Reinforcement Structure
Electrical grade glass fibers bundled into yarns and interlaced at right angles provide the primary mechanical strength for printed circuit substrates. The density and thickness of an e-glass weave affect the dielectric consistency of the board because the glass has a higher permittivity than the surrounding resin. Thin weaves create a more uniform surface for fine line etching.
Manufacturers specify the glass style by a four digit code that defines the yarn count and weight.
Thermal Stability
Dimensional changes during the lamination process are constrained by the high modulus of the glass filaments. Using a tight e-glass weave reduces the expansion of the laminate in the horizontal directions, which helps maintain the alignment of stacked vias. Heavy glass styles like 7628 offer better stiffness but may be harder to drill than lighter alternatives like 1080.
The thermal expansion of the board is a balance between the properties of the fiber and the polymer.
Signal Integrity
Non-uniformity in the distribution of glass and resin creates periodic variations in the dielectric constant that can cause timing skew in differential pairs. If one trace of a pair sits over a glass bundle while the other sits over a resin rich area, the signals travel at different speeds. Specifying a spread e-glass weave helps to minimize these effects by flattening the yarns to fill the gaps.
High speed digital designs rely on this uniformity to maintain phase alignment at high frequencies. This material choice is a fundamental step in designing for millimeter wave applications where wavelength and feature size are comparable.
