Structural Void
Structural voids between the woven yarns of a glass fabric reinforcement create localized regions filled entirely with the polymer matrix. The resin window gap is a physical feature of traditional glass weaves that results in a lower dielectric constant compared to the regions where the glass bundles are present.
Dielectric Influence
Dielectric influence is a concern for designers of high-speed differential pairs, as one trace of the pair may sit over a glass bundle while the other sits over a gap. This imbalance in the resin window gap leads to phase skew and signal distortion that can corrupt data at high bit rates.
Mitigation Technique
Mitigation techniques include the use of spread-glass fabrics or mechanical glass spreading, which flattens the yarns to minimize the size of the resin window gap. Manufacturers also suggest routing traces at an angle to the weave or using a zig-zag pattern to ensure that both traces of a pair experience the same average dielectric constant. Measuring the size of these gaps with high-resolution microscopy helps engineers select the right laminate for their specific frequency requirements.
If the gaps are too large, the board may be unsuitable for signals above 10 gigahertz. This consideration is a requirement for modern server and networking hardware.
