50ohm microstrip

[michaelym]

So I need to design a PCB with a bunch of 50ohm traces. I understand my best option is to use microstrips. I was playing around with a PCB calculator and understand that it is not an option to use a 2 layer board because it's require 100mil traces. So the only option is a 4 layer board. How does it work with the PCB manufacturer do I need to find out from them ahead of time what the thickness of the prepreg is? I saw on seeed studio there is an option for controlled impedance traces and it says "We will modify the stack-up and width of the traces accordingly to meet the impedance requirements" what?? they will change the width of the traces? where do I start

[Zero999]

I don't see why a two layer board can't be used. It just means to use thinner traces, the board also has to be thinner than the standard 1.6mm, or whatever is normal.

[michaelym]

Unfortunately I can't use a PCB less then 1.6 it will be too flimsy.
I was wondering what does it actually mean when you pay the PCB manufacturer for "controlled impedance traces".
Isn't it all dependent on the distance between layers which is a set value for the particular manufacturer

[bd139]

It means that the board substrate has consistent thickness and permittivity between batches.

Generic FR4 substrate is not well defined in this area so your traces don’t always come out at 50 ohms on susbsequent board runs.

It’s easier to do a 4 layer board in this case with transmission lines on the surface and a ground plane on inner 1 layer. That gives you the mechanical stability and the thin layer.

Two layer will work however but there’s a headache or two there. Particularly if you have route other signals across transmission lines. On 4 layer just do it on the back!

TBH a little mismatch isn’t going to kill anything hard unless you are really hitting the MHz.