Electronics > PCB/EDA/CAD

Controlled impedance for single and differential pair (45 ohm and 90 ohms)


So I have a 6 layer PCB:

Used this PCB manufacturer for values.

As you can see, my USB data lines go on 3rd layer from top - "SignalH".
Single strip ought to have 45Ohm, differential - 90 Ohms.

In this website for impedance calculation:

What values do I put in for "H" and small "h"?

I'm guessing small "h" is the distance to closest ground plane, which is 0.2mm in my case? What about big "H"? is it distance from "SignalH" to "Bottom" layer?

Also don't understand why the same PCB manufacturer has "Symmetric Stripline", when they don't allow for manufacturing in such way.
According to them, signal trace must lie between two ground layers - exactly in the middle, but that's not possible, because they have different thickness for "core" and "prepreg" per their stackups (see 6 layer, 1mm thickness).


I am happy to guess :)

--- Quote ---because they have different thickness for "core" and "prepreg" per their stackups (see 6 layer, 1mm thickness)
--- End quote ---

The core and prepreg is 0.02mm difference, which might be considered within manufacturing tolerances. Couldn't spot what the core dielectric constant is, though.

But, in any case, they do say:

--- Quote ---In other special cases or Advanced PCBs (you need to choose " Additional Options" and custom stackup or control impedance)
--- End quote ---

so I would assume that if you need a stackup to suit your stripline requirements they are willing to discuss the cost with you.

When calling out diff pairs I always reach out to the board house to ask for the trace/space/stackup for a given differential impedance. They use SI8000 that has their finely tuned process parameters baked in. It is usually better to ask and design to the nominal values the board house provides. After all, they are the ones that are going to build it and guarantee the tolerance on their COC. I only ever don't ask if I don't want to pay for controlled impedance at which time I break out the calculator. If you can get a pressed stackup from your vendor then that is great.

Does anyone know if I have a signal layer between two ground planes/layers.

If I run diff pair traces and remove GND copper polygon from the layer below those traces - can I pretend that I have edge embedded for impedance calculations? Instead of assymetric stripline?
Like this.
Bottom layer is also GND polygon.


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