Power semiconductor module

Details Power semiconductor module Power semiconductor module

2000-09-07

2003-01-14

Ho, Hoai (Department: 2818)

Active solid-state devices (e.g., transistors, solid-state diode

Housing or package

For plural devices

C257S723000, C257S718000, C257S719000

Reexamination Certificate

active

06507108

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a power semiconductor module having a baseplate on which at least one substrate is arranged which is fitted with power semiconductor chips and can be pressed via pressure elements against the baseplate.
BACKGROUND
When manufacturing power semiconductor modules, it is generally necessary to electrically insulate the power semiconductors or power semiconductor chips from the base body or heat sink which is used to dissipate heat. Ceramic isolators in disk form and with a metallized surface are widely used for this purpose. For example, direct copper bonding (DCB) substrates are used on one of whose surfaces the power semiconductor chips are essentially arranged, while the other surface is thermally coupled to the heat sink. The current-carrying capacities of the power switches required for higher-power applications are normally achieved by connecting individual power semiconductor chips in parallel to form a power switch. In addition, a number of power switches can be accommodated in a common can, thus simplifying the design from the assembly point of view by reducing the number of components, for example as is evident with regard to a three-phase bridge circuit for three-phase rectification or inversion, which may thus consist of only a single component instead of up to six controllable power switches and six non controllable power switches configured as discrete components. Examples of power semiconductor modules are widely known, see, for example, EP-A-0 265 833, EP 0 277 546B1.
Various methods are known for fabricating such power semiconductor modules.
By way of example, conductive connectors soldered onto the DCB substrate, in particular lugs, conductive connectors pressed onto the DCB substrate, in particular pressure contacts, and conductive connectors located in an additional insulator, for example a can frame, are used for electrical connection for the connections, to which conductive connectors the electrical connection is made for example by wire bonding, which is dependent on the insulator being fitted prior to this process.
With regard to thermal coupling to the cooling medium—typically via the underneath of the component—that surface of the DCB ceramic to which the chips are not fitted forms the underneath of the component in a module which has a DCB bottom—see DE35 21 572A1. As a rule, thermal coupling is achieved by pressing onto a typically metallic heatsink, from which heat is extracted by a cooling medium, such as air or cooling liquid, with the boundary surface between the DCB bottom and the heat sink advantageously having to be provided with an intermediate layer composed of a thermally linking medium, for example thermally conductive paste.
In a module having a bottom plate, that surface of the DCB ceramic to which the chips are not fitted is thermally coupled to a bottom plate which is generally composed of a metal or composite material; the coupling can be achieved by pressing—see DE41 31 200C2, DE41 11 247C2 and DE 41 22 428C2 typically, once again using a thermally linking medium, for example thermally conductive paste, by solder—see DE43 38 107C1 —or by other integral material joints, the latter in particular in the case of bottom plates composed of composite materials. The bottom plate for its part is then cooled which can be done, for example, by pressing against a heat sink or by applying a flow of a cooling medium, such as air or cooling liquid, subject to a suitable geometric configuration.
This prior art is subject to a number of disadvantages, as follows:
As stated above, power semiconductor modules were originally used to increase the integration level of power electronic circuit arrangements. If this aim is continued, it is necessary to. integrate power electronics together with peripheral functions such as actuation, intermediate circuit capacitors and the like in a controller close to the load: for a single-wheel drive using a three-phase motor, such a controller could be in such a form that it is placed against the motor housing, using the same cooling medium as the motor and, as electrical interfaces, could have a DC voltage feed and a bidirectional bus for information transmission. Power semiconductor modules according to the prior art in the described embodiments are not consistent with such greater integration since they are designed as enclosed units with a dedicated housing which, owing to requirements such as mechanical robustness, occupies more volume than would be necessary for greater integration; this fact is disadvantageous both from the point of view of costs and spatial economy and with regard to the electrical behavior, inter alia since it is influenced by parasitic supply line inductances; in addition, for example, accessibility to the electrical potentials that are present in the power semiconductor module is made more difficult through the housing, necessitating a not inconsiderable design cost for connection of the module.
The reliability of the electrical connections, particularly with soldered external connections, can leave something to be desired due to thermal fatigue. Efforts are being made to counteract this situation by suitable design of the connections, for example using expanding bars or special soldering methods—see, for example, DD 283 236A5. On the other hand, if the electrical connections are produced using pressure contacts with metallic springs, then they can become soft, that is to say they can become increasingly plastically deformable, under the influence of the heat produced by the heat losses from the surrounding semiconductor chips, so that the contact pressure on the substrates will decrease. Furthermore, owing to their hardness, metallic alloys which are better for mechanical purposes often have a higher electrical resistance than materials optimized for good conduction behavior. Contact springs, which avoid these disadvantages, are generally expensive due to their particular material composition and are thus used little, for financial reasons. In any case, there is a risk that the ends of the contact springs or the contact surfaces opposite them will be ground away by scraping movements resulting from thermally dependent work by virtue of the design.
Ensuring the transfer of heat is associated with difficulties, involves complex design and is often not guaranteed over the required life of the power semiconductor module: a description of weaknesses and solutions proposed can be found, for example, in DE39 40 933 —Flexing of the soldered bottom plate—,DE43 38 107C1 —Forming for a bottom plate—,DE19 707 514A1 —Introduction of weak points in the bottom plate—DE35 08 456A1 —Contact-pressure apparatus for the substrate by means of adjusting screws and spacers—or DE4 131 200 C2 —Exerting the pressure by spring elements, which may make adjustment necessary.
Power semiconductor modules constructed according to the prior art using the pressure-contact method are frequently distinguished by complex designs; for example, a design with a specially constructed bridge element is described in DE4 131 200C2. It can be seen that such a design must be matched to the specific circuits; circuit changes lead to tool costs with corresponding delivery times, which is disadvantageous with regard to more stringent requirements for prices and delivery times, even for new application-specific power semiconductor modules.
SUMMARY
The object of the invention is thus to provide a power semiconductor module of the type mentioned initially whose design is reduced to the essential parts depending on the application and which can nevertheless be integrated better in a peripheral, for example, a controller. In the process, the greatest possible spatial economy and optimum accessibility to the electrical potentials together with thermal resistance of the connections should be ensured. Furthermore, the module to be designed should be capable of being manufactured economically using simple production processes and parts while still offering even greater flexibility for application-speci

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