[SOLVED] microstrip impedance without considering the solder mask?

[swen_s]

How big impedance error do I get if I calculate a microstrip impedance without adding the solder mask dielectric constant to the calculation?

If I should consider the solder mask also the impedance would be different. But how much? Maby more on thin routes.

 

[thylacine1975]

I've agonised over this in the past and found that the solder mask has made *zero* difference to the expected performance of my designs (well, variations were within what I considered acceptable manufacturing tolerances anyway). I can't think of a single case where I could attribute a resonant frequency variation/impedance offset solely to an unmodelled solder mask.

This has been my experience on commercial PCBs (Er ~ 3.5) up to 5 GHz, and with coplanar waveguide/microstrip track widths down to 0.2mm at any rate. It's up to you, but I don't bother stressing anymore

[Not to mention some practical difficulties: the RF characteristics of solder mask are hard to find, and I've found (via microsectioning) that there are huge thickness variations (e.g. 3 - 20 um) in the mask across a board from a number of manufacturers.]

 

[marce]

Second that, as microstrip parameters are generally calculated on the basis that half the wave is travelling through air and half through the PCB substrate, a little thickness of solder resists dosn't seem to make that much difference.