IGBT Modules - Technologies, Driver and Application (Second Edition) - page 375

that with fine graining. The first case leads to the effect that nometal-to-metal
contact is established and theR
is higher than it wouldbewith finer graining.
A further aspect when selecting the thermal grease is its viscosity. If that is too low it
may lead to a separation process – as already described. If it is too high though, the
paste will not disperse fully into the voids and themodule will float. The result is that no
metal-to-metal contact is established and theR
will beunfavourable.
Fig. 10.8
Influence of the viscosity of the thermal grease
Directly cooledmodulesmostly show a standard nickel-plated copper baseplate or they
may be equipped with a so-called Pin-Fin structure
on the bottom of the
baseplate. In both cases the baseplate is in direct contact with the cooling fluid of the
cooling system. For this, heatsinks are used which are open at the top and which now
no longer have to be made frommetal. The module is mounted onto this. If synthetic
(plastic) coolers are used, an EMC suitable connection of the baseplate needs to be
established with ground potential. In particular, when connecting IGBT modules in
parallel a commonground potential is advantageous.
When direct cooling is used it is critical that the seal (O-ring) between module and
heatsink assures under all operating conditions and years of operation that no leakages
will occur. Also the water pressure as well as the area and gauge of the baseplate
needs attention. If the pressure is too high, it may lead to the bowing of the baseplate
which results in crackingof theDCB.
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