Meaning
Insulating substrates provide the physical foundation and electrical isolation for planar transmission lines. A microstrip dielectric separates the conductive trace on the top layer from the ground plane on the bottom layer. Thickness and chemical composition dictate the impedance and velocity of the signal.
Common materials include woven glass epoxy and ceramic-filled polymers.
Permittivity Stability
Dielectric constants must remain consistent across the entire surface of the circuit board. The microstrip dielectric governs the electromagnetic field distribution, so any variation in the material property creates impedance mismatches. High-frequency boards require materials with low moisture absorption to prevent permittivity shifts.
Consistent thickness is equally vital for maintaining a 50 ohm environment. Batch-to-batch consistency is a major requirement for high-volume production.
Thermal Conductivity
Heat generated by power amplifiers must pass through the substrate to reach a heat sink. While most microstrip dielectric materials are thermal insulators, specialized ceramic fillers improve the path for heat dissipation. Excessive heat can cause the board to expand, which alters the physical dimensions of the traces.
Expansion changes the resonant frequency of filters and antennas.
Manufacturing Process
Etching and plating procedures rely on the chemical resistance of the base material. The microstrip dielectric must survive the high temperatures of lead-free soldering without delaminating or warping. Designers choose the resin system based on the mechanical requirements of the final enclosure.
Rigid-flex designs use polyimide to allow for bending.
