Trace Uniformity
The allowable variation in the characteristic electrical resistance of an outer-layer transmission line on a printed circuit board governs signal reflection behavior. Maintaining a tight microstrip impedance tolerance, typically plus or minus ten percent, is required for high-speed digital links to prevent data corruption. This margin is dictated by the signal’s rise time and the noise margins of the transceiver chips.
When the impedance deviates too far from the target value, signals reflect back to the source, causing signal attenuation.
Dimensional Deviation
Minor changes in trace width, copper height, or etching profiles directly alter the resulting impedance. During the chemical etching process, the acid can over-etch or under-etch the copper, creating a trapezoidal trace shape instead of a perfect rectangle. This shape shift modifies the trace’s cross-sectional area and alters its capacitive coupling to the reference plane.
Fabricators manage this by using automated trace-width control systems and adjust their photolithography masks to compensate for expected lateral etch loss. Tight control over trace dimensions is the primary method used to keep the impedance within specification.
Dielectric Effect
Varying thickness and dielectric constant of the underlying prepreg layer also influence the final microstrip impedance. If the resin flows unevenly during the press cycle, the distance between the trace and the reference plane will vary across the board surface. Fabricators must specify high-quality materials with uniform resin distribution to minimize this source of impedance variation.
