SPRAAR7J November   2018  – February 2023 66AK2G12 , AM1806 , AM1808 , AM2431 , AM2432 , AM2434 , AM3351 , AM3352 , AM3354 , AM3356 , AM3357 , AM3358 , AM3358-EP , AM3359 , AM3871 , AM3874 , AM3892 , AM3894 , AM4376 , AM4377 , AM4378 , AM4379 , AM5706 , AM5708 , AM5716 , AM5718 , AM5726 , AM5728 , AM5729 , AM5746 , AM5748 , AM5749 , AM620-Q1 , AM623 , AM625 , AM625-Q1 , AM625SIP , AM62A3 , AM62A3-Q1 , AM62A7 , AM62A7-Q1 , AM62P , AM62P-Q1 , AM6411 , AM6412 , AM6421 , AM6422 , AM6441 , AM6442 , AM6526 , AM6528 , AM6546 , AM6548 , BQ24392-Q1 , HD3SS6126 , LP8727 , OMAP-L137 , OMAP5912 , TMS320C6745 , TMS320DM355 , TMS320DM365 , TMS320DM368 , TMS320DM369 , TMS320DM6441 , TMS320DM6443 , TMS320DM6446 , TMS320DM6467 , TMS320DM8127 , TMS320DM8147 , TMS320DM8148 , TMS320DM8165 , TMS320DM8167 , TMS320DM8168 , TMS320VC5506 , TMS320VC5507 , TMS320VC5509A , TS3USB221A-Q1 , TS3USBA225 , TSU5511 , TSU5611 , TSU6111 , TSU6111A , TSU6721 , TSU8111

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
    1. 1.1 Scope
    2. 1.2 Critical Signals
  4. 2General High-Speed Signal Routing
    1. 2.1 PCB Fiber Weave Mitigation
    2. 2.2 High-Speed Signal Trace Lengths
    3. 2.3 High-Speed Signal Trace Length Matching
    4. 2.4 High-Speed Signal Reference Planes
  5. 3High-Speed Differential Signal Routing
    1. 3.1  Differential Signal Spacing
    2. 3.2  High-Speed Differential Signal Rules
    3. 3.3  Symmetry in the Differential Pairs
    4. 3.4  Crosstalk Between the Differential Signal Pairs
    5. 3.5  Connectors and Receptacles
    6. 3.6  Via Discontinuity Mitigation
    7. 3.7  Back-Drill Stubs
    8. 3.8  Increase Via Anti-Pad Diameter
    9. 3.9  Equalize Via Count
    10. 3.10 Surface-Mount Device Pad Discontinuity Mitigation
    11. 3.11 Signal Bending
    12. 3.12 Suggested PCB Stackups
    13. 3.13 ESD/EMI Considerations
    14. 3.14 ESD/EMI Layout Rules
  6. 4References
  7.   A Device Layout Parameters
  8.   Revision History

2.1 PCB Fiber Weave Mitigation

When routing differential signals across common PCB materials, each trace of the pair will experience different dielectric constants and corresponding signal velocities due to the differences in static permittivity (Ɛr) of the fiberglass weave (Ɛr is approximately 6) and epoxy (Ɛr is approximately 3) that comprise a PCB. As signals travel faster when Ɛr is lower, an interpair skew can develop if a signal in a differential pair travels over a higher ratio of fiberglass or epoxy than does its companion signal. This skew between the differential signals can significantly degrade the differential eye diagram as presented to the receiver, cause significant AC common-mode voltage noise, and cause EMI issues. The extent of this problem will depend on the bus speed, the length of the traces, the trace geometries, the type of fiberglass weave used, and the alignment of the traces to the weave pattern of a PCB. Problems from fiber weave alignment vary from board to board. This variance makes issues difficult to diagnose.

#SPRAAR72802, #SPRAAR77655, and #SPRAAR78805 show the three most common methods to minimize the impact of PCB fiber weave in a board design. The goal of each method is to ensure that both signals of the differential pair will share a relatively common Ɛr across the length of the pair routing.

GUID-7FE025F5-9DC3-45B2-9CD8-35B08C0F8DFF-low.gif
The entirety of the signaling image plane is rotated 10° to 35° in relation to the underlying PCB fiber weave.
The PCB manufacturer can effect this rotation without making changes to the PCB layout database.
Figure 2-1 Rotation of the PCB Image
GUID-DB7BB60C-4770-4DC0-8224-CFDBE261C90C-low.gif
Only the high-speed differential signals are routed at a 10° to 35° angle in relation to the underlying PCB fiber weave.
Figure 2-2 Routing Angle Rotation
GUID-2F4E4227-7764-4DE1-9E3D-CA9B6D4D2A8F-low.gif
The high-speed differential signals are routed in a zig-zag fashion across the PCB.
Figure 2-3 Zig-Zag Routing

Because the ratio of fiberglass to epoxy is the primary contributor to the Ɛr disparity, choose a PCB style with a tighter weave, less epoxy, and greater Ɛr uniformity across longer trace lengths. Before sending your design out for fabrication, specify a PCB style that can best accommodate high-speed signals. For examples of common PCB styles, see #SPRAAR79005.

GUID-CBC6B050-86F2-4B51-927E-B8F9B6405224-low.gifFigure 2-4 PCB Fiberglass Style Examples