Conference proceedings article
The influence of environmental conditions on the properties of housing materials for power electronics

Publication Details
Böttge, B.; Bernhardt, R.; Klengel, S.; Wels, S.; Claudi, A.
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Miniaturization of power electronic devices is
inevitably related to increasing internal electrical stresses in the
insulation materials utilized for the package and housing systems. On
the other hand, application conditions resulting from the growing use of
power electronics for outdoor applications, e.g. off-shore energy
generation or automotive applications result in higher external loading
due to environmental factors like humidity and temperature. As a
consequence, knowledge of the insulating properties, e.g. the dielectric
strength, of power electronics housing materials as a function of
electrical field, humidity and temperature as well as of a function of
stressing and ageing time is of major importance to meet the reliability
requirements of the related systems. In contrast to the packaging of
microelectronic devices typically using silica particle-filled epoxies
power electronics housing materials are mostly based on glass
fiber-reinforced thermoplastics, such as e.g. polybutylene terephthalate
(PBT). For these thermoplastics, the level of understanding of the
reliability behavior as a function of the internal and external
stressing factors is yet rather limited.

Therefore, aim of the present study is to
enable investigating the dielectric strength and life time properties of
different thermoplastic insulating materials used for power electronics
as a function of the electrical field strength, humidity and
temperature. However, performing dielectric breakdown experiments at
high voltages and electrical field strengths that are characteristic for
power electronic devices requires specifically designed testing methods
and setups. In particular, a careful consideration of the electrical
field and of any related potential conductive paths affecting the
breakdown results is needed. As a consequence, use of specifically
adapted sample designs is required for these investigations.

The study presents results of the development
of a new specimen design that enables electrical testing of engineering
plastics, in particular stressing the materials with high electric field
strengths under various defined environmental conditions over an
extended period of time.

For part of the samples tested, the general
degradation and failure modes of dielectric breakdown have been analyzed
using microstructure analysis. Within this part of the study particular
focus was given to the mechanisms of humidity absorption, water
permeability and water binding as well as the effect of ion content in
correlation to the microstructure of different thermoplastic insulating
materials. Furthermore, also root causes for varying results for the
same thermoplastic material that have been provided from different
suppliers are discussed. In particular, the results for moisture
absorption analyses show that nominal identical materials can differ
significantly in their electrical insulating properties. This behavior
can be explained by different adhesion properties of the glass fibers
and the polymer matrix determining the formation of conductive paths.

The results of the study allow defining a new
methodology for a precise characterization of the insulation reliability
of thermoplastic materials used for electronics. Furthermore, essential
factors affecting the results related to humidity stressing of these
materials have been analyzed and related to the microstructure. Both
results will support the material selection and the design of
miniaturized power electronics packages with increased reliability for
harsh environment applications.


Last updated on 2019-25-07 at 14:55