IGBT Modules - Technologies, Driver and Application (Second Edition) - page 188

Fig. 4.16
Examples ofmaterials used
4.2.1 Thermal InterfaceMaterial (TIM)
Thermal bonding of an IGBTmodule with its thermal sink is one of the most neglected
topics in power electronics. While the manufacturer's responsibility finishes with the
baseplate or DCB, the user's responsibility starts with the heatsink. In between is a no-
man's-land, with the result that staff have applied thermal compounds with a "painter's
roller" without taking account of optimal thermal coupling, operating life, reliability or the
cost of the baseplate, DCB and/or heatsink. Module manufacturers did not focus their
attentionon thismatter until just a few years ago.
In principle, a thermal compound or TIM (thermal interface material) is a thermal
insulator. Nevertheless, the TIM is still a better thermal conductor than air, which has a
thermal conductivity
W 0261 .0
. That is the reason for using a TIM: It closes the
air spaces and voids between the IGBTmodule and the coolingmedium.
The main ingredients of TIM compounds are silicon oil and zinc oxide. Aluminium,
copper or silver particles can also be used instead of zinc oxide. The size of the grains
is the key to a quality compound. The smaller the particles, the better the thermal bond.
Silicon-free compounds are also available, but these are not recommended for power
electronics as they can volatise.
"Pumping of the baseplate", an effect caused by fluctuations in temperature, can
displace compounds. Compounds that contain silicon return to below the baseplate,
whereas silicon-free compounds are pumped out with the result that, after a certain
operating time, only the solid particles or dried up areas of the compound remain below
the IGBTmodule, and the thermal resistance increases markedly. This can sometimes
cause themodule –and therefore theapplication – to fail.
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