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Transfer molding compounds for power electronic applications - a qualification methodology for HT capable materials

: Braun, Tanja

Japan Institute of Electronics Packaging -JIEP-; Institute of Electrical and Electronics Engineers -IEEE-:
13th International Conference on Electronics Packaging, ICEP 2013. Proceedings. CD-ROM : Held from April 10 to 12, 2013 in Osaka, Japan
Osaka, 2013
International Conference on Electronics Packaging (ICEP) <13, 2013, Osaka>
Fraunhofer IZM ()

A current trend within power electronics is the use of alternative PowerIC materials as SiC that need an increased operation temperature for optimum performance. The temperature range for the operation of such PowerICs is up to 250 °C, being a challenge to todays packaging technology. Following the trend towards smart power modules the components inside such a module are PowerICs, capacitors, resistors, driver ICs and more, assembled on organic substrates, ceramic substrates or leadframes forming a truly heterogeneous package.
And as power electronics typically involves power loss and thus needs suitable thermal management, such packages need an exposed heat spreader that leads to asymmetrical package geometry a challenge when package warpage is an issue. And a low warpage is needed here for good thermal interconnect between heat spreader and cooler. The package sizes are in the range of 30x30 mm² footprint with differing height, package volume can be estimated to be larger than 2000 mm³, i.e. a big volume package with inhomogeneous internal structure and long flow path for the encapsulant needed to protect the whole assembly.
Encapsulants used for protection of such modules are typically epoxy based molding compounds with low CTE, high rigidity and good dielectric strength. Such Epoxy Molding Compounds (EMC) are processed with transfer molding technology, an encapsulation process which is typically used for high volume production and yields packages with high reliability, e.g. the standard BGAs, QFPs and TSOPs. Permanent operating temperatures of such materials are typically limited to 150 °C, selected materials withstand 200 °C and manufacturers roadmaps announce materials stable at 250 °C in the near future.
Within this paper a reference application is described, integrating power and control logic inside a leadframe based molded package. Taking into account the challenges mentioned above, a detailed description of material selection for this module will be given, including material analysis as rheology, reactivity, change in dielectric loss and r and thermo-mechanical properties as f(t,T) and of media storage. Particular attention is paid to rheological analysis of the molding compounds, as flow behaviour is of ample importance to high quality module encapsulation, especially for large volume as described within this paper. The material rheology models generated are fed into process simulation to identify critical geometries inside the package. Concluding rules for encapsulant material selection and package setup are provided.