Quantifying Differential Phase Skew Caused by Reinforcement Fiber Bundle Periodicity
Reinforcement fiber bundle periodicity causes localized dielectric constant variation, driving phase skew that requires spread glass or rotated routing to control.
Glass Weave
Substrate Composition
Reinforcement fabric made of woven filaments provides the mechanical strength and dimensional stability required for rigid circuit boards. In the production of prepreg and laminates, glass weave acts as the skeletal framework that holds the epoxy resin in place. It defines the board.
Various styles of cloth offer different weights and thicknesses to suit specific board designs.
Signal Impact
Non-uniform distribution of glass and resin creates localized variations in the dielectric constant. High-speed signals traveling over a glass weave experience different propagation delays depending on whether a trace sits over a bundle of glass or a resin-rich pocket. This phenomenon, known as the fiber weave effect, causes timing skew in differential pairs and jitter in serial links.
Using a tighter or spread cloth style mitigates these variations by providing a more homogenous environment for the conductors. Designers may also choose to route traces at an angle to the fabric orientation to average out the dielectric properties along the signal path. This technique reduces the likelihood of bit errors in multi-gigabit data streams.
Mechanical Property
Tensile strength and rigidity are the primary benefits of adding mineral fibers to the resin system. The orientation of the glass weave influences the warp and twist of the board during thermal processing. A balanced construction with identical layers on either side of the center prevents the laminate from bowing.
Thicker glass bundles provide better support for heavy components but are more difficult to drill cleanly without leaving protrusions.