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Breakdown voltage of FR4?

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  • Breakdown voltage of FR4?

    Hi every one!
    Hope every one doing great!

    Need to know the breakdown voltage of FR4? And need to make sure that it will stand 150V, any help?
    Is standard 4-layer PCB able to stand 150V, or I need to use custom stack-up?

    I am using SaturnPCB toolkit as Robert recommended, but can find a location to set and check this parameter!

    Thank you in-advance!

  • #2
    Hi,
    Unfortunately FR-4 isn't specific enough. As I understand, FR-4 is a fire rating. Any PCB material that meets that fire rating is essentially FR-4. No direct correlation to dielectric strength.

    Various PCB material manufacturers' materials have various dielectric strength. What they usually specify is volts per material thickness, so that translates to voltage between a circuit on one layer and a circuit on another layer, with this material in between.

    Check out Isola
    https://www.isola-group.com/products...ls/attributes/

    They have various materials, some of them are FR4. If you already have a favorite fab shop, ask them for the specs of the material they use when the fab drawing only specifies FR4 and doesn't call out a particular material part number.

    Hope this helps.

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    • #3
      Hi

      check this notebook from Sierra Circuits
      http://www.magazines007.com/pdf/High...e-PCDesign.pdf

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      • #4
        goncaloc , thank you. But I have checked it, not helping. At page 9 breakdown voltage is mentioned as 300V/mils, while the SaturnPCB toolkit is reporting much lower values. Do not sure which one I can trust.

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        • #5
          Originally posted by JohnsonMiller View Post
          goncaloc , thank you. But I have checked it, not helping. At page 9 breakdown voltage is mentioned as 300V/mils, while the SaturnPCB toolkit is reporting much lower values. Do not sure which one I can trust.
          This is because the breakdown voltage isn't related to "FR4" status of a material. FR4 material can vary in breakdown voltage depending on who makes the material and what other properties the material has.

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          • #6
            If I would need to use high voltage, I would definitely not do it in the same area of PCB and using different layers to separate the voltage (I would consider this to be too dangerous). Instead, I would use different areas in PCB which would be separated by a wide gap or with an air slot between these areas with high voltage difference.

            PS: I am not expert for high voltage, but I believe the isolation gap depends on several factors ... and one is the environment around your board (for example can the PCB get dirty, what will be the humidity, etc ). For some of my projects I have used Saturn PCB to decide on the gap - I used the widest one which was suggested for the worse case. In some cases I used the widest gap which is specified by the components which are crossing these two voltages e.g. if there were optocouplers I would use the gap as wide as the distance between the pads of optocoupler.

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            • #7
              150V on a 4 layer FR4 is no problem, as long as you do not try to use very thin prepregs.
              For clearances, both IPC 2221 and IEC 60664 have tables for clearance. IEC 60664 also includes the pollution degree and materials group.
              Also consider the over voltage level (IEC 60664).

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              • #8
                Clearances per standard are going to remain the same regardless of material, but that's going to be for same-layer objects with a voltage between them.

                Where the PCB material is of concern is when controlling objects on different layers with a voltage between them. For this, you have to go by the dielectric strength of the material, which varies according to material manufacturer. If you have a favorite assembly shop, or fab house, ask them what they like to use for their run of the mill jobs. Look at the specs of that material. It will be volts per 0.001" or volts per mm. Make sure your stackup is specified appropriately. Materials are often specified in thousands of volts per mil, so even if it's 150VAC (higher peak voltage), you shouldn't have to worry for typical thicknesses.

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