Patent application title: HEAT SPREADER WITH MECHANICALLY SECURED HEAT COUPLING ELEMENT
Konrad Pfaffinger (Deggendorf, DE)
IPC8 Class: AF28F700FI
Class name: Heat exchange heat transmitter
Publication date: 2012-04-12
Patent application number: 20120085527
A heat spreader for dissipating heat generated by at least one
heat-generating power semiconductor device. Such a heat spreader
comprises a base plate (11) which is connectable in a heat-conducting
manner to the at least one power semiconductor device (2), and 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 base plate (11) on the other hand and comprises at least one
elastic layer (5). The heat coupling element (4) comprises at least one
holding element for mechanically fixing the heat coupling element (4)
relative to a plane defined by the base plate (11).
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); 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 base plate (11) on the other hand and comprises at least one elastic
layer (5), wherein the heat coupling element (4) comprises at least one
holding element for mechanically fixing the heat coupling element (4)
relative to a plane defined by the base plate (11).
2. The heat spreader according to claim 1, wherein the at least one holding element is configured such that the elastic layer is deformable in a direction transverse to the plane defined by the base plate (11).
3. The heat spreader according to claim 1, wherein the at least one holding element is formed by a press-in bolt (12).
4. The heat spreader according to claim 3, wherein each heat coupling element (4) is held by two press-in bolts (12).
5. The heat spreader according to claim 4, wherein at least one heat coupling element (4) has a respective rectangular base area and the two press-in bolts (12) are arranged in corners of the base area that are opposite each other.
6. The heat spreader according to claim 1, wherein the heat coupling element (4) comprises at least one metal core (9) which is made from copper, aluminum or magnesium.
7. The heat spreader according to claim 1, wherein the base plate (11) is made from a metal.
8. The heat spreader according to claim 1, wherein the base plate (11) and/or the heat coupling element (4) is/are provided with a surface coating, preferably a nickel layer or an eloxal layer.
9. 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 coupling element.
10. The heat spreader according to claim 9, wherein the interlayer comprises a latent-heat storage material (15).
11. The heat spreader according to claim 1, wherein the base plate (11) is connectable to a heat sink (8) or to a heat-discharging 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 in the following, 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. 7 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. 7 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. 7 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. 7 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, 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 conventional 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 affords the compensation of tolerances. 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 assembly which is described in EP 1791177 A1 has the drawback that the elastic layer 5 is connected to the heat spreader only via an adhesive bond and that a thermal load or a load caused by vibrations poses the risk that it will 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.
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 the heat spreader comprises a base plate which is connectable in a heat-conducting manner to at least one power semiconductor device. Furthermore, the heat spreader comprises a heat coupling element which is connected in a heat-conducting manner to the at least one power semiconductor device on the one hand and to the base plate on the other hand and comprises at least one elastic layer. The elastic layer serves to compensate for tolerances because it compensates different constructional heights of the power semiconductor devices by deformation.
 The heat coupling element further comprises at least one holding element for mechanically fixing the heat coupling element with respect to a plane defined by the base plate. According to the present invention mechanical fixation stands for a glueless holding or mounting provided in addition or as an alternative to bonding. In contrast to the classic structure in which the plasticizing adhesive may lead to a slipping of the heat coupling element with respect to the position of the chip, the mechanical mounting according to the invention offers the advantage that even in vibration-prone surroundings and/or at an elevated temperature the required positional accuracy can be guaranteed.
 According to an advantageous embodiment the holding element is configured such that the elastic layer is still deformable in a direction transverse to the plane defined by the base plate and can thus compensate for tolerances in said direction without being hindered.
 The mechanical lock according to the invention can be achieved in a particularly simple way in that the holding element is formed by a press-in bolt without thread. Of course, recesses in the base plate or suitable frame constructions can also fulfill the desired purpose.
 To be more specific, the heat coupling element can be held by two press-in bolts. For a heat coupling element having a rectangular base area these two press-in bolts are advantageously arranged in corners of the base area that are opposite each other. Of course, the mechanical lock may also be arranged at any desired other place, e.g. on one or plural edges.
 Due to the provision of a heat coupling element which is connected in a heat-conducting manner to the power semiconductor device on the one hand and to the base plate on the other hand, an adaptation to existing tolerances of the semiconductor device and of the heat spreader can be implemented. At the same time the heat coupling element also serves as a buffer to absorb temperature peaks.
 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.
 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.
 To ensure a further improved heat coupling between the power semiconductor device and the heat coupling element, 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. Moreover, the base plate and/or the heat coupling element may be provided with a nickel layer or an eloxal layer.
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 top view on the heat spreader of FIG. 1;
 FIG. 3 is a side view of the heat spreader of FIG. 1;
 FIG. 4 is a further top view on the heat spreader of FIG. 1;
 FIG. 5 is a further side view of the heat spreader of FIG. 1;
 FIG. 6 is a perspective illustration of a heat spreader according to a further advantageous embodiment;
 FIG. 7 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. 7 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.
 The base plate 11 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.
 The heat spreader assembly shown in FIG. 1 is equipped with three heat coupling elements 4. A first larger-area heat coupling element 13 is here suited for thermal coupling with a CPU. Two further heat coupling elements 14 with a slightly smaller base area are configured for heat dissipation 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 13, 14 is formed by a three-layered structure. A copper core 9 is thermally coupled with the base plate 11 via an elastic layer 5. Said elastic layer may e.g. be a thermally conductive interlayer according to U.S. Pat. No. 5,679,457 of the firm "The Bergquist Company".
 The elastic layer 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. According to the invention the heat coupling elements 4 are mechanically fixed by means of press-in bolts 12 to the base plate 11. The press-in bolts 12 are advantageously dimensioned in their length in such a manner that an adequate deformability of the elastic interlayer 5 is possible upon contact with the chip.
 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 U.S. Pat. No. 6,197,859 B1.
 Due to the layered or stacked structure the heat coupling element 4 is also called thermal stack. During manufacture the press-in bolts 12 and the spacers 21 are first fastened with the help of corresponding tools to the base plate 11. The thermal stacks are joined, as shown in FIG. 1, they are subsequently positioned under guidance of the press-in bolts 12 acting as centering pins and are fixed via the adhesive bond with the elastic layer 5 to the base plate 11.
 FIGS. 2 to 5 are further views of the heat spreader 3 according to the invention as shown in FIG. 1. FIG. 2 shows the surface 22 of the heat spreader 3 facing away from the power semiconductor devices. This surface 22 is configured in the illustrated embodiment such 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, can also be provided on the surface. The respective configuration is left to the skilled person's discretion and depends on the desired application environment.
 FIG. 6 shows a further advantageous embodiment of a heat spreader in which the heat coupling element 13 for the CPU comprises a latent-heat storage element 15 with a hexagonal base area, similar to the base area of the latent-heat storage interlayer 15 on the heat coupling elements 14.
 In comparison with standard head-spreader plates it is ensured in the solution according to the invention that a lateral escape of the heat coupling element 4 or also only of the elastic layer 5 is prevented by means of the pin holder. Also under a mechanically strong load due to vibrations and impact it can be ensured even at elevated temperatures that all elements of the heat coupling element remain in an accurate position at their place of installation.
 According to an advantageous embodiment the heat coupling elements are made from copper or are coated with a nickel layer. Of course, further materials, such as aluminum or magnesium or multilayered structures, can also be used.
Patent applications in class HEAT TRANSMITTER
Patent applications in all subclasses HEAT TRANSMITTER