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If I understood correctly, when designing PCB with USB, the 90-ohm differential pair of D+/D- trace if for matching the USB cable differential impedance. Otherwise, there will be a signal reflection where the impedance is not continuous.

For those USB devices WITHOUT cable, for example,

  • USB storage
  • Wi-Fi module with USB interface, like this

Is it still necessary to trace D+/D- as 90-ohm differential pair?

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Is it still necessary to trace D+/D- as 90-ohm differential pair?

Absolutely yes. It doesn't matter whether there is a cable or not, the link between two ends must be one continuous transmission line, with impedance as uniform as possible. If the trace length (distance) between your host controller and peripheral device (be it a port receptacle or even embedded IC) is longer than 1/5 or 1/10 of the 1-bit signal propagation time, your traces must maintain 90-Ω differential impedance. For SuperSpeed Gen1 the bit interval is 200ps, which means the distance of aboout 1.2" (30-35mm) on FR4 substrate. This means that the impedance-unmatched trace segments longer than 5-7mm will negatively effect the signal. For USB 2.0 High-speed the bit time is 2.08 ns, or about 6" flight distance, and the mismatched segment should be no more than 1" long. It is very advisable to keep USB traces as clean as possible to avoid angry reports from users.

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