Does the presence of solder mask on PCBs impact impedance calculations, and is it important to take into account the effects of conformal coating while carrying out impedance calculations?

Normally you calculate your “effective” dielectric constant by more-or-less averaging together the constant of the underlying laminate (FR4, etc.) with that of air. So a 4.3 dielectric for FR4 when half the circuit is facing the laminate and half is facing the air might work out to something like 3.0 instead of the 4.3 we started with. You then proceed from where with all your various calculations.

However, If you replace the closest 1 or 2 mils of air with soldermask then you must now re-write your first calculation to account for this and average the original dielectric of the laminate with x amount of mils of soldermask, and Y amount of mils of air and arrive at a new “Effective”

dielectric constant.