Securing heat sinks to electronic components

Details Securing heat sinks to electronic components Securing heat sinks to electronic components

2001-05-18

2002-10-08

Thompson, Gregory (Department: 2835)

Electricity: electrical systems and devices

Housing or mounting assemblies with diverse electrical...

For electronic systems and devices

C165S185000, C165S080300, C174S016300, C257S718000, C361S710000

Reexamination Certificate

active

06462951

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the securing of heat sinks to electronic components.
BACKGROUND OF THE INVENTION
In the electronics industry, printed circuit boards form parts of electronic circuits which include electronic components carried by the boards. These components generate heat during usage. Some components, e.g. integrated circuits, require heat to be removed from them to permit operation within acceptable operating ranges of temperature, i.e. temperatures not sufficiently high to render the components non-functional. In one method of removing heat, heat sinks are used. In use, a heat sink needs to be held in heat conductive relationship with a corresponding electronic component and a heat emitting projection or projections of the heat sink are located in a cooling air flow path for removal of the heat generated by the component.
Various types of design structures are employed for securing heat sinks with bases of the heat sinks in positions in heat conductive relationship with their electronic components. In a first conventional design structure for securing a heat sink in position, a clip having a leaf spring portion is positioned with the leaf spring portion extending across the base of the heat sink. Arms at opposite ends of the leaf spring portion extend down opposite edges of the base and around a substrate to one side of which the electronic component, in the form of an integrated circuit, is attached. A screw extends in screwthreaded engagement through the leaf spring portion of the clip substantially midway between its ends. The screw is tightened against the base of the heat sink to cause the leaf spring portion to flex away from the base thereby drawing the arms inwards towards the substrate. This action causes inwardly directed free ends of the arms to engage the side of the substrate remote from the electronic component. Increased screw tightening then results in tightly holding the substrate, the electronic component and the base of the heat sink between the ends of the arms and the screw.
A problem which exists in use of the above design structure is that the screw needs to have a certain minimal length for tightening and deforming the leaf spring portion of the clip which is flexed outwards from the base of the heat sink during the tightening operation. While the heat sink itself may be sufficiently small to fit comfortably together with the substrate and integrated circuit within narrow gaps (e.g. 9 mm to 10 mm), such as provided between adjacent printed circuit boards, the minimum length of screw prevents the screw from being accommodated in such a narrow gap. This clip structure also has the disadvantage that the leaf spring portion requires space to be positioned across the base, which space could more advantageously be occupied by a heat emitting projection or projections of the heat sink. As a result, optimal heat removal and dispersion cannot be achieved. Further, another reason for detracting from optimizing heat removal and dispersion is that the leaf spring portion of the clip is arched, in its tightened position, away from the heat sink base and extends along its length across cooling air flow passages between the heat emitting projections thereby effectively reducing flow passage area and also providing a resistance to the flow of cooling air.
In addition to this, the clip is formed from an electrically conductive material, i.e. a metal. In cases where the substrate has terminals or conductors on its side remote from the electronic component (particularly in the case of a ball grid array structure on the substrate), then the ends of the clip arms in engaging the remote side of the substrate are in danger of approaching too close to the terminals or conductors. Thus, shorting of the circuitry is a possibility.
Also, the screw incorporated in the above structure offers certain weaknesses in that a specific torque is required to tighten the clip to a desired degree to provide a required compressive contact between the base of the heat sink and the electronic component. Special torque applying tools may be available for this purpose. However, actual torque applied may be at the whim of the assembler who may not, in some cases, use a torque applying tool. It is also possible that a torque applying tool is incorrectly adjusted. If insufficient torque is provided during assembly, repeated temperature cycling in use, attended by shrinkage and expansion of materials, may result in loosening of the screw and slackening in compressive contact of the heat sink base with the electronic component. Vibration may also assist in loosening of the screw. In any event, loosening of the screw results in reduction in heat removal from the electronic component and could also result in complete detachment of the clip and heat sink from the component. Alternatively, the application of too much torque during assembly could increase strain on components, particularly during temperature cycling, and may result in cracking or breaking apart of the component, substrate, or heat sink.
In a second conventional design structure, a heat sink has a screwthreaded cylindrical metal base which is screwthreadedly attached through a hole in a plastic clip as a replacement for the screw and clip design discussed above. In this second structure, one end of the base is in heat conductive contact with its corresponding heat generating electronic component. At the other end of the base, the heat sink has a heat emitting projection in the form of a radially extending heat emitting flange. This latter design is shorter than the screw and clip of the first conventional design structure discussed above and may be suitably located in narrow gaps such as those referred to above. However, the heat emitting flange needs a specific dimension radially of the screwthreaded base in order to transfer the required heat from the electronic component into the cooling air flow. Unfortunately, this results in the flange overshadowing and extending beyond the boundaries of an electronic component for which it is suitably designed. Hence, when in use, for instance upon a printed circuit board, the flange also overshadows areas of the board which could otherwise be used for circuitry or attachment of circuitry components. Thus, freedom for circuitry design becomes limited.
In addition, the screwthreaded base of the heat sink is substantially large. This results in the length of the plastic clip on either side of the base being short and limited in flexibility. This may prevent the clip from being flexed to allow the arms of the clip to move apart sufficiently to pass down opposite edges of the heat sink and substrate to allow for engagement of the inwardly extending free ends of the arms with the remote side of the substrate for holding purposes. In this case the clip needs to be slid laterally onto the substrate during assembly. To achieve this in practice, i.e. with the component and its mounting substrate upon a printed circuit board, the board needs to be free of circuitry elements adjacent an edge of the substrate which first receives the clip. This provides a further limitation to freedom for circuitry design upon the printed circuit board. Also, because of the need for lateral application of the clip, positioning of the clip upon the electronic component relies entirely upon the judgment of the assembler. If the clip, and thus the heat sink, are not correctly positioned, this could reduce heat removal from the component.
In addition, while the latter heat sink and clip design may be located within minimally narrow gaps, the heat emitting flange may not provide sufficient distance from an adjacent article (e.g. printed circuit board) to ensure an adequate cooling air flow across the flange for the required cooling purposes. Further to this, the clip of this second conventional design structure provides cooling air flow passage obstruction problems and potential screw loosening problems similar to those discussed above with regard to the first conventional design structure. In the

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