GB2126014A - Heat sink for electronic circuit assemblies - Google Patents

Abstract

A heat sink arrangement for circuit packages (2) which are mounted in rows on a circuit board (1) provides the sole means of support for the circuit board (1) and comprises a ladder-like heat conducting structure (5) having cross members (7) which extend between the circuit packages (2) and the circuit board (1) and to which the undersides of the circuit packages (2) are bonded. Side members (6) of the heat conducting structure (5) are attached to a larger heat dissipating member, such as for example, a lid of a metal box in which the circuit board (1) is contained. <IMAGE>

Description

SPECIFICATION Electronic circuit assemblies The present invention relates to electronic circuit assemblies and in p#articular to such assemblies which include heat sink arrangements.

It has previously been proposed to provide a heat sink arrangement for so called dual-in-line circuit packages positioned in rows on a circuit board. Such circuit packages have a row of downward facing connection pins on opposite sides which are soldered to through hole mountings on the circuit board. The packages contain integrated circuit chips which are attached to the bottoms of the packages and thus it is at the bottoms of the packages that most of the generated heat occurs. As the bottoms of the packages are adjacent the circuit board, they are not readily accessible and, due to manufacturing tolerances for example, may be spaced at different distances from the circuit board.

Previous heat sink arrangements have included strips of heating conducting material in contact with the undersides of the circuit packages, heat from the packages being conducted by the strips to transfer bars connected to the ends of the strips and secured to the circuit board. The transfer bars may be part of a heat conducting path leading to a larger heat dissipating member, such as part of a machine frame, for example. However, to be effective such an arrangement requires that good thermal contact between the circuit packages and the heat conducting strips is provided and maintained. In practice this is difficult to achieve due to the aforementioned manufacturing tolerances and also to differential thermal expansion of the component parts of the arrangement.Furthermore such tolerance problems and also thermal expansion can result in undesirable stresses being produced in the assembly.

According to the present invention an electronic circuit assembly includes; a circuit board; a plurality of circuit packages mounted on the circuit board; a heat dissipating support member; and a heat conducting structure comprising at least one side member secured to the heat dissipating support member and having a plurality of cross members extending therefrom, each cross member being formed to provide a flat surface to which at least one of the circuit packages is bonded, the heat conducting structure being effective conduct heat from the circuit packages to the heat dissipating support member and to provide the sole means of support for the circuit packages and the circuit board.

The heat conducting structure may include a pair of spaced apart parallel members with the cross members extending therebetween.

The side member or members may lie in a first plane and the cross members may lie in a second plane spaced from the first plane.

The heat dissipating support member may include the wall of a metal box structure inside which the remainder of the circuit assembly is contained.

The circuit packages may be dual-in-line circuit packages, the cross members of the heat conducting structure extending between the circuit board and the circuit packages. In this case the cross members of the heat conducting structure may be arranged so as to provide an air space between the circuit board and the cross members.

An electronic circuit assembly including a heat sink arrangement will now be described, by way of example, with reference to the accompanying drawing, in which; Figure 1 shows a plan view of a circuit assembly including a heat sink arrangement, and Figure 2 shows an end view of the assembly attached to the lid of a box structure.

Referring to the drawings, a circuit board 1 carries a row of dual-in-line circuit packages 2 containing integrated circuit chips (not shown).

The integrated circuit chips are attached to the bottoms of the packages 2, i.e. those parts of the packages adjacent the circuit board 1, and connections to the circuit chips are made via downward facing connection pins 3 positioned along two sides of the circuit packages 2. The connection pins 3 are soldered to through hole mountings 4 to provide connections to conductors (not shown) on the circuit board 1.

A heat conducting structure 5 is formed in a ladder-like configuration having side members 6 linked by cross members 7. The cross members 7 are formed as shown in Figure 2, so that they provide flat surfaces 8 which lie in a plane spaced from the plane of the side members 6. The heat conducting structure 5 is made from a material that possesses good heat conducting properties, such as for example aluminium, and is preferably formed as a unitary structure. If desired, however, the side members 6 and the cross members 8 may be formed separately and joined together by any means which will provide effective heat conducting joints.

The cross members 7 pass between the circuit board 1 and the circuit packages 2 and the undersides of the circuit packages 2 are bonded one to each of the surfaces 8 using an elastomeric adhesive having a low thermal resistance.

The side members 6 of the heat sink arrangement 5 are secured to the inside face of a metal box lid 9 which co-operates with a box 10 for containing the circuit structure. The box 10 is made from a material having good heat conducting properties and it must be possible to achieve an effective thermal contact between the lid and the box. The box may be diecast using an aluminium alloy, for example.

The side members 6 may be secured to the box lid 9 with screws, for example, or by any method which provides a good heat conducting joint.

It will be seen that the described arrangement provides good heat conducting paths from the parts of each of the circuit packages 2, at which the heat is generated, to the box containing the circuit assembly. Thus the heat generated by the circuit packages 2 is dissipated into the atmosphere by the surfaces of the box. In addition the heat conducting structure 5 serves as a support bracket which secures the circuit board 1, through the circuit packages 2, to the box.

A method of manufacturing the circuit assembly will now be described. The circuit board 1 is first fitted into an assembly jig with the side to which the circuit packages 2 are to be attached uppermost. The jig is arranged to provide support under the site of each circuit package and also clearance for component leads which pass through the circuit board. A thin removable spacer is positioned on each circuit package site and the heat conducting structure 5 is aligned and placed so that the cross members 7 rest on the spacers. A small amount of elastomeric adhesive is spread evenly over the top surface of each cross member 7 and the circuit packages 2 are positioned so that they straddle the cross members 7 with their connection pins 3 entered into the through hole mountings 4.Care must be taken to ensure that the circuit package connection pins 3 and the circuit board 1 are not contaminated by the adhesive. Pressure is applied individually to each circuit package 2 by weights or spring loaded plungers to ensure that each package is pressed down on to its associated cross member. The pressure is maintained until the adhesive has cured and, after removal of the pressure means, all other components are fitted to the circuit board.

After this operation the circuit board is removed from the assembly jig and placed in a flow solder jig carrier where the circuit packages and other components are soldered in position. Finally, the spacers are removed from beneath the cross members and the assembly is cleaned and secured to the box lid.

It should be noted that the circuit board is supported solely by the circuit package connection pins and is not attached to any other part of the structure. Thus, the described assembly is free from stresses induced by tolerance clashes which could exist if the circuit board and the heat sink were each mounted independently. The remaining stresses, due to differential thermal expansion between the circuit board and the heat sink, are minimal due to the flexibility of the package pins and the elastomeric adhesive.

Tests performed on a dual-in-line circuit package mounted on a circuit board inside a diecast alloy box without the heat conducting structure showed that the temperature at the junction of the integrated circuit chip was 107CC when the circuit was dissipating 1 watt. The thermal resistance under these conditions was calculated to be 820C/watt. With the heat conducting arrangement, however, the temperature of the chip junction was only 500C when 1 watt was being dissipated and the thermal resistance was calculated to be 29.5 C/watt. It was also shown that with the heat conducting arrangement the circuit could dissipate 3 watts before the temperature of the chip junction reached 1000C.

It will be realised, although the described arrangement shows only one circuit package bonded to each cross member of the heat conducting structure, that by dimensioning the cross members accordingly, more than one circuit package could be bonded to each cross member, for example.

It will also be realised that it is not an essential requirement of the invention that the circuit packages and circuit board are mounted inside a metal box. For example, the heat conducting structure could be mounted on to any suitable heat dissipating member while still retaining the advantages of a stress free efficiently cooled structure.

Furthermore, although the heat conducting structure described is of a ladder-like configuration comprising a pair of side members with cross members extending therebetween, it will be understood, for example, that a structure having only one side member to form a comb-like configuration will also be within the scope of the invention.

It will also be understood that, the heating conducting structure may be formed with the side member or members and the cross members lying in the same plane and not in different planes as described.

Claims (7)

1. An electronic circuit assembly including; a circuit board; a plurality of circuit packages mounted on the circuit board; a heat dissipating support member; and a heat conducting structure comprising at least one side member secured to the heat dissipating support member and having a plurality of cross members extending therefrom, each cross member being formed to provide a flat surface to which at least one of the circuit packages is bonded, the heat conducting structure being effective to conduct heat from the circuit packages to the heat dissipating support member and to provide the sole means of support for the circuit packages and the circuit board.

2. An electronic circuit assembly as claimed in Claim 1, in which the heat conducting structure includes a pair of spaced apart parallel side members with the cross members extending therebetween.

3. An electronic circuit assembly as claimed in Claim 1 or 2, in which the side member or members lie in a first plane and the cross members lie in a second plane spaced from the first plane.

4. An electronic circuit assembly as claimed in Claim 1, 2 or 3, in which the heat dissipating support member includes a wall of a metal box structure inside which the remainder of the circuit assembly is contained.

5. An electronic circuit assembly as claimed in any preceding claim, in which the circuit packages are dual-in-line circuit packages and the cross members of the heat conducting structure extend between the circuit packages and the circuit board.

6. An electronic circuit assembly as claimed in Claim 5 in which the cross members of the heat conducting structure are arranged so as to provide an air space between the circuit board and the cross members.

7. An electronic circuit assembly constructed as hereinbefore described with reference to the accompanying drawing.