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About stackup and impedance control

mulfycrowh , 02-09-2021, 01:09 PM
Hello everyone,

I am starting to lose my hair.
Third time I ask my manufacturer for a 4 layers stackup with impedance control.
The purpose is to get 120 ohm differential impedance on bottom layer and 50 ohm single impedance on each of 4 layers.
First time I got a far much too wide track for 50 ohm.
I gave the width of 0,25 mm.
Second time I got a width below 0.25 mm.
So I would need for the third time ...

Is there anybody having a 4 layers stackup ready with all parameters?

goncaloc , 02-10-2021, 02:43 AM
every manufacturer has their own standard stackups, can you share what standard stackups you have at your manufacturer?
The problem with a 4 layer stackup is the big distance between the outer layer and the next layer, this causes your tracks to be much to wide, so you want to reduce this distance between layers, this will probably mean using a non standard 4 layer stackup, increasing the price.
The best/cheaper way would be to go with a 6 layer stackup
ibocakir06 , 02-11-2021, 12:01 AM
How did you decided 0.25 mm number?
Which tool did you use to calculate?
What is you core/prepreg thickness?
Probably you use standart FR4, what is Er, 4.2?
robertferanec , 02-15-2021, 12:38 AM
There is probably a problem with that - e.g. too small track, too wide space etc why that combination can't be done for the stackup your PCB manufacturer uses.

I would recommend you to play with some impedance calculators. Here is an example from JLCPCB:

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