Z Axis Permittivity Variations in Standard High Speed Multilayer Stackups
Out-of-plane permittivity in glass-reinforced multilayer stackups runs lower than datasheet figures, requiring Z-axis test methods to hold impedance.
Flat Glass Fabric
Fiber Reinforcement
Woven glass reinforcements featuring spread or flattened fiber bundles reduce the microscopic resin-rich pockets between the weave yarns. Using flat glass fabric minimizes the variation in dielectric constant across the surface of the laminate. This structural uniformness prevents the skew of high-speed differential signals that occurs when one trace runs over glass and the other runs over resin.
Electrical Performance
High-frequency signals suffer from phase mismatch when differential traces encounter non-uniform dielectric environments. Replacing standard round-yarn bundles with flat glass fabric ensures that the fiberglass is spread evenly, presenting a homogeneous medium to the copper traces. This layout mitigates the glass-weave effect, which can degrade signal rise times and cause severe jitter in multi-gigabit digital systems.
When traditional fabrics are used, the alternating glass bundles and resin lanes create localized shifts in transmission line impedance. This flattened glass style offers a more predictable dielectric constant, allowing designers to calculate more accurate impedance values during the layout phase.
Mechanical Properties
Structural stability and dimensional consistency of the laminate are also improved by the uniform distribution of these flat yarns. It reduces the occurrence of micro-warpage and provides a more stable substrate for micro-via drilling. This laminate choice is widely adopted in high-layer-count line cards and motherboard designs where signal integrity requirements are stringent.