Sizing up soft substrate laminates

The basic performance parameters of several soft substrates for microwave and RF circuitry were evaluated experimentally with some custom built resonators. The trials were run with high and low dielectric constant substrates to quantify their variability over a wide range of operating temperatures. The low dielectric constant substrates were made of polytetrafluoroethylene (PTFE) loaded with either chopped or microfiber glass filler. The material was hot-pressed between a thin copper foil sheet and thick Al ground sheet. The high dielectric constant substrates were impregnated with a TiO2 ceramic powder. Tests measured insertion losses in 50 ohm lines from 1-18 GHz and the Q and dielectric constant at 3 GHz with half-wave resonators. The resonators were formed on the substrates with various conditioning treatments and were also examined for adhesion strength. The adhesion did not degrade until heated past 150 C. The substrate properties remained intact after numerous thermal cycles up to 250 C. High dielectric constant soft substrates did maintain good contact with the Cu foil up to 250 C.