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SiC power devices for harsh environments
SiC power devices have emerged as strong alternatives to traditional silicon-based power devices in high-efficiency, high-voltage, and high-frequency power conversion systems. Thanks to their superior material properties, SiC devices have enabled high-performance systems that meet the highest efficiency standards without compromising reliability. Wolfspeed (formerly Cree, Inc.) has been using its SiC diodes in various applications for over a decade, with these devices accumulating more than 2 million operational hours in major industrial (indoor) settings. The failure rate is less than 1 in 1 billion hours, showcasing their exceptional reliability.
In recent years, outdoor applications such as renewable energy systems and electric transportation have highlighted the need for SiC power devices to optimize system performance in terms of size, weight, efficiency, and cost. Although SiC devices have met cost-performance expectations, they face unique challenges when deployed in outdoor environments, particularly under high humidity conditions, which can affect semiconductor performance.
**The Humidity Challenge**
Humidity under electrical bias remains a long-standing issue for all electronic components. Conventional silicon power devices are prone to higher voltages on exposed chip surfaces, increasing the risk of degradation. Materials and active regions become more sensitive to damage as humidity increases, leading to electrochemical migration and corrosion. These failures can result in a loss of high-voltage blocking capability due to ion flow.
With the use of SiC, the problem is amplified because of the stronger electric fields, which accelerate these failure mechanisms. To address this, Wolfspeed engineers have invested significant time and resources into researching and developing new processes and designs for their W-Series power modules, ensuring enhanced reliability in harsh environments.
**New Certification Test Standard**
The JEDEC standard for industrial module testing has traditionally relied on the High Humidity, High Temperature Reverse Bias test (H3TRB), also known as the Temperature and Humidity Bias test (THB). This involves exposing devices to 85% relative humidity and 85°C for 1000 hours at 100 V bias. While this test ensures acceptable performance, it is not sufficient for outdoor applications where operating voltages are much higher.
To better simulate real-world conditions, researchers developed the High-Voltage, High-Humidity, High-Temperature Reverse Bias test (HV-H3TRB), or THB-80. This test applies up to 80% of the rated blocking voltage—such as 960 V for a 1200 V device—making it more relevant for outdoor use. The SiC power module WAS300M12BM2, part of Wolfspeed’s 62mm package, passed the 1000-hour HV-H3TRB test and continued functioning beyond 2000 hours, demonstrating its robustness in high-humidity environments.
**Full SiC Power Module for Harsh Environments**
The WAS300M12BM2 features Wolfspeed’s next-generation MOSFETs and Schottky diodes designed for extreme conditions. During the bare-chip certification process, 25 samples from three different production batches successfully passed the HV-H3TRB test, confirming statistical significance and consistent reproducibility.
This module offers the same electrical performance as the existing industrial-grade CAS300M12BM2, with an on-state resistance as low as 4.2 mΩ and switching losses 20% lower than comparable IGBT modules. It uses an aluminum nitride substrate and optimized assembly techniques to meet thermal cycling and power cycling demands in industrial applications.
Six randomly selected modules were tested under HV-H3TRB conditions. After every 500-hour cycle, they were removed and tested for stability and compliance. One module failed after 500 and 1000 hours, respectively, but visual inspection revealed no signs of early failure. All other modules continued functioning beyond 2000 hours.
Additionally, leakage current (IDSS) was monitored throughout the test, serving as an early indicator of potential failure. As shown in the results, all modules maintained stable IDSS waveforms, with voltage drift below 5% and leakage current drift under 50%, meeting JEDEC standards.
Visual inspections of the failed modules showed no signs of oxidation or electrochemical migration, confirming that the failures were not related to humidity-induced degradation. The remaining four modules remained operational for over 2000 hours, proving the reliability of the design.
**Conclusion**
Wolfspeed’s WAS300M12BM2 power module demonstrates excellent performance under high-humidity conditions. It is the first full SiC power module to pass the HV-H3TRB test with no detectable defects after stress testing. These results open new possibilities for the use of Wolfspeed’s W-Series SiC power modules in outdoor applications like renewable energy and transportation.
**Editor: Yan Zhixiang**