WO2014140098A1

WO2014140098A1 - Heat spreader with flat pipe cooling element

Info

Publication number: WO2014140098A1
Application number: PCT/EP2014/054835
Authority: WO
Inventors: Pavel CADEK
Original assignee: Congatec Ag
Priority date: 2013-03-12
Filing date: 2014-03-12
Publication date: 2014-09-18
Also published as: DE202013002411U1

Abstract

The present invention refers to a heat spreader for dissipating heat generated by a heat-generating power semiconductor device. The heat spreader comprises a base plate adapted to be thermally connected to the power semiconductor device, and a heat transmitting element, which is mounted on the base plate and adapted to dissipate the generated heat. The surface of the heat transmitting element is dimensioned so as to extend at least over the entire surface of the at least one power semiconductor device.

Description

HEAT SPREADER WITH FLAT PIPE COOLING ELEMENT

The present invention refers to a heat spreader for dissipating heat generated by at least one heat-generating power semiconductor device. The present invention particularly relates to heat spreaders, which provide for an improved heat coupling between a chip and a heat sink by means of a heat transmitting element mounted on the base plate, wherein the heat transmitting element has a surface dimensioned so as to extend at least over the entire surface of the at least one power semiconductor device.

Power semiconductors can be mounted on printed circuit boards which have additionally provided thereon further components that vary in response to the respective functionality of the printed circuit boards. Due to the high power densities that nowadays prevail particularly in connection with embedded computers, optimized cooling solutions are needed for preventing destruction of the components due to overheating. Printed circuit boards with the semiconductor devices are here subject to specific tolerances that should be compensated during the mounting of the cooling device. Such a compensation of tolerances is particularly important in semiconductor units that are installed in so-called baseboards. Such units are normally called "computer on modules", COM solutions.

Besides the application described above, the present invention can of course also be used for heat dissipation in any other electronic components, for cooling semiconductor switches in rectifiers and inverters, so-called power modules, and also in any desired other fields of application in which excessive heat is to be conducted away.

COM solutions are highly integrated CPU modules that, although they do not represent independently operable computers, contain the most important functional elements of a computer. It is only with the installation of the modules in a baseboard that they are given their functionality, e.g. as a measuring device, control computer or for another application. System extension and adaptation are exclusively made possible via the baseboard, resulting in a partly customer-specific integrated solution. The baseboard contains all of the necessary connections for connecting the system to peripheral devices, such as hard disks, mouse or screen. A COM module contains e.g. the processor, a processor bus and one or more memories (RAM) and can also perform, depending on the manufacturer, a certain number of standard peripheral functions. COM modules can be installed as plug-in cards into a baseboard or can be connected in a planar configuration via corresponding connectors to the baseboard.

COM modules normally have a specified interface for connection to suitable cooling solutions. This interface shall be called heat spreader hereinafter, but other terms are also common, such as heat spreading plate, heat distributor or also heat sink. The heat spreader is used for removing heat in the COM module from the active heat-generating components, such as CPU or chip, and for transferring the heat to another cooling solution, such as a heat sink or a housing wall. One of the strong points of COM modules is their free exchangeability. Thus the thermal connection of the module to a further cooling solution is always carried out via the heat spreader surface at the same geometric position. To ensure exchangeability within the system environment without any mechanical adaptations, COM modules are therefore exactly specified in their dimensions, such as height, width and length. Tolerances of the components and specified deviations during the assembling process, e.g. due to the soldering process, make a position-accurate connection of the baseboard to the heat spreader more difficult. In the case of unfavorable tolerances undesired mechanical loads are created on the circuit carrier, e.g. a printed circuit board, and on the heat spreader, resulting in a bending or sagging of the printed circuit board in the worst case. It is therefore known that the heat spreader is equipped at least in part with a flexible heat coupling element which is flexibly arranged with respect to a base plate of the heat spreader to compensate for tolerances in the dimensions of the power semiconductor device.

Fig. 4 is a schematic sectional illustration of a semiconductor unit 10 comprising a heat spreader 3, as is e.g. known from EP 1791 177 A1 . The semiconductor unit of Fig. 4 is normally installed in a baseboard to achieve an application-specific integrated solution.

A chip 2 with an active and a rear surface is here the executing part of the CPU module, the active surface of the chip 2 comprising a processor core 6. The rear surface of the chip 2 is mounted on a printed circuit board 1 . It is e.g. possible to use small solder balls 7 so as to connect the chip electrically and firmly to the printed circuit board. The chip 2 may e.g. be configured as a surface mounted device. The surface mount technology (SMT) is nowadays used as a rule because it offers numerous advantages over former methods. The reflow soldering used therein is a soft soldering method in which the components are directly mounted on a printed circuit board by way of soft solder and soldering paste. However, apart from this, all of the techniques known to the skilled person for mounting a chip on a printed circuit board are possible.

During operation of the COM module heat is generated around the chip 2 and the processor core 6. The generated heat must be removed from the chip 2 to prevent possible heat damage to the printed circuit board 1 and the chip 2. A heat spreader 3 is mounted above the chip 2 and dissipates heat from the chip 2 on the one hand and protects the chip 2 against potential damage on the other hand. The heat spreader 3 is detachably connected to the printed circuit board.

For instance, the heat spreader 3 can be mounted on the printed circuit board by corresponding fastening devices with screws (not shown). However, all mounting options that are known to the skilled person and permit a detachable fastening are also conceivable. These include e.g. rivet, clip or adhesive fastenings (also not shown). Such a detachable mounting offers the advantage that the heat spreader 3 can be exchanged. Furthermore, the heat spreader 3 can be disassembled for repair work on the chip 2, whereby the chip 2 is more easily accessible. To connect the chip 2 thermally to the heat spreader 3, a thermally conductive heat coupling assembly 4 is used. This assembly is arranged in Fig. 4 between the active surface of the chip 2 and the heat spreader 3, so that a direct thermal connection is established between the heat generating chip 2 and the heat spreader 3.

The semiconductor unit is installed as a standardized unit into a baseboard and is normally connected to further cooling solutions of the system. Such a cooling solution is illustrated in Fig. 4 by way of example as a plurality of cooling fins 8. Other cooling solutions are however also possible. All of the heat sinks and cooling processes known to the skilled person can here be used, such as e.g. fan or water cooling. Furthermore, a heat dissipation by means of additional heat pipes is also conceivable. Also the size of the contact surface with respect to the heat spreader 3 can be chosen in an appropriate way, depending on the system environment. Preferably, the heat sink 8 abuts on the sides of the heat spreader 3 facing away from the chip 2. The used system cooling solution 8 is without any great significance to the function of the present invention.

A standardized connection to the system (cooling solution, baseboard) requires exactly specified dimensions of the semiconductor unit. Although components are built as true to size as possible, they are nevertheless subject to certain tolerance variations. Normally these tolerances are in the range of + 0.2 mm. Moreover, tolerances are created e.g. during the assembly of the chip 2 on the printed circuit board 1 by the soldering process. These building and mounting tolerances make a connection to the baseboard more difficult on the one hand and also the mounting of the semiconductor unit on the other hand.

Unfavorable tolerance combinations of the components may result in mechanical loads that lead to a bending of the printed circuit board 1 . For instance, high mechanical loads may arise in the case of great positive tolerances of the soldering process and/or of the components and in the case of an uninventive heat connection to the heat spreader 3. Upon connection to the cooling solution 8 of the system, here by way of example a cooling fin at the same mechanical position, a mechanical force is exerted via the heat spreader 3 and the uninventive heat connection on the chip 2 and thus on the printed circuit board 1 . The mechanical load will bend the printed circuit board 1 , which can definitely lead to damage.

If a high negative tolerance of the soldering process prevails, it is true that no mechanical force is exerted on the printed circuit board, but an optimal heat connection of the chip to the system cooling solution is not guaranteed. Gaps may arise between the uninventive heat connection and the heat spreader, the gaps being detrimental to heat dissipation.

The heat coupling assembly 4 consists of a multilayered heat-conducting block and an elastic layer 5. This elastic layer 5 permits tolerance compensation. The mechanical loads which, as has been explained above, may arise in the case of unfavorable tolerances of the components and the soldering process are compensated by the elastic layer 5. The mechanical force provides for a compression of the elastic layer. There is therefore no transfer of the force to the printed circuit board 1 and thus also no bending of the printed circuit board 1 . Since the elastic layer 5 is compressible, it can be configured such that it is always slightly thicker than necessary. This always provides for an uninterrupted and thus unrestricted heat connection to the heat spreader 3 even if, as has been explained above, negative tolerances arise. In this case the elastic layer 5 would be compressed less than in the case of positive tolerances of the soldering process and of the components.

The elastic layer 5 should have a surface with respect to the heat spreader 3 that is as large as possible, and it should be as thin as possible because the elastic layer 5 normally exhibits a poorer thermal conductivity. With a decreasing thickness, however, the layer is less elastic and flexible, whereby a greater force would be needed to compensate for tolerances. That is why a compromise must be found between thermal conductivity and flexibility.

The elastic layer 5 can here consist of different materials as long as these show good heat conduction properties and are adequately elastic. Graphite-filled silicones should here be mentioned by way of example. However, other elastic materials that are known to the skilled person can be used. For a better thermal connection of the heat coupling assembly 4 the assembly may be connected to the chip 2 via a thin layer of heat conducting paste (not shown), which has a low thermal transition resistance.

Furthermore, it is known that a layer 9 of the heat conducting block 4 consists of copper or a material of a similarly good thermal conductivity. The amount of thermal energy which the copper layer can absorb depends on the volume thereof. The copper layer 9 abuts either directly or via a thin layer of heat conducting paste on the chip 2 and on the processor core 6, respectively, and absorbs the heat thereof and discharges it to further layers of the heat conducting block 4, in Fig. 4 to the elastic layer 5. The layer, in turn, conducts the heat further to the heat spreader 3 which is cooled with a further cooling technique 8 of the system.

However, the assemblies which are described in EP 1791 177 A1 have the drawback that it is not possible to dissipate enough heat from the semiconductor device to absorb particularly high heat peaks. Moreover, the amount of heat that can be dissipated per time unit is often no longer adequate for modern semiconductors 2. The object underlying the present invention consists in providing improved heat dissipation from a heat generating power semiconductor device, such as a CPU or a chip set, to the heat spreader, and the heat spreader should here be producible at low costs.

This object is achieved according to the present invention by a heat spreader comprising the features of claim 1 .

According to an advantageous embodiment of the present invention a heat spreader for dissipating heat generated by at least one heat-generating power semiconductor device, comprises a base plate adapted to be thermally connected to at least one power semiconductor device, and a heat transmitting element mounted on the base plate and adapted to dissipate generated heat, wherein surface of the heat transmitting element is dimensioned so as to extend at least over the entire surface of the at least one power semiconductor device. In this manner the heat generated by the power semiconductor device can be effectively transferred to the heat transferring element and spread away from the hot spot in the surroundings of the power semiconductor device. Advantageously the heat transmitting element may comprise a plurality of flat heat pipes parallel to each other so as to cover at least half of a surface of the base plate.

According to an advantageous embodiment the heat spreader may comprise at least one heat coupling element connected in a heat conducting manner to the at least one power semiconductor device through a first surface and to the heat transmitting element through a second surface opposing the first surface.

Due to the provision of an additional heat coupling element which is connectable in a heat- conducting manner to the power semiconductor device on the one hand and to the heat transferring element on the other hand, a transition between the different surfaces of the semiconductor device and of the heat pipe can be achieved. At the same time the heat coupling element also serves as a buffer to absorb temperature peaks.

To achieve an optimal heat transition on the one hand and a minimal mechanical load on the chips on the other hand, the contact pressure of the heat coupling element on the heat transferring element and/or on the semiconductor power device can be adjusted in a definite way via an elastic layer according to an advantageous embodiment of the present invention. According to an advantageous embodiment the at least one heat coupling element comprises an elastic layer, the heat coupling element being mounted to the heat transferring element such that the elastic layer contacts the heat transmitting element. The elastic layer resiliency supports the heat coupling element relative to the base plate. The heat spreader may further comprise a retention frame adapted to be mounted to the base plate so as to partially cover the heat transmitting element.

Advantageously, the retention frame may comprise at least one opening, wherein the at least one heat coupling element is arranged on the heat transmitting element so as to be thermally connected to the heat transmitting element and to extend through the at least one opening. The retention frame avoids sideways movements of the heat coupling elements with respect to the base plate, when the heat coupling elements are pressed against the corresponding power semiconductor device.

According to a further advantageous embodiment the heat transmitting element may have a flattened cross section. In the heat spreader according a further advantageous embodiment, the base plate may have a recess adapted to receive the heat transmitting element, the cross section of the heat transmitting element being dimensioned such that the heat transmitting element has a thickness smaller or equal to a thickness of the base plate frame.

Alternatively, the heat spreader may further comprise a protection frame mountable on the base plate so as to surround the heat transmitting element, wherein the cross section of the heat transmitting element is dimensioned such that the heat transmitting element has a thickness smaller or equal to a thickness of the base plate frame.

In the heat spreader according to a further advantageous embodiment, the heat transmitting element is made of metal, preferably copper to assure good heat conduction. In the heat spreader according to a further advantageous embodiment the base plate, the heat transmitting element and/or the heat coupling element is/may be provided with a surface coating, preferably a nickel layer. In this manner, the individual components can be made solderable. In the heat spreader according to a further advantageous embodiment a heat-conducting interlayer may be provided for coupling the at least one power semiconductor device to the heat spreader. This ensures a further improved heat coupling between the power semiconductor device and the heat spreader. According to a further advantageous embodiment the interlayer may comprise a latent-heat storage material. In addition or alternatively the interlayer may be mechanically secured on the heat coupling element.

This interlayer may further comprise a latent-heat storage material. Latent heat storage means the storage of heat in a material which is subjected to a phase change, predominantly solid/liquid (phase change material, PCM).

Apart from the phase change solid/liquid, solid/solid phase changes can in principle also be used. These, however, normally exhibit much lower heat storage densities. When heat is stored into the storage material, the material starts to melt when the temperature of the phase change is reached, and will then no longer raise its temperature despite the further storing of heat until the material has completely melted. It is only then that a further increase in the temperature occurs. Since there is no increase in temperature for a long period of time, the heat stored during the phase change is called hidden heat or latent heat. This effect makes it possible to even out temperature variations and to prevent temperature peaks that might damage the semiconductor. The latent-heat storage material is chosen in response to the temperature range. Various salts and their eutectic mixtures are used most of the time.

The base plate is configured in an advantageous manner such that it is connectable to an additional heat sink or a housing.

For a better understanding of the present invention said invention shall now be explained in more detail with reference to the embodiments illustrated in the following figures. Like parts are here provided with like reference numerals and like component designations. Furthermore, some features and feature combinations taken from the illustrated and described embodiments can also per se represent independent inventive solutions or solutions according to the invention. Figure 1 is a perspective exploded view of a heat spreader according to an embodiment of the present invention;

Figure 2 is cross sectional a side view of the heat spreader of figure 1 ;

Figure 3 depicts a top view and a bottom view of the heat spreader of figure 1 ; Figure 4 is a schematic sectional illustration of a semiconductor unit.

Figure 1 is a perspective view showing the heat spreader assembly 3 according to the invention, which can be used in the semiconductor unit 10 of Fig. 4 in an advantageous way for cooling the power semiconductor devices 2. Here, the heat spreader 3 comprises a base plate 1 1 , which can e.g. be made from aluminum. In order to avoid any disturbing oxide layer that would prevent soldering e.g. with further circuit components or a heat sink, the surface of the base plate 1 1 may be nickel-plated. It is however clear to a skilled person that the heat-spreader base plate 1 1 can be made from any other materials that are standard for a skilled person, e.g. also from a highly heat-conducting ceramic material.

The base plate 1 1 is connectable via spacers 21 to a printed circuit board 1 . Said spacers 21 may e.g. be formed by press-in bolts with internal thread.

A heat transferring element 13 is mounted on a face of the base plate 3 facing the power semiconductor devices 3. The heat transferring element 13 may be made of a material with good heat conduction characteristics, such as nickel-plated copper and can be mounted on the base plate by adhesive bonding. In the embodiment illustrated in figure 1 , the heat transferring element 13 has a flattened cross-section and consists of a plurality of parallel heat pipes.

In an advantageous embodiment the heat pipes are formed as a single main body to form a flat pipe cooling element. The flat pipe cooling element 13 will be described in more detail with reference to figure 2. The flat pipe cooling element or heat transferring element 13 can be soldered to the base plate 1 1 and has a flat surface that extends so as to cover a substantial portion of the base plate 3, so that when the heat spreader assembly is connected to a printed circuit board, the entire surface of the power semiconductor devices mounted thereon is thermally connected to the heat transferring element.

According to an advantageous configuration an area of a face of the flat pipe cooling element 13 respectively facing the base plate 1 1 is dimensioned so as to cover at least half of the surface of a face of the base plate 1 1 facing the power semiconductor devices. Advantageously the flat pipe cooling element may cover up 80% or more of the face of the base plate 1 1 . In this configuration the heat transferring element 13 is in thermal contact with a substantial portion of the base plate to which heat can be transferred. Accordingly, the heat produced by the power semiconductor devices can be effectively and quickly spread through the flat pipe cooling element 13 away from the hotspot zones above the power semiconductor devices.

The heat spreader assembly 3 according to the embodiment depicted in figure 1 further includes a protection frame 12 for the heat transferring element 13. The protection frame 12 is mechanically mounted to the base plate 1 1 by means of fixing elements, such as e.g. assembling screws 16. The protection frame 12 is mounted so as to encircle the flat pipe 13 and has a thickness bigger or equal than the thickness of the flat pipe 3. The flat pipe 13 has a flattened cross section and is accommodated within the protection frame so as not to protrude above the protection frame 12.

In an alternative embodiment, not shown in the figures, the base plate 3 may have a recess formed e.g. through a milling operation so that the heat transferring element 13 can be arranged therein. Similarly to the embodiment described before the cross section of the flat pipe can have a thickness smaller than or equal to a thickness of the base plate so that the flat pipe can be almost completely accommodated in the recess. Advantageously, the thickness of the flat pipe is such that it does not to protrude from the recess. To further increase the thermal contact with the chip (not shown in Figs. 1 and 2), the heat spreader 3 according to the present invention is equipped with heat coupling elements 4. These, however, are optional. According to an advantageous embodiment the heat coupling elements are made of copper and are coated with a nickel layer. Of course, other materials, such as aluminum or magnesium, or multilayered structures, can also be used.

The heat spreader assembly 3 shown in figure 1 is equipped with 2 heat coupling elements 4. Clearly the number of the heat coupling elements 4 is not fixed and can vary according to the application for which the heat spreader assembly 3 is intended. A first larger-area heat coupling element may be suited for thermal coupling with a CPU. The other heat coupling element, which has a slightly smaller base area, may be used for dissipating heat from an interface controller hub and a graphic memory controller hub. Of course, the principles according to the invention can also be employed for any other heat-generating components.

Each of the heat coupling elements 4 is formed by a three-layered structure. A core 9 is thermally coupled with the heat transferring element 13 via an elastic layer 5. The core 9 may be made of copper or any other suitable thermally conducting material. The elastic layer may e.g. be a thermally conductive interlayer according to US patent 5,679,457 of the firm "The Bergquist Company".

The elastic layer 5 compensates for tolerances by virtue of its deformability. The copper core permits a temporary storage and rapid discharge of generated heat and can be coupled with the power semiconductor device via an additional latent-heat storage interlayer 15. The heat coupling elements 4 may be fixed to the base plate 1 1 by means of an adhesive bonding. Furthermore, the heat coupling element 4 comprises an interlayer which is preferably formed from a latent-heat storage material 15 and gets into abutment with the chip. This latent-heat storage element 15 serves as an additional heat buffer for attenuating temperature peaks and can be secured e.g. by means of an adhesive bond and/or a further mechanical fixation on the heat coupling element 4. Such latent heat-storage materials, as can be used in connection with the present invention, are e.g. described in US patent specification 6,197,859 B1 . Even if the embodiment depicted in figure 1 shows two heat coupling elements 4 including the interlayer 15, a skilled person would recognize that this element is not essential and that also different types of heat coupling elements can be used in connection with the present invention. Due to the layered or stacked structure the heat coupling element 4 is also called thermal stack. During manufacture the thermal stacks are joined, as shown in figure 1 , subsequently positioned under guidance of a retention frame 14 acting also as centering means and are fixed via an adhesive bond with the elastic layer 5 to the heat transferring element 13. Fixing the heat coupling elements only by means of adhesive bonding may have the drawback that due to thermal load or to a load caused by vibrations the heat coupling elements can slip out of the desired position within a plane defined by a base plate of the heat spreader. In the worst case an adequate contact with the chip 2 will then no longer be given and the chip will get damaged due to overheating. In the present invention, the retention frame is mechanically fixed to the base plate, for instance through fastening means, such as bolts or the like, and has also the function of securing the coupling elements 4 so that they can not move sideways along a direction parallel to the plane of the base plate, when mechanical loads are applied to the heat coupling elements.

Figure 2 shows a cross sectional a side view of the heat spreader assembly 3. From the enlarged particular of the cross-sectional view of the heat spreader assembly 3 it is possible to see a section of the heat transferring element (flat pipe cooling element) 13 used in the heat spreader assembly 3 of the present invention. The flat pipe cooling element is a closed space composed of a plurality of parallel flat pipes partly filled with a liquid. The wall material and the kind of liquid depend substantially on the temperature range in which the flat pipes operate. When heat is supplied at a place of the flat pipe (the heat zone), the liquid evaporates and the vapor is distributed over the whole interior. The vapor condenses at a heat sink (of the condensation zone) and releases heat in this process. The working liquid must be transported back to the heating zone. This can be done by gravity if the flat pipe is arranged such that the heating zone is at the bottom and the heat sink is further above. The liquid, however, can also be transported back by capillary forces, for which purpose the inner wall is provided with grooves, grids or a porous layer. The flat pipe is distinguished in that heat can be transported from a heat source, i.e. the heat-generating semiconductor device, to a more distant heat sink and/or a heat sink with a larger area with an only slight temperature drop. The plurality of chambers forming the flat pipe need not be cylindrical but can have almost any desired cross-section. The heat coupling element 4 is mounted to the heat transferring element through the elastic layer 5. Advantageously, the elastic layer 5 comprised in the heat coupling element for thermal coupling with a CPU may have an uncompressed thickness of 1 mm and can be compressed under mechanical load to a thickness of 0.7 mm. Clearly this is only an example and the elastic layer can be chosen to have any other thickness that can assure an efficient transfer of heat and good elastic properties. Power semiconductor devices other than a CPU, such as interface controller hubs and graphic memory controller hubs generate less heat than a CPU. Therefore, a thickness of an elastic layer 5 comprised in a the heat coupling element for thermal coupling with a interface controller hub and a graphic memory controller hub may have a thickness of 2 mm and may be compressed up to 1 .6 mm under mechanical load.

Figure 3 shows a top view (a) and a bottom view (b) of the heat spreader assembly according to the invention.

Figure 3 (a) shows a surface 22 of the base plate 1 1 that, when the heat spreader assembly is mounted to a PCB, faces away from the PCB and from the power semiconductor devices. The surface 22 is configured in the illustrated embodiment in such a manner that a heat sink or also a housing can be brought into direct heat-discharging contact with the heat spreader 3. Alternatively, however, a structured geometry, e.g. a cooling fin surface for improved air cooling, may also be mounted on the surface 22. The corresponding design is left to the skilled person's discretion and depends on the desired application environment.

Figure 3 (b) shows a side of the heat spreader assembly 3 that, when the heat spreader assembly is mounted to a PCB, faces the PCB and the power semiconductor devices mounted thereon. From this figure it is possible to see how the protection frame 12 is directly mounted by means of fastening means, such as assembling screws 16 or the like, to the base plate 1 1 . In the mounted state, the protection frame 12 encircles the heat transferring element 13 so that the heat transferring element does not protrude from the protection frame 12. The retention frame 14 is arranged on the assembly including the protection frame 12 and the heat transferring element and is fixed to the protection frame 12 and to the base plate 1 1 by means of fastening means. In the embodiment of figure three the retention frame has two openings adapted to encircle respective heat coupling elements 4. The heat coupling elements 4 are mounted on the heat transferring element 13 so as to be placed within respective openings on the retention frame 14. Since the heat coupling elements are arranged within respective openings of the retaining frame 14, a sideways displacement along a line parallel to the surface of the base plate facing the power semiconductor devices can be prevented. In the embodiment of figure 3 the retaining frame 14 only partially covers the heat transferring element 13. The portion of the heat transferring element that is not covered by the retaining frame 14 defines a condensation zone. The vapor generated in the hot spot above the power semiconductor devices condenses at the condensation zone and releases heat in this process. The working liquid is then transported back to the heating zone according to one of the methods described above with reference to figure 2.

With the help of the computer-processor heat transferring element, such as the flat pipe cooling element, according to the invention optimal heat dispersion can be obtained. In comparison with a standard heat-spreader plate the flat pipe in this solution extends so as to be in thermal contact with a substantial portion of the base plate so that heat is efficiently and quickly transferred from the chip to the heat spreader plate. This enhances the heat transport from the processor environment of the heat spreader in an advantageous way and distributes the heat over the whole heat spreader surface. Furthermore, the heat coupling element including the elastic layer provides for an optimal contact pressure of the cooling solution on the processor chip. The retention frame supports the heat coupling elements so as to prevent a sideways movement of the heat coupling elements when they are under mechanical load, for instance when the power semiconductor devices are pressed against the heat coupling elements.

Claims

1 . A heat spreader (3) for dissipating heat generated by at least one heat-generating power semiconductor device, the heat spreader (3) comprising: a base plate (1 1 ) adapted to be thermally connected to at least one power semiconductor device, and a heat transmitting element (13) mounted on the base plate (1 1 ) and adapted to dissipate generated heat, wherein surface of the heat transmitting element (13) is dimensioned so as to extend at least over the entire surface of the at least one power semiconductor device.

2. The heat spreader (3) according to claim 1 , wherein the heat transmitting element (13) comprises a plurality of flat heat pipes parallel to each other so as to cover at least half of a surface of the base plate (1 1 ).

3. The heat spreader (3) according to claim 1 or 2, further comprising at least one heat coupling element (4) connectable in a heat conducting manner to the at least one power semiconductor device through a first surface and to the heat transmitting element (13) through a second surface opposing the first surface.

4. The heat spreader (3) according to claim 3, wherein the at least one heat coupling element (4) comprises an elastic layer (5), the heat coupling element (4) being mounted to the heat transmitting element (13) such that the elastic layer (5) contacts the heat transmitting element (13).

5. The heat spreader (3) according to any one of claims 1 to 4, further comprising a retention frame (14) adapted to be mounted to the base plate (1 1 ) so as to partially cover the heat transmitting element (13).

6. The heat spreader (3) according to claim 5, wherein the retention frame (14) comprises at least one opening, wherein the at least one heat coupling element (4) is arranged on the heat transmitting element (13) so as to be thermally connected to the heat transmitting element (13) and to extend through the at least one opening.

7. The heat spreader (3) according to any one of claims 1 to 5, wherein the heat transmitting element (13) has a flattened cross section.

8. The heat spreader (3) according to any one of claims 1 to 7, wherein the base plate (1 1 ) has a recess adapted to receive the heat transmitting element (13), the cross section of the heat transmitting element (13) being dimensioned such that the heat transmitting element (13) has a thickness smaller or equal to a thickness of the base plate (1 1 ).

9. The heat spreader (3) according to any one of claims 1 to 7, further comprising a protection frame mountable on the base plate (1 1 ) so as to surround the heat transmitting element (13), wherein the cross section of the heat transmitting element

(13) is dimensioned such that the heat transmitting element (13) has a thickness smaller or equal to a thickness of the protection frame.

10. The heat spreader (3) according to any one of the preceding claims, wherein the heat transmitting element (13) is made of metal, preferably copper.

1 1 . The heat spreader (3) according to any one of the preceding claims, wherein the base plate (1 1 ), the heat transmitting element (13) and/or the heat coupling element (4) is/are provided with a surface coating, preferably a nickel layer.

12. The heat spreader (3) according to any one of the preceding claims, wherein a heat- conducting interlayer (15) is provided for coupling with the at least one power semiconductor device to the heat spreader (3).

13. The heat spreader (3) according to claim 12, wherein the heat-conducting interlayer (15) comprises a latent-heat storage material.

14. The heat spreader according to claim 14 or 15, wherein the heat-conducting interlayer (15) is mechanically secured on the heat coupling element (4).

15. The heat spreader (3) according to any one of the preceding claims, wherein the base plate (1 1 ) is connectable to a heat sink (8) or a heat-releasing surface structure, preferably cooling fins.