Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – By pressure alone
Reexamination Certificate
2000-01-21
2001-11-20
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
By pressure alone
C257S726000, C257S727000, C257S718000, C257S719000, C257S690000, C257S602000, C257S177000, C361S787000, C361S789000
Reexamination Certificate
active
06320268
ABSTRACT:
TECHNICAL FIELD
The invention relates to the field of power electronics. It is based on a power semiconductor module according to the preamble of the first claim.
DISCUSSION OF THE BACKGROUND
Such a power semiconductor module has already been described in Laid-Open Specification DE 195 30 264 A1. This is a so-called pressure-contact semiconductor module, in which a plurality of semiconductor chips are arranged with their first main electrode on a base plate. The second main electrodes of the chips make electrical contact with a plurality of contact stamps. The base plate is connected to first main connection, and the contact stamp is connected to a second main connection.
It is difficult to solder the individual chips at the same level, and even harder to keep them plane-parallel. In the abovementioned Laid-Open Specification, the position of each individual contact stamp is set individually on the basis of the distance between the semiconductor chip with which contact is to be made, and the second main connection. This allows the requirements for plane-parallelity of the chips surfaces to be reduced. The position of the contact stamps, which are mounted such that they can move and are of a constant length, is set by means of a spring which is arranged in a hole provided to accommodate the contact stamps. The force acting on the module is transmitted to the individual semiconductor chips via these springs.
The solution specified in the abovementioned Laid-Open Specification has the disadvantage that the current must flow through the spring. The electrical conductivity of the spring itself and, in particular, that of the junction between the spring and the contact stamp or the second main connection, is, however, often inadequate.
SUMMARY OF THE INVENTION
The object of the present invention is to specify a power semiconductor module in which all the semiconductor chips have the same pressure applied to them irrespective of the distance between them and the second main connection, and in which the electrical conductivity of the means provided for making contact is improved. This object is achieved by a power semiconductor module having the features of the first claim.
The essence of the invention is to separate the means which are provided for the electrical connection between the semiconductor chips and the second main connection from the spring elements which are responsible for the standard contact force for all the semiconductor chips. An electrical connection for the second main connection of the module is provided for each semiconductor chip, and is distinguished by good conductivity and low contact resistances.
To this end, contact elements are provided which have two planar contact surfaces. The distance between these contact surfaces is bridged by a connecting element. This connecting element must be flexible and is preferably in the form of a clip or a wire-like conductor.
Each contact element is spread by a spring element. In this case, contact surfaces are pressed on the one hand against the cover plate and on the other hand onto the semiconductor chip, thus allowing a low contact resistance. The contact force is transmitted by the stressed spring element, which is arranged between the contact surfaces and contributes little or nothing at all to electrical conduction.
Power semiconductor modules having semiconductor chips with which contact is made by pressure are in general resistant to short circuits, since there are no thin bonding wires which can melt as a result of overloading. However, when contact is made by pressure, the sensitive semiconductor chips, in particular IGBT (Insulated Gate Bipolar Transistor) chips are subjected, without special precautionary measures, to unacceptable mechanical loading or mechanical overloading, which can lead to destruction of the chips. Damage can occur in particular when modules connected in series are clamped in a stack. In one preferred embodiment of the present invention, this is avoided by providing supporting elements between the first and the second main connections, in order to absorb any overloading. These supporting elements limit the spring movement and thus the maximum contact force transmitted by the spring element. As soon as the cover plate is in contact with the supports, any further load is absorbed by them, and the contact pressure on the semiconductor chips is decoupled from the external clamping-in load.
According to a further preferred embodiment, the units formed from the spring and contact elements can be prefabricated, and can easily be placed on the chips while the semiconductor module is being fitted. The said unit is even more compact and easier to handle if the spring element is prestressed by using suitable restraining means to prevent it from being completely relieved of stress. This allows, in particular, more economic and thus simpler production of the semiconductor modules.
The contact with the chips by means of the combination of a contact element and spring element according to the invention ensures a permanent, low impedance contact resistance between the casing contacts and the chip. In consequence, if a chip fails, the entire nominal current is passed via the defective chip without damaging the corresponding contacts.
Further advantageous embodiments are evident from the dependent claims.
REFERENCES:
patent: 4769744 (1988-09-01), Neugebauer et al.
patent: 5247425 (1993-09-01), Takahasi
patent: 5842512 (1998-12-01), Guerrero
patent: 5942796 (1999-08-01), Mosser et al.
patent: 6058014 (2000-05-01), Choudhury et al.
patent: 31 52 040 (1982-08-01), None
patent: 195 30 264 (1997-02-01), None
Bucher Benno
Frey Toni
Lang Thomas
ABB (Schweiz) AG
Crane Sara
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Parekh Nitin
LandOfFree
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