Patent Application
20060263584
The invention relates to a composite material or a composite raw material according to the preamble of claim 1 and to an electric circuit or an electric module according to the preamble of claim 32.
A “composite material” or “composite raw material” according to the present invention is generally a material comprising several material components, for example in a common matrix or also at least partially in at least two adjacent material sections that are bonded together.
A “component for thermal dissipation” or a “heat sink” according to the invention are generally components that are used in electronics and particularly in power electronics and are used to dissipate heat loss and to cool electric or electronic components, for example base plates and/or thermal dissipation or cooling plates in electric circuits or modules, substrates for electric or electronic components, housings or housing elements of electric components or modules, and also, for example, coolers, heat pipes or elements of such active heat sinks through which a coolant flows, such as water.
In many areas of technology, composite materials are used as materials for constructions, components, etc., particularly when material properties are required that cannot be achieved with a single material component. The desired properties for the composite material can be optimized by carefully selecting the individual components and the physical and/or chemical properties of these components, for example the thermal properties.
“Materials for Thermal Conduction”, Chung et al., Appl. Therm. Eng., 21, (2001) 1593-1605, gives a general overview of materials for thermal conducting or thermal dissipating materials. The article outlines the properties of possible individual components and relevant examples for the composite materials.
Ting et al reports in J. Mater. Res., 10 (6), 1995, 1478-1484 on the manufacture of aluminum VGCF (Vapor Grown Carbon Fiber) composites and their thermal conducting properties. U.S. Pat. No. 5,814,408 Ting et al is the resulting patent specification for the AL-VGCF MMC.
Composites with Carbon Fibrils™, a defined CVD carbon fiber, in both a metal and polymer matrix are mentioned in U.S. Pat. No. 5,578,543 Hoch et al.
Ushijima et al describes in U.S. Pat. No. 6,406,790 the manufacture of a composite material with a special variant of CVD grown carbon fibers as a filling material by means of pressure infiltration of the matrix metal.
Houle et al report in U.S. Pat. No. 6,469,381 on a semiconductor element that dissipates the heat produced during operation by including carbon fibers in the substrate.
The use of coated carbon fibers in composite materials with a metallic matrix is described by Bieler et al in U.S. Pat. No. 5,660,923.
Al2O3 fibers in an Al matrix and the manufacture of corresponding fiber-reinforced composite materials is described in U.S. Pat. No. 6,460,497, McCullough et al.
Due to the improved electrical properties, the use of metal-ceramic substrates as printed circuit boards, for example substrates made of aluminum oxide (Al2O3) or increasingly also substrates made of aluminum nitride (AlN) is known in particular for power modules, which are increasingly being used in electric drive systems, for example in traffic and automation technology. Layers or base plates made of copper, which have high thermal conductivity and therefore are suitable for dissipating power or heat loss and also for thermal spreading, have heretofore been used for the substrate or transition layer to a heat sink, which often has to dissipate a considerable power loss from such a power module.
The disadvantage of this is the high fluctuation in the thermal expansion coefficients of the materials used, namely of the ceramic, the copper and also of the silicon of the active electric or electronic components of such a module. Such power modules and their components are subjected to a considerable change in temperature not only during the manufacturing process, but also during operation, for example during the transition from the operating phase to the dwell or not operating phase and vice versa, and also when the module is switched during operation. Due to the differing expansion coefficients, these temperature changes cause mechanical stress in the module, i.e. mechanical stress between the ceramic and the adjacent metallizations or metal layers (such as base plate on one side of the ceramic layer and strip conductors, contact surfaces, etc. on the other side of the ceramic layer), and also between metal surfaces and the electric or electronic components located thereon, in particular semiconductor elements. Frequent mechanical alternating stress causes material fatigue and therefore failure of the module or its components.
This problem is compounded by the additional factor of miniaturization and by the ensuing increase of the power density of power modules. The thermal expansion coefficients of the material components of a power module with a copper-ceramic substrate are within the range of
=16.8×10−6K−1 for copper and
=3×10−6K−1 for silicon.
Reference is also made to the following table, in which the thermal conductivity and the thermal expansion coefficient
are specified for various materials.
Since the thermal conductivity for dissipation of the power loss is necessary, the metals used especially in semiconductor modules or their substrates for the metallizations, the base plate, etc. must be able to conduct heat sufficiently. At present, materials with a copper or aluminum base, such as Cu—W, Cu—Mo or Al—SiC are preferred especially for heat sinks.
The method of manufacturing the metallization required for strip conductors, connections, etc. on a ceramic, for example on an aluminum-oxide ceramic, using the direct copper bonding technology is known in the art, the metallization being formed by metal or copper foils or metal or copper sheets, featuring on the top side a layer or coating (melt layer) from a chemical bond with the metal and a reactive gas, preferably oxygen. In the method described for example in U.S. Pat. No. 3,744,120 or in DE-PS 23 19 854, this layer or coating (melt layer) forms a eutectic with a melting temperature below the melting temperature of the metal (e.g. copper), so that when the foil is placed on the ceramic and all layers are heated they are bonded together, namely through melting the metal or copper essentially only in the area of the melt layer or oxide layer.
This DCB method then comprises the following steps:
oxidation of a copper foil in a manner that results in an even copper oxide layer;
placing of the copper foil on the ceramic layer;
heating of this composite or strukture to a process temperature between approximately 1025 and 1083° C., e.g. to approximately 1071° C.;
cooling to room temperature.
The object of the invention is to provide a composite material, which while maintaining a high degree of thermal conductivity, which is greater than or at least equal to that of copper or copper alloys, has a thermal expansion coefficient significantly lower than that of copper. This object is achieved by a composite material according to claim 1. An electric circuit or an electric module is exemplified according to claim 32.
The composite material according to the invention, which is suitable for example for electrical engineering applications and therefore for applications as a substrate or as a component for dissipating heat in electric power modules, consists therefore essentially of three main components, namely of at least one metal or at least one metal alloy, of at least one ceramic and of nanofibers, which have a thickness between 1.3 nm and 300 nm, and the length/thickness ratio for a majority of the nanofibers contained in the composite material being greater than 10. The ceramic content can be replaced partially or entirely by glass, for example by silicon oxide.
The nanofibers used effect the desired reduction of the thermal expansion coefficient of the composite material in at least two perpendicular spatial axes, preferably in all three perpendicular spatial axes.
In the embodiment of the composite material according to the invention, the following measures are possible in further embodiments of the invention:
The nanofibers are distributed isotropically in their orientation at least in the at least two spatial axes.
At least some of the nanofibers are for example nanotubes, which are especially stable in axial direction, so that they contribute very effectively to the desired reduction of the thermal expansion coefficient.
The nanofibers preferably are made of an electrically conductive material, so that the composite material comprising the nanofibers or the part of the composite material comprising the nanofibers can also be used for electric strip conductors or contacts, etc., i.e. it possesses the necessary electric conductivity for this application.
The nanofibers are preferably such made of carbon and/or of boron nitride and/or of tungsten carbide. Other materials or composites that are suitable for the manufacture of nanofibers are also conceivable, in particular nanofibers made of carbon and coated with boron nitride and/or tungsten carbide.
The ceramic used for the composite material according to the invention is preferably an aluminum oxide ceramic or an aluminum nitride ceramic, in which case the aluminum nitride ceramic is characterized by especially high electric strength and by increased thermal conductivity.
The metal component used for the invention is preferably copper or a copper alloy. This applies in particular in the event that the composite material is to be used for substrates or printed circuit boards or as a component for thermal dissipation in electric circuits or modules. Copper, and also copper alloys, are relatively easy to process, in particular when this material component of the composite material contains the nanofibers.
It is also possible to provide the nanofibers in the at least one metal or the at least one metal alloy and/or in the ceramic and/or in the glass, for example in a matrix formed by the metal or metal alloy.
The nanofiber content of the composite material is for example between 10 and 70 percent by volume, preferably between 40 and 70 percent by volume, in relation to the total volume of the material component of the composite material containing said fibers.
If the nanofibers are contained in the metal or in the metal alloy of the composite material, then a wide range of methods are available for implementing this special design. It is possible, for example, to first form a perform from the nanofibers, for example in the form of a three-dimensional latticework, a fleece-like structure, a hollow body or tubular structure, etc., wherein the at least one metal or the at least one metal alloy is incorporated into this perform. A wide variety of methods is conceivable for this design in particular, for example through chemical and/or electrolytic precipitation, through melt infiltration, etc.
According to one embodiment of the invention, the composite material is a fiber-reinforced ceramic-glass composite as a substrate for electric or electronic applications and consists of a carrier substrate based on ceramic and/or glass materials and at least one fiber-reinforced metal layer applied to one side. The fibers in the metal layer are then for example carbon nanotubes, with a thickness of 1.3 to 300 nm and a length/thickness ratio >10, and the nanofibers are present in the metal matrix of the metal layer with a content of 10 to 70 percent by volume. If the carrier substrate also contains nanofibers, then they have a high nitride and/or tungsten carbide content.
Furthermore, it is possible to apply the metal and the nanofibers to a perform or a substrate made of metal and/or ceramic, for example through chemical and/or electrolytic precipitation.
Other methods for manufacturing the matrix of the at least one metal or the at least one metal alloy with the nanofibers are conceivable, for example, the so-called HIP technology, in which the at least one metal or the at least one metal alloy is inserted into a capsule in powder form mixed with the nanofibers, after which the capsule is tightly sealed with a cap. Afterwards, the interior of the capsule is evacuated and sealed so that it is gastight. Then pressure is applied to the entire capsule (e.g. gas pressure using inert gas, for example argon, or hydrostatic pressure) and therefore also to the material contained in the capsule, while simultaneously heating it to a process temperature between 500 and 1000° C.
In a further process step, after cooling, the capsule and the metal blank containing the nanofibers are separated, so that the blank can be further processed, for example through machining or cutting, sawing and/or rolling to manufacture boards or foils, which then are bonded with a ceramic layer to manufacture a metal-ceramic substrate or a printed circuit board.
Especially for use in electric or electronic components the composite material according to the invention is designed as a laminate, namely with at least two bonded material sections or layers, where one material section or one layer is made of the at least one metal or the at least one metal alloy and the other material section or the other layer is made of ceramic. The nanofibers are then contained for example in the at least one material section made of the metal or the metal alloy. Generally it is also possible that the nanofibers are likewise contained in the ceramic, for example to reinforce the mechanical stability of the ceramic and/or to improve the thermal conductivity of the ceramic.
If the composite material consists of at least one material section made of the at least one metal or the at least one metal alloy and of the material section made of ceramic, then both material sections or layers are bonded together, for example by soldering, preferably by the active soldering process, or using the known direct bonding technology.
Especially in the possible embodiment of the composite material as a metal-ceramic substrate or printed circuit board, a metallization is provided on at least one surface of one ceramic layer, which (metallization) is formed by the at least one metal or the at least one metal alloy and contains the nanofibers. This metal layer is then for example the base plate of such a substrate or is bonded with such a base plate, with which the substrate is bonded with a passive heat sink, for example in the form of a cooler body or with an active heat sink, for example in the form of a cooler through which a coolant flows, also a micro cooler.
Strip conductors and/or contact surfaces and/or fixing or fastening surfaces for components of an electric circuit or module, for example, are then provided on the other surface of the ceramic layer. The metal or metal alloy forming these strip conductors, contact surfaces, etc. can also contain the nanofibers, in which case the structured metallization of the strip conductors etc. is effected in the normal manner, namely in that after application of a metal layer, this layer is formed into the structured metallization, for example through an etch-masking process.
Therefore, the invention is used to create a composite material in which considerably higher conductivity (e.g. >380 W(mK)−1) is achieved through the dispersion of the nanofibers into the metal matrix, for example copper matrix, combined with reduced thermal expansion. Furthermore, especially the use of copper for the metal matrix ensures easy processing of the metal containing the nanofibers, so that all standard processing methods, such as drilling, milling, punching, and also chemical processing, are possible.
The composite material according to the invention can be used for solutions in the field of thermal management that previously presented major difficulties, e.g. also in laser technology, where in particular the differing thermal expansion coefficients between the semiconductor material of a laser bar and the metal of a heat sink considerably decreases the service life of laser diodes or laser diode arrays. The improved thermal conductivity can be used to achieve higher power densities than previously possible in electric and electronic power modules, namely with the possibility of miniaturization of electrical and electronic modules and assemblies and with the possibility of additional applications especially also in such technical fields in which the miniaturization and ensuing reduction of mass and weight is of consequence, as in air and space technology.
The composite material according to the invention makes it possible to combine in one material properties that were previously less than optimally compatible. If the nanofibers are provided in the metal matrix, then they serve as reinforcing components, which, with their high thermal conductivity (greater than 1000 W(mK)−1) and their negligible thermal expansion coefficient, significantly reduce the expansion coefficient of the overall composite material and Improve its thermal conductivity.
The invention is described below in detail based on exemplary embodiments with reference to the drawings, wherein:
The component 3 is a power semiconductor component, e.g. a transistor for switching high currents, e.g. for controlling an electric motor or a drive. Other power semiconductor components are also conceivable, for example laser diodes. The thickness of the base plate 4 in the axis direction perpendicular to the planes of the metallizations 6 and 7 is a multiple of the thickness of the foils used for these metallizations 6 and 7.
The two metallizations 6 and 7 are bonded two-dimensionally with one surface of the ceramic layer 5 using a suitable method, for example by means of DCB technology or the active soldering process. Furthermore, the metallization 6 is structured in the required manner, preferably using the etch-masking method known to persons skilled in the art, in order to form strip conductors, contact surfaces, fasting surfaces for fastening or soldering of components 3, of shielding surfaces or strips functioning as inductors, etc. Other methods are also conceivable, for example in the manner that the structuring is produced by mechanical processing of the foil forming the metallization 6, for example following or preceding the application of the metallization 6 to the ceramic layer 5. The foil forming the metallization 7 is not structured in the depicted embodiment. In the depicted embodiment, this foil covers a large part of the bottom of the ceramic layer 5, wherein the edge area of the ceramic layer 5 is kept free from the metallization 7 in order to increase the electric strength, i.e. the edge of the metallization 7 ends at a distance from the edge of the ceramic layer 5. Furthermore, the base plate 4 in the depicted embodiment is designed so that its perimeter clearly protrudes beyond the perimeter of the copper-ceramic substrate 2. The base plate 4 is for example the base plate of a housing of the power module not further depicted.
The metallization 7 is connected two-dimensionally on its surface facing away from the ceramic layer 5 with the base plate 4, using a suitable method, such as soldering, brazing or active soldering, or likewise using DCB technology. The base plate 4 in the depicted embodiment is likewise made of a metal or a metal alloy, for example of copper or a copper alloy, wherein the metal or the metal alloy of the base plate 4 likewise contains the nanofibers with a content of 10-70 percent by volume relative to the total volume of the base plate 4, preferably with a content of 40-70 percent by volume. The nanofibers in the metallizations 6 and 7 and in the base plate 4 are distributed isotropically or nearly isotropically with respect to their orientation at least in the two perpendicular spatial axes that define the planes of the metallizations 6 and 7 and the plane of the top of the base plate 4 connected with the metallization 7.
The nanofibers have a thickness between 1.3 nm and 300 nm, wherein the greater part of the nanofibers contained in the metal matrix has a length/thickness ratio>10. The nanofibers in this embodiment have a carbon base or are made of carbon, for example in the form of nanotubes. Generally it is also possible, however, to replace these nanofibers made of carbon in whole or in part with nanofibers made of another suitable material, for example of boron nitride and/or tungsten carbide. Generally the nanofibers can be distributed Isotropically with respect to their orientation in all three perpendicular spatial axes, i.e. in the two axes defining the planes of the metallizations 6 and 7 and the top of the base plate 4 and in the axis extending perpendicular to the other two axes.
The use of nanofibers in the matrix of the metal or metal alloy significantly reduces the thermal expansion coefficients of the metallizations 6 and 7 and in particular also of the base plate 4, especially in the axes of the preferred orientation of the nanofibers, namely in the axes defining the planes of the metallizations and the planes of the top of the base plate, to a value of <5×10−6K−1, especially also in the relevant temperature range for substrates of semiconductor modules, i.e. between room temperature (approximately 20° C.) and 250° C. The electric conductivity especially of the strip conductors formed by the metallization 6 corresponds to the electric conductivity of copper or of a copper alloy without the nanofibers.
The thermal conductivity of the metallizations 6 and 7 and of the base plate 4 is greater than that of copper and is for example on the order of
=600 W(mK)−1 or greater. Due to the extremely reduced thermal expansion coefficient
as compared with pure copper or a copper alloy, it is clearly adapted to the thermal expansion coefficient of the silicon of the semiconductor component 3, and also clearly adapted to the thermal expansion coefficient of the ceramic of the ceramic layer 5. This significantly reduces thermal stress, as a result of temperature changes in the power module 1, between the metallization 6 and the silicon body of the components 3 and the ceramic of the ceramic layer 5, and in particular also thermal stress between the metallization 7 reinforced by the base plate 4 and the ceramic layer 5. Such temperature changes are caused by the switching states of the power module 1, and also by changes in power during operation of the power module, for example by corresponding control of this module.
The improved thermal conductivity as compared with copper significantly improves the thermal dissipation of the heat loss produced by the semiconductor component 3 and also significantly improves the thermal spreading through the metallization 7 and improves the transfer of the power loss to the base plate 4. The latter is then connected with a passive heat sink, for example with a cooler or radiator, which is located in a current of a medium dissipating the heat loss, in the simplest case an air current, or the base plate 4 is connected to an active heat sink, for example with a micro cooler, through which a coolant flows, for example a gaseous and/or vaporous and/or liquid coolant, for example water. Furthermore it is possible to provide the base plate 4 on a so-called heat pipe for the especially effective dissipation of the heat loss from this base plate 4 to a passive or active cooler.
As an alternative to the embodiments described above, it is also possible to design the base plate 4 as a cooler, in particular as an active cooler, e.g. micro cooler, through which the coolant flows, or also as a heat pipe. In these cases it is also advantageous to manufacture a part of the cooler or heat pipe, which (part) is connected to the metallization 7, from the metal containing the nanofibers or from the corresponding metal alloy.
Mixing additives can also be added to the mixture 8, especially in order to maximize the portion of nanofibers and to achieve an even distribution of these fibers, inter alia, to reduce the adhesion between the nanofibers. Furthermore, to improve the bond between the metal, for example copper, and the carbon of the nanofibers, it can be advantageous to use nanofibers with a fishbone surface structure, which improves the mechanical bond. It can also be advantageous to coat the nanofibers with reactive elements, which cause a chemical bond, and/or to charge the nanofibers with the metal and/or with ceramic and/or boron nitride and/or tungsten carbide, for example by means of vapor deposition, for example.
In a further process step (position b) a cap 10 is placed on the upper opening of the capsule 9 and is bonded tightly with the capsule, for example by welding.
In a further process step, the interior of the capsule 9 is evacuated by means of a connection 11 provided on the cap 10 and the interior of the capsule 8 is then sealed so that it is gastight.
In a further process step (position d) the ductile, sealed capsule 9 is subjected to high pressure on all sides at a process temperature between 500 and 1,000° C., for example. This pressurization on all sides of the capsule 9 takes place in a closed chamber 12 by means of hydrostatic pressure acting on the capsule 9, as indicated in position d by the arrows there. This actual HIP process causes a reduction in volume, resulting in deformation of the capsule 9. As a rule, the loss of volume occurring during this deformation is approximately 5-10%, but can also be greater, for example as high as 20%. The capsule 9 and the corresponding cap 10 and the connection between these two elements is such that the capsule is not damaged. In order to calculate the reduction behavior, the capsule 9 has a simple geometric shape and thin walls.
After the HIP process, the capsule 9 and the starter material manufactured for example as a block in the HIP process are separated, so that the starter material can be further processed in a suitable manner.
The capsule 9 and its cap 10 serve several functions in the HIP process, namely as an enclosed space during the evacuation for reduction of the open porosity in the powdered starter material, for transfer of the hydrostatic pressure during the actual HIP process and also for shaping the end product produced by the method.
The starter material obtained from this process is then used directly as a layer containing the metal or metal alloy together with the nanofibers in a laminated embodiment of the composite material according to the invention, for example for the metallizations 6 and 7 or the base plate 4 of the power module 1 of
In deviation from the above description, it is possible in the process of
For the perform in the process in
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 on which the invention is based.
For example, it is possible with the power module 1 of
| Number | Date | Country | Kind |
|---|---|---|---|
| 103208380 | May 2003 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DE04/00824 | 4/20/2004 | WO | 10/26/2005 |
