Temperature dependence of transmission characteristics of flexible printed circuits in microwave band

The relative complex permittivities of dielectric substrates for flexible printed circuits (FPCs) at 10 GHz were measured by the perturbation method using a cylindrical cavity resonator for temperatures ranging from 27 to 100 C. The temperature dependencies of the ε_γ and tan δ values of polyimide (PI) substrates were greatly changed by the moisture absorption properties. The ε_τ and tan δ of a liquid crystal polymer (LCP) substrate were not affected by temperature and were lower than those of the PI substrate below 70 °C. The temperature dependencies of the dielectric losses of microstrip lines (MSLs) for FPCs were evaluated. These losses were found to be more stable and lower for an LCP substrate than for a PI substrate.