SNLA426 june   2023 DS320PR1601 , DS320PR410

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. Introduction
  5. PCIe Gen3, Gen4, and Gen5 Loss Budget
  6. Minimum Eye Width
  7. Cross Talk Mitigation
  8. Humidity and Temperature Insertion Loss
  9. Critical Signals
  10. General High-Speed Signal Routing
  11. PCB Grain and Fiber Weave Selection
  12. PCB Material Loss Budget
  13. 10High-Speed Signal Trace Impedance
  14. 11High-Speed Signal Trace Length Matching
  15. 12Differential Trace Routing Guidelines
  16. 13Differential-Inter-Pair Matching
  17. 14Intra-pair Length Matching
  18. 15Trace Bends
  19. 16Minimum Differential Trace-To-Trace Distance
  20. 17Serpentine Guidelines
  21. 18High-Speed Differential Signal Quick Rules
  22. 19High-Speed Differential Pair Reference Plane
  23. 20Via Staggering
  24. 21Via Stubs
  25. 22Via Pads
  26. 23Via Discontinuity Mitigation
  27. 24Back-Drill Stubs
  28. 25AC Coupling Capacitors Placement
  29. 26AC Coupling Capacitor Physical Placement
  30. 27Auxiliary Signal AC Match Termination
  31. 28Suggested PCB Stack-ups
  32. 29Summary
  33. 30References

Serpentine Guidelines

Serpentine routing needs to be avoided when possible. However, if serpentine routing is needed, keep the following in mind:

  • The difference in trace lengths for a differential pair changes geometry from closely coupled to loosely coupled for a length or 3xW or 3xRPH. As trace width increases, impedance drops. To compensate for the drop in impedance, increase trace width. This new trace width depends on the board geometry or board stack-up. Alternatively, if the board stack-up is known, the trace width can be calculated.
  • The length of serpentine must be at least 3xW or 3xRPH – whichever is larger.
GUID-20230511-SS0I-DRNF-S84C-3MGRD2G64FDT-low.svgFigure 17-1 Serpentine Guidelines