Repeating Pattern
Electromagnetic environments inside a circuit board vary at regular physical intervals because of the grid like structure of the fiberglass reinforcement. This spatial dielectric periodicity means that a trace will periodically pass over a heavy knot of glass and then a pool of resin. The electrical properties cycle between low and high values every few hundred microns.
This creates a predictable but unwanted variation in impedance along the signal path. If the frequency of the signal matches the physical period, resonance issues can occur.
Fiber Bundle
Weaving processes create intersections of horizontal and vertical fibers that are denser than the surrounding areas. In environments with high spatial dielectric periodicity, the distance between these knuckles controls the frequency at which the signal will be disrupted. Traces that run parallel to the weave are particularly vulnerable to long segments of mismatch.
Small zig zag movements in the layout can break the pattern. Designers specify spread fabrics to minimize the amplitude of these dielectric swings.
Jitter Generation
Rapid changes in local capacitance as the signal crosses from glass to resin and back again lead to erratic timing shifts. High frequency digital links experience increased data error rates when spatial dielectric periodicity is allowed to persist unchecked. These subtle shifts accumulate over the entire length of a long backplane.
Fabricators select glass styles with smaller gaps between the fibers to smooth out the transition. Consistency is the primary defense against frequency specific signal attenuation.
