IGBT Modules - Technologies, Driver and Application (Second Edition) - page 370

Fig. 10.2
Warping of an IGBTmodule baseplate
To achieve the best possible connection, characterised by a low thermal resistance
between module and heatsink, it is necessary to allow for the maximum of direct
contact between both components. In reality, however, this has its restrictions as both
components show an inherent unevenness of their contact surfaces. For example the
baseplate of an IGBT module is not 100% plane, and even after being mounted to a
heatsink shows a certain warping as detailed in
with a baseplate
measurement. Similar can be said of a heatsink, which too, is not 100% even. As a
consequence there will be direct metallic contact areas but also, however, air filled
voids. Whilst the contact areas show a very low thermal transfer resistance this
resistance is extremely high for the voids. Operation of an IGBT module under these
conditionsmay only be carriedout with restrictions.
The voids of the IGBTmodule baseplates are system-borne and find their origin in the
joint between the baseplate and the substrate. With a conventional soldering process
the baseplate distorts as a result of different coefficients of expansion (CTE). This effect
stems from thematerials used and it is set by themanufacturers in such a way that the
baseplate is de-tensioned when the IGBT modules have reached their operating
temperature. The voidsmeasured in cold conditionwill then reduce. This effect is called
pumping and it causes thermal greasewith low viscosit
to be easily pumped out of the
area between baseplateandheatsink.
Further, each baseplate has a different unevenness depending on the number, the
surface areaand the thickness (gauge) of theDCB.
Viscositymeans the viscidity (runny or pasty) of a fluid substance. The greater the index of viscosity themore
pasty is the substance.
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