High-frequency PCBs are crucial in communication electronics and advancing sectors like air transportation and satellites. Selecting PCB substrates involves prioritizing low and stable dielectric constants (Dk) for faster signal transmission, minimizing dielectric loss, and considering material properties such as consistent copper foil thermal expansion, low water absorption, and robust mechanical characteristics. Teflon is preferred above 5GHz, while FR-4 suits 1-10GHz, with material choice ultimately tailored to specific needs like high stability and signal quality maintenance under varied conditions.