GB2164499A

GB2164499A - Heat sinks for electronic components

Info

Publication number: GB2164499A
Application number: GB08522583A
Authority: GB
Inventors: Thomas Munn Pollock
Original assignee: HOWWOOD IND Ltd
Current assignee: HOWWOOD IND Ltd
Priority date: 1984-09-18
Filing date: 1985-09-12
Publication date: 1986-03-19
Also published as: GB2164499B; GB8423618D0; GB8522583D0

Abstract

A heat sink structure for cooling electronic components (4) mounted on circuit boards (1) consists of a thermal bridge (12) which lies over the component (4) in close proximity to the upper surface (15) thereof, and which is coupled to a thermal plane (9) located on one side (5) of the circuit board to cool the electronic component by providing heat transfer from the upper side of the electronic component to the thermal plane. In one embodiment the thermal bridge is fastened to the thermal plane and in another embodiment the thermal bridge is extruded and shaped to receive a circuit board and to interlock with other thermal bridges to provide a compact thermal management system for multi-layer circuit boards. <IMAGE>

Description

SPECIFICATION Heat sink structures The present invention relates to heat sink structures for use on circuit boards to cool electronic components and to a method of cooling such electronic components.

The trend in modern electronic hardware towards higher performance has resulted in increased component densities and a greater use of available circuitry on circuit boards. A direct consequence of this has been an increase in the heat produced per unit area. The increase in the amount of heat produced can result in excessive device temperatures if they are not cooled. Inadequately cooled electronic circuits suffer from malfunctioning, decreased reliability and increased cost. Therefore, cooling is vital and it may also be necessary to control the temperature distribution across the circuit board which is usually in the form of a printed circuit board, hereinafter referred to as a PCB.

Thermal management of electronic equipment requires that heat is taken from the device and transferred to an ultimate heat sink, usually the atmosphere. Heat is transported via at least one of three modes, namely; conduction, convention and radiation, all of which are used in various thermal management systems. One system of thermal management which uses the conduction mode of heat transport is a PCB bonded thermal plane.

In this kind of system a layer of thermally conductive material, usually copper or aluminium, is sandwiched between the electronic components and the PCB. The thermal plane is interrupted by holes and slots to receive pins and other component connections which are connected to etched conductors on the underside of the PCB. Heat produced by the electronic components is transferred to the thermal plane and conducted to the edges of the plane where it is dispersed by a wider thermal management system.

Thermal management using a thermal plane offers a number of advantages over some other commonly used systems. Some of these advantages are, low relative cost small space requirements, and medium to high thermal efficiency. Furthermore, the thermal plane does not require any specialist equipment and fits into existing enclosures. However, at present, the thermal performance of thermal planes limits their use to cases where the heat output is not greater than about 1 watt per chip on a typical PCB assembly.

The thermal efficiency of a thermal plane is limited by two main causes. Firstly, there is no heat flow path of low thermal resistance provided to take heat from the upper surfaces of electronic devices. Secondly, physical discontinuities such as holes and slots in the thermal plane provide thermal discontinuities which can hinder the flow of heat from the electronic device to the edge of the thermal plane.

It is an object of the present invention to avoid or reduce one or more of the above disadvantages.

Accordingly there is provided a heat sink structure which is adapted to be located over a component in proximity to an upper portion of the component and coupled to a thermal plane for conducting heat generated from the component to the thermal plane.

Accordingly, in one aspect of the present invention there is provided a heat sink structure for use with a circuit board having an electronic component mounted thereon and extending to a predetermined height above said circuit board, said heat sink structure comprising a thermal plane in the form of a plate of material having a high thermal conductivity, said plate being disposable, in use, between the circuit board and said electronic component, said plate having a plurality of apertures passing therethrough for receiving electrical conductor leads for connecting said electronic component to a circuit on said circuit board, and said plate also being coupled to a thermal bridge formed of a material having a high thermal conductivity and being in substantially direct contact with said thermal plane, said thermal bridge having a bridging height equal to or slightly greater than the height at said component whereby, in use, heat generated from an upper surface of said component is conveyed from said upper surface of said component to said thermal plane via said thermal bridge.

Additionally, the heat sink structure may include at least one passive cooling device such as a fin radiator element and/or active cooling device such as a heat pipe, a vortex cooling jet or a thermoelectric heat pump.

With a heat sink structure of the present invention improved heat dissipation and cooling of electronic components is achieved by removing heat from the upper surfaces of electronic components and improving transfer of heat through the thermal plane via the thermal bridge.

The thermal bridge and the thermal plane may be made from any highly thermally conductive material, such as,for example, copper or aluminium. In general, the heat bridge is in the form of an inverted generally U- or Cshaped channel with its two legs fastened to the surface of the thermal plane. The fastening mechanism may take the form of a bolted, screwed, brazed, welded, soldered, snap fitted or adhesive joint. Advantageously, the bridging height of the thermal bridge is such that the bridge is in intimate contact with the electronic component. In order to ensure a low thermal resistance between a component's upper surface and the underside of the bridge a thermal coupling means such as thermal grease or some other highly thermally conductive insert may be used.

The overall dimensions of the thermal bridge are dictated by the layout of the circuit requiring cooling, disposition of electronic components and pattern of the thermal plane. Thermal bridges may be made to fixed dimensions for particular PCB/thermal plane assemblies or may be made to be adjustable to fit a variety of circuit boards.

In a further aspect the present invention provides a heat sink structure mounted on a circuit board and a method of cooling an electrical component mounted on a circuit board having a thermal plane disposed between the board and the component, said method comprising locating a thermal bridge of a high thermal conductivity in substantially close proximity to said upper surface of said component, and coupling said thermal bridge to said thermal plane, whereby, in use, heat generated from said upper surface of said compo- nent is transferred to said thermal plane via said thermal bridge.

Further features and advantages of the invention will appear from the following detailed description of two embodiments illustrated with reference to the accompanying drawings in which: Fig. 1 is a partly sectioned elevation and view of a heat sink structure of one embodiment of the invention mounted on a printed circuit board.

Fig. 2 shows a view, similar to Fig. 1, of an alternative heat sink structure; and Fig. 3 shows an end view of thermally managed assembly consisting of a plurality of interlocked heat sink structures as shown in Fig.

2.

Reference is first made to Fig. 1 which shows a printed circuit board (PCB) 1 comprising a plurality of copper conductor tracks 2 on either side of a non-conducting substrate 3. An electronic component such as a dual-inline package (DIP) 4 is supported above one side 5 of the PCB 1 via connector leads 6 whose distal ends 7 are soldered 8 to respective conductor tracks 2', 2".

Side 5 of the PCB is provided with a thermal plane in the form of a copper plate 9 secured thereto with the aid of an electrically non-conductive layer 10 of prepreg (glass fibre with an epoxy adhesive). The plate 9 has a plurality of small apertures 11, one of which is shown, through which the distal ends 7 of the DIP connector leads 6 pass, whilst being spaced from the plate 9, for connection to the conductor tracks of PCB 1.

A thermal bridge in the form of a generally inverted U-shaped channel 12 of copper is secured at the ends 13 of its limbs 14 to the thermal plane 9, e.g. by brazing, so as to bridge across the DIP 4. The bridge 12 is proportioned so that limbs 14 are disposed clear of the DIP connector leads 6 and any other conductors but its base 14a is located in close proximity to the upper surface 15 of the DIP 4. Indeed, in order to maximise thermal transfer from the DIP to the bridge, as indicated by the arrows in the drawing, the latter could be formed and arranged so as to physically contact the upper surface 15. Alternatively, as shown in the drawing, a small gap 16 remains between the bridge 12 and DIP 4, this gap could be filled with a thermal grease or other thermally conductive insert.

In addition to functioning as a cooling means for the upper surface by drawing heat away from the upper surface, the bridge can also function to convey heat from one part of the PCB, for example, where there is greater heating, to another part where, for example, an active or passive cooling device is provided. This is useful in cases where the normal heat transfer route via the thermal plane has for any reason a significantly reduced thermal capacity, for example, because a large number of apertures for passing connector leads between the PCB and components disposed above the thermal plane.

Reference is now made to Fig. 2 of the drawings which shows an extruded aluminium heat sink structure 20 capable of being interlocked with other like structures to provide a thermally managed assembly as shown in Fig.

3.

The structure 20 is generally of an inverted U-shape having a long generally horizontal portion 22 integrally connected to shorter, downward extending side-legs, generally indicated by reference numeral 24. For clarity, only one of the side-legs will be described although it will be appreciated that they are allochiral and like numerals will refer to like parts on the other side-leg.

One side-leg 24 defines with a downwardly disposed flange 26 a channel 28 of generally T-shaped cross-section for receiving fasteners for attaching the structure 20 to a PCB board.

The leg 24 has an upwardly disposed flange 30 which defines with an edge of the portion 22 a generally L-shaped channel 32 for receiving outermost leg portions 34 of another structure as will be described. The portion 34 has an inwardly projecting ledge 36 for abutting interior surfaces 38 and 40 of channel 32 to provide preset spacing between adjacent structures and to ensure close fit and stability of the assembled unit.

Reference is now made to Fig. 3 and it will be seen that there is a stack of four structures 20 as shown in Fig. 2, each structure carrying a PCB board 1 with a plurality of electronic components 4. Each PCB is mounted to a respective structure using bolts 29, and captive nuts 31 which are located within channel 28. When each PCB is so mounted leg surface portions 42, 44 abut the thermal plate 9 to provide a thermal path from the horizontal portion 22.

An assembly as shown in Fig. 3 is created by sliding the legs 24 of one structure into the channels 32 of another structure, and so on, until the desired amount of PCBs are assembled. The assembly is, of course, locatable in an appropriate housing, and the interlocked structures can be aligned in the housing by abutment against a stop, not shown, to prevent relative sliding movement of the structures. In the resultant assembly the upper surface 15 of each electronic component 4 is in close proximity to a horizontal portion 22 of the structure to which it is mounted. However, in Fig. 3 there is clearance 46 between the upper portion 15 and the horizontal portion, a suitable thermal conducting coupling medium such as a copper spring strip 48 can be inserted as shown.The copper spring strip 48 is proportioned so that a spring surface 50 contacts each component and other spring linking surfaces 52 are in direct contact with portions 22 to increase thermal transmission from the components.

It will be appreciated that use of copper spring strips is optional and these can be omitted so that each structure of the assembled unit functions like the structure shown in Fig. 1. An advantage of the interlocking unit is that the interlocked legs provided a large thermal heat sink which can readily be coupled to a further passive or active heat sink such as those described above, also no soldering, brazing or mechanical fastening is required resulting in rapid and efficient assembly of the structure. Also, interlocking of the structures allows thermal management of as many cards of components as desired in an efficient and cost effective manner.

It will be appreciated that various modifications may be made to the above described embodiments without departing from the scope of the present invention. For example, with regard to the first embodiment, aluminium or any other highly thermally conductive material could be used in place of copper in the thermal bridge, and in the second embodiment the aluminium extrusion could be replaced by an extrusion of any other suitable thermally conducting material. In addition, the interlocking structure need not be extruded and could, for example, be rolled, formed and pressed to a desired shape. It will also be appreciated that the shape of the interlocking heat sink structure shown in Fig. 2 is exemplary and many different shapes and different materials can be used for different applications. In addition, it will be appreciated that the present invention is equally applicable with respect to other forms of PCB including, for example, multi-layer PCBs.

Claims

1. A heat sink structure for use with a circuit board having an electronic component mounted thereon and extending to a predetermined height above said circuit board, said heat sink structure comprising, a thermal plane in the form of a plate of material having a high thermal conductivity, said plate being disposable, in use, between the circuit board and said electronic component, said plate having a plurality of apertures passing therethrough for receiving electrical conductor leads for connecting said electronic component to a circuit on said circuit board, and said plate also being coupled to a thermal bridge formed of a material having a high thermal conductivity and being in substantially direct contact with said thermal plane, said thermal bridge having a bridging height equal to or slightly greater than the height at said component whereby, in use, heat generated from an upper surface of said component is conveyed from said upper surface of said component to said thermal plane via said thermal bridge.

2. A heat sink structure as claimed in claim 1 wherein said thermal plane is copper.

3. A heat sink structure as claimed in claim 1 or claim 2 wherein said thermal bridge is copper.

4. A heat sink structure as claimed in any preceding claim, said structure is thermally coupled to an auxiliary cooling device, said auxiliary cooling device being a passive or active cooling device.

5. A heat sink structure as claimed in any preceding claim, said structure is thermally coupled to an auxiliary cooling device, said auxiliary cooling device being a combination of an active and a passive cooling device.

6. A heat sink structure as claimed in any preceding claim, wherein said thermal bridge is directly fastened to said thermal plane.

7. A heat sink structure as claimed in any preceding claim, wherein said thermal bridge is brazed to said thermal plane.

8. A heat sink structure as claimed in any preceding claim, wherein said thermal bridge physically contacts the upper side of said electronic component.

9. A heat sink structure as claimed in any one of claims 1 to 7, wherein said thermal bridge is spaced from the upper side of said component to define a gap therebetween.

10. A heat sink structure as claimed in claim 9, wherein said gap is provided with high thermal conductivity coupling means to thermally couple the upper side of said component to said thermal bridge.

11. A heat sink structure as claimed in claim 10, wherein said thermal conductivity coupling means is a copper spring strip proportioned so that one surface of the copper spring strip abuts the upper surface of said component and another linked surface of said copper spring strip abuts said thermal bridge.

12. A heat sink structure as claimed in claim 10 wherein said thermal coupling means is a high thermal conductivity grease.

13. A heat sink structure as claimed in any one of claims 1 to 5, wherein said thermal bridge is substantially of an inverted U-shape having a pair of spaced leg means for thermally coupling to said thermal plane, and an integral bridging portion connected to said leg means, the integral bridging portion being in substantially direct contact with the said electronic component.

14. A heat sink structure as claimed in claim 13, wherein each of said legs is shaped and proportioned to define an upper and a lower channel, each lower channel being adapted to receive circuit board fastening means for fastening the circuit board and a thermal plane with said electronic component mounted thereon to said thermal bridge, such that said electronic component is spaced from said integral bridging portion to define a gap therebetween, and surface portions of said leg means abut said thermal plane to provide a thermal pathway from said upper surface of said electronic component to said thermal plane, and said upper channels being proportioned to receive and interlock with respective lower portions of leg means of another like thermal bridge to provide a stacked interlocked assembly of thermal bridges and circuit boards.

15. A heat sink structure as claimed in claim 14, wherein said legs of said other thermal bridge are slidable into said upper chan nels.

16. A heat sink structure as claimed in claims 14 and 15,wherein said thermal bridge is an aluminium extrusion.

17. A heat sink structure as claimed in any one of claims 14 to 16 wherein a copper spring strip is located in said gap, said copper spring strip being proportioned so that one surface of the copper spring abuts the upper surface of said component and the other surface of said spring abuts the integral bridging portion of said thermal bridge.

18. A method of cooling an electronic component mounted on a circuit board having a thermal plane disposed between the board and the component, said method comprising locating a thermal bridge of a high thermal conductivity in substantially close proximity to said upper surface of said component, and coupling said thermal bridge to said thermal plane, whereby, in use, heat generated from said upper surface of said component is trans- ferred to said thermal plane via said thermal bridge.

19. A method as claimed in claim 18 including locating thermal coupling means between said upper surface and said thermal bridge.

20. A heat sink structure substantially as hereinbefore described with reference to Figs.

1 or 3 of the accompanying drawings.