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SiC power devices for harsh environments
SiC power devices have emerged as strong alternatives to traditional silicon-based power components in high-efficiency, high-voltage, and high-frequency power conversion systems. Thanks to their superior material properties, SiC devices deliver top-tier performance without compromising reliability. Wolfspeed (Cree, Inc.) has been utilizing 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 exceptionally low—less than 1 in 1 billion hours.
In recent years, outdoor applications such as renewable energy systems and electric vehicles have highlighted the need for SiC power devices to achieve optimal system performance in terms of size, weight, efficiency, and cost. While SiC has met cost-performance expectations, outdoor environments pose unique challenges, particularly in terms of high humidity, which can affect semiconductor reliability.
**The Humidity Challenge**
Humidity under electrical bias has long been a concern for electronic components. Conventional Si power devices are especially vulnerable due to higher voltages on exposed chip surfaces, making them susceptible to degradation from increased moisture. This leads to electrochemical migration and corrosion, ultimately resulting in loss of voltage blocking capability. These issues become even more pronounced with SiC due to the large-scale electric fields involved, which accelerate failure mechanisms.
To address this, Wolfspeed engineers have conducted extensive research into these failure modes, developing advanced processes and designs that support the new W-Series power modules. Their efforts have led to significant improvements in reliability under harsh conditions.
**New Certification Standards**
Traditionally, the JEDEC standard for industrial module testing has relied on the H3TRB test, which involves exposing devices to 85% humidity and 85°C for 1000 hours at 100 V bias. While this test meets basic requirements, it falls short for outdoor applications where voltages are significantly higher.
To better reflect real-world conditions, Wolfspeed introduced the HV-H3TRB test, also known as THB-80. This test applies 80% of the rated blocking voltage—up to 960 V for a 1200 V device—making it far more rigorous. A 1200V/300A half-bridge power module, the WAS300M12BM2, was successfully tested under this protocol and exceeded 2000 hours of operation without failure.
**Full SiC Modules for Harsh Environments**
The WAS300M12BM2 features Wolfspeed’s latest MOSFET (CPM2-1200-0025A) and fifth-generation Schottky diodes designed for extreme conditions. During certification, 25 randomly selected samples from three different batches passed the HV-H3TRB test, demonstrating consistent and reproducible performance.
This module offers comparable electrical performance to existing industrial-grade modules, with an on-state resistance as low as 4.2 mΩ and switching losses 20% lower than similar IGBT modules. It uses an aluminum nitride substrate and optimized assembly techniques to meet demanding thermal cycling and power cycling requirements.
Six units were tested under HV-H3TRB conditions. After each 500-hour cycle, all samples were checked for electrical stability. One module failed after 500 and 1000 hours, but visual inspection found no signs of early failure. The remaining four continued testing beyond 2000 hours without issue.
Leakage current (IDSS) was monitored throughout the test, serving as an early indicator of potential failure. All samples maintained stable IDSS levels, with voltage drift below 5% and leakage current drift under 50%, well within JEDEC standards.
Post-test microscopic analysis of the two failed modules showed no signs of oxidation or electrochemical migration, confirming the robustness of the design.
**Conclusion**
Wolfspeed’s WAS300M12BM2 power module demonstrates exceptional performance under high-humidity conditions. As the first full SiC module to pass the HV-H3TRB test with no detectable defects, it opens new possibilities for outdoor applications like renewable energy and transportation. These results confirm the reliability and durability of SiC technology in challenging environments.
*Editor: Yan Zhixiang*