IGBT Modules - Technologies, Driver and Application (Second Edition) - page 85

mixed, the original liquidmixturewill usually hardenwithin a fewminutes to a few hours,
depending on its composition and the temperature. Depending onwhich resin system is
chosen, the thermal stability of the heat-hardened final product can be more than
250°C. By contrast, the glass transition temperature
of cold-hardened systems is
around 60°C. Unlike with polyester resin, if epoxy resin is blended in, then usually the
resin/hardener ratiomust be observed. Otherwise, parts of the epoxy or
hardener do not have a co-reactant, with the result that the surfaces of the final product
are sticky and not as rigid as they should be. This effect cannot always be avoided, but
is undesirable in the context of modern manufacturing of power electronics. Therefore,
manufacturers are clearly moving away from epoxy resins to soft casting
resins/insulating compounds like the silicone gels.
In contrast to epoxy resin, silicone gel is not rigid or unworkable after hardening, but
remains flexible. The gel has very good electrical insulating properties and can be used
within a wide range of temperatures, from -100°C to more than 200°C. This feature is
particularly welcome to power electronics manufacturers, because in borderline cases,
e.g. short circuit, temperatures over 200°C can occur in the power semiconductor chip.
Thanks to its gel-like structure, silicone gel is very flexible under thermal and
mechanical loads, whichmeans that there is no distortion of thematerials between the
different package components. On the other hand, its lack of mechanical rigiditymeans
that when gels are used, the plastic housing is solely responsible for fulfilling that
One disadvantage of silicone gels is that they liberate highly volatile silicones. This
hinders the direct use of IGBTmodules in silicone free environments such as automatic
spray painting lines. Silicone gels also tend to absorb moisture from the air, including
any sulphur-containing substances in it (e.g. hydrogen sulphides). If this combination of
contaminated moisture reaches the copper surface of the DCB, electrically conductive
copper sulphide (CuS) is created and spreads along the electrical field. Nickel-plating
the copper finishes of the DCB inhibits this chemical process, which would otherwise
inevitably lead to the failure of power electronics components due to a short circuit.
Fig. 2.7
Growth of copper sulphidebetween two electric potentials on aDCB
Lastly, it should bementioned that silicone gels and epoxy resins are not hazardous to
human healthonce they havebeenprocessedandhardened.
The glass transition temperature T
also shows the greatest deformation ability of the final product. Below T
the product is brittle; above, it is soft.
Stoichiometrydescribes thequantifiable composition of chemical compounds.
1...,75,76,77,78,79,80,81,82,83,84 86,87,88,89,90,91,92,93,94,95,...548
Powered by FlippingBook