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Impedance Calculations

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  • Impedance Calculations

    How do I calculate the impedance of a trace on the top layer when the solder mask has a dk of 4.5 and a thickness of 25um and the core beneath the trace has dk of 3.5 and a thickness of 2.54mm. Attached is the closest calculator from the Saturn PCB Toolkit but it doesn't allow me to specify two dielectric constants.

  • #2
    This is exactly one of the reasons why it is important to ask PCB manufacturer to provide the final stackup and track geometry. They can calculate the values more precisely.

    In this situation, I sometimes use the Er which is in the middle between the two Ers or I use the Er value of the thinner dielectricum which is placed closer to the reference plane. But the result is never exactly the same value as specified by PCB manufacturer. PCB Manufacturers use different calculators and they also include the technological process variables into the calculations (e.g. pressing the material may change it's thickness, melting the material and flowing between tracks can change thickness, ...).

    So, the best is to ask PCB manufacturer.

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    • #3
      I spoke to my PCB manufacturer today and he said that he has never been asked that question. He basically said that as the thickness of the solder mask is much thinner in comparison to the core and the effect on impedance would be negligible. I wouldn't mind calculating this for myself so that I could verify as I don't know how truthful his response was.

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      • #4
        Honestly, I do not normally use solder mask Er for my calculations. However, if you have a look at standard pictures generated for example by Polar, you will see they have cells to enter the values. Here are some examples:

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        • #5
          Thanks you for your insight, that looks like a great tool but I don't we will be able to afford it at this point in time.

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          • #6
            I don't think you would need to buy the software. Even we do not have it - the screenshots are from our PCB manufacturer. We just suggest the stackup and geometry based on the simple and free tools and the PCB manufacturer re-calculate it in the professional software and send us the calculations and stackup back.

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