Adhesive bonding and miscellaneous chemical manufacture – Surface bonding means and/or assembly means therefor
Reexamination Certificate
2002-08-27
2004-11-30
Wilczewski, Mary (Department: 2822)
Adhesive bonding and miscellaneous chemical manufacture
Surface bonding means and/or assembly means therefor
C257S706000, C257S707000
Reexamination Certificate
active
06823915
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a heat-conducting adhesive joint between two workpieces and a method for producing a heat-conducting adhesive joint between two work-pieces.
BACKGROUND OF THE INVENTION
Electronic components, in particular power semiconductor components such as IGBTs, MOS-FETs, diodes, thyristors and so on produce high power losses during operation, which have to be dissipated efficiently in order not to exceed the maximum operating temperature.
For semiconductor chips up to at most 2×2 cm in size, soft soldering with tin, lead and their alloys on supporting elements made of ceramic or metal has become completely widespread in the technology. Other methods, such as those with gold solders, glass pastes and so on have found only a very narrow field of use, for cost reasons.
Development trends are leading, on one hand, to ever higher operating temperatures until close to the melting points of the solders, with the reliability increased at the same time, but on the other hand lead should be superseded for environmental protection reasons, in particular also by law.
Adhesive bonding, which is otherwise very common in chip assembly technology, suffers from poor thermal conductivity and also poor electrical conductivity of the adhesives.
This poor thermal conductivity of the adhesives can be improved by means of particles which conduct heat well and which are suspended in the poorly conducting adhesive. For example, DE-A-195 29 627 (95P1762 DE) discloses improving the thermal conductivity of the adhesive by adding a heat-conducting powder, for example nickel powder.
In practical terms, this document describes a heat-conducting, electrically insulating adhesive joint between two workpieces, said joint having a layer of ceramic material and a layer of adhesive.
The layer of ceramic material has two flat surfaces which face away from each other and, on each flat surface, there are openings defined by voids in the layer, and the layer is arranged between the two workpieces in such a way that one of the two flat surfaces of one of the two workpieces, which is constructed in the form of a heat sink, has flat contact made with it. Furthermore, at least the openings on the other flat surface, which faces away from the one flat surface, is filled with electrically insulating material.
The layer of adhesive is arranged between the layer of ceramic material and the other workpiece, which forms an electronic power component, and has two flat surfaces that face away from each other. One of these surfaces makes flat contact with the other workpiece and adheres to the latter. The other surface makes flat contact with the other flat surface of the layer of ceramic material and adheres to said layer.
In order to improve the thermal conductivity of the layer of adhesive, a heat-conducting powder, for example nickel powder, is added to said layer.
This known adhesive joint is produced as follows:
The ceramic layer is produced on one workpiece by means of thermal spraying, the openings defined by voids in the layer being produced automatically on the flat surfaces of this layer.
At least the openings on the other flat surface facing away from the one flat surface and the one workpiece are filled with electrically insulating material.
The layer of the ceramic material which comprises the electrically insulating material is joined to the other workpiece by means of the adhesive layer to which the heat-conducting powder is added.
DE 34 14 065 A1 (84 P 1304) and EP 0 242 626 A2 (86 P 1242) in each case reveal a different type of adhesive-free joint between a workpiece in the form of an electronic component and a workpiece in the form of a substrate, which has a layer of heat-conducting material in the form of a sintered metal powder and is therefore both thermally conductive and electrically conductive.
The layer of the sintered metal powder has two flat surfaces which face away from each other and each of which has openings defined by voids in this layer.
The layer is arranged between the two workpieces in such a way that one of the two flat surfaces is sintered onto one of the two workpieces and the other flat surface is sintered onto the other workpiece.
The sintered metal powder of the layer is coherent from one of the flat surfaces in the direction of the other flat surface.
The production of the adhesive-free joint according to DE 34 14 065 A1 is carried out by the following steps:
A paste is applied to one workpiece and/or the other workpiece, said paste being composed of a mixture of a metal powder which can be sintered at a specific sintering temperature and a liquid.
The two workpieces are brought together in such a way that the paste is located between the two workpieces and makes contact with both workpieces.
The paste is dried and the dried powder is sintered by heating to the sintering temperature. This sintering is carried out in a non-oxidizing atmosphere, for example N
2
or H
2
, and the sintering temperature in this case is about 400° C. During the sintering operation, a mechanical pressure, for example 80 to 90 N/cm
2
, can be exerted.
The production of the adhesive-free joint according to EP 0 242 626 A2 is carried out by the following steps:
A paste is applied to a workpiece, said paste being composed of a mixture of a metal powder that can be sintered at a specific sintering temperature and a liquid.
The paste is dried.
The other workpiece is placed on the dried powder.
The entire arrangement is then heated to sintering temperature with the simultaneous exertion of a mechanical pressure of at least 900 N/cm
2
. The sintering temperature is about 230° C. to 250° C.
In the thesis by Sven Klaka: “Eine Niedertemperatur-Verbindungstecknik zum Aufbau von Leistungshalbleiter-modulen” [A low-temperature joining technique for the assembly of power semiconductor modules], Cuvillier Verlag, Göttingen 1997, in this connection the sintering operation in the case of silver powder is examined at low sintering temperatures between 100° C. and 250° C. and it is established that this powder can form sintered bridges between 200° C. and 250° C.
SUMMARY OF THE INVENTION
The invention is based on the object of providing a heat-conducting adhesive joint between two workpieces which exhibits a higher thermal conductivity than a joint with a layer of adhesive to which a heat-conducting powder is added.
This object is achieved by the features of claim
1
.
According to said claim, the heat-conducting adhesive joint according to the invention comprises:
a layer of heat-conducting material,
which has two flat surfaces facing away from each other,
which, on each flat surface, has openings defined by voids in the layer,
which is arranged between the two workpieces in such a way that one of the two flat surfaces (
31
) makes flat contact with one of the two workpieces, and the other flat surface makes flat contact with the other workpiece, and
whose heat-conducting material is coherent from one of the flat surfaces in the direction of the other flat surface,
and
an adhesive
which fills the openings in the layer and
which adheres to both workpieces.
The term “coherent” is to be understood in such a way that, in the layer of heat-conducting material, this material coheres from one of the flat surfaces in the direction of the other flat surface of the layer outside the voids of this layer, or forms a unit, at least in such a way as in the case of a layer of sintered powder of heat-conducting material. Such coherence, beneficial for the thermal conductivity, is not present in a thermally and electrically poorly conducting layer of adhesive to which heat-conducting powder is added, since heat-conducting paths are formed in this layer only at comparatively few points of contact between the particles of the added powder.
The higher the thermal conductivity of the heat-conducting material of the layer, the more beneficial this is for the heat-conducting adhesive joint according to the invention.
One advantage of the adhesive joint according to the invention is that it can b
Lewis Monica
Siemens Aktiengesellschaft
Wilczewski Mary
LandOfFree
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