Dear
I want to have impedance control.
there are two ground traces around the signal trace and a ground plane under the signal trace.
the thickness of the Board is 1.575mm so I should use 2.95mm trace for the signal trace.
Which size of via should I use to have 50 ohm impedance?
in my design I want to use via as follow: 0.3mm hole size and 0.6mm diameter.
Can you post a screenshot of what you are actually trying to do? Because this doesn't make a lot of sense. If you have a 4-layer board, the distance from the track to the ground plane should be much less than 1.575mm, so I suspect that you are not calculating the impedance correctly. Depending on the type of buildup (ask your manufacturer) the thickness will probably be somewhere in the range of 0.25mm to 0.5mm. So your trace should be about 0.5mm to 1mm wide if it was a regular microstrip. If you also have ground traces on either side, that's called a 'coplanar waveguide with ground', which requires an even smaller track width.
Via size is not the only thing that will determine the impedance, there are many factors to consider:
- Hole diameter (higher diameter = lower inductance, lower impedance)
- Pad diameter (higher diameter = higher capacitance, lower impedance)
- Spacing between via and the hole in your ground plane (higher spacing = lower capacitance, higher impedance)
- Board thickness (higher thickness = higher inductance (via) and lower capacitance (pads), higher impedance)
Via plating thickness is pretty much irrelevant. You could even fill the via completely with copper, it's not like there's going to be any significant electromagnetic field on the inside of the via.
If you really want to accurately calculate which dimensions you need to use to maintain an impedance of 50 ohm, you will have to model the entire thing in a 3D electromagnetic simulator like CST or HFSS. In practice very few people bother with this, because:
- For most applications, the mismatch caused by a via isn't significant.
- For those applications where it is significant, you generally try to avoid vias in RF signals entirely.
What kind of frequency are we talking about here? These things usually only become relevant if you are really dealing with RF signals in the GHz range.