differential trace impedance calculation

[Veronika]

Hello!

I am confused about the correct PCB stack-up for 100 Ohm differential trace impedance.
Can you describe me the basic rules for this and advise relevant links?

For example, I have differential stripline signals, which should be 100 Ohm impedance.
Dielectric thickness may vary, but at the moment I have 4mils.
Trace thickness is 0.6mils.

So I need to choose the correct trace width/spacing, am I right?
Previously I was used 5mils width and 6mils spacing differential traces.
But when I put all these values to some calculator it gives me only 72 Ohm of differential impedance.
What could be wrong and what should I change in my values in order to have about 100 Ohm impedance?
Should I trust to such kind of calculators?

Thank you in advance!

Veronika

 

[sivamani]

It depends on
trace width
trace thickness
trace to trace spacing
dielectric constant of the material
dielectric height from the signal to reference plane

use

https://www.saturnpcb.com/pcb_toolkit.htm

https://www.hughescircuits.com/index.php/en/support/impedance-calculator

 

[marce]

How many layers is the board, and what is the allowable tolerance on the impedance figure as a percentage.

 

[Veronika]

How many layers is the board, and what is the allowable tolerance on the impedance figure as a percentage.

The PCB will have 16 Layer in total.

About the tolerance I have read only this requirements:
"The data signal traces should be designed for controlled impedance. Single ended signals are 50 Ohm and LVDS signals are 100 Ohm differential."

 

[marce]

Usually 10-20% depending on interface, to get 10% or lower you have to involve your PCb manufacturer