Signal Timing
Differential signal propagation delay along paired printed circuit board traces results from microscopic variations in glass bundle density across the dielectric substrate. Fiber weave skew alters the arrival time of high-speed differential signals when one trace runs over glass bundles while the other runs over resin-rich areas. Woven fiberglass fabrics like style 1080 or 7628 consist of dense glass yarn bundles separated by resin-filled windows.
The dielectric constant of E-glass is approximately six, whereas epoxy resin measures near three. Signals traveling over glass bundles move slower than signals moving through surrounding resin matrix areas.
Propagation Velocity
Signal velocity varies inversely with the square root of the local relative dielectric constant. High-speed differential pairs operating above ten gigabits per second experience skew when phase alignment breaks down. Phase skew converts differential signal energy into common mode noise, degrading eye diagrams and increasing jitter.
Differential timing mismatches cause signal integrity failures in high-speed digital designs.
Substrate Anisotropy
Routing traces at diagonal angles relative to the substrate weave orientation mitigates dielectric variation effects. Spreading glass yarn bundles during laminate manufacturing creates more uniform dielectric distribution across the board layout. Mechanically rotated board layouts ensure both conductors in a differential pair traverse equal proportions of glass and resin.
Advanced low-skew laminates use flat glass fabrics to minimize dielectric variations across high-frequency circuit boards.
