SNLA426 june 2023 DS320PR1601 , DS320PR410
- 1
- Abstract
- Trademarks
- 1 Introduction
- 2 PCIe Gen3, Gen4, and Gen5 Loss Budget
- 3 Minimum Eye Width
- 4 Cross Talk Mitigation
- 5 Humidity and Temperature Insertion Loss
- 6 Critical Signals
- 7 General High-Speed Signal Routing
- 8 PCB Grain and Fiber Weave Selection
- 9 PCB Material Loss Budget
- 10High-Speed Signal Trace Impedance
- 11High-Speed Signal Trace Length Matching
- 12Differential Trace Routing Guidelines
- 13Differential-Inter-Pair Matching
- 14Intra-pair Length Matching
- 15Trace Bends
- 16Minimum Differential Trace-To-Trace Distance
- 17Serpentine Guidelines
- 18High-Speed Differential Signal Quick Rules
- 19High-Speed Differential Pair Reference Plane
- 20Via Staggering
- 21Via Stubs
- 22Via Pads
- 23Via Discontinuity Mitigation
- 24Back-Drill Stubs
- 25AC Coupling Capacitors Placement
- 26AC Coupling Capacitor Physical Placement
- 27Auxiliary Signal AC Match Termination
- 28Suggested PCB Stack-ups
- 29Summary
- 30References
17 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.
Figure 17-1 Serpentine Guidelines