Internal Plane

[mulfycrowh]

Thanks for reply.
I know that this development doesn't imply high speed signals.
I would like to make it to prepare the next developments that are far much complicated.
What disturbs me is that I asked the PCB manufacturer to compute the impedances (50 and 120 Ohm for this first project) and they told us they do not perform any impedance calculations.
They suggest us to use the following free calculators to do it:

http://www.hp.woodshot.com/
http://www.saturnpcb.com/pcb_toolkit/
https://www.polarinstruments.com/support/cits/IPC1999.pdf
https://www.eeweb.com/tools/microstrip-impedance

[Paul van Avesaath]

I use the PCB saturn tool for basic impedance caluclations. it gives you a good indication, but if you need 5% or 10% accuracy in your design the manufacturer should be able to control the impedance.. because they have all the exact numbers and material specs that might deviate from normal numbers.. you pay for this option it is not for free because they have to do extra TDR measurements after production on the test coupons they produce in the same batch.. also they might have to scrap a batch because they did not meet the spec.. so they have risk of less yield per batch.

There is a difference here though..
If you are talking to the manufacturer of the pcb (e.g. Fineline global) then they should be able to suggest a stackup and impedance report, (if not they choose a different one).
If you are talking to you EMS then i can imagine you get an answer like that because they get the PCB and only place components on it.

if you are ordering a PCB through a EMS then they should be able to forward your request or bring you into contact with the correct persons.

hope this helps..

[robertferanec]

I asked the PCB manufacturer to compute the impedances (50 and 120 Ohm for this first project) and they told us they do not perform any impedance calculations.

- If you need impedance controlled boards, do not use this PCB manufacturer. Every serious PCB manufacturer will provide you with the calculations. Of course, there are some factors - as they need to spend their time on the calculations, they would like to be sure, they will manufacture your PCB. Otherwise they may not be very keen to help.

Other option could be to check PCB manufacturer websites. Some PCB manufacturers may have "default" stackups with all the impedances calculated. For example try to have a look at Sierra circuits - they are expensive, but maybe their calculator could help you (I used their calculator long time ago, but I think their calculator should give you accurate numbers based on materials and their PCB manufacturing process): https://www.protoexpress.com/hdi/hdi-tools.jsp

[mulfycrowh]

I totally agree with you.
I have a RS 485 link with a length of about 1m40. The impedance has to be 120 Ohm.

[robertferanec]

I have a RS 485 link with a length of about 1m40. The impedance has to be 120 Ohm.

For RS485 it may not be so critical. Usually what I do is that I keep the transceiver very close to the connector and then even if these short tracks (the tracks between transceiver and connector) are not controlled impedance, it is fine.