Dielectric modeling of polyimide exposed to environmental stress

The effects of high temperature and humidity exposure on the complex permittivity of PMDA-ODA polyimide (PI) are reported. The dielectric properties of a Cr-PI-Cr capacitor are measured over a frequency range from 12 Hz to 100 kHz for ambient relative humidity (RH) values from 5 to 85% RH (at room temperature). The capacitor is then aged at 85°C/85% RH for six weeks. A low-frequency dispersion in the capacitance and dissipation factor of the aged devices is observed. In addition, there is a low-frequency loss peak at high humidity values after aging that was not observed in the virgin device. The observed changes in the dielectric behavior after aging are not fully reversible via annealing at the cure temperature, though the dispersion is somewhat reduced. The authors propose a modified Debye model with two relaxation times, and its dual lumped parameter circuit model which includes a parallel conductance for the frequency response of PI films before and after aging at 85°C/85% RH. The model shows good agreement with the experimental observations