Meaning
Timing discrepancies occur in high speed differential signals when the two conductors of a pair travel over different densities of fiberglass reinforcement. Because the dielectric constant of the glass bundles differs from that of the surrounding resin, glass weave skew causes one signal to arrive slightly before the other. This arrival time difference degrades signal integrity by converting differential energy into common mode noise.
It is a critical concern for designs using PCI Express or 10-gigabit Ethernet interfaces. The phenomenon is purely a function of the material geometry.
Physical Cause
Printed circuit boards use a woven glass fabric to provide structural strength. The weave pattern consists of bundles of glass fibers separated by pockets of resin. If one trace of a pair sits on a glass bundle while the other sits over a resin pocket, they experience different propagation velocities.
This effect becomes more pronounced as data rates increase into the gigabit range.
Differential Degradation
The phase shift between the positive and negative signals closes the eye diagram. Jitter increases and the receiver struggles to recover the clock from the data stream. Total signal loss can occur if the skew reaches a large portion of the bit period.
The resulting common mode radiation also creates challenges for electromagnetic compatibility testing.
Engineers Strategy
Designers specify specialized glass styles with a spread or flattened weave to minimize the resin gaps. Routing traces at an angle relative to the weave direction ensures that both conductors see an average of glass and resin. High performance materials use resin systems with dielectric constants closer to that of glass.
These strategies reduce the timing variance to acceptable levels.
