Flex-Rigid PCB RF design

Hello!

I have a RF reference circuit at 2.4Ghz designed with transmision lines (balun) in 0.8mm separated 2 layer pcb. We are going to design this circuit in a flex-rigid PCB. Instead of having

copper/FR4/copper

we have

Copper/kapton/FR4/copper/FR4/Copper/FR4/Copper

We can mantain 0.8mm of thickness but the 4.2 permittivity needed, is "disturbed" by 60um of kapton (permittivity 3.5).

Anyone knows how to calculate the effective permittivity of the PCB? Do you think that this kapton material can vary a lot the RF performance?

The equivalent value for this sandwich is t=0.86mm with permittivity 4.14
The deviation in permittvity is small compared to the large tolerances of FR4 permittivity.
For the change in substrate thickness, you might want to check in your circuit simulator / EM simulator.

Manufacturer say us that the final tickness is going to be is exactly 0.8mm. How do you calculate the total permittivity? Thanks!

I have an in-house tool for that, because it is frequently used for my work on RFIC technologies.
Calculation is based on parallel plate capacitance and the equivalent permittivity of cascaded (series) parallel plate capacitors.

Thanks for the help! And what about tangent loss? Does it change a lot?

I think that everithing is going to work ok because the change is less than material tolerance...

Good question. I did a quick web search and learned that Kapton for PCB is "Dupont Pyralux".
http://www2.dupont.com/Pyralux/en_US...ad_H-73241.pdf
According to the data sheet, the loss tangent is less (better) than FR4.

So I think your design is fine, without modifications.

Hi! Thanks for the help. Just for demonstrate my decision... is there any way to easily calculate the multilayer permittivity? Thanks!

Here's the code:

totalThickness := 0;
totalErel := 0;
for n:=first to last do begin

end;
totalErel := totalThickness/totalErel;