Woven Specification
Glass fabric used in printed circuit board fabrication consists of woven fiberglass filaments configured to provide mechanical stability and electrical insulation. The 7628 glass style represents a standard heavy-weave fabric specification commonly employed in rigid laminates.
Resin Absorption
Resin absorption during the prepreg manufacturing phase determines the final dielectric constant and board thickness. Because of the thick yarn diameter, the 7628 glass style has a relatively low resin content by weight, typically around 40 percent. This lower resin ratio results in a higher overall dielectric constant compared to styles woven with finer glass yarns.
Fabricators must balance this resin volume against the need to fill copper patterns during lamination to prevent voids.
Thickness Variance
Thickness variance across the finished laminate arises from the knuckle-crossover points of the heavy weave. These high spots create localized regions of high glass density and low resin density. In high-speed signal propagation, this non-uniformity causes propagation delay differences known as the fiber-weave effect.
Mitigating this issue requires specifying alternative materials or routing traces at an angle relative to the glass weave direction, though this increases fabrication complexity and cost. For this reason, multi-layer designs with tight impedance tolerances often avoid using a single layer of the coarse weave.
