50R conductor impedance

[chipeater]

Hello,

I need to layout a 4 layers board with a 868Mhz RF module and a SMA connector for the antenna.
I will use JLCPCB service and their JLC7628 stackup.

To compute the trace width between the RF module and the SMA connector, I used online calculator and Saturn PCB design software.
With a dielectric constant of 4.6 and a 7.1th prepreg thickness, the calculator gives me a conductor width around 12th.

But I was told it was wrong without further explanation, the trace must be much larger.
Does anyone have any experience in this ?

Thank you very much 

[TheUnnamedNewbie]

The thickness all depends on the type of transmission line (CPW? GCPW? Microstrip? Stripline? Slotline? ...) you want. Without stating that we cannot help you.

[chipeater]

I don't know what I want/need. I don't have the skills to make that choice.

The layout is super simple so I have some flexibility.
I have a full ground plane on inner 1, the RF module and the SMA connector are on top layer.
I tried with a microstrip configuration and  JLCPCB gives me a 11.55th trace width for JLC7628 stackup.
Saturn PCB gives me 12.45th with no plating thickness ( there is soldermask on this trace ).

Then I tried with coplanar ware configuration and with 55th of conductor gap between top layer ground plane and the RF trace, Saturn PCB gives me 14.95th



I find that both results are in the same order of magnitude but I was told that the trace must be much larger.
I'm looking for information so I don't look too stupid 

Thank you for your time

[NickZA]

According to JLC, their 7628 stack-up has a prepreg thickness of 0.21mm, so in AppCad I came up with the following:

0.4mm trace with 0.4mm clearance gives you close to 50ohm impedance.

[M0HZH]

What are you designing?

I did some 868Mhz modules on 4-layer JLC7628 stack as well, used their calculator for impedance (11.55mil for 50ohm) and PCB traces weren't matched properly, luckily the traces were short and didn't matter much.

[Miek]

fwiw, the JLCPCB calculator for their 7628 stackup has been pretty much spot on in my experience. I did some TDR tests a while back and posted them here: https://github.com/miek/usb3-fixtures/tree/main/microstrip-test#microstrip-test