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Assume that you having one track that goes from 0.11mm in width to 0.09mm in width.

I want to compute the impedance, but I need to select the width. What option should I select?

  1. The largest width of 0.11mm
  2. The smallest width of 0.09mm
  3. The average width of 0.1mm

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  • \$\begingroup\$ It depends what is the purpose. If most of the track is wider and you have only 1 cm of narrow track, then the impedance of the narrow track may be irrelevant. The width change can also be for other compensating reasons to keep a constant impedance due to where the track goes to or if there are other changes as well, such as changes in the ground plane underneath the track. \$\endgroup\$
    – Justme
    Commented May 25 at 10:43

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The calculator needs the width as it affects the impedance. Therefore you should not change the width of the track mid-way if you want the impedance to be continuous.

Any discontinuity will cause reflections. Every time you change the width, a discontinuity is created, unless you change something else too, to compensate for the change.

Other important factor is that the reference plane (often GND) needs to be continuous in geometry too. You can't for example cut the plane underneath the track without affecting the impedance.

So the answer is you need to calculate the impedance for each width along the track.

Also, widths below 0.1 mm become trickier to manufacture, check with your circuit board manufacturer what the design rules should be.

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  • \$\begingroup\$ or use 0.09mm instead all way long? \$\endgroup\$
    – euraad
    Commented May 25 at 11:28
  • \$\begingroup\$ @euraad It might not be possible to reach the target impedance with 0.09mm width, as then you need a different PCB stackup with less distance between signal and ground planes. \$\endgroup\$
    – Justme
    Commented May 25 at 11:48
  • \$\begingroup\$ @Justme Well, I'm locked to a specific stack. And I need to have 50 ohm as close as possible. It's for between MCU and Ethernet PHY. \$\endgroup\$
    – euraad
    Commented May 25 at 12:03
  • \$\begingroup\$ @euraad then you can go with 0.09 mm, if your PCB manufacturer is ok with it. Some designs skip the layer directly below (by placing no copper pour where the track is) and use 3rd layer as the reference plane to get the geometry right. \$\endgroup\$
    – Ralph
    Commented May 25 at 12:06
  • \$\begingroup\$ @Ralph But can't I compute the average width? I mean, if I have 0.11 and 0.09, the middle is 0.1mm. Can't I go with that? \$\endgroup\$
    – euraad
    Commented May 25 at 12:12

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