Abstract:
A heat sink for cooling parts, subassemblies, modules, or similar components, for cooling electrical or electronic components. The heat sink includes at least one cooling element which forms at least one cooling area for connecting the component that is to be cooled and which is made of a metallic material in the cooling area.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a heat sink and to an assembly or module unit or arrangement. 
     It is generally a standard and necessary practice to cool electrical or electronic components or assemblies, in particular power components or assemblies or modules to dissipate heat loss, namely by means of at least one heat sink (cooler) comprising at least one cooling element. For this purpose, the existing art uses in particular heat sinks with cooling elements in which at least one, preferably highly branched cooling channel structure is provided, through which a liquid and/or gaseous and/or vaporous heat-transporting or medium or cooling medium, for example water, can flow. 
     For optimal cooling, it is advantageous in many cases to connect such components or assemblies by means of a solder bond to an outer cooling surface of the cooling element of the heat sink. The cooling element in this case, at least in the area of its outer cooling surface, is made of a metal material with high thermal conductivity, in particular, a copper or aluminum. The solder bond between the component or assembly and the cooling element features the advantage, for example, that both components can be manufactured separately and connected with each other after being manufactured. 
     Problematic, however, is the fact that the solder bond or solder layer between the respective cooling element and the part of the constructional or modular unit comprising the at least one electric component, due to the generally widely differing thermal expansion coefficients of the components connected with each other by the solder layer, is subjected to considerable mechanical stress caused by thermal factors. This is especially pronounced in case of frequent changes in temperature, such as in the case of a constant load variation in the electrical component or electrical assembly, as is the case with electric drive controls, for example. This thermally related mechanical stress causes premature aging of the solder bond and in extreme cases even partial or total separation of the solder bond and therefore loss of the required cooling of the component or assembly. 
     The “DCB process” (direct copper bond technology) is known in the art, for example for connecting metal layers or sheets (e.g. copper sheets or foils) with each other and/or with ceramic or ceramic layers, namely using metal or copper sheets or metal or copper foils, the surfaces of which are provided with a layer or coating (melt-on layer) resulting from a chemical bond between the metal and a reactive gas, preferably oxygen. In this method, which is described for example in U.S. Pat. No. 3,744,120 and in DE-PS 23 19 854, this layer or coating (hot-melt layer) forms a eutectic with a melting temperature below the melting temperature of the metal (e.g. copper), so that the layers can be bonded to each other by placing the foil on the ceramic and heating all layers, namely by melting the metal or copper essentially only in the area of the hot-melt layer or oxide layer. 
     This DCB method then comprises the following steps:
         oxidation of a copper foil so as to produce an even copper oxide layer;   placing of the copper foil on the ceramic layer;   heating the composite to a process temperature between approx. 1025 and 1083° C., e.g. to approx. 1071° C.;   cooling to room temperature.       

     Also known is the so-called active soldering method (DE 22 13 115; EP-A-153 618) for bonding metal layers or metal foils forming metallizations, in particular also of copper layers or copper foils, with ceramic material. In this process, which is used especially for manufacturing a metal-ceramic substrate, a bond is produced at a temperature of 800-1000° C. between a metal foil, for example copper foil, and a ceramic substrate, for example aluminum-nitride ceramic, using a hard solder, which in addition to a main component such as copper, silver and/or gold also contains an active metal. This active metal, which is at least one element of the group Hf, Ti, Zr, Nb, Ce, creates a bond between the solder and the ceramic through a chemical reaction, while the bond between the solder and the metal is a metallic hard solder bond. 
     It is an object of the invention is to present a heat sink that eliminates the above disadvantages. 
     SUMMARY OF THE INVENTION 
     The compensating layer which is provided on the at least one cooling surface of the cooling element and which is applied directly to the metal of the cooling element achieves or provides for effective compensation of the differing thermal expansion coefficients between the cooling element and the functional elements or components of an assembly or module connected with it by means of the solder bond, in particular between the metal cooling element and a metal-ceramic substrate connected with the former by means of the solder bond, or another substrate or intermediate carrier made of a material differing from that of the cooling element, for example a material or metal that is softer than the cooling element or has a reduced thermal expansion coefficient as compared with the cooling element. 
     Further embodiments, advantages and applications of the invention are disclosed in the following description of exemplary embodiments and the drawings. All characteristics described and/or pictorially represented, alone or in any combination, are subject matter of the invention, regardless of their being summarized or referenced in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described below in detail based on exemplary embodiments with reference to the drawings, in which: 
         FIG. 1  is a schematic representation of an electronic power module provided on a cooling element of a heat sink; 
         FIGS. 2 and 3  show the temperature curve of the power module based on the time at switching on or activation of the module ( FIG. 2 ) and switching off or deactivation of the module ( FIG. 3 ), namely using different cooling methods; 
         FIG. 4  is a graphical representation of the thermal expansion coefficient for various substrates; and 
         FIGS. 5-7  are schematic representations of further embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1 , an electric or electronic power module  1 , made up of a ceramic-metal substrate  2 , namely of a DCB substrate made of a ceramic layer  3 , is provided on both sides with a metallization  4  or  5 . The metallizations  4  and  5  are formed respectively by copper foils, which are applied full-surface to the respective top surface side of the ceramic layer  3  by means of DCB technology. The ceramic layer  3  is for example made of an aluminum oxide (Al 2 O 3 ) ceramic or an aluminum nitride (AlN) ceramic. The thickness of the ceramic layer  3  is for example between 0.2 and 2 mm. 
     The metallization  4  of the top side of the ceramic layer  3  is structured for forming conductors, contact surfaces, etc. Electronic components are attached to the metallization  4 , namely for example a power component  6 , e.g. in the form of an electronic switch element (IGBT) and further controlling components  7 . The components  6  and  7  are housed in a closed housing  8 , which is made of plastic, for example. The interior  9  of the housing  8  is compound-filled with a suitable material. Corresponding connectors  10  lead through the top side of the housing  8  for the power supply and control of the module  1 . 
     For cooling of the module  1 , said module is provided on a cooling element  11  in  FIG. 1 , which functions as a heat sink for dissipating heat loss generated by the module  1  and with which the metallization  5  is bonded in an effective manner to ensure good heat transfer, namely by means of a solder layer  12 . The cooling element  11  is plate-shaped or cube-shaped, namely with a top side  11 . 1 , a bottom side  11 . 2 , with two longitudinal sides  11 . 3  and with face sides  11 . 4  and  11 . 5 , which together with the longitudinal sides  11 . 3  form the peripheral surface of the cooling element  11 . 
       FIGS. 2 and 3  show conceptually the temperature/time curve of the module  1  and therefore also of the ceramic-metal substrate  2  forming the base of this module  1  during switching on or activation of the module ( FIG. 2 ) and during switching off or deactivation of the module  1  ( FIG. 3 ), for an air-cooled cooling element  11  (curve LK) and a liquid- or water-cooled cooling element  11  (curve WK), respectively. 
     As shown in  FIG. 2 , in an air-cooled cooling element  11  the temperature T rises to the operating temperature in the time t with a delay, while in a water-cooled cooling element  11 , the temperature rise is relatively steep; the temperature gradient, i.e. the change of the temperature in the time t (temperature-time differential) is relatively abrupt. Analogously, the temperature curve during deactivation of the module  1 , i.e. for an air-cooled cooling element  12 , is such that the temperature T drops relatively slowly and constantly after deactivation, while in a liquid-cooled cooling element  11 , the temperature change is very abrupt, i.e. also during deactivation, the temperature gradient (change of temperature T dependent on time t) is considerably greater than in an air-cooled cooling element  11 , although the absolute cooling efficiency of a water-cooled cooling element  11 , of course, is much higher. 
       FIG. 4  shows the thermal expansion coefficient, stated as e×10 −6 /° K, for various materials, namely for aluminum, silicon, copper, aluminum nitride ceramic (AlN), aluminum oxide ceramic (Al 2 O 3 ), for DCB substrates with aluminum oxide ceramic (Al 2 O 3 -DCB substrates) and for DCB substrates with an aluminum nitride ceramic (AlN-DCB substrates). Since cooling elements corresponding to the cooling element  11  generally are made of metals with high thermal conductivity, i.e. copper and aluminum, the representation in  FIG. 4  clearly shows that in the case of the modular construction or modular unit of  FIG. 1 , made up of the module  1  and the cooling element  11 , the different thermal expansion coefficient e of the substrate  2  and the cooling element  11 , which is made of copper, for example, is sufficient to cause tensions within the modular unit, which (tensions) essentially affect the solder layer  12 , i.e. are absorbed and even partially compensated by the latter. To achieve an optimal cooling effect, the solder layer  12  is as thin as possible. The thickness of the solder layer is, for example, only 0 to 300 mμ. 
     If the module  1  is not operated continuously, but rather in switching mode or intermittently, as is generally the case with a module for controlling or switching drives, for example, the solder layer  12  is subjected to very strong, constantly changing mechanical tensions, which especially also in the case of a water- or liquid-cooled cooling element  11  cause a high shock load to the solder layer  12 . This can destroy the solder bond between the module  1  and the cooling element  11  and, as a result of insufficient cooling, can also ultimately destroy the module  1 . 
     The stress on the solder layer  12  due to the different thermal expansion coefficients of the adjoining ceramic-metal substrate  2  and of the cooling element  11  increases with the reduction of the thickness of the solder layer and is also dependent on the composition of the solder in the solder layer  12 . The stress on the solder layer  12  is especially high if lead-free solder is used for this layer, which is increasingly being required to reduce environmental impact. Examples of such lead-free solders are SnAg5 and SnCu3. 
     To prevent this disadvantage, as shown in  FIG. 5 , the cooling element  11  is provided on its top side  11 . 1  or cooling surface to be bonded with the substrate  1  with a compensating layer  13 , which is made of a material with high thermal conductivity and a thermal expansion coefficient e that is lower than that of copper and aluminum, i.e. of a material with a thermal expansion coefficient e less than 10×10 −6 /° K. The compensating layer  13 , which has a thermal conductivity greater than 100 W/m° K and the thickness of which is for example between 0.05 and 2 mm, is applied without any intermediate layer, i.e. directly to the cooling element  11  or to the metal (for example copper) of said cooling element  11  and is made for example of Mo, W, Mo—Cu, W—Cu, Cu-diamond and/or Cu—CNF (copper with carbon nanotubes or carbon nanofibers). 
     The intermediate or compensating layer  13  achieves equalization of the thermal expansion coefficients of the ceramic-metal substrate and the cooling element  11  in the area of the bond between these components, i.e. on both sides of the solder layer  11 . Since the thermal expansion coefficient e of the ceramic-metal substrate  2  depends on the thickness of the ceramic layer  3 , the thickness of the compensating layer  13  is also adapted to the thickness of the ceramic layer  3 , preferably so the ratio of “thickness of the compensating layer  13 /thickness of the ceramic layer  3 ” is between 1.3 and 0.25. In a preferred embodiment of the invention, the thickness of the compensating layer  13  is between 0.05 and 3 mm. 
     The application of the compensating layer  13  to the metal surface of the cooling element  11  is achieved with a suitable surface process, for example cladding, e.g. explosion cladding, by metal cold spraying, by thermal metal spraying, for example molten bath spraying, flame shock spraying, flame spraying, electric arc spraying, plasma spraying, etc. 
     The compensating layer  13  achieves equalization of the thermal expansion coefficients of the components provided on both sides of the solder layer  12  and therefore a reduction of the stress on the solder layer  12 , especially in stop-and-go operation of the module  1  and also a reduction of the resulting constant temperature change of the module  1  and of the ceramic-metal substrate  2 . This reduction is advantageous due to the high temperature gradient in the case of an active heat sink, i.e. a heat sink that comprises cooling channels within its cooling element  11  through which a gaseous and/or vaporous and/or liquid medium can flow and which is for optimal cooling, for example, so that the inner heat exchange or cooling surface that is in contact with the coolant is considerably larger, for example at least by a factor of 2 or 4, than the outer cooling surface that is in contact with the module  1 . 
     To achieve a symmetrical design, especially also with respect to the temperature curve, the cooling element  11  is also provided on its bottom side facing away from the module  1  with an additional layer  13   a  corresponding to the compensating layer  13 , the additional layer then having a thickness that is greater than the thickness of the compensating layer  13 . 
       FIG. 6  shows in a simplified representation an arrangement  14 , which is made up of the ceramic-metal substrate  2 , which is part of a module not further depicted in this drawing, and of the cooling element  11  that is bonded with the ceramic-metal substrate  2  by means of a solder bond (solder layer  12 ), the cooling element  11  being made of copper at least on its top surface side that is bonded with the ceramic-metal substrate. The intermediate or compensating layer  13  is applied to the cooling element  11 . In divergence from the embodiment in  FIG. 5 , the layer  13  is provided with a further intermediate layer  15  made of nickel or a nickel alloy, for example of a nickel-silver alloy or another alloy, which contains at least one metal that is also a component of the solder of the solder layer  12  adjoining the intermediate layer  15  and therefore bonding the cooling element  11  with the ceramic-metal substrate. 
       FIG. 7 , in an enlarged partial representation, shows the cooling element  11  together with a laser bar  12 , which is oriented with its longitudinal extension perpendicular to the plane of projection of  FIG. 7  and comprises a plurality of laser light emitting emitters, which are provided offset from each other in the longitudinal direction of the laser bar. The cooling element  11  is again plate-shaped or cube-shaped, namely with the top side  11 . 1 , the bottom side  11 . 2 , the longitudinal sides  11 . 3  and the face sides  11 . 4  and  11 . 5  The laser bar  12  is provided on the top side  11 . 1  in the area of a face side  11 . 5 , namely so that it is oriented with its longitudinal extension parallel to said face side and to the top side  11 . 1 , i.e. it is oriented perpendicular to the plane of extension of  FIG. 7  and lies with its laser light emitting side approximately flush with the face side  11 . 5 . 
     At least in the area of the face side  11 . 5 , a compensating layer  13  is applied to the top side  11 . 1  and the bottom side  11 . 2 . The laser bar  16 , which is provided with a plate-shaped intermediate carrier  17 , is soldered onto the compensating layer  13  on the top side  11 . 1 , namely by means of the solder layer  18  provided between the compensating layer  13  and the intermediate carrier  17  (submount). The bond between the laser bar  16  and the intermediate carrier  17  is formed by a solder layer  19 , so that the laser bar lies with its laser light emitting side flush with a longitudinal side or longitudinal edge of the intermediate carrier  17  extending along the entire length of the laser bar  16 , the intermediate carrier however projecting with its other longitudinal side over the back of the laser bar  16 . 
     Due to the compensating layer  13 , in this embodiment, equalization is achieved between the different thermal expansion coefficients of the cooling element  11  made of copper or aluminum and of the intermediate carrier  17  made of Cu—Mo and therefore reduction of the stress on the solder layer  18 . To maintain a symmetrical design, with respect to the thermal aspects, the compensating layer  13  is provided with a corresponding layer  13   a  to the bottom side  11 . 2 , namely so that the thickness of the layer  13   a  is greater than the thickness of the compensating layer  13 , but less than the sum of the thicknesses of the compensating layer  13  and of the intermediate carrier  17 . 
     The invention was described above based on exemplary embodiments. It goes without saying that numerous modifications and variations are possible without abandoning the underlying inventive idea upon which the invention is based. 
     For example, the compensating layer  13  and/or counter-layer  13   a  can also be made of sputtered ceramic or high-strength metals. 
     It is also possible to manufacture the compensating layer  13  and/or counter-layer  13   a  as composite layers, namely as single or multi-ply layers, in which case the single plies are made of several different materials, for example metals or alloys of different metals, or different plies of different materials or material mixtures (e.g. metal alloys), which then for example are applied using different processes. It is possible, for example, to apply a metal ply (e.g. Cu ply) by cold spraying and a further ply (e.g. ceramic ply) by plasma spraying. 
     Special layers or plies made of diamond, carbon and/or carbon nanofibers can be applied by chemical vapor deposition (CVD), in which case these layers or plies can then be coated with Cu powder cold gas. 
     The cooling element  11  can also be part of a heat pipe, in which case the layers  13  and/or  13   a  also serve to seal risk zones against leaking and for this reason alone already contribute to improving the service life of a constructional of modular unit. 
     REFERENCE LIST 
     
         
           1  module 
           2  ceramic-metal substrate, especially ceramic-copper substrate 
           3  ceramic layer 
           4 ,  5  metallization, for example copper layer 
           6 ,  7  electronic component 
           8  module housing 
           9  interior of housing 
           10  connection 
           11  cooling element or heat sink 
           11 . 1  top side 
           11 . 2  bottom side 
           11 . 3  longitudinal side 
           11 . 4 ,  11 . 5  face side 
           12  solder layer 
           13  compensating layer 
           13   a  additional layer 
           14  arrangement 
           15  intermediate layer 
           16  laser bar 
           17  subcarrier or submount 
           18 ,  19  solder layer