Substrate Copper Roughness Impacts on High Frequency Cavity Performance
Copper surface roughness increases high frequency cavity conductor attenuation by extending skin current path length and degrading unloaded quality factor.
Phase Velocity Distortion
Signal Degradation
The alteration of a high-speed signal’s wave shape due to different frequency components traveling at different speeds through a dielectric medium occurs in long transmission lines. This phenomenon, known as phase velocity distortion, spreads out the signal pulses as they propagate along the board tracks. It causes inter-symbol interference, which increases bit-error rates in high-speed digital communications.
This effect is a primary limitation on trace lengths in multi-gigabit backplanes.
Material Influence
Variations in the dielectric constant of the laminate drive this speed discrepancy. High-speed signals are composed of a wide range of frequencies, and if the dielectric constant of the laminate varies across this range, the phase velocity distortion will be severe. In composite materials like FR-4, the non-uniformity of the fiberglass weave also contributes to this problem.
When a trace passes alternately over glass bundles and resin-rich regions, the effective dielectric constant changes along the trace length. This local variation leads to phase skew between differential signal pairs, which degrades signal integrity.
Mitigation Strategy
Minimizing this distortion requires selecting laminates with flat dielectric constant curves and uniform glass styles. Designers specify materials with low loss resins and spread-glass weaves to ensure that the trace experiences a uniform dielectric constant. In critical designs, routing the traces at an angle to the glass weave reduces the impact of the weave pattern.
This routing technique ensures that each line averages the dielectric properties of the glass and resin evenly over its length.