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What variables does Mentor Layout constraint manager use when calculating impedances of inner layer striplines?

Question asked by g_mal on Aug 26, 2020
Latest reply on Aug 31, 2020 by daniel.lange



I would like to know what variables impedance calculator uses by default and what it "assumes" automatically.

Variables I know that it uses to calculate impedance are:

- In Stackup, Dielectric layer thickness on both sides of stripline or one reference side when using microstripline

- In Stackup, Loss Tangent and Dielectric constant

- In Stackup, Copper thickness

- In Constrain Manager, Spacing and Trace width


In real world also technology type between prepreg or core can have effect.


Now, my question is
How mentor handles transmission line in inner layers?

Does it assume that in inner layers I always use Stripline by having two planes (GND) both sides which makes ideal stripline. What if I only use one reference ground only below and have no planes or traces anymore above the transmission line?

I know that when I delete Reference ground above, it does not automatically change the trace width or spacing.

For example 4 Layer Board:
1 GND plane (ref)

2 Signal (controlled impedance)

3 Signal

4 Signal or GND plane