Heat dissipation in electrical apparatus

Abstract: The invention relates to the provision of a cooling effect on components (6) of electrical apparatus which are mounted on an upper surface (8) of a printed circuit board (4) within a housing (2). The invention provides for the mounting of a material layer (12) which can absorb or otherwise dissipate heat and the invention allows the mounting of the said material (12) beneath the printed circuit board (4) within the housing (2) in position to provide improved cooling effect without contacting the components (6) themselves.

Description: The invention which is the subject of this application relates to a device and apparatus for use in the dissipation of heat which is generated by components within electrical apparatus when the apparatus is in operation. The generation of heat by components such as integrated circuits when electrical apparatus is in operation is a well known problem and as a result many forms of apparatus include fans or other powered devices which are operated when the electrical apparatus is powered, to attempt to cool the components within the electrical apparatus housing. When one considers that it is. believed that the life span of the components can be doubled for every 10 degree Celsius drop in operating temperature which can be achieved it will be appreciated that the temperatures in which the components operate is an important factor in product reliability. However a problem with the use of powered fans and the like to cool the air around the components is that they generate a noise which in some apparatus such as for example, broadcast data receivers for use with televisions, is unacceptable as it affects the use of the apparatus. The use of thermally conductive material is also well known wherein said material is applied to the inside surface of the housing in an attempt to provide some reduction in temperature by using the material to fill in air gaps in the housing but this material is expensive and can add considerably to the cost of the apparatus and is a result use of this form of material is still somewhat limited. Another use of thermally conductive material is to inject the same in a liquid form into a gap between a heat sink arrangement an the component to be cooled. This method is disclosed in W096/23397 which illustrates the heat sink arrangement positioned on the same side of the Printed Circuit Board as the said component or on the opposing side. However in each case the thermally conductive material is required to fill the gap with the component such that when positioned on the opposing side apertures have to be provided in the PCB and steps taken to ensure that the injected thermally conductive material reaches the component. The aim of the present invention is to allow the use of thermally conductive material to provide an effective cooling effect on components within a housing of electrical apparatus and to do so in an economical manner which minimises the amount of the material which is required to be incorporated while maximising the effect of the same. In a first aspect of the invention there is provided electrical apparatus, said apparatus comprising a housing of sheet material and at least one electrical or electronic component contained in said housing, said component mounted on a first side of a printed circuit board and powered by an electricity supply which when operational and, when powered, generates heat, said apparatus including a pad or block of thermally conductive material mounted thereon to dissipate the heat generated, the thermally conductive material is mounted on at least one raised platform to position the thermally conductive material in contact with or adjacent the opposing side of the printed circuit board from the component and located with respect to the component to reduce heat with respect to the said component, characterised in that the at least one raised platform is frustoconical in shape and has a plurality of slots formed at spaced intervals. Preferably the component is on an upper surface of the printed circuit board and the thermally conductive material is on a lower surface. Further preferably the thermally conductive material does not contact the component itself. More typically the apparatus will comprise a series of components and if one thermally conductive material position is provided, that position will be determined with respect to design, heat generation and/or efficiency considerations. Alternatively, a number of locations can be provided, again, said location determined with reference to the area in which the heat is generated and/or the component locations. In one preferred embodiment the platform is formed so as to protrude upwardly from the surface of the housing which in normal use forms the base of the electrical apparatus and this allows the material to be positioned under the component or components at a required distance from the same. In one embodiment the sheet material of the housing is sheet metal. According to the invention as defined in claim 1 the platform which is formed is frustoconical in shape and has a series of slots formed at spaced intervals, said slots allowing two advantages, firstly the reduction in stresses on the sheet material caused by the formation of the platform and secondly the provision of air flow from externally of the housing into the same to improve the cooling effect. Preferably solder resist material normally coated on a part or whole of the electrical apparatus and/or at least one electrical or electronic component is removed and/or not applied in the area of the thermally conductive material, such that solder is picked up in the area during the solder wave process. The extra solder on or surrounding the thermally conductive material increases the thermal conductivity and improves the Electro Magnetic Compatibility (EMC) performance in high current/ high speed regions of the electrical apparatus and/or components. In an alternative embodiment, if the sheet material is a moulded plastic material then the platform can be formed as part of the mould process. Preferably the thermally conductive material in the pad is in a gel form. In a further aspect of the invention there is provided a method of providing a cooling effect on a component or components mounted on a first side of a printed circuit board within a housing for electrical apparatus, said housing formed from sheet material, said method including the steps of identifying from tests the heat generated by the components when in operation, identifying the hottest areas within the housing and forming in the housing at least one raised platform with a thermally conductive pad or block mounted on the platform, said platform of a size so that the thermally conductive material is located at or adjacent the side of the printed circuit board opposing the side on which the component is mounted, and locating the thermally conductive material at or adjacent to the identified hottest area so as to maximise the cooling effect of the material but without contacting the component or components, characterised in that the at least one raised platform is frustoconical in shape and has a plurality of slots formed at spaced intervals. The provision of the platform allows the use of a minimum of thermally conductive material, provided in a sheet form, to maximum effect as the material can be positioned at the hottest areas in the housing rather than either being placed on the conventional housing inner surface which is removed from the hottest areas or alternatively the sheet material being provided in a relatively thick block to enable the same to reach the hottest areas which is prohibitively expensive. Specific embodiments of the invention are now described with reference to the accompanying drawings; wherein Figures 1 - 3 illustrate perspective views of the apparatus according to one embodiment; and Figure 4 illustrates an elevation of apparatus in accordance with the embodiment of the invention. Referring to the Figures there is illustrated one embodiment of the invention wherein there is provided electrical apparatus which comprises a housing 2 and part of the base of which is shown although it should be appreciated that in practice the housing has a base, a top surface, side walls and front and rear walls. Enclosed within the housing there is provided a printed circuit board (PCB) 4 on which there are typically provided a series of components which are interconnected to perform the functions of the apparatus but for illustrative purposes only one component 6 in the form of an integrated circuit component is shown. The component is mounted on the top side 8 of the board. In contact with or adjacent to the underside 10 of the board there is located a layer of thermally conductive material 12 which is mounted on a platform 14 to raise the thermally conductive material to a designated height and position with respect to the component 6. The platform 14 is, in this embodiment formed as part of the housing base and is formed in a frustoconical shape as shown with a series of slots 16 at spaced intervals around the wall of the same and typically is stamped or pressed as the housing is shaped and formed prior to the assembly with the PCB to form the electrical apparatus. In use, in the design of the apparatus, analysis is undertaken to assess the position of the components within the housing and, in operation, the location of the hottest area or "hot spots" within the apparatus. When the hot spots are identified the platform or platforms can be formed so as to position the thermally conductive material at or adjacent to the hot spots underneath the printed circuit board. In one example, if a hot spot lies 10mm above the housing base then the platform 14 is raised to a height of 7mm which leaves a 3mm gap which is filled by the 4mm thick thermally conductive material 12 in the form of a gel pad. This pad which is under slight compression acts as a thermal interface between the printed circuit board and the housing. In one test undertaken the reduction in temperature achieved was found to be 23 degrees Celsius. Thus, the thermal impedance of the heat path of the PCB is significantly reduced by providing a gel pad and metal dome heatsink. The gel pad does not contact the component directly and only comes into contact with a ground plane of the underside of the PCB. The conductive gel pad is typically of a ball grid array type. Thermal conductivity/contact between the PCB and the thermally conductive gel pad can be increased by removing solder resist material from the area of the bottom of the PCB adjacent and/or surrounding the pad. During the solder wave process in assembly of the PCB, the area of the PCB not having solder resist picks up solder, thereby reducing the thermal impedance of the heat path in the PCB. This in turn increases the reliability and performance of the PCB and electrical apparatus. For example, in some large integrated circuits, such as ST 5512, a number of power and ground pins and employed in the centre of the circuit silicon chip to provide a thermal conductive path from the centre of the chip to reduce junction temperature. Removal of solder resist in this area to allow solder to be picked up during the solder wave process results in low thermal resistance from the PCB to the gel pad, both by eliminating the low but finite thermal resistance of i.e., the thin layer of green epoxy resist material, and by providing better contact of the gel pad to the thermal vias and surrounding plane of the PCB. Conduction of heat from the main ground plane of the PCB will also benefit heat dissipation and reduce electrical noise. Trailing edges of the components of the PCB can be tapered to also assist in controlled solder thickness on the PCB, thereby avoiding "blobbing" of the solder. Although described with reference to the thermally conductive material placed underneath the PCB the invention is equally applicable if placed to the top of the printed circuit board to allow the same to be cooled at the top of the component so that heat is dissipated through the upper surface of the component. Thus the present invention provides a means of reducing the internal temperature of an integrated circuit by transferring heat via the gel pad and dissipating the heat into the. housing of the apparatus surrounding the integrated circuit.

Claims: Electrical apparatus, said apparatus comprising a housing (2) of sheet material and at least one electrical or electronic component (6) contained in said housing (2), said component mounted on a first side of a printed circuit board (4) and powered by an electricity supply which when operational and, when powered, generates heat, said apparatus including a pad or block of thermally conductive material (12) mounted thereon to dissipate the heat generated, the thermally conductive material (12) is mounted on at least one raised platform (14) to position the thermally conductive material (12) in contact with or adjacent the opposing side of the printed circuit board (4) from the component and located with respect to the component to reduce heat with respect to the said component (6), characterised in that the at least one raised platform (14) is frustoconical in shape and has a plurality of slots (16) formed at spaced intervals. Electrical apparatus according to claim 1 characterised in that the apparatus comprises a series of components and if one thermally conductive material position is provided, that position is determined with respect to design, heat generation and/or efficiency considerations. Electrical apparatus according to claim 1 characterised in that a number of locations are provided, said locations determined with reference to the areas in which heat is generated and/or the component locations. Electrical apparatus according to claim 4 characterised in that the platform (14) is formed so as to protrude upwardly from the surface of the housing (2) which forms the base of the electrical apparatus. Electrical apparatus according to claim 1 characterised in that solder resist material is not applied to and/or is removed from the printed circuit board at the location of the thermally conductive material (12). Electrical apparatus according to claim 1 characterised in that the thermally conductive material (12) includes a gel pad. A method of providing a cooling effect on a component or components (6) mounted on a first side (8) of a printed circuit board (4) within a housing (2) for electrical apparatus, said housing formed from sheet material, said method including the steps of identifying from tests the heat generated by the components (6) when in operation, identifying the hottest areas within the housing (2) and forming in the housing (2) at least one raised platform (14) with a thermally conductive pad or block (12) mounted on the platform said platform (14) of a size so that the thermally conductive material (12) is located at or adjacent the side (10) of the printed circuit board (4) opposing the side on which the component is mounted, and locating the thermally conductive material at or adjacent to the identified hottest area so as to maximise the cooling effect of the material (12) but without contacting the component or components (6), characterised in that the at least one raised platform (14) is frustoconical in shape and has a plurality of slots (16) formed at spaced intervals.

Inventors: BRISTOW ANTHONY, CROSS STEPHEN, FULLER COLIN, STURGE DAVID, WOODHOUSE DAVID, BRISTOW, ANTHONY, CROSS, STEPHEN, FULLER, COLIN, STURGE, DAVID, WOODHOUSE, DAVID
Applicants: PACE MICRO TECH PLC, PACE MICRO TECHNOLOGY PLC
Application year: 2000
Main classification: H05K7
IPC codes: H05K7, H01L23