The present invention relates to a method of attaching a body, in particular a heat sink, made of metal matrix composite (MMC) material or copper to a ceramic member, in particular a ceramic circuit board, wherein the ceramic member has a surface which is coated with a metal layer and bears upon the MMC body or the copper body.
In the following description, the term xe2x80x9cMMCxe2x80x9d or metal matrix compositexe2x80x9d will denote a material which is comprised of two components. The first of these components is a metal, also designated as matrix metal. The second component is a reinforcement material which is embedded in the matrix metal and predominantly made of a ceramic material in a wide variety of material configurations, such as for example particles, fibers or coarser pieces or in the form of a singular piece or few single-piece parts of porous material. In either case, the matrix metal surrounds or permeates the reinforcement material to realize a good mixture of both components.
Composite materials of this type exhibit especially desirable properties such as for example, a high degree of mechanical strength, good heat conductivity, good resistance against temperature fluctuations, a low expansion coefficient, and the like. The extent to which certain compositions of these composite materials exhibit those afore-described properties and the actual values of their mechanical strength, heat conductivity or others can thus be controlled. Accordingly, a matrix metal material composite (MMC) can be manufactured based on the choice of these properties by selecting a particular matrix metal and a particular reinforcement material in a given ratio.
As the present invention is not directed to the fabrication of MMC materials and the artisan is aware of methods as to the manner of producing MMC materials, a detailed description of particular production methods for MMC materials is omitted for the sake of simplicity.
Such MMC-material bodies find many applications and are used in a great variety of fields and industries. One of the many applications of MMC-bodies is in the field of electric/electronic circuits, where MMC-bodies are used as heat sinks for electric/electronic circuits, in particular also circuit boards, to provide a cooling function. By attaching the MMC-bodies to the printed circuit board of the electric/electronic circuit to be cooled they effect the desired cooling function.
Circuit boards for electronic circuitry, which include for example high capacity unit components such as for example, rectifying circuits, which also comprise IGBTs that are utilized for providing direct current for direct current machines are produced increasingly from ceramic material. In general, the strip conductors that are attached on the printed circuit board are made from copper and are attached to the circuit board surface preferably by means of a Direct Copper Bonding process (DCB-process).
In accordance with the prior art, attachment of a heat sink of MMC material to the ceramic circuit board is carried out by a soldering process, whereby the ceramic circuit board is soldered together with the heat sink. To carry out this process, the surface of the ceramic member that is placed adjacent at the MMC-body of the heat sink is coated with a metal, which can be soldered to the matrix metal of the heat sink. Normally, this metal is copper. Subsequently, the MMC heat sink and the metal-coated ceramic circuit board are being attached to each other by means of solder.
Use of solder for so attaching an MMC-body to the ceramic substrate has however considerable drawbacks. For example, the soldering process requires the use of additional material and furthermore, the solder has to be introduced into the space between the two bodies that are to be attached which, from a production-technical viewpoint, requires an additional production step. Another drawback is that solder, generally known to be a low thermal conductor thereby prevents an optimal heat flow to the heat sink. The following are examples showing the heat conductivity of various solders: Au80Sn20=58W/mK; Au88Ge12=44W/mK; Sn66Pb36Ag2=54W/mkK.
Furthermore, solder compounds that are conventionally used for the attachment of heat sinks onto ceramic substrates contain lead, which presents a health hazard to those working in the production line due to their exposure to the toxic vapors that develop when applying the solder, in addition to also presenting a general threat to the environment.
Aside from the afore-described drawbacks, a further drawback of the method known in the prior art is that the attachment of a copper body or, respectively a copper layer onto a ceramic substrate in particular on a substrate of aluminum nitride has been unsatisfactory.
It would therefore be desirable and advantageous to provide an improved method to obviate the above-stated shortcomings and to provide a method by which an attachment of a body formed from MMC-material or from copper onto a ceramic member can be realized, which is technically simple to carry out and, at the same time, has environment-friendly properties.
According to one aspect of the present invention, a method is provided for attachment of a body made from an MMC-material or from copper to a ceramic substrate that includes that the ceramic member is coated with a first metal layer and then placed on the body of MMC-material or on the copper body so that the ceramic substrate is resting with the first metal layer against the MMC-body or the copper body. The matrix metal of the MMC-body, respectively the copper body and the first metal of the ceramic substrate member are then heated above the eutectic temperature of a system formed from the matrix metal of the MMC-body, respectively the copper-body, and the first metal of the ceramic substrate and subsequently cooled to room temperature whereby an intimate bond is formed between the ceramic member and the MMC-body respectively the copper body.
Advantageously, the method according to the invention of attaching a body of MMC-material or of copper to a ceramic substrate without the use of any solder whatsoever avoids all shortcomings and drawbacks associated with a soldering process.
It is a further advantage of the method according to the invention that the bond between the MMC body, respectively the copper body, and the ceramic member form a eutectic bond, which is of greater stability than a bond formed merely by solder. Moreover, the bond of this type exhibits a higher melting point than a solder connection, so that the bond formed according to the method of the invention has a wider range of application in instances where exposure to higher temperatures is required.
The bond formed between a copper body and ceramic substrate member according to the process of the invention, for example a ceramic substrate from AlN (aluminum nitride) is considerably better than the bonds formed by the known prior art methods.
In accordance with a preferred method according to the invention, prior to placing the ceramic member on the body of MMC-material, a second metal is provided at the surface of the MMC-body, which is placed adjacent the ceramic member. The ceramic member and the MMC-body are then heated above the eutectic temperature of a system formed of the matrix metal of the MMC-body, the first metal deposited on the surface of the ceramic member and the second metal deposited on the surface of the MMC-body, and subsequently cooled to room temperature.
By selecting and coordinating these three metals i. e. the first metal of the ceramic member, the second metal at the MMC-body and the matrix metal of the MMC-body in an optimum manner, a ternary eutectic system can be realized, which forms a particularly intimate bond between the ceramic member and the MMC-body.
According to one feature of the method according invention, the matrix metal of the MMC-body is aluminum or an aluminum alloy. Bond systems using this type of MMC-body are especially suitable for use as heat sinks due to their particularly good thermal conductivity.
According to another feature of the method according to the invention, the reinforcement material of the MMC-body is SiC. An MMC-body comprising this reinforcement material exhibits a particularly high mechanical strength. Especially in combination with an aluminum containing matrix metal, a good thermal conductivity is realized making the MMC-body suitable for forming a heat sink.
According to yet another feature of the method according to the invention, the surface of the ceramic member placed adjacent the MMC-body is coated with copper.
Furthermore, according to yet another feature, the surface of the MMC-body placed adjacent the ceramic member is coated with copper.
The MMC-body and the ceramic member are separated by three metals, i. e. the matrix metal of the MMC-body, the first metal on the surface of the ceramic member and the second metal on the surface of the MMC-body. The second metal, which is, located on the MMC-body suitably reacts with the first metal on the ceramic member, thereby forming a eutectic. At the same time an additional reaction takes place between the second metal of the MMC-body with the matrix metal of the MMC-body, also forming a eutectic. These bonds can then react further among each other, whereby optimally, a ternary eutectic is being formed. Copper is particularly suitable as a component in a variety of material combination in order to form such a ternary system, so that the use of copper is universally applicable in the method according to the invention.
It has been shown that coating the surface of the ceramic, which is placed adjacent the MMC-body, is advantageously carried out by means of the DBC-process. With this process, a particularly intimate bond of the copper layer to the ceramic member can be realized.
According to another feature of the invention, for bonding a copper body with a ceramic member, the surface of the ceramic member, which is placed adjacent the copper body is coated with aluminum, wherein in a further variation, a copper layer can be added to the aluminum layer, for example by means of an electrochemical method, preferably by a galvanic process.
At 548xc2x0 C., aluminum and copper form a eutectic, so that by applying an aluminum coating to the ceramic member, pre-conditions for the correct reaction sequence of the method according to the invention are provided.
In accordance with yet another feature of the invention, the surface of the MMC-body that is placed adjacent the ceramic member, is coated with zinc, tin, germanium or similar material. These materials are especially suitable for completing a ternary system wherein an MMC-body of aluminum containing matrix metal and a copper coating exhibit a eutectic point.
In such a system, depositing the second metal at the surface of the MMC-body, which is placed adjacent the ceramic member, is carried out by means of a galvanic process. This coating process is well known, which is why it is suitable for controlling the good results that can be realized thereby.
Alternatively, the second metal which is deposited at the surface of the MMC-body placed adjacent the ceramic member is carried out by means of a current-free process, such as for example, by means of a tampon process. In this case the surface of the MMC-body is purposefully and selectively coated with metal.
As another alternative, the second metal can be provided at the surface of the MMC-body placed adjacent the ceramic member by depositing the second metal in the form of a metal foil. This type of process is technically especially simple because it requires only one tool for cutting the metal foil and requires no further installations, such as for example a galvanic bath or an oven or similar.
According to another feature of the process according to the invention, the MMC-body of the copper body and the ceramic member can be heated under normal atmospheric condition. Thus, the process according to the invention can be carried out with few resources. In particular, aside from providing heat for heating the two bodies, no special conditioning of the atmosphere in the oven is required.
Alternatively, the copper body and the ceramic member can also be heated in a protective gas atmosphere such as a nitrogen gas atmosphere, which has been proven particularly suitable. Furthermore, the MMC-body or the copper body can also be heated under a vacuum. While heating under a vacuum, there is virtually no oxygen surrounding the two bodies, so that no oxides are able to form in the bonding layer forming between the two bodies.
In a preferred embodiment of the method according to the invention, at least the MMC-body or the copper body, and in an even more preferred embodiment also the ceramic member are being held by graphite brackets during heating.
Graphite reacts with materials in its surrounding environment only at very high temperatures that are not reached during the process according to the invention, so that no compounds are being generated, which would negatively impact on the properties of the bond forming between the ceramic member and the MMC-body.
These aspects, and others which will become apparent hereinafter, are attained in accordance with the present invention by providing a method for attaching an MMC-body or a copper body, as a heat sink to a ceramic substrate such as a circuit board by depositing a first metal on the surface of a ceramic member, which is placed adjacent the metal matrix composite body; placing the surface of the ceramic member with the deposited first metal on the MMC-body; heating the two bodies to above a eutectic temperature for a system formed from the matrix metal of the MMC-body and the first metal deposited at the surface of the ceramic member; and subsequently cooling the system to room temperature.