FR-4 remains the most mainstream substrate material for conventional low-to-medium frequency printed circuit boards thanks to its affordable price, mature processing technology and stable mechanical property under room temperature, yet inherent dielectric characteristics make it incompetent for high-frequency circuit working above several gigahertz. The core disadvantage of FR-4 concentrates on unstable dielectric constant (Dk) and high dissipation factor (Df) as operating frequency rises. Standard FR-4 owns Dk value fluctuating from 4.2 to 4.7 within low-frequency range, while its dielectric constant drifts drastically with frequency elevation beyond 1GHz, breaking pre-calculated controlled impedance and causing signal waveform distortion during high-speed transmission.

High dielectric loss factor becomes another fatal defect limiting FR-4 in high-frequency scenarios. Df of regular FR-4 climbs rapidly alongside frequency growth, generating substantial dielectric loss that converts electrical energy into heat energy inside substrate. For millimeter-wave, RF antenna and microwave communication PCB working at 5G/6G frequency band, massive power attenuation reduces transmission efficiency, lowers antenna radiation gain and increases circuit working temperature to trigger performance drift. High-frequency signal demands low-loss substrate like PTFE, ceramic-filled hydrocarbon to cut dielectric consumption effectively.

Thermal and dimensional variation further disqualify FR-4 from precision high-frequency layout. FR-4 features relatively high thermal expansion coefficient; temperature swing during component soldering or long-term equipment operation produces substrate expansion deformation, shifting trace impedance and damaging high-frequency matching circuit parameters. Besides, FR-4 resin absorbs ambient moisture easily after long-term storage, and absorbed water molecules alter local dielectric property unevenly across PCB surface, introducing random signal fluctuation in radio frequency channels.

Cost and processing differences cannot offset FR-4’s high-frequency performance defects. Though FR-4 is far cheaper than specialized high-frequency base materials, later debugging cost caused by unstable high-frequency electrical parameters far exceeds initial material savings. High-frequency PCB manufacturers select low-Dk, low-Df specialized substrates to guarantee consistent circuit performance across full working frequency range instead of adopting economical FR-4 material.