Geometric Correlation
Spatial relationship between the width or pitch of printed circuit board traces and the periodic spacing of the reinforcing glass weave fibers dictates the uniformity of the dielectric constant experienced by a traveling electromagnetic wave. The trace-weave pitch ratio determines whether a trace runs consistently along a fiber bundle or repeatedly crosses between fiber bundles and resin-filled gaps. It is a metric used to evaluate the risk of localized impedance variations and timing differences between parallel transmission lines.
Skew Generation
When the trace-weave pitch ratio is close to one, a trace can become perfectly aligned with either a fiberglass bundle or a resin-rich channel over a long distance. One line of a differential pair might run entirely over glass, while the other runs entirely over resin, creating a measurable difference in their signal propagation speeds. This asymmetry introduces phase skew and converts differential signals into common-mode noise, which degrades signal integrity and increases electromagnetic interference.
To avoid this mismatch, high-frequency layouts avoid traces that run parallel to the warp and fill directions of the woven fiberglass.
Design Avoidance
Rotating the signal trace routing by a small angle relative to the substrate fabric prevents alignment over long distances. This angular routing alters the effective trace-weave pitch ratio across the board, ensuring that both lines of a differential pair experience the same average dielectric constant.
