Patent application title: HEAT SPREADER WITH FLEXIBLY SUPPORTED HEAT PIPE
Konrad Pfaffinger (Deggendorf, DE)
IPC8 Class: AF28D1504FI
Class name: Liquid fluent heat exchange material utilizing change of state utilizing capillary attraction
Publication date: 2012-04-12
Patent application number: 20120085520
A heat spreader for dissipating heat generated by at least one
heat-generating power semiconductor device which, apart from an improved
heat coupling between a chip and a heat sink, also comprise a flexible
coupling assembly which can compensate for tolerances in the components
and in the assembling process. The heat spreader has a base plate (11)
which is connectable in a heat-conducting manner to the at least one
power semiconductor device (2). The base plate (11) has at least one
recess (12) in which at least one heat pipe (13) is arranged for
dissipating the generated heat, and wherein the at least one heat pipe
(13) is movably supported in the recess (12).
1. A heat spreader for dissipating heat generated by at least one
heat-generating power semiconductor device (2), the heat spreader (3)
comprising a base plate (11) which is connectable in a heat-conducting
manner to the at least one power semiconductor device (2), wherein the
base plate (11) has at least one recess (12) in which at least one heat
pipe (13) is arranged for dissipating the generated heat, and wherein the
at least one heat pipe (13) is movably supported in the recess (12).
2. The heat spreader according to claim 1, further comprising at least one heat coupling element (4) which is connected in a heat-conducting manner to the at least one power semiconductor device (2) on the one hand and to the heat pipe (13) on the other hand.
3. The heat spreader according to claim 2, wherein a contact pressure of the heat coupling element (4) on the heat pipe (13) and/or on the power semiconductor device (2) is adjustable via at least one spring element (14).
4. The heat spreader according to claim 3, wherein the spring element (14) includes a spiral spring having a defined spring constant, which resiliently supports the heat coupling element (4) relative to the base plate (11).
5. The heat spreader according to claim 2, wherein the heat coupling element (4) is made from copper, aluminum or magnesium.
6. The heat spreader according to claim 1, wherein the heat pipe (13) has a flattened cross-section.
7. The heat spreader according to claim 6, wherein the cross section of the heat pipe (13) is dimensioned such that the heat pipe (13) has a thickness smaller than or equal to a thickness of the base plate.
8. The heat spreader according to claim 1, wherein the recess (12) is formed at least in part by an opening extending through the base plate (11).
9. The heat spreader according to claim 1, wherein the recess (12) is formed at least in part by a groove in the base plate (11), so that the heat pipe (13) is in heat-conducting communication with a wall of the base plate.
10. The heat spreader according to claim 1, wherein the recess (12) is formed by milling, die casting or eroding.
11. The heat spreader according to claim 1, wherein the base plate (11) is made of a metal.
12. The heat spreader according to claim 1, wherein the at least one heat pipe (13) is made of metal.
13. The heat spreader according to claim 1, wherein the base plate (11), the heat pipe (13) and/or the heat coupling element (4) is/are provided with a surface coating.
14. The heat spreader according to claim 1, wherein a heat-conducting interlayer is provided for coupling the at least one power semiconductor device (2) to the heat spreader (3).
15. The heat spreader according to claim 14, wherein the interlayer comprises a latent-heat storage material (15).
16. The heat spreader according to claim 14, wherein the interlayer is mechanically secured on the heat coupling element (4).
17. The heat spreader according to claim 1, wherein the base plate (11) is connectable to a heat sink (8) or a heat-releasing surface structure.
BACKGROUND OF THE INVENTION
 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, apart from an improved heat coupling between a chip and a heat sink, also comprise a flexible coupling assembly which can compensate for tolerances in the components and in the assembling process.
 Power semiconductors are for instance 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.
 The present invention, however, 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 1791177 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 evolves on the chip that must be removed from the chip 2, including processor core 6, 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, great 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 great 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.
 In one embodiment the heat coupling assembly 4 consists of a multilayered heat-conducting block 4. According to the invention the heat coupling assembly comprises an elastic layer 5. This elastic layer 5 permits a 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 1791177 A1 have the drawback that on the one hand upon a heat load there is the risk in the case of an elastic material that vapors of the elastic plastic 5 deposit at an undesired place especially in cases where silicone is used. On the other hand, the configuration with resilient parts of the base plate has 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.
SUMMARY OF THE INVENTION
 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. Furthermore, a connection of the COM module to the system cooling solution that is as position-accurate as possible should be ensured together with mechanical loads that are as small as possible.
 According to the present invention a heat spreader comprises a base plate which is connectable in a heat-conducting manner to at least one power semiconductor device. The base plate comprises a recess in which at least one heat pipe is arranged for dissipating the generated heat. According to the invention said at least one heat pipe is movably supported in the recess of the base plate.
 Heat pipes are generally known and e.g. shown in EP 0191419 B1. Such a heat pipe is a closed space partly filled with a liquid. The wall material and the kind of liquid depend substantially on the temperature range in which the heat pipe is to operate. When heat is supplied at a place of the heat 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 heat 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 heat 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 chamber formed by the heat pipe need not be cylindrical; a heat pipe can have almost any desired cross-section.
 With the movable support of the heat pipe according to the invention relative to the base plate it is possible that despite existing manufacturing tolerances in the structural height of the power semiconductor devices an optimal heat transition to the heat pipe is achieved. The provision of a heat pipe affords a much better dissipation of the heat into areas where in a mounted state e.g. heat sinks can be arranged.
 Due to the provision of an additional heat coupling element which is connected in a heat-conducting manner to the power semiconductor device on the one hand and to the heat pipe 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 pipe and/or on the power device is adjusted in a definite way via at least one spring element according to an advantageous embodiment of the present invention. The spring element according to an advantageous embodiment can e.g. comprise a spiral spring with a defined spring constant. The spiral spring resiliently supports the heat coupling element relative to the base plate. Alternatively, however, spring rings or spring steel sheets may be used.
 Said heat coupling element can be made from copper in an advantageous way. This material has the advantage that it shows a particularly good thermal conductivity. Of course other metallic or other materials of good heat conduction can also be used.
 According to an advantageous embodiment of the present invention the heat pipe has a flattened cross-section that is dimensioned such that the heat pipe has a thickness smaller than or equal to the thickness of the base plate. Moreover, such flattened heat pipes can be easily deflected in the desired direction to be able to compensate for tolerances between base plate and chip.
 Since the recess is formed at least in part by an opening extending through the base plate, it can be ensured in a simple way that there is enough space for deflecting a heat pipe.
 On the other hand, in order to ensure that the condensation area of the heat pipe is coupled with the base plate in a highly heat-conducting manner, if possible, the recess is formed in these areas by a groove in the base plate so that the heat pipe is in heat-conducting communication with at least a wall of the base plate.
 The recess can be formed in a particularly simple and inexpensive way in the base plate by milling, during the manufacturing process of the plate by die casting or by erosion.
 An inexpensive material for the base plate that shows good heat conduction at the same time is aluminum. Of course, it is however also possible to use other heat-conducting materials. Furthermore, the at least one heat pipe is made in an advantageous way from copper, and of course all of the other standard materials can be used as well.
 To make the individual components solderable, the base plate, the heat pipe and/or the heat coupling element are coated with a nickel layer according to an advantageous embodiment.
 To ensure a further improved heat coupling between the power semiconductor device and the heat spreader, an additional heat-conducting interlayer may be provided.
 This interlayer may further comprise a latent-heat storage material. Latent heat storage generally 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, exhibit much lower heat storage densities as a rule. 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.
BRIEF DESCRIPTION OF THE DRAWINGS
 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.
 FIG. 1 is a perspective illustration of a heat spreader according to an advantageous embodiment;
 FIG. 2 is a tilted view of the heat spreader of FIG. 1;
 FIG. 3 is a perspective illustration of a heat spreader according to a further advantageous embodiment;
 FIG. 4 is a schematic sectional illustration of a semiconductor unit.
 FIG. 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. 3 in an advantageous way for cooling the power semiconductor devices 2. Here, the heat spreader 3 comprises a base plate 11, which can e.g. be made from aluminum. On the surface of the base plate 11, in order to avoid any objectionable oxide layer that would prevent soldering e.g. with further circuit components or a heat sink, the surface of the base plate 11 is e.g. nickel-plated. It is however clear to a skilled person that the heat-spreader base plate 11 can be made from any other materials that are standard for a skilled person, e.g. also from a highly heat-conducting ceramic material.
 Recesses 12 which accommodate heat pipes 13 are provided in the base plate 11 according to the invention. The recesses 12 are here e.g. formed in the base plate in a milling operation.
 The heat pipes 13, in the illustrated case two pipes for dissipating the heat of a CPU chip and one for dissipating the heat generated by an interface controller hub, consist e.g. of nickel-plated copper and have a flattened cross-section, so that they are almost completely accommodated within the thickness of the base plate 11.
 It is however clear to a skilled person that all other suitable materials and suitable geometric configurations can be used under the principles of the present invention.
 According to the above-explained functional principle the heat pipes 13 have a heating zone 18 and a condensation zone 20.
 As can be seen in connection with FIG. 2, the recesses 12 are configured as continuous openings in the area of the heating zones in which the mechanical contact with the chip is established, so that there remains enough space for deflecting the heat pipes 13 if this should be required for tolerance compensation. By contrast, in the area of the condensation zones 20 the recess 12 is configured as a groove in the base plate 11, so that the condensation zones 20 of the heat pipes 13 are here in a particularly large-area contact with the base plate 11. This can achieve a particularly efficient cooling.
 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.
 According to an advantageous embodiment of the present invention these heat coupling elements 4 are not rigidly connected to the deflectable heat pipe 13, but are fixed via an elastic support by means of spiral springs 14 to the base plate 11. Countersunk screws 16 and spacers arranged in the interior of the spiral springs 14 secure the heat coupling elements 4 on the base plate 11.
 In the illustrated embodiment the heat pipes are moreover arranged within the base plate plane in a bent configuration so as to save space.
 FIG. 2 shows the surface 22 of the heat spreader 3 which faces away from the power semiconductor devices 2. This 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.
 According to a further advantageous embodiment, which is shown in FIG. 3, a further interlayer, e.g. latent-heat storage element 15, can additionally be provided on the heat coupling element 4 to serve as an additional heat buffer for attenuating temperature peaks. This latent-heat storage element 15 can be secured mechanically e.g. by means of a bolt on the heat coupling element 4. Such latent heat-storage materials, as can be used in connection with the present invention, are described in U.S. Pat. No. 6,197,859 B1.
 With the help of the computer-processor heat-pipe cooling according to the invention with a defined spring constant, as is implemented according to the illustrated invention, an optimal contact pressure can be achieved between heat spreader 3 and power semiconductor device 2. In comparison with a standard heat-spreader plate the flattened heat pipe in this solution is used for transmitting heat 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 spiral springs 14 with a defined spring constant provide for the optimal contact pressure of the cooling solution on the processor chip. The heat pipe supports the optimal contact pressure in its flexible height. Known solutions have the problem that due to the different processor height as a result of the soldering process an exact contact pressure of a rigid cooling solution on the processor chip is not possible and that the processor can thus not be cooled in an optimum way.
Patent applications in class Utilizing capillary attraction
Patent applications in all subclasses Utilizing capillary attraction