I am designing fast digital lines for the first time. I want to change the PCIe connector of the CM4 IO board to mini PCIe.

I reviewed the AN307: Hardware Design Considerations for PCI ExpressTM and SGMII. Pages 35 - 44 give the design and layout considerations.

  1. What does the "via sites" in "Stitching vias (to GND) at all diff pair via sites" mean?
  2. "Vertical and horizontal routing increases dielectric loss" - by "vertical and horizontal" do they mean "traces that go upwards/downwards and left/right?
  3. There is no reference to PCB stackup. Doesn't it matter?
  4. Are there other considerations I should take into account? (I'd appreciate a recommended app-note).

1 Answer 1


While routing and layout are very important, PCI-e is a very robust protocol given its high speeds.

  1. The "via site" is just referring to any point where you route the differential pair through a pair of vias to a new layer. The arrow in the accompanying figure is supposed to be pointing this out, but it's not very clear!

Via site

  1. There is interaction with the FR4 weave. If you route solely vertically/horizontally, you will cross bundles and resin rich areas harmonically. For long traces, it is recommended to route off the XY axes to distribute these losses. For short traces, it shouldn't be an issue.

From: https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/639/7851.PCIe_5F00_designGuides.pdf

  1. This is not explicitly called out, but on page 36, there is the statement Solid reference GND plane (strongly recommended). This is obviously an adjacent plane, so you won't go wrong with the normal 4-layer stackup (SIG-GND-GND/PWR-SIG). Look at the CM4IO layout as a reference.

  2. Usual high speed differential things (not a specific app note, but plenty of information freely available):

    • Don't cross gaps in reference planes
    • Keep things as short as possible
    • Use as few vias as possible
    • Route differentially as far as you can (minimise differences at connectors etc)
  • 1
    \$\begingroup\$ thank you for the full answer \$\endgroup\$
    – metsik
    Commented Feb 9, 2022 at 9:11

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