Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1996-06-21
2001-02-06
Echols, P. W. (Department: 3726)
Metal working
Method of mechanical manufacture
Electrical device making
C029S847000
Reexamination Certificate
active
06182358
ABSTRACT:
BACKGROUND OF THE INVENTION
Metal-ceramic substrates and especially also copper-ceramic substrates are known in the most varied versions and are used in particular for electrical or electronic circuits and thus especially for power circuits (power modules).
In the simplest case these substrates consist of a single ceramic layer which is provided on both surface sides with metallic coating or a metal layer and thus preferably with a copper layer, the metal layer on the one surface side of the ceramic layer being preferably configured, i.e., forming printed conductors and/or contact surfaces, etc., of which during operation of the electrical circuit at least one part has a voltage potential which is different from the voltage potential of the other metal layer on the other surface side of the ceramic layer. By means of the ceramic layer fundamentally a relatively high dielectric strength between the metal layers on the surface sides of the ceramic layer is ensured. However, often these metal-ceramic substrates in certain areas are also often formed such that the metal layers on the two surface sides of the ceramic layer, for example, on the edge of the ceramic layer or in the area of the recesses, openings, interruptions, etc. provided in this ceramic layer are electrically separated from one another only on an air path along the edge of the ceramic layer so that sufficient dielectric strength or sufficient insulation distance between the metal layers cannot be achieved there on the two surface sides of the ceramic layer for higher voltages. Furthermore, cracking occurs preferably at these points, especially when the substrates are subjected to repeated temperature changes. Cracking is caused by the corresponding mechanical stresses which are the result of different coefficients of expansion.
Generally, in these cases the dielectric strength is improved by using insulating compounds. In existing metal-ceramic substrates, dielectric strengths cannot be reached which are sufficient for higher voltages, i.e. for voltages, for example, higher than 1000 V. Occurrence of thermomechanically induced cracks cannot be prevented in this way.
It addition, it is also known that ceramic-metal substrates are produced by attaching (U.S. Pat. No. 4,500,029) metal foils or metal boards flat for forming metal layers on the surface side of the ceramic layer by means of the so-called direct bonding process (DB process) or eutectic bonding process, in which it is also known in these substrates that a metal layer is formed on one surface side of the ceramic layer in at least one sub-area in the manner of a bridge, such that this metal layer has a distance from the plane of the pertinent surface side of the ceramic layer there. A printed conductor provided on the same surface side of the ceramic layer is routed through the bridge-like section or area in this known configuration of a metal-ceramic substrate.
In addition, attaching a metal strip on the top of a ceramic layer by means of the DB method so that this strip projects with a sub-area above the border of the ceramic layer is also known (DE 25 08 224). The metal strip is first oxidized on one surface side for this purpose. The oxide is then removed on the sub-area with which the metal strip will project above the edge of the ceramic layer. Then the metal strip with its remaining oxidized surface side is placed on the ceramic layer and then by heating the ceramic layer and metal strip to a process temperature which exceeds the eutectic temperature of the metal oxide, but which is below the melting point of the metal of the metal strip, the bond is produced.
SUMMARY OF THE INVENTION
The problem of the invention is to devise a process for producing a metal-ceramic substrate which has improved dielectric strength or improved insulation spacing in those areas in which at least one of the metal layers provided on a ceramic layer projects above one border or one edge of the ceramic layer, and prevents formation of thermomechanically induced cracks.
To solve this problem, a process is devised in which an initial substrate is deformed so that a metal layer has an edge disposed a distance from one surface of a ceramic layer and the edge is raised over a border area of predetermined width from the ceramic layer.
In the process according to the invention an initial substrate is first produced which has at least one ceramic layer and on the surface sides of this ceramic layer one metal layer each, at least one metal layer provided on one surface side of the ceramic layer projecting above one edge of the ceramic layer. In a subsequent process step then this projecting sub-area of at least one metal layer is permanently mechanically deformed such that the metal layer on the edge is at a distance from the surface side of the ceramic layer and along the edge in one border area with a stipulated width. This selective and precisely controlled deformation and separation of the metal layer from the ceramic layer greatly increase the distance decisive for the insulation resistance and dielectric strength between the one metal layer on one surface side of the ceramic layer and the other metal layer on the other surface side of the ceramic layer and thus greatly improve dielectric strength, especially in ceramic layers as are used conventionally for producing metal-ceramic substrates for electrical circuits.
In order to remove metal residues which may be adhering on the ceramic or ceramic layer the substrate is treated after deformation of at least one metal layer or sub-area of this metal layer in a suitable etching agent. For copper a solution of 10% sodium persulfate in 10% sulfuric acid is suitable as the etching agent, for example.
REFERENCES:
patent: 3487541 (1970-01-01), Boswell
patent: 4295184 (1981-10-01), Roberts
patent: 4500029 (1985-02-01), Yerman
patent: 4527330 (1985-07-01), Sturm et al.
patent: 4737236 (1988-04-01), Perko et al.
patent: 4831723 (1989-05-01), Kaufman
patent: 5029386 (1991-07-01), Chao et al.
patent: 5353499 (1994-10-01), Hattori et al.
patent: 4004844 (1991-03-01), None
patent: 3931551 (1991-04-01), None
Echols P. W.
Hoffman Wasson & Gilter PC
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