SLAAE45 September   2021 TMUXHS4412


  1.   Trademarks
  2. 1Introduction
  3. 2Practical PCB Design Rules
    1. 2.1 PCIe® Specific Standard
    2. 2.2 PCIe® High-Speed Signal Layout Guidelines
    3. 2.3 Vias, Stub, and ESD/EMI Layout Guidelines
    4. 2.4 Power and Grounding Layout Guidelines
  4. 3Layout Examples
  5. 4Summary
  6. 5References

Vias, Stub, and ESD/EMI Layout Guidelines

  1. The use of vias is essential in most routings, but vias add additional inductance and capacitance, and reflections occur due to the change in the characteristic impedance. Vias also increase the trace length.

    If possible, avoid routing high-speed traces through the vias.

    If it is impossible to avoid vias:

    • Ensure that the via count on each member of the differential pair is equal and that the vias are as equally spaced as possible. A maximum of two vias are recommended for high-speed trace over 5Gbps.
    • Be careful with the return current when changing the layers. Use ground vias around the signal via to make sure that the return current can flow as close as possible to the signal (see Figure 2-3)
      GUID-20210923-SS0I-LXRL-PL65-KBHPDCBJ7Q6F-low.png Figure 2-3 Differential Pair Via Return Path With GND Vias
    • Longer via stubs resonate at lower frequencies and increase insertion loss, keep these stubs as short as possible. TI recommends keeping via stubs to less than 15 mils. Longer stubs must be back-drilled (see Figure 2-4).
      GUID-20210923-SS0I-J4CS-XH7X-Q2RTGVQSG0NG-low.png Figure 2-4 Long Vias With Back-Drilled Stubs
  2. Do not place probe or test points on any high-speed differential signals.
  3. Do not route high-speed traces under or near crystals, oscillators, clock signal generators, switching power regulators, mounting holes, magnetic devices, or integrated circuits (IC) that use or duplicate clock signals.
  4. When choosing ESD/EMI components, TI recommends selecting devices that permit flow-through routing of the differential signal pair because they provide the cleanest routing.
    • Incorporate voids under the ESD/EMI component signal pads to reduce losses (see Figure 2-5).
      GUID-20210923-SS0I-TNQ2-7MVS-0BLBD4062F6G-low.png Figure 2-5 Void Below Surface Mount Devices
    • Use 0402 0-Ω resistors for common-mode filter (CMF) no-stuff options because larger components will typically introduce more loss that the CMF itself.
    • Keep the overall routing of AC coupling capacitors+ CMF+ ESD protection as short and as close as possible to the connector.