Tuan,

If I model a microstrip with a circuit thickness of .0021 and soldermask
thickness(over the trace) of .0009 .005 trace .004 dielectric(Er 4.0) I
get a Zo of 54.06 ohms
If I remove the soldermask I get 59.75 ohms so there is your 6 ohms.

note I used 3.8 for the Er of the mask and 1 for air

This is the very reason that you don't see many RF boards with
soldermask over the circuits.

Regards,

Alan B. Cochrane

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