IGBT Modules - Technologies, Driver and Application (Second Edition) - page 185

The heat transmission coefficient or heat transfer factor
determines the intensity of the
heat transfer at the interface between the cooler and the cooling medium. The heat
transmission coefficient is a specific index, derived from the geometry of the interfaces,
the interface properties, the type, direction and speed of flow and the thermophysical
properties of the interfacematerials. The greater the heat transmission coefficient
, the
better the thermal conduction.
4.2 Materialsand their thermal properties
As described in the previous chapters, the type of material used is extremely important
in the heat dissipation of power electronic components. As is so often the case in power
electronics, several mechanisms come into play here. Perfect thermal properties do not
necessarily also mean high electrical insulation/dielectric strength, or even high
reliability. The right mix of materials and boundary layers, and the right way of joining
them together, enablea reasonable compromiseas regards:
Thermal properties
Thermomechanical properties
(Power Cycling andThermal Cycling
Electrical insulation properties
(Dielectric strength and coupling capacity)
Cost ofmaterials
It is well known that metals such as copper, silver, gold, iron and aluminium are good
thermal conductors. However, these good thermal conductors all lack any electrical
insulating properties. On the other hand, plastics and ceramics have these properties,
but they are poor thermal conductors. As described in chapter
power electronics
combines these materials to achieve the desired effect. Unfortunately, when metals,
ceramics and/or plastics are combined to create a suitable thermal and electrical
composite, a new effect occurs: The thermomechanical effect or, if only metals are
used, the bi-metal effect. Materials with different coefficients of thermal expansion joint
homogenously tend to expand or contract unequally when the temperature changes.
The result of this difference of expansion is delamination of the compound. The atomic
lattices that were bonded together earlier are irreparably mechanically broken up. One
example of this type of compound of several materials is a DCB, or direct copper
bonding, which is joined to the baseplate of the IGBT module by soldering. Once
destructed atomic lattices bear influence on the behaviour of the entire system by
reducing thermal conductivity.
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