Power cycling tests under driving ΔTj = 125 °C on the Cu clip bonded EV power module
Introduction
Eco-friendly automobiles has become a global campaign in response to climate change and the building of a sustainable planet. In this context, the governments of France and the UK announced plans from 2017 to ban the sales of all new diesel and gasoline vehicles by 2040 [1]. For this reason, recently a powertrain of eco-friendly vehicles consisting of a battery-inverter-motor has academically and industrially attracted a lot of attention. Amongst the powertrain systems, the power module of the power conversion systems is a key component due to the fact that it converts the direct current (DC) power of the battery into alternating current (AC) by switching in order to drive the motor in eco-friendly vehicles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs) [2]. The power losses accompanied in this switching operation are converted as heat energy, and most electronic device failures are caused and form heat. The larger the junction temperature swing (∆Tj) and the maximum junction temperature (Tjmax) are applied, the shorter the power cycling lifetime is induced [3]. The main failure mechanism is the thermo-mechanical stress caused by the coefficient of thermal expansion (CTE) mismatch between various interfacial materials [3]. Therefore, a thermal design and a thermal management of next-generation power modules driven by high voltage/high temperatures are a crucial for performance and reliability.
In particular, a wire bonding technology is a major design factor that can determine the power density, heat dissipation performance, and reliability [4], [5]. Al wires are widely used for its excellent wiring formability [6], but the limitation of the current density and chronic issues of failure modes such as a heal-crack and lift-off are remained as major hurdles for next-generation high-voltage/high-temperature applications [7], [8], [9]. As an alternative, Cu wiring [10], double side heat dissipation type (wireless) [11], clip bonding [12], etc., have been proposed. Nevertheless, there are still difficulties in high-temperature operations due to problems such as a heat dissipation, delamination, and thermal deformation [12].
Here we designed and manufactured a transfer-molded EV power module with Cu clip bonding, aimed at commercial products. The power module was subjected to power cycling tests under Tjmax = 175 °C up to 97,000 cycles. The process of degradation according to the number of cycles was investigated through SAT and cross-sectional analysis, and the failure mechanism was revealed in detail with the assistance of finite element method (FEM) simulation in detail.
Section snippets
Power module configuration
The 3D modelling of the transfer-molded power module with Cu clip bonding was designed as shown in Fig. 1(a). The actual power module manufactured and the description of P, O and N and signal pins are shown in Fig. 1(b). The power module composed of 4 EA Si insulated gate bipolar transistors (IGBTs) and 4 EA Si diodes which can be distinguished with high side and low side. The IGBT die size is 10.9 mm × 11.0 mm × 0.1 mm, and the diode die size is 10.8 mm × 5.8 mm × 0.1 mm, respectively. Both
Power cycling reliability
As shown in the switching configuration in Fig. 3, the Si diode device 1 and 2 were switched. S1 and S2 are in parallel. ΔTj was calculated as the average of device 1 and 2. We used a total of 10 samples for reproducibility. The initial thermal resistance of this power module was measured to be about 0.15 °C/W. The thermal resistance showed no significant increase for up to about 45,000 cycles. From 60,000 cycles, the thermal resistance gradually grew until it reached 70,000 cycles, when it
Conclusion
This study systematically revealed the power cycling reliability and its failure mechanism of a transfer-mold type power module with Cu clip bonding. Operation at Tjmax = 175 °C did not cause cracks in the Pb-5Sn-2.5Ag solder layer. Intermetallic compounds such as Cu3Sn were observed to grow with increasing thickness from the value of 0.43 μm at the initial stage to 1.43 μm at 97,000 of PCT cycles. A major factor of the increase in thermal resistance is cracking in the die, but another factor
CRediT authorship contribution statement
Dongjin Kim: Methodology, Writing - Original draft preparation, Resources, Visualization.
Byeongsoo Lee: Validation, Investigation.
Tae-Ik Lee: Methodology, Validation, Investigation.
Seungjun Noh: Investigation, Project administration.
Chanyang Choe: Visualization, Data curation.
Semin Park: Conceptualization, Methodology, Supervision.
Min-Su Kim: Conceptualization, Writing - Review & Editing, Supervision, Funding acquisition.
Declaration of competing interest
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript.
Acknowledgments
This study has been conducted with the support of the Korea Institute of Industrial Technology as “Development of root technology for multi-product flexible production (KITECH EO-22-0006)” and HYUNDAI MOBIS Co., Ltd.
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