DRC: USB3 TX-RX isolation violations near connector/IC pads

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Problem

The USB3 SuperSpeed TX (SSTX+/-) and RX (SSRX+/-) differential pairs converge at the USB 3.0 connector (U7) and CYUSB3014 IC pad breakout areas on F.Cu, with clearances as low as 0.25mm.

The custom DRC rule USB3 TX-RX isolation (defined in #35) enforces 0.5mm minimum clearance between TX and RX pairs. This is correct for parallel routing in the field but physically impossible at connector/IC pads where the pairs must converge.

Expected violations

41 clearance violations, all on F.Cu between SSTX+/- and SSRX+/- near U7 and the CYUSB3014. Actual clearances range from 0.25mm to 0.42mm at breakout areas.

Options

1. Add a pad/breakout exception in .kicad_dru — exempt the rule within a certain distance of pads (cleanest)
2. Relax the rule to 0.3mm globally (still provides some isolation but fewer violations)
3. Accept and exclude — add DRC exclusions for the unavoidable pad-area violations
4. Reroute — increase separation at breakout if board space permits

Dependencies

• Requires DRC: Only default net class defined — no signal-specific classes #34 (net classes) and DRC: No custom design rules file (.kicad_dru) #35 (.kicad_dru file) to be implemented first
• The isolation rule is defined in DRC: No custom design rules file (.kicad_dru) #35's .kicad_dru content

Location

TAPRX-888 KiCad/TAPRX-888.kicad_dru — rule "USB3 TX-RX isolation"

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Using a design rule that only looks at the tracks and not the pads would fix the worst of it:

  (rule "USB3 TX-RX isolation"
      (condition "A.Type == 'track' && B.Type == 'track' &&
          (A.NetName == '/SSTX+' || A.NetName == '/SSTX-') &&
          (B.NetName == '/SSRX+' || B.NetName == '/SSRX-')")
      (constraint clearance (min 0.5mm))
  )

But so would the standard review-and-exclude method...

I looked at my own USB3 Fiber Converter and I essentially got to avoid this issue since the IC route to the USB3 B connector was all spaced 0.5mm end to end.