Sensor technology is a key technology in the modern world. Precise temperature detection and control ensure safe, efficient, and sustainable operation in a wide range of applications such as power generation, health care, and mobility. As the shift towards electromobility continues, temperature measurement is emerging as a key innovator once again.
The performance, speed, and efficiency of every electric vehicle are determined by the layout and capabilities of the voltage converter and inverter units. Increased switching frequencies, higher power levels, and operation at elevated temperatures allow for greater driving ranges and more dynamic driving modes. However, higher operating temperatures demand new materials and new joining techniques.
Simplified design by eliminating etched trenches
The sinterable Pt1000 temperature sensor in an SMD package was developed to optimize power modules (Fig. 1). The electrical isolation between the sensing layer on the top side and the back side metallization enables potential-free positioning of the temperature sensor adjacent to the heat source. The sensor and other components can be installed at the same electrical level and on the same substrate. The need to mount the sensor chip on a separate “island” is eliminated. Omitting the additional etched trench that is required for the potential-free mounting of through-contact components (NTC type) reduces the design effort on the substrate level. Reducing the size of the substrate contributes to smaller components and supports the overall trend towards miniaturization.
Connection of the sensor element can be achieved through standard thick-wire bonding; connection to the PCB is feasible with standard silver sinter processing, which allows for seamless integration in standard production processes. Fig. 2 shows a comparison of the design options for SMD and NTC sensors.
The sinter connection is the key to high-temperature operation, opening the operation window far beyond 200 °C. While the Pt1000 sensor element is currently specified with an upper operating limit of 200 °C, ongoing development activities target higher temperatures where the limits of the sinter connections can be further utilized.
Improvement of measuring accuracy and response time
To better understand the impact of sensor positioning on the printed circuit board, a simplified model was employed to investigate heat distribution and response times in state-of-the-art silicon-based power modules as well as next generation silicon-carbide-based setups. The chosen design geometry is independent of the material selection. The material properties and the operating temperature have been adjusted to resemble Si- and SiC-based designs.
The model calculations show that the deviation between the junction temperature (150 °C or 200 °C) and the measured temperature is strongly influenced by the distance between the sensor and the power semiconductors. Fig. 3 shows the dependency of the temperature drop on the distance. As a result of the electrical isolation between the sensing area and the back side metallization optimized for sinter connections, the SMD-PT sensor can be placed on any available position on the power module PCB. The reduced distance between the heat source and the sensor element results in higher accuracy of the measurement results.
The response time is also influenced by the positioning of the sensor. A greater distance leads to a much slower response and a pronounced delay after the switch-on step. The time to reach equilibrated conditions is best described by the delay to reach 90% of the equilibrium temperature t90 (Fig. 4). Comparing the t90 times for sensor position 1 and 2 with 1.0 and 1.3 seconds reveals a substantially more dynamic detection with a 30% faster detection for the position close to the power dies.
The shorter distance between the Pt1000 SMD-type temperature sensor and the heat source not only improves the measurement accuracy but also significantly reduces the time to reach thermal equilibrium, resulting in a much shorter temperature measurement response time.
Summary
The sinterable Pt1000 temperature sensor in an SMD package offers a variety of benefits for solving temperature sensing challenges in state-of-the-art and next-generation power modules: The layout of the sensor with intrinsic isolation between the sensor and the contact layer allows for new designs and elimination of the etched trench. The reduced distance between the heat source and the sensor element results in higher accuracy and shorter temperature measurement response times. Overheating effects and temperature spikes can thus be avoided, and the overall life expectancy is significantly increased.
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