Failure Mechanism Analysis and Reliability Enhancement of Bead-Type NTC Thermistors for Temperature Sensing

Purpose: This study aimed to enhance the reliability of bead-type negative temperature coefficient (NTC) thermistors by identifying failure mechanisms and developing improvement measures. Comparative life tests were conducted on thermistors before and after improvements to assess reliability improvements.BR/Methods: The relationship between temperature and resistance was evaluated using a temperature recorder and resistance meter. Failure analysis was conducted using field emission scanning electron microscopy (FE-SEM), X-ray, and optical microscopy. The findings revealed that thermal shock caused expansion and contraction of the internal thermal conductive epoxy, inducing mechanical stress on the thermistor component that led to fatigue failure. The primary failure mechanism was identified as grain boundary fractures, rather than intragranular fractures, caused by thermal shock, which resulted in gradual resistance increases.BR/Results: To mitigate thermal stress, an epoxy with a 57% lower thermal expansion coefficient was used. Comparative life testing confirmed a 64% increase in the relative lifespan of the improved samples, validating the hypothesized failure mechanism. The thermal stress from epoxy with a high thermal expansion coefficient was identified as the primary cause of failure of bead-type NTC thermistors under fluctuating temperature conditions.BR/Conclusion: Applying a thermally conductive epoxy with a thermal expansion coefficient closer to that of the thermistor effectively mitigated thermal stress, extending the device’s average lifespan. This adjustment shifted the failure mechanism to wear failure, significantly enhancing overall reliability.