Many designers use the built-in impedance calculator now found in most CAD tools. Regardless of which ad you use, the calculation will be different between them. Therefore, the CAD tool is not a finite impedance calculator, but it gives you a very good indication of the impedances based on your inputs. BUT, there will always be adjustments when your data comes to a PCB manufacturer.
As mentioned so many times before, there are quite a few adjustments that a CAM Engineer makes based on his knowledge, long experience and factory process parameters.
The CAM Engineer will prioritise that you get your expected impedances and will make adjustments of Dk, based on the material’s epoxy content, percentage of copper distribution on the different layers, remaining copper thickness after processing, tolerances on this purchased batch of materials and even more.
As an example, just take a look at IPC’s standard for finished copper thickness, print and etch of straight forward inner layers, IPC-600J-Class 2.
Start copper thickness can be:
18um – finished processing 11.4um
35um – finished processed 24.9um.
The pressed prepreg thickness will also be calculated and changed whether there is 20% or 80% copper on a layer.
Therefore, what you get out of impedance calculators is a good indication, but the manufacturer is the one who can give you that impedance closest to your requirements and expectations.
My tip and advice before summer is. Please trust the CAM Engineer when he sends you a stackup for approval that is not looking exactly as yours.
Happy summer to you all!