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

Hello Veronica,

Before I can answer you question, lets go a little bit in the the theory.

A signal travels along the trace and is influenced by the path it is traveling (the trace) and the neighborhood that influences the path (the stackup, the material, other traces or planes, etc)

To describe what a signal "sees", one practical value is the impedance. If the signal travels together with another they build a differential pair and we speak of a differential impedance.

So the impedance is dependent

1) on the material characteristics (the trace material (copper, aluminum, other metals), the prepreg above or below, the core material above or below, the air above or.... etc...) and

2) on the geometries (trace width, height, distance to the next plane, distance to the other trace(for differential pairs) and distance from other traces, etc...)

The stackup defines the most important parameters of the geometries: the trace height, the distance to planes and traces. And also it defines the material characteristics. A prepreg has a different characteristic than a core.

So the first step to calculate a normal or diff. impedance is to know how is the stackup build and what material is you board made of. Ask the manufacturer to give you the detailed stackup with measures and the material parameters that are used for the calculations. They are typically the dielectric constant and the tangent loss. They vary for different thickness of the material.

The seconds step is to use any tool to calculate the needed trace widths and insulations. You can trust most of today's impedance calculators if you give in all the right values.

Remember that usually at the end you will want to do a small rounding. IF for example the calculator tells you to have a trace of 3.87548234mils, you can simplify it to 3.9mils. If you do the reverse calculation you will see that the error is small. And remember that etching can not be so precise either, so you usually accept a tolerance of about 10%. IF you need better then make special arrangements with your board producer.

The third step is to use this in your design.

Remember that if the signal changes layer then the parameter will change. You have to calculate the trace and space for every layer that you use in the design.

In you example you said that you used to use 5mils width and 6mils spacing differential traces for your design. What you didn't tell was what is your stackup, what is the material used, what is the layer on which this trace and space is valid, and so on. The same trace and space with the same stackup but with different material will have different impedances.

I hope this will clarify some. IF you have more questions feel free to ask and to read some other books or take some classes on this.

Welcome in the world of signal integrity.

Bye

Matija