Electronic – How critical is PCB layout for USB Full-Speed

I've noticed the same problem when routing boards with the FTDI FT232 chips. Here's what the CY7C68013A datasheet says about routing those signals:

• DPLUS and DMINUS trace lengths should be kept to within 2 mm of each other in length, with preferred length of 20 to 30 mm.
• Maintain a solid ground plane under the DPLUS and DMINUS traces. Do not allow the plane to split under these traces.
• Do not place vias on the DPLUS or DMINUS trace routing.
• Isolate the DPLUS and DMINUS traces from all other signal traces by no less than 10 mm.

I would suggest the zig-zag method, but only if it that little detour around the pin added more than 2 mm to the trace length.

Assuming you're only using USB-low-speed or full-speed, you should be fine.

Generally, layout considerations only really have to be taken if you're going long distances (many inches), or using USB-2.0. Even then, USB is surprisingly tolerant.

• USB 1.1 or USB2.0 low/full speed

  • You really don't need to worry. There are (possibly apocraphyal) stories of people running USB2.0 low-speed of 50' of CAT-5 wire. As long as you keep your wire-runs a few inches or less, I wouldn't worry.
  • The fastest edge you will need to worry about in low/full speed applications is 12 Mhz. As such, you're not really approaching the point where making sure your traces/wiring is properly transmission-lined/impedance-controlled is that important, at least as long as your overall uncontrolled-impedance sections are less then, say ~6".
  • As I said, most USB controllers are impressively tolerant of USB devices that are widely out-of-spec. If this is something for production, I would spend the effort to do it properly (there is one guy out there who has a motherboard that throws a hissy-fit if anything connected to it deviates from the spec by a tiny amount), but if it's just a test-board, I'd say just lay it out neatly, and don't worry about it.

• USB2.0 High-speed.

  • Here layout becomes more important. USB2.0 High-Speed has a maximum edge rate of 480 Mhz. As such, even short traces start to approach the wavelength of the data, and as such proper impedance-control becomes important.
  • Assuming you EDA package has proper impedance-controlled routing options, just set your differential-pair impedance to be ~90Ω, and you should be fine. Be careful to make sure you have a contiguous ground-plane, though

• USB3.0

  • So you hate yourself?