solder mask effect on microstrip impedance

how can i take into consideration the effect of the plating to microstrip circuits?

is soldermask advisable?

thanks

Yes, the extra thickness lowers the characteristic impedance of the trace

See http://www.sigcon.com/Pubs/edn/Passi...SolderMask.htm

The soldermask will increase the losses.

how can i take into consideration the effect of the plating to microstrip circuits?

is soldermask advisable?

thanks

it is not good to do it.
do u know th thiknes of the plating ? ( or the glue)

2 u can compare the diffrent betwen stripline and microstrip

Hi all,

by coating your microstrip lines with soldermask you will increase the
dielectric constant. This will change the impedance of your line. You can
take this in account and use smaller traces.
The problem is, that you will have differences between two prints
manufactured by the same manufactuerer, but not in the same lot...
(coating material, coating thickness, ...) Making the traces resistent by
silver or gold is (in my opinion) the better idea.

Bye

This has me thinking:

If you dont plate or soldermask it, and the copper trace oxidizes..does that make a difference to the characteristics as well?

Hi twinsen,

Yes of course. An oxidated copper-trace has (also for lower frequencies)
another impedance, because you have less free electrons that can carry
energy.
So an oxidated microstrip-line will not have your desired impedance and
can cause a reflection...

Bye

well if you do plating with some chemicals that make silver or gold... there is problem with crystal growth. border betwen them is not ppure ohmic contact, but it can be semiconducting :)

If you cower the line with solder mash, there will be lower impedance line, thets mean, the microstrip "effective widerness" will become bigger.

If cooper oxidize, the cover is kind of ceramic. that mean low loss and high dielectric constant. (imagine metals like dielectrics with infinite epsilon). becouse line will still be physicaly the same, the impedance would even become higher(becouse ceramic has "lower epsilon than metal"). the thickness of oxide is very low, the change would be neglible :)

keep the lines clean, do not leave fingerprints or resins, solder only there where you must (Pb an Sn have higher resistance than cooper!) the surface. if you must clean the surface use organic solvent, or grind with the grinding paper 1200(gratings) or even some kind of polishing.this is becouse, the length of current must be short as possible(skin effect!).

so whats the best approach to protect the copper traces but not degrading the circuit perfomance much?
are simulation software accurate enough to predict the effect of plating the copper lines?

It may decrease the Microstrip line's impedence,there is a software to can
calculate this Si6000 from
www.polarinstruments.com,,there is also
some useful article about microstripline
and stripleline.