GB2471497A

GB2471497A - Double sided multi-layer metal substrate PCB with SMD components mounted to top traces and lead wire components mounted to opposite side for heat dissipation

Info

Publication number: GB2471497A
Application number: GB0911403A
Authority: GB
Inventors: Martin Coates
Original assignee: TDK Lambda UK Ltd
Current assignee: TDK Lambda UK Ltd
Priority date: 2009-07-01
Filing date: 2009-07-01
Publication date: 2011-01-05
Anticipated expiration: 2029-07-01
Also published as: GB0911403D0; GB2471497B

Abstract

A heat sink arrangement for electronic components mounted on both sides of a double sided multi layered PCB, such as an insulated metal substrate (IMS) board (fig 3) or directly bonded copper substrate (DBC) board. A preferred embodiment has on a first side of the IMS board, surface mount components 11 arranged contacting copper tracks 4 formed on a insulating layer 3 on an metal substrate 2. One or more wire lead components 5, such as a SO220 package (figs 2a,2b) is mounted on to the second opposite side of the board via an adhesive pad 17 having adhesive on both sides 19, 20. The metal tab 8 of the so220 package 5 is in thermal contact via the pad 17 with the metal substrate 2. (figure 4 showing how heat spreads into this substrate from the components) The leads 7 from the component 5 preferably extend away from the PCB; connector leads 25 may extend away from the traces and the SMD components. The leads 7 and 25 may be connected to a further PCB main board / motherboard (not shown).

Description

HEAT SINK

[001] The present invention relates to a heat sink arrangement for electronic components, and more particularly to a heat sink arrangement for a power supply unit.

[002] Power supply units use a variety of components with high power dissipation, including both leaded and surface mount components. These include snubber resistors, capacitors and diodes, rectifier diodes, rectifier FETs (Field Effect Transistors), switching FETs, power transistors and linear regulators.

[003] Leaded components are mounted on a printed circuit board (PCB) using wire leads, which are inserted into through holes pre-formed in the PCB and soldered in place. Surface mount components, on the other hand, are mounted directly onto the surface of a PCB, without the need for through holes. The different configuration of surface mount and leaded components means that they require different cooling arrangements.

[004] Surface mount components can be effectively cooled by mounting them on certain types of PCB, which incorporate a layer that acts as a heat sink. For example, an insulated metal substrate (IMS) . As illustrated in figure 1, an IMS 1 comprises a metal base plate 2 formed of copper or aluminium, covered by a thin electrically insulating layer 3 and a top layer of copper 4 which forms the electrical interconnections between the components mounted thereon. In use, heat generated by the components is conducted through the top layer of copper 4 and the ceramic layer 3, and dissipated by the base plate 2.

[005] However, PCBs of this type are not suitable for use with leaded components, due to the electrically conductive metal base plate, and because only a relatively small area on the surface of the component comes into thermal contact with the board. Accordingly, where cooling of leaded components is required, these are bolted to a separate metal sheet, which acts as a heat sink.

[0061 Front and rear views of a 10-220 package for a leaded component 5 are shown in figures 2a and 2b. The 10-220 package comprises a body 6 from which a number of leads 7 (typically 3) extend. The package is provided with a metal tab 8 with a hole 9 formed therein, which is used for mounting the component to a metal sheet with a nut and bolt.

As the metal tab is often electrically connected to the internal circuitry of the component, an electrically insulating washer may be provided between the metal tab and the heat sink, in order to electrically isolate the component from the heat sink.

[007] There is a commercial requirement for smaller power supply units, motivated by the trend towards miniaturization of electronic equipment in general. The concept of power density is widely referred to in this context. Power density is defined as the amount of output power per unit volume of product, and is measured in Watts/inch3 or Watts/cm3. It is commercially desirable to reduce the volume of the device as far as possible without reducing the output power, so as to maximise the power density of the device.

[0081 However, the heat generated by the various electronic components which make up the power supply must be effectively dissipated to prevent the power supply unit overheating. The need for effective thermal management of these components is critical, and places limits on the extent to which the volume of the device can be minimised. In particular, it will be appreciated that the device must be large enough to accommodate heat sink arrangements with a sufficiently large surface area for both the surface mount and the leaded components.

[0091 In principle, the need for separate heat sink arrangements could be avoided by sourcing components which are either all housed in surface mount packages, or all housed in leaded packages. However, whilst surface mount components generally take up less space than equivalent leaded components, using leaded components can be more economical in terms of space and cost in cases where the component is required to accommodate high currents flowing into and/or out of the device. In this respect, the leads of a standard leaded component package can accommodate high current levels, whereas the interconnecting pins used to connect the circuitry on an IMS to a motherboard have lower current ratings. Consequently, a large number of pins (which occupy additional space) or a high current connector (which is expensive) would be required to accommodate high current levels. Special surface mount packages (eg, I2PAK) are available. These are designed to be soldered to the copper tracks on an IMS as with normal surface mount components, but have additional leads which extend from the body of the package to carry higher currents to and from the motherboard.

In practice, however, such packages are rare, and thus expensive. Consequently, a combination of both surface mount and leaded components are generally required.

[0010] According to one aspect of the present invention, there is provided a power supply unit which comprises:-one or more surface mount components; one or more leaded components; and a circuit board comprising a first layer in which one or more electric circuit interconnections are formed, and one or more second layers for dissipating heat generated by said components; wherein the one or more surface mount components are mounted to a first surface of the circuit board, formed by the first layer thereof, and the one or more leaded components are mounted to a second surface of the circuit board, opposite the first surface thereof.

[0011] Thus, the surface mount and leaded components are respectively mounted to the outer major surfaces of the circuit board. With this configuration, the second layer(s) of the circuit board act as a heat sink for both the surface mount component(s) and the leaded component(s), which avoids the requirement for separate heat sink arrangements for the two different types of component. Moreover, by utilising an area of the board that does not have electric circuit interconnections (metal tracks) formed thereon, and is thus not populated by surface mount components, the leaded components occupy space that would have been empty in a conventional power supply. This allows the overall volume of the power supply to be reduced, and thus allows greater power densities to be achieved.

[0012] The second surface of the circuit board may be a surface of one of the one or more second layers. The second surface of the circuit board may be a substantially planar surface.

[0013] The ability of the second layer(s) to dissipate heat generated by the mounted components depends on the thermal conductivity of the material from which they are formed. The material from which the or each of the one or more second layers is formed preferably has a thermal conductivity of approximately 1OW/m-K or higher.

[0014] The one or more second layers preferably include one or more metal layers. For example, a layer formed of aluminium, which has a thermal conductivity of approximately 250 W/m-K.

[0015] Alternatively, or in addition, the one or more second layers may include one or more ceramic layers. For example, a layer formed of alumina (AL2-03), which has a thermal conductivity of approximately 18 W/m-K.

[0016] Where the one or more second layers include one or more metal layers, these metal layer(s) may be electrically isolated from the first layer of the circuit board by an electrically insulating layer. For example, a thin layer of epoxy resin filled with ceramic filler.

[0017] The term "circuit board" as used herein, refers generally to a layered structure or substrate on which electronic components may be mounted.

[0018] For example, the circuit board may comprise an Insulated Metal Substrate (IMS) . An IMS comprises a thin electrically insulating ceramic layer between a layer of copper in which electric circuit interconnections are formed, and a metal base plate which acts as a heat sink.

[0019] Alternatively, the circuit board may comprise a Direct Bonded Copper Substrate (DBC) . A DBC comprises an electrically insulating ceramic tile with a sheet of copper bonded to a major surface thereof by a high temperature oxidation process. Electric circuit interconnections are formed in the metal sheet, either by pre-forming the sheet before it is bonded to the ceramic tile, or by chemical etching after the sheet is bonded to the tile. The ceramic tile is sufficiently thermally conductive to act as a heat sink in itself. However, additional layers may be glued, soldered or otherwise mounted or bonded to the ceramic tile, to assist in the dissipation of heat.

[0020] The leaded components are preferably mounted to the second surface of the circuit board, such that they are electrically isolated therefrom. Thus, whilst the surface mount components are electrically connected to the electrical interconnections formed in the first layer of the circuit board, as well as being in thermal contact with the board, the leaded components are only in thermal contact with the board, and are not connected to electric circuit interconnections formed on the circuit board.

[0021] In a preferred embodiment, the one or more leaded components are mounted to the second surface of the circuit board by means of an adhesive layer.

[0022] By mounting the leaded component(s) to the circuit board using an adhesive layer, rather than using nuts and bolts as is conventional for mounting a leaded component to a heat sink, the space available for mounting surface mount components on the other side of the circuit board is maximised. The dimensions of the circuit board, and thus the overall volume of the power supply unit can therefore be minimised.

[0023] The cost of the power supply unit is also reduced, as compared with conventional devices, because mechanical fixings, including nuts, bolts and insulating washers, are not required in order to mount the leaded components. The additional cost of machining holes and/or placing threaded inserts in the heat sink is also avoided. Furthermore, the assembly time of the device is reduced, due to the simplified structure.

[0024] The adhesive layer preferably comprises an electrically insulating material for electrically insulating the leaded components from the circuit board. This is particularly important where the leaded components are mounted directly to a metal surface of the circuit board, such as the metal base plate of an IMS or a metal base plate bonded to a DBC substrate, and where the external casing of the leaded components is electrically connected to its internal circuitry.

[0025] The area covered by the adhesive layer preferably encompasses the whole mounting surface of the leaded component, where the mounting surface of the leaded component is the surface thereof which is mounted to the circuit board.

This ensures that no part of the leaded components comes into electrical contact with the second surface of the circuit board. Again, this is important where the second surface of the component is a metal surface, and where the external casing of the leaded components is electrically connected to its internal circuitry.

[0026] The area covered by the adhesive layer may be large enough to encompass the mounting surface of more than one leaded component, so that more than one leaded component can be mounted to the circuit board by a single section of the adhesive layer. Alternatively, separate components may be mounted using separate sections of the adhesive layer.

[0027] The adhesive layer preferably has a thermal conductivity of approximately of 0.5 W/m-K or higher, and preferably has a thickness of 0.3mm or less (more preferably 0.127mm) . This ensures good thermal coupling between the leaded component(s) and the circuit board.

[0028] In preferred embodiments of the present invention, the adhesive layer may comprise two layers of an adhesive material, separated by a central layer. For example, a central, non-adhesive layer with an adhesive coating on both sides. Where the adhesive layer comprises a central layer, this is preferably formed of a material which is electrically insulating and/or thermally conductive.

[0029] For ease of manufacture, the adhesive layer may be provided as a double-sided adhesive pad or double-sided adhesive tape. For example, Bond-Ply � 100, which is sufficiently thermally conductive to transfer heat efficiently from the leaded component to the circuit board, and uses acrylic adhesive, which is able to withstand the high temperatures generated by the components.

[0030] In other embodiments, the adhesive layer may consist of a single layer of an adhesive material. In this case, the adhesive material itself is preferably electrically insulating and/or thermally conductive.

[0031] The leaded component(s) are preferably mounted to the circuit board by a body portion thereof, more preferably a planar surface of the body portion. This is desirably a major surface of the body portion, ie, a larger planar surface of the body portion, preferably the largest planar surface thereof.

[0032] In this way, a large proportion of the surface area of the leaded component is in thermal contact with the second layer(s) of the circuit board, such that heat is transferred efficiently from the component to said second layer(s) [0033] The leaded component(s) may be positioned on the circuit board such that their leads project beyond an edge of the circuit board. This allows the leads to be mounted directly to a motherboard.

[0034] The components mounted to the circuit board may include components which have a power loss density of approximately 20mW/mm2 or higher. Such components are generally required to be mounted to a heat sink, in order to avoid overheating.

[0035] The one or more surface mount components mounted to the first surface of the circuit board may include any or all of:-snubber resistors, capacitors and diodes; rectifier diodes, rectifier FETs, switching FET5, power transistors and linear regulators.

[0036] The one or more leaded components mounted to the first surface of the circuit board may include any or all of:-rectifier diodes; rectifier FET5; switching FET5; power transistors and linear regulators.

[0037] According to a second aspect of the present invention, there is provided a method of manufacturing a power supply unit, the method comprising:-providing one or more surface mount components and one or more leaded components; providing a circuit board having a first layer in which electric circuit interconnections are formed, and one or more second layers for dissipating heat generated by said components; mounting the one or more surface mount components to a first surface of the circuit board formed by the first layer thereof; and mounting one or more leaded components to a second surface of the circuit board, opposite the first surface thereof.

-10 - [00381 The method preferably further comprises applying an adhesive layer to the second surface of the circuit board and mounting the one or more leaded components to the circuit board by means of the adhesive layer.

[00391 The power supply unit may be a switched mode power supply unit.

[0040] An embodiment of the present invention will now be described with reference to the accompanying drawings in which:-Figure 1 shows an insulated metal substrate (IMS); Figures 2a and 2b show front and rear views of a TO-220 electronic component package; Figure 3 shows, in exploded view, a heat sink arrangement for use in a switched mode power supply in accordance with an embodiment of the present invention; Figure 4 illustrates the dissipation of heat by the metal base layer of an IMS.

[0041] Figure 3 shows, in exploded view, a heat sink arrangement 10 for use in a switched mode power supply unit.

[0042] The heat sink arrangement 10 of figure 3 comprises an IMS 1, similar to that shown in figure 1. The IMS comprises an aluminium base plate 2, an electrically insulating layer 3, and a layer of copper 4 which is chemically etched to form copper tracks, ie, the interconnections of an electric circuit.

[0043] The insulating layer 3 is formed of epoxy resin filled with a ceramic filler, which has a thermal conductivity of approximately 2 W/m-K. The aluminium base plate 2 has a relatively high thermal conductivity of approximately 250 W/m-K.

-11 - [0044] A plurality of surface mount components 11 are mounted on the copper layer 4 in a manner that would be readily apparent to a person skilled in the art. Thus, a first surface 12 of the IMS 1 is populated by the surface mount components.

[0045] The heat sink arrangement 10 of figure 3 further comprises a leaded component 5 in a TO-220 package, similar to that shown in figures 2a and 2b. The leaded component comprises a metal tab 8, which is inset into the body 6 of the component to lie flush with a first (rear) surface 14 thereof, and extends outwardly from a second (top) surface of the body. The leads 7 of the leaded component extend outwardly from a third (bottom) surface 16 of the body, opposite to said second surface.

[0046] The leaded component 5 is mounted to a second surface 13 of the IMS 1, opposite to the surface 12 populated by the surface mount components 11, by means of a double-sided adhesive pad 17.

[0047] The adhesive pad 17 is cut from a sheet of or roll of 0.127mm thick Bond-Ply � 100, supplied by The Bergquist Company (www.bergquist.com) . Bond-Ply � 100 is a thermally conductive, fibreglass reinforced, double sided, pressure sensitive adhesive tape, which uses acrylic adhesive. The tape has a thermal conductivity of approximately 0.8 W/m-K and a dielectric breakdown voltage of approximately 3000.

[0048] The adhesive pad 17 comprises a layer 18 of a fibreglass reinforced material, which is electrically insulating and thermally conductive. Both sides of layer 18 are coated with a layer of a heat resistant acrylic adhesive on both sides to form first and second adhesive layers 19, -12 -20.

[0049] A first surface 21 of the adhesive pad 17 is adhered to surface 13 of the IMS 1 by means of the first adhesive layer 19.

[0050] The adhesive pad 17 covers an area of the IMS 1 which is at least as large as the rear surface 14 of the leaded component 5.

[0051] The rear surface 14 of the leaded component 5, which is formed in part by the metal tab 8, is adhered to a second surface 22 of the adhesive pad 17 by means of second adhesive layer 20.

[0052] The leaded component 5 is positioned on the second side 13 of the IMS 1 such that leads 7 extend beyond an edge 23 of the IMS, for mounting in a motherboard (not shown) Similarly, a surface mount connector 24, which comprises connecting leads 25 is mounted on the first side 12 of the IMS, such that leads 25 extend beyond edge 23, for mounting in the motherboard.

[0053] In use, heat generated by the surface mount components 11 is conducted through the copper layer 4 and the electrically insulating layer 3, and spread by the aluminium layer 2, as indicated by solid line arrows in figure 4.

[0054] Although layer 3 has a relatively low thermal conductivity, it is sufficiently thermally conductive, and sufficiently thin, that heat is effectively conducted through this layer to the aluminium base plate 2. Due to the relatively high thermal conductivity of the aluminium layer 2, the heat is spread outwards from the area directly beneath the component.

-13 - [00551 The heat is then radiated from a relatively large area of the exposed surface of the aluminium plate, as indicated by dashed line arrows in figure 4.

[00561 Similarly, heat generated by the leaded component 5 is conducted through the thermally conductive adhesive pad 17 and spread by the aluminium layer 2, as indicated by solid line arrows in figure 4.

[0057] Although the adhesive pad has a relatively low thermal conductivity, it is sufficiently thermally conductive, and sufficiently thin, that heat is effectively conducted through the pad to the aluminium base plate 2.

[00581 The heat is then radiated from a relatively large area of the exposed surface of the aluminium plate, as indicated by dashed line arrows in figure 4.

[00591 The aluminium base plate 2 of the IMS 1 thus acts as a heat sink for the leaded component 5 as well as the surface mount components 11.

[00601 To manufacture a power supply unit in accordance with an embodiment of the present invention, the copper layer 4 of an IMS 1 is chemically etched to form a predetermined pattern of electrical interconnections. A plurality of surface mount components 11 are then mounted at appropriate positions on this first surface 12 of the IMS. The plurality of surface mount components include a surface mount connector 24 which is positioned close to an edge 23 of the IMS, such that its leads 24 project beyond edge 23 of the IMS.

[0061] An adhesive pad 17 of a size suitable for covering -14 -the whole of a rear surface 14 of a leaded component 5 is selected, and applied to a second surface 13 of the IMS 1.

The adhesive pad is positioned on the IMS such that one edge thereof is aligned close to (within 0-5mm of) an edge 23 of the IMS.

[0062] The rear surface 14 of the leaded component 5 is then applied to a second surface of the adhesive pad 17, and positioned such that the whole of the rear surface is separated from the IMS 1 by the adhesive pad, and such that leads 7 project beyond edge 23 of the IMS.

[0063] Alternatively, the adhesive pad may be first applied to the rear surface 14 of the leaded component 5, and subsequently applied to the IMS 1.

[0064] In either case, the resulting assembly 10 is then connected by means of leads 7 and 25 to a motherboard assembly, in a similar manner to conventional heat sink arrangements.

[0065] The present invention has been described in terms of an embodiment wherein the leaded components are mounted to the IMS by means of an adhesive layer. However, it will be appreciated that other means of attachment are possible. For example, clips or other mechanical retaining means may be provided for holding the leaded components to the second surface of the IMS. In these embodiments, an electrically insulating layer may be provided between the metal base plate of the IMS and the leaded components.

[0066] The present invention has been described in terms of an embodiment wherein the circuit board to which the components are mounted is an IMS. However, other types of circuit board may be used. For example, a DBC or other -15 -metal bonded substrate. In particular, although the heat dissipating layer 2 described above is a metal layer, the heat dissipating layer may be formed of a different material, such as alumina (AL2-03) . This has a thermal conductivity of approximately 18 W/m-K, and is suitable for spreading and radiating (ie dissipating) heat in the manner illustrated in figure 4 in relation to the aluminium base plate.

[0067] Moreover, there may be more than one heat dissipating layers which act together to dissipate heat generated by the components. For example, a metal layer bonded to a ceramic layer.

Claims

-16 -CLAIMS1. A power supply unit which comprises:-one or more surface mount components; one or more leaded components; and a circuit board comprising a first layer in which one or more electric circuit interconnections are formed, and one or more second layers for dissipating heat generated by said components; wherein the one or more surface mount components are mounted to a first surface of the circuit board, formed by the first layer thereof, and the one or more leaded components are mounted to a second surface of the circuit board, opposite the first surface thereof.
2. A power supply unit according to claim 1 wherein the second surface of the circuit board is a surface of one of the one or more second layers.
3. A power supply unit according to claim 1 or 2 wherein the material from which the or each of the one or more second layers is formed has a thermal conductivity of approximately 1OW/m-K or higher.
4. A power supply unit according to any preceding claim wherein the one or more second layers include one or more metal layers.
5. A power supply unit according to claim 4 wherein said metal layer(s) are electrically isolated from the first layer of the circuit board by an electrically insulating layer.
6. A power supply unit according to any preceding claim wherein the one or more second layers include one or more ceramic layers.-17 -
7. A power supply unit according to any preceding claim wherein the circuit board comprise an Insulated Metal Substrate (IMS)
8. A power supply unit according to any one of claims 1 to 6 wherein circuit board comprises a Direct Bonded Copper Substrate (DBC) 9. A power supply unit according to any preceding claim wherein the leaded components are mounted to the second surface of the circuit board, such that they are electrically isolated therefrom.10. A power supply unit according to any preceding claim wherein the one or more leaded components are mounted to the second surface of the circuit board by means of an adhesive layer.11. A power supply unit according to claim 10 wherein the adhesive layer comprises an electrically insulating material for electrically isolating the leaded components from the circuit board.12. A power supply unit according to claim 10 or 11 wherein the area covered by the adhesive layer encompasses the whole mounting surface of the leaded component, where the mounting surface of the leaded component is the surface thereof which is mounted to the circuit board.13. A power supply unit according to any preceding claim wherein the adhesive layer has a thermal conductivity of approximately 0.5 W/m-K or higher.14. A power supply unit according to any preceding claim wherein the adhesive layer has a thickness of 0.3mm or less.-18 - 15. A power supply unit according to any preceding claim wherein the adhesive layer comprises two layers of an adhesive material, separated by a central layer.16. A power supply unit according to any preceding claim wherein the adhesive layer comprises a double-sided adhesive pad or double-sided adhesive tape.17. A power supply unit according to any one of claims 1 to 14 wherein the adhesive layer consists of a single layer of an adhesive material.18. A power supply unit according to any preceding claim wherein the leaded component(s) are mounted to the circuit board by a body portion thereof.19. A power supply unit according to claim 18 wherein the leaded component(s) are mounted to the circuit board by a planar surface of said body portion.20. A power supply unit according to claim 19 wherein said planar surface is a major surface of the body portion.21. A power supply unit according to any preceding claim wherein the leaded component(s) are positioned on the circuit board such that their leads project beyond an edge of the circuit board.21. A power supply unit according to any preceding claim wherein the components mounted to the circuit board include at least one component which has/have a power loss density of approximately 20mW/mm2 or higher.22. A method of manufacturing a power supply unit, the method comprising:- -19 -providing one or more surface mount components and one or more leaded components; providing a circuit board having a first layer in which electric circuit interconnections are formed, and one or more second layers for dissipating heat generated by said components; mounting the one or more surface mount components to a first surface of the circuit board formed by the first layer thereof; and mounting one or more leaded components to a second surface of the circuit board, opposite the first surface thereof.23. A method according to claim 22 further comprising applying an adhesive layer to the second surface of the circuit board and mounting the one or more leaded components to the circuit board by means of the adhesive layer.24. A power supply unit substantially as hereinbefore described with reference to the accompanying drawings.25. A method of manufacturing a power supply unit, substantially as hereinbefore described with reference to the accompanying drawings.AMENDMENTS TO CLAIMS HAVE BEEN FILED AS FOLLOWS1. A heat sink arrangement for electronic components which comprises:-one or more surface mount components; one or more leaded components; and a circuit board comprising a first layer in which one or more electric circuit interconnections are formed, and one or more second layers for dissipating heat generated by said components; wherein the one or more surface mount components are mounted to a first surface of the circuit board, formed by the first layer thereof, and the one or more leaded components are mounted to one of said one or more second layers, to be located on a second surface of the circuit board, opposite the first surface thereof.2. A heat sink arrangement according to claim 1 wherein the material from which the or each of the one or more second layers is formed has a thermal conductivity of approximately 1OW/m-K or higher.3. A heat sink arrangement according to claim 1 or 2 wherein the one or more second layers include one or more metal layers.4. A heat sink arrangement according to claim 3 wherein said metal layer(s) are electrically isolated from the first layer of the circuit board by an electrically insulating layer.5. A heat sink arrangement according to any preceding claim wherein the one or more second layers include one or more ceramic layers.6. A power supply unit according to any preceding claim wherein the circuit board comprise an Insulated Metal Substrate (IMS) 7. A heat sink arrangement according to any one of claims 1 to 5 wherein circuit board comprises a Direct Bonded Copper Substrate (DBC) 8. A heat sink arrangement according to any preceding claim wherein the leaded components are mounted to the second surface of the circuit board, such that they are electrically isolated therefrom.
9. A heat sink arrangement according to any preceding claim wherein the one or more leaded components are mounted to the second surface of the circuit board by means of an adhesive layer. (010. A heat sink arrangement according to claim 9 wherein the adhesive layer comprises an electrically insulating material for electrically isolating the leaded components from the circuit board.11. A heat sink arrangement according to claim 9 or 10 wherein the area covered by the adhesive layer encompasses the whole mounting surface of the leaded component, where the mounting surface of the leaded component is the surface thereof which is mounted to the circuit board.12. A heat sink arrangement according to any preceding claim wherein the adhesive layer has a thermal conductivity of approximately 0.5 W/m-K or higher.13. A heat sink arrangement according to any preceding claim wherein the adhesive layer has a thickness of 0.3mm or less.14. A heat sink arrangement according to any preceding claim wherein the adhesive layer comprises two layers of an adhesive material, separated by a central layer.15. A heat sink arrangement according to any preceding claim wherein the adhesive layer comprises a double-sided adhesive pad or double-sided adhesive tape.16. A heat sink arrangement according to any one of claims 1 to 13 wherein the adhesive layer consists of a single iLayer of an adhesive material.17. A heat sink arrangement according to any preceding claim wherein the leaded component(s) are mounted to the circuit board by a body portion thereof. (018. A heat sink arrangement according to claim 17 wherein 20 the leaded component(s) are mounted to the circuit board by a planar surface of said body portion.19. A heat sink arrangement according to claim 18 wherein said planar surface is a major surface of the body portion.20. A heat sink arrangement according to any preceding claim wherein the leaded component(s) are positioned on the circuit board such that their leads project beyond an edge of the circuit board.21. A heat sink arrangement according to any preceding claim wherein the components mounted to the circuit board include at least one component which has/have a power loss density of approximately 20mW/mm2 or higher.22. A power supply unit comprising a heat sink arrangement according to any preceding claim.23. A method of manufacturing a heat sink arrangement, the method comprising:-providing one or more surface mount components and one or more leaded components; providing a circuit board having a first layer in which electric circuit interconnections are formed, and one or more second layers for dissipating heat generated by said components; mounting the one or more surface mount components to a first surface of the circuit board formed by the first layer thereof; and mounting one or more leaded components to one of said one or more second layers, to be located on a second surface of the circuit board, opposite the first surface thereof. (024. A method of manufacturing a power supply unit, the method comprising:-providing one or more surface mount components and one or more leaded components; providing a circuit board having a first layer in which electric circuit interconnections are formed, and one or more second layers for dissipating heat generated by said components; mounting the one or more surface mount components to a first surface of the circuit board formed by the first layer thereof; and mounting one or more leaded components to one of said one or more second layers, to be located on a second surface of the circuit board, opposite the first surface thereof.25. A method according to claim 23 or 24 further comprising applying an adhesive layer to the second surface of the circuit board and mounting the one or more leaded components to the circuit board by means of the adhesive layer.26. A heat sink arrangement substantially as hereinbefore described with reference to the accompanying drawings.27. A power supply unit substantially as hereinbefore described with reference to the accompanying drawings.28. A method of manufacturing a heat sink arrangement, substantially as hereinbefore described with reference to the accompanying drawings.29. A method of manufacturing a power supply unit, substantially as hereinbefore described with reference to the accompanying drawings. L() (\J