for fully electric vehicles

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1 Reliability of new SiC BJT power modules for fully electric vehicles Alexander Otto 1, Eberhard Kaulfersch 2, Klas Brinkfeldt 3, Klaus Neumaier 4, Olaf Zschieschang 4, Dag Andersson 3, Sven Rzepka 1 1 Fraunhofer ENAS, Micro Materials Center, Chemnitz, Germany, alexander.otto@enas.fraunhofer,de, sven.rzepka@enas.fraunhofer.de 2 Berliner Nanotest und Design GmbH, Berlin, Germany eberhard.kaulfersch@nanotest.org 3 Swerea IVF AB, Mölndal, Sweden, klas.brinkfeldt@swerea.se, dag.andersson@swerea.se 4 Fairchild Semiconductor GmbH, Aschheim, Germany, klaus.neumaier@fairchildsemi.com, olaf.zschieschang@fairchildsemi.com Abstract Wide-bandgap semiconductors such as silicon carbide (SiC) or gallium nitride (GaN) have the potential to considerably enhance the energy efficiency and to reduce the weight of power electronic systems in electric vehicles due to their improved electrical and thermal properties in comparison to silicon based solutions. In this paper, a novel SiC based power module will be introduced, which is going to be integrated into a currently developed drive-train system for electric commercial vehicles. Increased requirements with respect to robustness and lifetime are typical for this application field. Therefore, reliability aspects such as lifetime-limiting factors, reliability assessment strategies as well as possible derived optimization measures will be the main focus of the described work. 1 Introduction Power electronics is gaining more and more importance in the automotive sector due to the slow but steady progress of introducing partially or even fully electric powered vehicles. The demands for power electronic devices and systems are manifold, and concerns besides aspects such as energy efficiency, cooling and costs especially robustness and lifetime issues. This is in particular true for commercial vehicles such as lorries or construction equipment, where in comparison to passenger cars higher performance requirements and harsher environmental conditions facing increased needs for total driving range (lifetime) and up-time.

2 2 The European joint research project COSIVU (project duration: October September 2015) addresses these issues by developing a novel electric drive-train system architecture. The goal in this project is to realize a smart, compact, and durable single-wheel drive unit including an integrated electric motor, a 2-stage gear system, an inverter with SiC based power electronics, a novel control and healthmonitoring system with wireless communication, and an advanced ultra-compact cooling solution (Fig. 1) [1]. Fig. 1. Overview of the currently developed COSIVU drive-train system including inter alia single-wheel e-motor, inverter with SiC power modules, cooling system and control electronics (VOLVO CE, Fraunhofer IISB) Furthermore, reliability assessment and optimization of critical components such as the power modules for the inverter will be addressed in this project. The underlying concepts as well as first results will be discussed more in detail in the following chapters. 2 New SiC BJT power module The main components of the inverter unit are three equal inverter building blocks (IBB), a power supply for the base driver, an inverter controller module and an inverter housing. Each IBB in turn consists of a cooling plate, DC-link capacitors, current sensor, base drivers and three paralleled half-bridge modules based on an automotive qualified power module package from Fairchild Semiconductor (FSC), as shown in Fig. 2. The half-bridge modules are composed of SiC bipolar junction transistors (BJT) from Fairchild (part number reference FSICBH017A120) and their anti-parallel SiC diodes from Cree (part number

3 CPW2-1200S050). These components are specified for 1200V and 50A (54A for the diode), which allows to drive motor currents up to 300A at system level. The power module construction is based on a direct bonded copper (DCB) substrate with aluminum nitride (AIN) used as the ceramic isolator due to its superior thermal conductivity. For the die attach a lead-free soldering process has been used, whereas the electrical connection of the chip topside pads is done with aluminum wire-bonds (300µm for emitter, 150µm for base). 3 Fig. 2. Photography of the FCS SiC BJT power module (left) and of a SiC BJT (middle) and schematic plan (right) For the encapsulation an epoxy molding compound (EMC) was applied. It has openings for threaded fasteners to assure an appropriate clamping force for an optimal heat transfer to the cooler. The overall size of the package is 44 mm by 29 mm by 5 mm. Furthermore, a thermistor for temperature indication is integrated into the module. However, in the COSIVU project the virtual junction temperature will be determined additionally to allow thermal impedance spectroscopy measurements for health monitoring purposes [2]. 3 Reliability aspects 3.1 Principle approach for lifetime assessment The harsh environmental conditions (passive and active temperature cycles, vibrations, shocks etc.), to which commercial vehicles are usually exposed to, clearly pose high risks for the power modules within the traction module. In addition, these systems have high safety requirements, particularly in the present case of multi-motor solutions. For this reason, the potentially critical reliability issues need to be assessed already during the concept and design stages, i.e., when real samples and demonstrators are not available yet. To face this challenge, a lifetime estimation process will be applied for the SiC power modules, as shown in Fig. 3.

4 4 Fig. 3. Overview on the lifetime assessment methodology Mission profiles, derived from the respective application scenarios for the power module together with relevant norms and standards, provide information about the frame conditions to perform reliability tests as well as the accompanying finite element (FE) simulations. The FE simulations are based on models covering the geometric, material, and load conditions. Practical reliability tests, which in the case of power electronics are conducted in terms of active power cycling tests, in turn provide information about failure mechanism types and their location as well as the statistical failure probability. Lifetime or damage models are linking these results with the simulation results to provide a lifetime estimation of the investigated product. The lifetime models, which need to be chosen or developed for the specific case, are usually based on the underlying physics of failure. Failures in the investigated power modules are mainly expected to be thermo-mechanically induced due to mismatches in the coefficients of thermal expansions (CTE) of the involved materials, such as degradation in the die-attach (solder joint) or in the chip metallization, wire-bond lift-off as well as delamination and cracks within the DCB substrate. However, further failure types such as chip cracking or gate oxide breakthroughs are also possible. Finally, with the knowledge about the weak point's, design rules can be set up to improve the power module reliability and to finally allow the COSIVU system to fulfill its mission under all condition. 3.2 Active power cycling test preparation Active power cycling (APC) test are state-of-the-art for performing accelerated lifetime tests on power electronics. They are performed by means of an internal heating of the device under test (DUT) due to a targeted power loss insertion at a

5 high cycle rate. For the SiC power modules, the available APC test bench [3] needs to be adapted in terms of mechanical sample fixation and electrical connection as well as in terms of the LabVIEW based control software. 5 Fig. 4. 3D CAD design of temperature controllable sample holder for performing ACP tests on COSIVU SiC modules (for single-sided as well as for double-sided cooling versions) In Fig. 4 the 3D CAD design of the temperature controllable sample holder system is shown, which is extended in a way to emulate also a double-sided cooling environment by using two independent cooling bodies with the DUT being placed in-between (cf. chapter 4). The thermal simulation results for the aluminum cooling body in single-sided cooling mode are shown in Fig. 5. The coolant temperature was set to 60 C and the power modules are for simplicity reasons represented as simple heat sources (130W at 50A), representing the worst-case scenario which will also be used in the subsequent APC tests. Fig. 5. Thermal simulation of APC cooling body It can be noticed that with the developed design an even temperature distribution over the cooling body can be achieved with only minor deviations for the out-

6 6 er DUT places, which is important for comparison of the test results of each DUT among each other. Next step within the project will include thermal simulation for the double-sided cooling constellation and adaption of the existing junctiontemperature determination methods to the SiC power modules. Subsequently, power cycling tests on single-sided as well as on double-sided power modules will be performed for benchmarking purposes and for comparison with the numerical simulation results. 3.3 First thermo-mechanical simulation results Besides experimental investigations numerical simulation is necessary to systematically analyze and evaluate the response of the device under given boundary conditions in order to generate design guidelines for lifetime prediction. These models need to reflect the physics behind the failure mechanisms and have to be reproduced consistently by experiments and simulations. FE simulations have been performed to investigate the stresses and strains induced by processing and operational internal and external thermal loads relevant for the envisaged field of automotive application. Prior to modeling, relevant materials were analyzed regarding temperature and process dependence. An FE model of the module has been generated and material data have been implemented. Fig. 6. Geometry and FE model of the SiC power module To evaluate influences of thermo-mechanical stresses and strains, extensive simulations of the package behavior and non-destructive evaluation have been performed. Simulation runs allow location and monitoring of mechanical stress concentration and accumulating plastic and creep strains over the process steps and through several thermal cycles. A typical process flow with soldering on DCB

7 substrate and transfer molding as well as thermal cycling (TC) between -40 and +150 C were subject of investigation by thermo-mechanical simulation based on FE models for the molded module, generated as depicted in Fig. 6 with colors denoting property regions. The model allows a process simulation with individual materials added analogue to the real technological process to evaluate the individual stress state due to thermal mismatch at various steps of production. Results of the warpage measurement and simulation for the surface profile across the DCB surface of a SiC power module are depicted in Fig. 7. Warpage is the primary information coming out of the calculations to evaluate production and test cycle results. It is obvious from the simulations that cycling is resulting in remaining module deformation (sim_150_zyk) originating from mold compound creep. Additionally, TC changes the warped profile for the module (not fixed to any cooling plate). The simulation slightly overestimates the warpage at low temperatures but still reproduces the deformation behavior well. 7 Fig. 7. Warpage measurement (top) and simulation results (bottom) on SiC power module for calibration of FE simulation model Furthermore, thermo-mechanical reliability investigations have to address mechanical stress concentrations as well as accumulating equivalent creep strains, the latter serving as a failure criterion for low cycle solder fatigue. Significant strains and stresses evolve in the die attach (Fig. 8). The results indicate that the creep

8 8 strains are primarily influenced by the high CTE mismatch between DCB and SiC dies. Accumulating die attach creep will lead to solder fatigue and therefore increased thermal resistance to the substrate. To visualize weak points of the design, parametric FE models have and will be successfully used in the regarding reliability investigations. By identifying process steps and loading conditions mainly contributing to stresses and strains in the module and altering respective parameters, deformations and intrinsic stresses will be minimized in order to avoid excessive stressing of the device. Fig. 8. Equivalent creep strains accumulated after one thermal cycle (-40 to 150 C) 4 New cooling concept based on double-sided cooling To gain further improvement in the power module lifetime a double-sided cooling concept ( 2 COOL) is investigated within the COSIVU project. In this process, two different types of cooling structures have been evaluated: a simple inline pinfin structure as well as a sponge-like structure (Fig. 9, left). Thermal computational fluid dynamics (CFD) analyses have been performed for different coolant flows (5 15 l/min at 20 C) for single-sided heating scenario with five heat sources (30W each, four on the corners, and one in the middle). The simulation results for a flow rate of 5 l/min are shown in Fig. 11. For the pin-fin structure the temperature has increased by around 12K at the in- and outlet and by 8 9K in the center, whereas for the sponge-like structure the increase in temperature was slightly lower at the in- and outlet and slightly higher in the center. Further simulation results have also shown a higher pressure drop for the sponge-like structure (approx. 50 mbar vs. 25 mbar for 5 l/min and 470 mbar vs. 155 mbar for 15 l/min) and a predominant flow direction from in- to outlet in both cases.

9 9 Fig. 9. The two investigated cooling structures (left) and the final module assembly (right) The simulation results of this initial analysis clearly pointed out that further improvements are required, such as a reduction of the structure height as well as an increase of the vertical mixing of the coolant. To reach the latter one, a tilted sponge structure to force the coolant flow more in the vertical direction will be realized. The double-sided cooling assembly with the new sponge structure design is depicted in Fig. 9, right. Fig. 10. CFD results for inline pin-fin (left) and sponge-like structure (right)

10 10 5 Conclusion This paper presented a newly developed SiC based power module, which is going to be integrated into a novel drive-train system for commercial vehicles within the frame of the currently running project COSIVU. Furthermore, reliability measures, including the general lifetime assessment process, latest status on power cycling test preparation and first simulation results, as well as investigation results on different cooling structures for double-sided cooling concepts have been discussed. 6 Acknowledgement The Authors would like to acknowledge the European Commission for supporting these activities within the project COSIVU under grant agreement number References [1] Rzepka, S., Otto, A., COSIVU Compact, Smart and Reliable Drive Unit for Fully Electric Vehicles, Micromaterials and Nanomaterials, issue 15, , [2] Hensler, A., Wingert, D., Herold, Ch., Lutz, J., Thoben, M., Thermal impedance spectroscopy of power modules, Microelectronics Reliability, issue 51, , [3] Otto, A., Vohra, A., Rzepka, S., Newly Developed Test Bench for Active Power Cycling Tests, Micromaterials and Nanomaterials, issue 15, , 2013.

11 8 Full Authors Information 11 Alexander Otto, Sven Rzepka Fraunhofer Institute for Electronic Nano Systems ENAS Department Micro Materials Center Technologie-Campus Chemnitz Germany alexander.otto@enas.fraunhofer.de sven.rzepka@enas.fraunhofer.de Eberhard Kaulfersch Berliner Nanotest und Design GmbH Volmerstrasse 9B Berlin Germany eberhard.kaulfersch@nanotest.org Klas Brinkfeldt, Dag Andersson Swerea IVF AB Argongatan Mölndal Sweden klas.brinkfeldt@swerea.se dag.andersson@swerea.se Klaus Neumaier, Olaf Zschieschang Fairchild Semiconductor GmbH Technology Development Center Einsteinring Aschheim Germany klaus.neumaier@fairchildsemi.com olaf.zschieschang@fairchildsemi.com Keywords Electric drive-train system, power electronics, silicon carbide, double-sided cooling, reliability