Differential Phase Skew Control through Glass Cloth Weave Flattening and Alignment
Differential phase skew control requires spread-glass fabrics or off-axis routing to eliminate local micro-scale dielectric variations across high-speed traces.
Fabric Style 3313
Substrate Weave
Fiberglass fabrics used as reinforcement in high-speed printed circuit board laminates are classified by their yarn thickness and thread count. A specific weave pattern known as fabric style 3313 is designed with flat, spread-yarn fibers to provide a highly uniform dielectric substrate. This balanced structure reduces the size of the resin-filled gaps between the woven glass bundles.
Laminates built with this fabric are commonly used for high-performance multi-layer boards that require precise impedance control.
Glass Uniformity
Physical thickness and resin content of the prepreg layer are determined by the underlying glass yarn selection. For fabric style 3313, the flat yarn shape allows for a thinner laminate profile with less resin pockets than traditional round-yarn weaves like style 1080. The reduced thickness variance across the board helps to maintain a consistent dielectric constant, which is critical for routing tight differential pairs.
During fabrication, the flat weave ensures that the drilling of microvias is more uniform because the laser encounters a consistent ratio of glass to resin.
Electrical Performance
Differential signal skew is a major failure mode in boards operating at microwave frequencies. When a differential pair is routed over a non-uniform weave, one trace may run over a glass bundle with a high dielectric constant while the other runs over resin with a low dielectric constant. Utilizing fabric style 3313 eliminates this glass-weave effect by ensuring both traces see an identical dielectric environment.
Board fabricators measure this skew with specialized high-frequency probes to verify signal alignment across the board surface. The resulting increase in signal integrity allows for longer transmission paths without the need for active re-drivers. Circuit designers specify this weave in their fabrication notes for any traces operating above ten gigabits per second to keep transmission delays within acceptable bounds.