MARC OSAJDA, BUSINESS DEVELOPMENT MANAGER, NXP SEMICONDUCTORS
The automotive industry is changing. Our vehicles are becoming automated and electrified. As this trend is accelerating, it’s having an impact on how semiconductor devices, including microelectromechanical systems (MEMS) sensors, are designed and qualified for the automotive industry. As automotive semiconductor designers carefully consider product definition, product validation and long-term reliability, MEMS sensor suppliers are responding to new opportunities created by automated and electrified vehicles, developing inertial measurement units (IMUs) for automated vehicles and lithium-ion (Li-Ion) battery pressure sensors for electrified vehicles.
IMUs
The IMU is probably the most complex MEMS device that will be used inside a vehicle. This type of IMU is a system-in-package (SiP) that comprises multiple gyroscope and accelerometer sensing elements and a signal processing application specific integrated circuit (ASIC), creating an inertial sensor able to measure up to 6 degrees of freedom (6DoF): yaw, rate and pitch for rotational movements, and lateral, longitudinal and vertical acceleration for linear movements.
Degrees of freedom (DoF) in a vehicle.
For vehicles with level 3 autonomy and above, as per SAE International’s definition1, the IMU is mandatory for taking over the trajectory control of the vehicle in case other sensors, such as the camera, radio detection and ranging (radar) or light detection and ranging (lidar), become impaired. Should such a failure occur, the IMU will function as a guidance sensor to bring the car to a safe stop within a short period of time and distance. The IMU is also used to control the regular movement of the car while driving in automated mode.
While IMU technology already exists for aerospace applications, there are significant challenges to adapting it for automotive ones. The automotive IMU requires high performance at costs that are compatible with the automotive industry. As vehicle lifecycles are long, MEMS sensor suppliers must produce the device at high volume for an extended period. They must also guarantee the sensor’s performance and reliability over a 10–15-year lifetime, with no maintenance or recalibration required. Only a few MEMS suppliers have the capability and willingness to embark on this kind of journey.
Li-Ion battery pressure sensors
The conversion from internal combustion engines to electrified propulsion is going to affect the powertrain MEMS market. For example, pressure sensors used in engine management for air pressure and fuel pressure will simply disappear with electrification. However, the use of large Li-Ion batteries in electrified vehicles has created a new application for MEMS sensors.
One of the risks of Li-Ion batteries is the small probability for a cell to enter thermal runaway, a process that sees the temperature in the cell rising incredibly fast so that the amount of heat generated exceeds the amount dissipated, potentially resulting in a fire2. Multiple cases of electrified vehicle batteries catching fire have been reported in the press. When it comes to thermal runaway events, every second counts. Detecting the event as early as possible enables the vehicle safety system to take all necessary measures to warn the vehicle occupants of an imminent fire. It also allows for the taking of timely counter measures to mitigate the impact of the fire, e.g., triggering a fire extinguisher and calling the fire brigade.
Thermal runaway effects.
Studies have shown that a rise in pressure inside the battery pack is a good indication that a thermal runaway is starting3. The outgassing of a battery cell, plus a sudden rise in temperature, will increase pressure inside the battery pack, which will generate a pressure pulse. To detect such a pressure pulse, a MEMS pressure sensor must permanently measure the pressure inside the pack. It must also report to the battery management system any suspicious change in pressure, independent of atmospheric pressure changes. It’s important to keep this kind of sensor on all the time to detect any pressure anomaly in the system, even when the vehicle is completely off. NXP has developed a pressure sensor to specifically address this new safety application in EVs, and several automotive manufacturers are already using it.
NXP battery pressure management sensor.
The quest for zero defects
While the automotive industry is targeting zero fatalities as its ultimate goal, the semiconductor industry and module suppliers are targeting zero defects for each and every semiconductor device. For safety-critical automotive MEMS sensors complying with the Automotive Electronics Council (AEC) Q100 qualification for semiconductors, it’s necessary but clearly not sufficient to guarantee a zero defects production launch and long-term reliability of the device.
To boost the reliability and robustness of automotive sensors, NXP has developed Above and Beyond (AaB), a new methodology that studies advanced reliability and robustness well ahead of the device’s qualification and production release4. Based on risk-mitigation analysis, AaB consist of extensive testing, such as test-to-fail, corner lot testing and new use-case testing combined with advanced statistics, all of which help NXP understand how these different parameters interact with each other.
Integrating AaB into project planning does increase time and cost, but this early investment pays off as long as weaknesses in the device can be detected and corrected before a production launch. Field failures, on the other hand, can lead to unplanned redesign and requalification of a device. Worst-case, they can lead to a recall campaign that costs a huge amount of money. We’re systematically using the AaB methodology at NXP for safety-critical MEMS sensors because its potential benefits far outweigh its costs.
NXP Semiconductors
References
1SAE International (2018). SAE International releases updated visual chart for its levels of driving automation standard for self-driving vehicles [press release]. December 11.Available at: https://bit.ly/3qEz24U
2Neill, P. (2020). Electric vehicle fires, should we be concerned? [article]. Air Quality News. October 9.Available at: https://bit.ly/3qD5rsF
3Koch, S., Birke, K. and Kuhn, R. (2018). Fast thermal runaway detection for lithium-ion cells in large scale traction batteries. Batteries, volume 4, issue 2, article no.16.Available at: https://bit.ly/3Hy5Wue
4Fritz Vos, S. (2021). Advancing quality and reliability of MEMS [PowerPoint presentation]. Technology Unites Global Summit: MEMS and imaging forum, February 17, digital event.Available at: https://bit.ly/324sItm