PCB High-Speed Signal Design Guidelines: Agilex™ 5 FPGAs and SoCs

ID 821801
Date 11/18/2024
Public
Document Table of Contents

3.3.7. PCB Vias

  • Vias impact high-speed channel loss and the timing budget, so use as few vias as possible for the high-speed differential channel.
  • Keep impedance continuity between the high-speed PCB via and trace. Vias usually have higher capacitance and lower impedance than traces.
  • Optimize via impedance, using a 3D electromagnetic (EM) field solver, by sweeping the anti-pad width, length, and radius for your specific stackup, drill size, and via stub. Keep in mind that:
  • Keep in mind that:
    • The smaller the drill size, the higher the via impedance.
    • The larger the anti-pad size, the higher the via impedance.
    • The shorter the via stub, the higher the via impedance.
    • The smaller the via top, bottom, and functional pads, the higher the via impedance.
  • Make sure that each high-speed signal via has a ground via for reference, and make sure that the two signal vias of a differential pair have symmetrical ground vias.
  • Remove non-functional pads for high-speed signal vias and ground vias to lower via capacitance.
  • Make the closest TX and RX signal via coupling length as short as possible through an appropriate layer assignment.