Electrochemical Impedance Spectroscopy (EIS) for Monitoring the Water Load on PCBAs Under Cycling Condensing Conditions to Predict Electrochemical Migration Under DC Loads

Humidity induced failures like metallic dendrite formation are a major problem for automotive electronic components. The harsh environment, where operating conditions in terms of temperature and humidity vary, can repeatedly provoke thin water layers on the surface of Printed Circuit Board Assemblies (PCBAs). The presence of a water film on electronics enables various corrosive processes. The understanding of the film formation and its effects is therefore crucial for assessing the humidity robustness of a specific setup. In this work, we conducted temperature and humidity load experiments with test boards containing interdigitated copper traces of different gap sizes on FR-4 substrate material. We repeatedly provoked condensation and evaporation conditions on the boards' surfaces by temperature cycling between 25 °C and 55 °C at 97 %rH. Electrochemical impedance spectroscopy (EIS) was employed as testing approach to detect the water film formation and respectively its evaporation. An AC excitation of 10 mV over a frequency range between 1 kHz and 100 kHz was used. Simultaneously, the commonly used SIR (Surface Insulation Resistance) test method was conducted at 5 V DC. This method lacks in delivering information on the actual water layer build up, but it detects the growth of dendrites, for which the DC voltage is required. The evaluated results of the EIS testing show, that the magnitude of water present can be depicted by the change in phase shift in the high frequency domain. We could also detect the water film closing for different gap sizes upon condensation. The DC measurements showed a correlation in terms of dendrite formation upon certain water load conditions.