Guided Structure
The electrical conductor fabricated directly on or within a dielectric substrate using printed circuit board processing methods provides a controlled path for high-frequency electromagnetic waves. This configuration, known as a planar transmission line, is most commonly realized as microstrip or stripline geometry. It is used to distribute radio frequency signals and high-speed digital clocks across the board with minimal loss and reflection.
Physical Geometry
Dimensional accuracy of the conductor width and the thickness of the dielectric layer directly determine the characteristic impedance of the line. In a microstrip configuration, the trace is located on the outer layer of the board, exposed to air on one side and the dielectric on the other. Stripline configurations place the trace on an inner layer, sandwiched between two reference planes, which provides better shielding against electromagnetic interference.
Because the electric field in a planar transmission line is partially or fully contained within the board material, the choice of substrate material has a major influence on the signal propagation speed.
Manufacturing Constraint
Etch tolerances during board fabrication represent a primary source of impedance variation in these structures. If the chemical etching process leaves the trace too wide or too narrow, the resulting deviation from the nominal impedance can lead to signal reflections. Designers must work closely with the board fabricator to ensure that the trace profile remains within the specified limits.
Applying soldermask can also change the effective dielectric constant of outer-layer lines, meaning that the fabricator must adjust the pre-etched trace width to compensate for this coating.
