Accelerated lifetime estimation and failure analysis of micromachined humidity sensors under high temperature and high humidity conditions

Accelerated Life Testing (ALT) is used to detect and understand failure mechanisms, as well as to calculate and evaluate the robustness and resulting reliability of electronic components such as sensors subjected to various influencing factors in different applications. Based on the successful lab and field robustness validation of the customized Humidity Sensor in Axle Bearings (HSAB) system utilized for the condition monitoring of lubricated rail components, an ALT methodology was designed for a micromachined humidity sensor, and respective results are presented. ALT was aimed at quantifying the robustness of the selected sensor under higher-than-normal use environmental loads (temperature and humidity), with a focus on its sensor element. Thus, ALTs under constant high temperatures combined with low- and high-humidity conditions were executed until all the tested sensor elements failed. Thereafter, the resulting failures were investigated using various methods. The solder joint failure of the sensor element was determined as a central failure mode. Based on the data obtained for the time to failure of the sensor elements, a two-parameter Weibull distribution function was fitted, in agreement with comparable scientific works on solder joint failures. As the aged sensor elements themselves did not seem to be significantly influenced or even damaged by the executed ALTs, their existing functionality was proven afterwards. For this purpose, they were first resoldered and then tested using a developed step-validation test program for temperature and humidity. After a statistical evaluation of the sensor signal deviations relative to a calibrated reference sensor, the amount of still operational sensor elements was assessed. As a result, it was determined that sensor elements aged at high temperatures in a high-humidity atmosphere failed to a significantly greater extent owing to the damaging effect of water. This indicates that water significantly affected not only the solder joints of the sensor but also the sensor element itself under the investigated test conditions.