Temperature dependence of complex permittivity for polyimide film used in flexible printed circuit in microwave band

Temperature dependence of the complex permittivity for polyimide (PI) used in flexible printed circuit at 10 GHz was measured by cylindrical cavity resonator method with temperature controller in temperature range between 27 to 100°C. The complex permittivity of PI absorbing moisture was larger than those of PI drying at 27°C. With an increase in temperature, the complex permittivity of PI absorbing moisture decreased and came close to those of PI drying over 70°C. The complex permittivity of PI in low temperature is affected by moisture absorption compared to high temperature. From measurement result, we discussed the effect of temperature and moisture condition on dielectric loss of microstrip line fabricated on PI.