PCB stack-up standard thickness

[Acidover]

Hi there,

I'm designing a 8 layer pcb with controlled impedances. Is the first time I have to tackle with a serious stack-up construction so I tried to deal with the PCB shop to help me on choosing the materials, thicknesses and traces geometry. They told me that since I'm the designer I have to do that job and send to them all my calculated stack-up and then if they can't do it they will apply corrections. Is like that how it usually works? Even if I'm paying them for a controlled impedance service?

Anyway, I assumed that and then I've downloaded their standard FR4 datasheet:
https://www.multi-circuit-boards.eu...ver-e-cu-104i_www.multi-circuit-boards.eu.pdf

Now, in the datasheet I see there are 2 different stantard thicknesses: one for (I guess) the final board, from 0.8mm to 3.2mm and then a second table with standard construction thicknesses from 75um to 1.2 mm. Well, now I wonder if do I have to choose such a set of cores and prepregs so the sum of all their thicknesses needs to match a standard final thickness. It's like, not only do I have to choose them to get the impedance I need but I only have to fit standard thicknes...

Please can someone clarify me how should I proceed?
Thanks

 

[keith1200rs]

You could look at the sample layer build-ups:

https://www.multi-circuit-boards.eu/en/pcb-design-aid/layer-buildup/series.html

Their suggested 8 layer build-up results in a nominal 1.6mm PCB I think.

Keith

 

[FvM]

I guess you are talking about pre-estimated rather than controlled impedances. Controlled impedance involves test structures that are measured after production of a sample board and adjustment of PCB process parameters to meet the specified impedance. The procedure is respectively costly.

Pre-estimated impedance means that trace widths are calculated based on nominal board geometry and material permittivity. The achieved accuracy is usually sufficient for high speed digital and standard RF boards.

Referring to a standard stack-up is reasonable unless you have special requirements.

 

[Acidover]

Well, thanks for replying!

So let's assume I make the calculations and send to them the stack-up which must present the pre-estimated impedances. So they "trust" my calculations and go ahead constructing the board? If after production they make measurements and realize that the impedance is not the right one, not even within acceptable margins, then what? Do they assume the costs of producing again the board with the proper modifications?

 

[marce]

Find a manufacturer that can help you, they have control of all the aspects of PCB manufacture, so go to another one, they use polar instruments to calculate the impedances. If you only require 15-20% tolerance do your own stack up.