Missions

The objective of this post-doctoral position is first to investigate the intrinsic electrical properties of FR4 preg insulating materials under high-voltage DC and AC conditions. Secondly, to evaluate the electrical aging of FR4 preg laminates, as part of representative PCB packaging assembly, during accelerating tests under combined voltage stress conditions (E, f, waveform, polarity…) in order to identify the underlying electrical degradation mechanisms. Finally, upon aging results, to specify the PCB packaging design routes that are capable to meet the upcoming WBG power device stress requirements.
Power electronic conversion is a key enabling technology to succeed the energy transition and reduce the global greenhouse gas (GHG) emission. It is at the heart of the transformation of our society, with, among other challenges, the electrification rise of transportation and homes, as well as the greater increase in the electric share of decarbonized energy production (i.e., solar, wind, nuclear). However, during the AC/DC energy conversion, some energy is inexorably lost. To improve the efficiency, the future generation of power converters will integrate wide bandgap (WBG) power
devices, such as gallium nitride (GaN) or silicon carbide (SiC), that offer lower losses, higher switching frequencies, higher operating temperatures, robustness in harsh environments and high breakdown voltages. The deployment of SiC (3kV MOSFETs [1]) and GaN (future JFET 1.2kV [2, 3]) will thus enable massive reductions in GHG emissions [4].
For instance, GaN-based converter technologies alone could enable global savings of several billion tons of GHG in the next 15 years (e.g., 1 billion tons in the USA and India alone) [4]. SiC and GaN power devices offer lower switching losses than Si, higher switching frequencies (10kHz) and higher power density, and they enable overall reductions in system cost, weight and size [5]. In addition, they can switch at a commutation speed dV/dt of 100V/ns or more, which greatly reduces switching losses. To fully benefit the GaN or SiC device switching speed potential, their packaging must have very low parasitic inductance (1nH) [6-8]. Therefore, the new packaging approaches, such as embedding the power devices within in multilayer laminated printed circuit board (PCB-embedded) packages, have demonstrated
to achieve the required performance at lower cost, while eliminating wire bonding that leads to device reliability issues [6]. However, this next generation of integrated power converters with extreme operating constraints of electric field
(2-3MV/cm), operating frequency (10kHz-1MHz), switching speed (100V/ns) and junction temperature (175°C), will result in the transfer of all or part of these combined stresses to the electrical insulation materials, i.e., the
impregnated insulating layer (FR4 preg) of the PCB-embedded packaging [9]. So far, little is known about the electrical limitations of PCB-embedded packaging [10]. It appears critical to lead research in this field to propose reliable design routes for the electrical insulation of PCB-embedded packaging that will integrate future WBG semiconductors.

Activities

The post-doctoral fellow will first investigate the electrical properties of FR4 preg insulating materials under high-voltage conditions. This will be made on different Cu-FR4-Cu laminate test-structures with a FR4 thickness ranging between 35 to 100 μm. A preliminary assembly process optimization will be performed (e.g., temperature of curing …). A first focus will be made on the dielectric and electrical intrinsic characteristics by performing broadband dielectric spectroscopy measurements to measure the permittivity and dielectric loss factor under high AC voltage, as well as
the electrical conductivity under DC voltage conditions. Different environmental conditions will be considered, i.e., temperature and humidity, and their dependencies on the electrical properties will be assessed. In addition, AC and DC breakdown voltage testing will be performed on the various laminates and the results will be statistically analyzed in order to derive the mean breakdown field values under various experimental conditions (temperature, thicknesses, number of layers …). Simplified FR4-packaged test-vehicles (vertical and lateral) will be used to evaluate the partial discharge inception voltage (PDIV) under AC conditions. Phase-resolved partial discharge (PRPD) patterns will be analyzed. Then, the post-doctoral fellow will perform accelerating electrical aging tests to identify the electrical degradation mechanisms under various high-voltage conditions (E, f, waveform, polarity…) below and beyond PDIV. This will be complemented by degradation and failure analyses over aging by using a set of characterization techniques (SEM, numerical microscopy, X-ray tomography…). It will be checked what the degradation mechanisms are and where they initially occur and then propagate up to breakdown within the PCB assemblies for the various conditions.
For leading this design of experiments (DoE), the post-doctoral fellow will take care of the sample preparation to conduct the experiments (lamination by thermocompression, …) by working closely on an internal Lab-platform.

Skills

To own a PhD in electrical engineering is required with a background in the field of dielectric/insulating materials or high-voltage engineering. The strong experimental character of this proposal will require to apprehend and perform numerous experiments and characterizations. The postdoctoral fellow will be part of a multidisciplinary project :curiosity, openness of mind towards material sciences and engineering applications is required. A proficient English level (C1, C2) is required to favour scientific diffusion.

Work Context

The postdoctoral fellow will develop her/his activity at the LAPLACE Institute, within the MDCE research group, located on the site of the University of Toulouse (UT campus), Toulouse, France.
The LAPLACE Institute, at the University of Toulouse, is the largest French Research Institute in the field of Electrical Engineering with 300 staffs. It seeks weave an “activity continuum” encompassing the production, the transportation, the management, the conversion and the use of the electricity while covering all the aspects right from the study of fundamental processes in solid and gas to the development of processes and systems. Within this widespread field, the major themes concern the plasma discharges as well as plasma applications, the study of the dielectric materials (polymers, ceramics, composites in particular) and their integration into electrical systems, the study and the design of electrical systems, the optimization of the control of power converters. The research topics by their multidisciplinary nature lean on a physical science base willing to study the basic phenomena and introduce new concepts emanating from the fundamental sciences but, evidently, strongly motivated by the constraints and the technological or the
environmental locks; they are therefore linked to industrial activities through various collaborations and participate in transfer of technologies, especially in the aeronautic domain.
The Dielectric Materials in Energy Conversion group (MDCE) is a worldwide recognized research entity in the field of insulating and dielectric materials and 3D integration technologies for electrical energy conversion. The group's research activity is driven by a strong need to reduce the volume and mass of energy conversion systems, as well as their losses. This need, in practice, leads to a desire to increase voltage levels and power density, reduce dimensions, and use new so-called "wide bandgap" components. Some of these devices are also required to operate under new and severe conditions, such as low pressure, high temperature, or very high voltage. In this context, the MDCE group work on polymer, ceramic, composite materials and the 3D integration technology (including PCB packaging and device interconnects) with advanced functionalities (electrical, thermal, mechanical) for enhancing electrical systems efficiency when they operate at high voltage.
The postdoctoral position is part of an ongoing research program [2] funded by the French research funding agency (ANR) that intends to develop novel vertical GaN power transistors (1200V) and the PCB-embedded packaging.

The position is located in a sector under the protection of scientific and technical potential (PPST), and therefore requires, in accordance with the regulations, that your arrival is authorized by the competent authority of the MESR.

Additional Information

References of related works:
[1] C. Langpoklakpam et al., Review of Silicon Carbide Processing for Power MOSFET, Crystals 12, 245, 2022.
[2] PEPR Électronique, VERTIGO Project. https://www.peprelectronique.fr/projet_cible_vertigo/
[3] F. Monflier, Le projet Vertigo promet de "verticaliser" les transistors de puissance en GaN pour atteindre 1200 volts, L'Usine Nouvelle, 31 mars 2023. https://www.usinenouvelle.com/article/le-projet-vertigo-promet-de-verticaliser-les-
transistors-de-puissance-en-gan-pour-atteindre-1200-volts.N2116331
[4] U. K. Mishra, What Will Win the Wide-Bandgap Wars ?, IEEE Spectrum, pp. 32-39, April 2023.
[5] M. Slovick, Use SiC and GaN Power Components to Address EV Design Requirements, DigiKey, 2019.
https://www.digikey.fr/en/articles/use-sic-and-gan-power-components-ev-design-requirements
[6] C. Buttay et al., Application of the PCB-Embedding Technology in Power Electronics – State of the Art and Proposed Development, Proc. Conference IEEE 3D-PEIM, 2018.
[7] R. Mrad, J. Morand, R. Pérrin, S. Mollov, A PCB based package and 3D assembly for high power density converters, Proc.
IEEE International Workshop on Integrated Power Packaging (IWIPP), pp. 73-77, 2019.
[8] D. Wöhrle, B. Burger and O. Ambacher, Power Module Design for GaN Transistors Enabling High Switching Speed in
Multi-Kilowatt Applications, Energy Technol. 11, 2300460, 2023.
[9] T. Huesgen, Printed circuit board embedded power semiconductors: A technology review, Power Electronic Devices and
Components 3, 100017, 2022.
[10] D. J. Kearney, S. Kicin, E. Bianda and A. Krivda, PCB Embedded Semiconductors for Low-Voltage Power Electronic Applications, IEEE Trans. Compon. Packag. Manuf. Technol. 7(3), 387-395, 2017.