For my 4-layer PCBs, I was suppling only this to the manufaturer:

enter image description here

And was considering a dielectric constant of 4.6 to calculate the 50 ohms RF trace.

enter image description here

And now I want to provide a more complete difinition to them:

enter image description here

IPC 4101/21 base material was copied from a stack of a development board of Nordic Semiconductor. The following website shows that for IPC 4101/21 the dielectric constant value to be used to calculate impedance of RF traces is 4.0, should I trust this value? https://www.lab-circuits.com/en/materials

I want to start specifying the stack and materials better to have better repeatability in PCB manufacturing, mainly PCBs with RF matching circuits.


  • 1
    \$\begingroup\$ You need to communicate with your PCB house. Do not guess or assume. If you specify the material, number of layers and copper weight, they will give you the stackup they will use. They can also give you the dielectric constants of the prepreg and laminate. \$\endgroup\$
    – qrk
    Commented Jun 21, 2021 at 2:19
  • \$\begingroup\$ dielectric constant is : 4,70 max, 4,35@ 500 MHz, 4,34 @1 GHz ... xxx@2GHz. To be measured or "known". \$\endgroup\$
    – user288518
    Commented Jun 21, 2021 at 7:23
  • \$\begingroup\$ this link can help you : by Stephen Mumby and Jih Yuan , researchgate.net/publication/… \$\endgroup\$
    – user288518
    Commented Jun 21, 2021 at 7:31

1 Answer 1


FR-4 is a flame rating- how well the material can burn. It has nothing to do with it's electrical, and even mechanical properties. It is highly unfortunate that the FR-4 name has "stuck" as a defacto standard for PCB laminate names, but that is the world we live in...

Even when specified further as IPC 4101\21, that standard lists FR-4 material as maximum permittivity to 5.4. What that is means is that many/most (?) compositions of epoxy/glass fibers (and dimensions of the glass fibers!) that gives the ratings listed in the IPC standard can be used. It is perhaps slightly better than just saying "FR-4" but I would belive that any "FR-4" materials on the marked are in accordance to IPC 4101\21, as I belive the FR-4 term originates from that standard.

If you want repeatability for FR-4 boards you should contact your fabricator, agree with them on a specific material vendor and then specify that - if they will let you. (rembemer; permittivity isn't a controlled property for FR-4, so manufactures are not monitoring that in the production line. Why should they? That would just add cost. So likely they are not able to give a statement saying that "our material has Er of 4.4 +/-10%).

So: If your design has critical requirements for permittivity (I.e; that your design cannot tolerate variations, and/or has frequencies of several GHz and above) you should use materials where a specific permittivity is listed in the datasheet. FR-4 is not the right material for you. Again, contact your fabricator, they will have several options.


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