In recent years, WBG (Wide Band Gap) semiconductors have been applied in the realms of battery energy and automotive electronics. As they are high voltage resistant, high temperature resistant with low consumption and high power properties, they can not only elevate component technologies, but also adapt to the wave of energy-saving and carbon reduction that is vigorously promoted around the globe. The key is to stay ahead of the curve of the ever-evolving technology field for future development.
On July 5th, DEKRA iST was invited to attend the "Power Technology Forum - High Performance Power Conversion Design" hosted by DIGITIMES. Max Chen, Senior Manager of the Component Engineering Department at DEKRA iST, shared with the audience "Operating Reliability Validation and Dynamic Measurement Methodology of Wide Band Gap Power Devices." Centered on the development and new editions of the major three international standards for automotive applications, he dissected the difference of static and dynamic reliability test techniques of power components as well as the importance of comparison. He also demonstrated the value of measurement technology platform.
It is common to see that the number of power components used in electric vehicles is 7~10 times more than those used in traditional fuel vehicles Therefore, whether it is the efficiency of components, requirements for reliability, and even standards of driving safety are factors that car manufactures really care about. Senior Manager Max Chen summarized the main points of his sharing as follows:
- Automotive standards related to the prospective progress of WBG:
- WBG dynamic reliability and case studies
- WBG dynamic measurement and thermal resistance application
Also, introduced three international automotive standards that are often take as reference.
- AEC Q-series：AEC Q101 defines stress test driven qualification requirements for discrete semiconductors. The qualified devices should achieve zero defects. Plenty of automakers require automotive discrete semiconductor devices to reach the AEC Q101 specification in order to meet the basic conditions for driving safety. Since it is fundamental, there are more issues that arise during actual application. In response to this, the Q101 test will outline more detailed specific test conditions and contents, such as HTGB and HTRB tests. Once real-time monitoring of leakage can be realized, one can obtain a deeper understanding of problems occurring in the process and shorten the R&D timeline more efficiently.
- AQG 324：It specifies the qualification of power modules for use in power electronics converter units in motor vehicles. As this standard is very close to actual conditions, most of the EV manufactures are more concerned if power modules are aligned with this standard. Simply put, car manufactures have referred to AQG 324 as their own validation requirement.
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ISO 16750：It provides guidance regarding environmental conditions commonly encountered by electrical and electronic systems installed in road vehicles. In the revision in July 2023, it extends frequency range from 20kHz (used for fuel cars) to 200kHz to fit the new electrical environment. What's more, it also considers the properties of WBG semiconductors. The change clearly reflects the consideration of the characteristics in the electric vehicle industry.
Senior Manager Max Chen mentioned that DEKRA iST attended 2024 AEC Workshop in Detroit, America in April. The agenda stated that stress conditions are different due to different ways of use of WBG and Silicon. However, the AEC-Q101 standard does not fully cover WBG semiconductors, so the AEC intends to expand/ edit Q101 or build a new, independent document. Senior Manage Max Chen compiled six important points about WBG semiconductors that can be possibly updated in the agenda: HAST/H³TRB、IOL/PTC、TC、GSS、BDOL、HV H³TRB. AEC specifically defines the tests required for dies, such as IOL and TC tests. Before conducting the TC test, it is necessary to confirm the packaging method with the die supplier. AEC also defines the criteria of HV H³TRB and removes the test voltage limit of 100V from AEC-Q101. Thus, in the future, laboratories will be able to use higher voltages for testing under high temperature and high humidity conditions.
- BDOL(body diode operating life)：It is an early life failure mechanism. Most failures occur in less than 100 hours, so one can use ELFR for screening. It is necessary to reach output current requirement of 20A or higher for the BDOL test, which makes air cooling insufficient for temperature control. In such cases, a water cooling system can be put in place to maintain the appropriate operating temperature for testing components.
- IOL(Intermittent Operating Life)：Parts are subjected to an power on/off cyclic which uses heats from device switching to a predefined temperature, and subsequently to an off state temperature. IOL uses air cooling to insure the junction ∆ TJ ≥ 100oC for cycling. With the increasing power trend of WBG devices over time, an IOL test is essential.
- PCT(Pulse Cycling Test) to test settings and monitor：It is a test for a high power module. DEKRA iST can perform cycling test up to 800A, and using a water-cooling system for temperature control. We monitor critical data such as △Tj, TJmax, and Von/Ron in real-time, and furthermore, by calculating the structure function of thermal resistance, the potential areas within the product where defects might occur can be non-destructively identified.
To move on with the requirements of AQG324 for automotive power modules, Senior Manager Max Chen highlighted some testing methods of dynamic reliability. One is dynamic reverse bias (DRB), such as the High Humidity, High Temperature Reverse Bias (H³TRB) involves applying a dynamic bias under humidity conditions, and verifying at a switching frequency greater than 25 kHz. Another is dynamic gate stress (DGS). Stress is applied to the gate through voltage changes, causing Vth and RDS(on) drifts in Wide Band-Gap semiconductors for validating the extent of efficiency loss of the modules.
Senior Manager Max Chen further explained that as high-speed computing technology advances, thermal issues are on the agenda to be addressed for AI or CoWoS power modules. DEKRA iST currently adopts the internationally standardized JEDEC 51-1 electrical test method for thermal resistance testing, which consists of four measurement steps: (1) Searching the appropriate sensing current, (2) Calibration (K factor), (3) Power conversion/extraction and (4) Structural analysis. Through structural analysis data, the thermal resistance values of each structural layer of a device can be determined. To speed up reliability testing, they often undergo stress tests such as TST, TCT, or IOL. Then, working with the thermal resistance measured together, one can quickly identify structural flaws, if any, within the device package. Besides, thermal resistance parameters can be used to analyze structural anomalies. What is also beneficial about them is to combine them with electrical parameters for calculating the Safe Operation Area (SOA) of the power device. These methods allow for non-destructive measurements, reducing sample loss and time cost, while lowering research and development cost and streamlining the technology modification timeline.
Senior Manager Max Chen stressed the importance of comparing reliability testing methods. The differences between static and dynamic measurements are similar to the use of an electrocardiogram (ECG) for diagnosing heart function. While static measurements might show no abnormalities, they can miss hidden or unexpected defects. By incorporating dynamic measurements, a more comprehensive picture can be obtained under various conditions, which enables to the identification of the root cause of any failure. Only through multi-dimensional analysis can meaningful data be acquired. Hence, using high-frequency switching instruments to perform dynamic measurements requires attention to four key indicators of MOSFETs: switching loss, time, overshoot, and switching speed. These indicators can help detect potential issues that may arise from high-frequency switching.
As the session comes to an end, Senior Manager Max Chen concluded his presentation and gave a prospect of the development trends of emerging energy. The WBG applications in the market will become much wider. For instance, the world has been fostering three green energy supply chains: solar energy, wind energy, smart energy storage and electric grid. With regards to how people can ensure high reliability of products, Senior Manager Max Chen left five takeaways for the audience：
- Vertically integrated production model from materials, chips to module assembly.
- Distinguish between international automotive standards for devices and modules, pass reliability validation, and accelerate mass production.
- Perform a precise anomaly analysis of power components/modules in both non-destructive and destructive ways.
- Develop module assembly techniques and accumulate experience in heat dissipation.
- Select static and dynamic measurement equipment and compare laboratory capacities.
In the global race for energy conservation and carbon reduction, WBG semiconductor manufacturers need to keep up with fast-paced technological changes, efficiently validate their products, and boost reliability. These game-changing moves will be the key ingredients for developing cutting-edge products in the future!
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