCB. The arrangement 10 comprises a PCB 12 against which is located a "cold wall" 14. A corrugated lining or finning system 16 is located against the "cold wall" 14 and the whole is housed in a container 18.

Conventionally, BGA devices have under-device connections and hence only take up the surface area of the device itself on the board. PCBs are normally cooled through the base of the board and heat is dissipated from a device through the board to its underside by making contact with the "cold wall" 14. This means that devices mounted on the PCB 12 are on the opposite side of the PCB to the "cold wall" 14.

As a result of the devices being on the other side of the PCB to the "cold wall", large gaps need to be provided between adjacent PCBs so that the devices do not become damaged in use. Moreover, where increased computing power is required, several PCBs are located in a stack with large gaps between adjacent pairs of PCBs. Naturally, this increases the amount of space required for a bank of PCBs, and this is a serious disadvantage where space is a premium, for example, in aircraft.

It has therefore been proposed to cool devices on PCBs through their tops or lids. This cooling technique is known as "lid-cooling". This technique has the advantage that the PCBs can be placed more closely together in a stack thereby reducing the gaps and maximising the number of PCBs which can be located in a stack of a predetermined size.

It has been established by exhaustive thermal analysis that lid-cooled BGAs can be effectively cooled by applying a machined plate over the surface of the component and, where necessary, over the entire PCB. In practice however, it is known that the component tolerance variation requires the establishment of a high performance thermal interface between the component lid and the plate.

Two techniques may be utilised to ensure that there is good thermal contact between the machined plate and the lids of the components to be cooled. One technique is to use a relatively thick compliant gap-filler type material that compresses to match and fill the gap between the component lid and the plate. One suitable material for this application is an aligned fibrous thermal interface. However, it has been shown that although the conductivity of the material is relatively high, the thickness required to take-up the range of possible gap sizes produces a relatively large temperature rise of the component itself. One alternative technique is to minimise the gap mechanically so that a direct dry contact is achieved between the component and the plate. In another alternative technique, a very thin highly conductive paste or film can be applied to the component to improve the contact. However, this tends to increase the mechanical complexity of assembling the PCB and its cooling components.

In accordance with the present invention, good thermal interface between the component lid and the plate is described in detail with reference to Figures 2 to 4. In Figure 2, a first cooling arrangement 20 in accordance with the present invention is shown. The arrangement 20 illustrates a PCB 22 on which is mounted a BGA (or other high power component) 24. A heat spreading plate 26 is mounted on the PCB 22 and has a recess 28 which fits over the BGA 24. The plate 26 has a boss 30 which is threaded to accept a screw of high thermal conductivity 32. The screw 32 carries an interface material 34 at its end which contacts the BGA 24. Located on top of the heat spreading plate 26 is a layer of finstock material 36 which assists in the dissipation of heat from the plate 26.

In this embodiment of the invention, heat is transferred from the BGA 24 via the interface material 34 and the screw 32 to the heat spreading plate 26. The finstock material 36 then transfers the heat from the plate 26 into a cooling gas or liquid flow (not shown) which is directed through and over the finstock material 36.

Although Figure 2 has been described with reference to a single high power component, it will readily be understood that the PCB may have more than one high power component and a thermal screw could be provided for each high power component.

Figure 3 illustrates another cooling arrangement 40 in accordance with the present invention. Components which have previously been described in

Figure 2 are referenced the same. In Figure 3, PCB 22 has a BGA 24 mounted on it as before. A heat spreading plate 42 is mounted on the PCB 22 and has a recess 44 which fits over the BGA 24. The plate 42 has a hole 46 formed substantially in the centre of the recess 44 and accommodates an aluminium disk 48. The disk 48 forms an interference fit with the hole 46 and contacts an interface material 50 located on the BGA 24 to make a thermal contact path from the BGA 24 to the heat spreading plate 42. Finstock material 52 similar to finstock material 36 in Figure 2 is located on top of heat spreading plate 42 and disk 48.

In the embodiment of Figure 3, the heat spreading plate 42 is preferably made of aluminium so that there is no thermal gradient between the disk 48 and the plate 42. Preferably, the disk 48 has a greater diameter than the hole 46 into which it is to be inserted. In order to ease the insertion of the disk 48 into the hole 46, the disk 48 can have its diameter temporarily reduced by reducing its temperature say, by immersion in liquid nitrogen.

Once the diameter disk 48 has been reduced to a size where it can be inserted in the hole 46, it is inserted into the hole 46 in the plate 42 so that it makes contact with the interface material 50 on the BGA 24. The assembly so formed is allowed to return to the temperature of the surroundings and as the disk heats up, it expands to fill the hole 46 and the interference fit with the hole 46 is formed. The force created by the expansion holds the disk 48 in place within the hole 46 and creates a joint with very low thermal resistance. In another embodiment (not illustrated), the disk 48 could be fitted such that it can be removed to facilitate a BGA component change.

The disk 48 and heat spreading plate 42 are preferably made from aluminium, but it will be readily understood that any other suitable metal can be used which provides the desired heat transfer characteristics. As before, the embodiment of Figure 3 can be used to accommodate more high power components and a disk is provided for each of such components.

Figure 4 illustrates a further embodiment of a cooling arrangement 60 in accordance with the present invention. Again, components previously described are referenced the same. The PCB 22 has a BGA 24 mounted on it as before. A heat spreading plate 62 is mounted on the PCB 22. The plate 62 has a cut-out 64 formed in the vicinity of the BGA 24 which has a shoulder 66 for receiving an insert 68. The insert 68 is joined to the plate 62 at shoulder 66 using thermally conductive adhesive 70.

As shown, the insert 68 is substantially larger than the disk 48 in Figure 3 and is larger than the BGA 24. The insert 68 has a recess 72 formed a substantially central region thereof for accommodating an interface 74 which makes contact with the BGA 24.

As before, finstock material 52 is located over the heat spreading plate 62 and covers the insert 68. The embodiment shown in Figure 4 allows the heat from the BGA 24 to spread much more in the insert 68 before it reaches the adhesive 70 and the joint with the plate 62. It also provides a greater joining area which reduces the dependency on the thermal efficiency of the joint between the insert 68 and plate 62 at shoulder 66. Again, the embodiment of Figure 4 can be modified to accommodate more high power components.

In each of the embodiments of Figures 2 to 4, the finstock is cooled by passing a cooling gas or liquid supply over and through the finstock. If the cooled PCB is mounted on an aircraft, it is preferred that the gas or liquid supply is provided by the onboard environmental control system, driven by bleed air from the aircraft engines. In this case, there is a temperature difference between the coolant and component of around 40°C with the coolant temperature being the lower of the two values.

Although aluminium has been described as a suitable material for the heat spreading plate and disk/insert, it will be appreciated that any other suitable material having a suitable thermal conductivity can be utilised. For example, it may be possible to utilise copper but only if weight is not critical. Non-metallic materials may also be used, for example, silicon carbide composite materials. However, it is to be noted that the material selected also needs to have suitable properties to enable the interference fit to be achieved if the embodiment of Figure 3 is utilised.

Aluminium is preferred for the finstock material as it is lightweight, easy to form and has a high thermal conductivity. (The finstock material normally comprises two plates which sandwich a sheet of material which has been shaped to form a similar profile to that shown in Figures 2 to 4).

If a plurality of PCBs which are cooled in accordance with the present invention are placed in a stack, a 2mm may be provided between adjacent PCBs and the pitch of the PCBs may be around 31mm for an embodiment thickness of 29mm. Naturally, the pitch will vary with the thickness of the particular embodiment.

The cooling arrangement of the present has the advantage that it can readily be used with electronic components which require cooling through the top and does not require that the PCB be thermally conductive. Moreover, as the overall size of a cooled PCB can be reduced, it is possible to maximise the use of the available space and provides a weight advantage.

It is also possible that a single finstock could be utilised for two PCBs but a higher rate of gas or liquid supply will be required to dissipate the heat at a rate similar to that for a single PCB.

Furthermore, it may be desirable not to have an interface material in the thermal path between the high power component and the heat distributor plate. In this case, the thermal screw (Figure 2), the disk (Figure 3) and the insert (Figure 4) will make direct contact with the high power component on the PCB.

Claims

1. A method of cooling an electronic component mounted on a printed circuit board, the method comprising the steps of:-

a) placing a heat distributor plate adjacent to the printed circuit board; b) creating a thermal path between the electronic component and the heat distributor plate; and c) cooling the heat distributor plate.

2. A method according to claim 1 , wherein step a) comprises shaping the heat distributor plate to accommodate the electronic component on the printed circuit board and to allow the plate to make contact with the printed circuit board.

3. A method according to claim 1 or 2, wherein step c) comprises passing a cooling gas or liquid flow through a finning arrangement in thermal contact with the heat distributor plate.

4. A cooling arrangement for an electronic component mounted on a printed circuit board, the arrangement comprising :-

a heat distributor plate mounted on the printed circuit board over the electronic component, the plate being shaped to accommodate the electronic component;

interface means in thermal contact with the electronic component and the heat distributor plate; and

cooling means for dissipating heat from the heat distributor plate.

5. An arrangement according to claim 4, wherein the interface means comprises biasing means for forming thermal contact between the electronic component and the heat distributor plate.

6. An arrangement according to claim 5, wherein the biasing means comprises a screw mounted in a boss formed in the heat distributor plate.

7. An arrangement according to claim 5, wherein the biasing means comprises a disk mounted in a hole in the heat distributor plate as an interference fit.

8. An arrangement according to claim 5, wherein the biasing means comprises an insert mounted in a cut-out in the heat distributor plate, the insert being sealed to the plate using a thermally conductive adhesive.

9. An arrangement according to anyone of claims 5 to 8, wherein the interface means further comprises an interface material and biasing means for biasing the interface material into thermal contact with the electronic component.

10. An arrangement according to any one of claims 4 to 9, wherein the cooling means comprises finstock in thermal contact with the heat distributor plate through which a cooling gas or liquid supply is directed.

11. A method of cooling an electronic component mounted on a printed circuit board substantially as hereinbefore described with reference to Figures 2 to 4 of the accompanying drawings.

12. A cooling arrangement for an electronic component mounted on a printed circuit board substantially as hereinbefore described with reference to Figures 2 to 4 of the accompanying drawings.