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I'm in the process of trying to make my first PCB with USB 2.0 on it.

Right now I'm trying to work out what the trace width and clearance for the differential pair should be to get the correct impedance.

According to this: http://www.usb.org/developers/docs/hs_usb_pdg_r1_0.pdf

they normally use 7.5 mil(0.19mm) traces with 7.5mil clearance on a 63 mil board (1.6mm)

That should give the traces around 90 Ohms differential impedance according to the PDF, but when I try to input it to my impedance calculator (Saturn PCB toolkit V7.03) it gives me almost 160 Ohm - and even if use 53mil conductor height or anything I cannot get the 90 Ohm differential impedance.

What am I doing wrong here?

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Those 60mil are NOT the board thickness but the distance to the next contiguous plane (GND or POWER).

I don't know your layer stack but if this is a 2 layer board and you want to keep that, you will need 1mm thick tracks which is probably a little .. well ...

You better want to stick to a 4-layer stack.

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  • \$\begingroup\$ Well, at a 2 layer PCB 1.6mm with 2x35um copper, it the distance between bottom and top layer is 60mil - but i assume that the PDF is talking about a 4 layer PCB with the traces way closer then? \$\endgroup\$
    – M.Egsdal
    Commented Jun 12, 2017 at 7:18
  • \$\begingroup\$ Whenever you require a specific impedance, the distance between the signal lines and the next plane is what you're looking for. If you cannot achieve the track thickness / spacing with your stackup, it's time to look at a different stackup. Some PCB fabricators specify default stackups. If yours doesn't, you can always request your desired stackup and specify the required impedances. The manufacturer will then calculate the track thickness and spacing for you (because the material in use is also relevant) and tell you those values. \$\endgroup\$
    – Tom L.
    Commented Jun 12, 2017 at 7:48

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