# Via Impedance Calculation

Discussion in 'General Electronics Chat' started by Management, Jun 8, 2009.

1. ### Management Thread Starter Active Member

Sep 18, 2007
306
0
I would like to calculate the impedance of a solid (filled) via and compare it with that of a plated via.

I know the dimensions of the via but I just don't know the equation for resistance. It is the same for that of a strip of copper on a PCD trace just the only thing different is the way you calculate area?

Stripline R: pL/A
where: p = resistivity; L = length; A = cross sectional area

Thanks.

Apr 27, 2009
97
1
If this problem would come up in one of my designs I would first calculate the via resistance. A plated via is a hollow cylinder. The metal area at the cylinder base is the difference between the area of the outside circle and the area of the inside circle. So, the formula would be:

Rvia = p * L / (pi * (D^2 - d^2))/4
where: p = resistivity; L = length; D outside diameter of the plated hole; d inside diameter of the plated hole.

When the plated via is filled up with solder there is another resistor in parallel with the plated via. This is a full cylinder with the base area a circle with the diameter d. The combined resistance is:

Rfilledvia = ((p * L / (pi * (D^2 - d^2))/4)^-1 + (p1 * L /(pi * d^2/4))^-1)^-1
where p1 is the solder resistivity

Last edited: Jun 8, 2009
3. ### Management Thread Starter Active Member

Sep 18, 2007
306
0

Thank you soooo much. That is what I was thinking. That I could use the same equation but was not sure.

I don't think one would fill a via with solder but if it was filled with Cu then it would just have a larger cross-sectional area and be one full cylinder of Cu, correct?

One question though. Lets say you staircase through some dielectric material, the connecting traces would be in series correct? And once you get to the outer layer you would have a pad (if directly under a BGA ball) so that pad is also in series and you can calculate it's R separately and sum it, correct?

Apr 27, 2009
97
1
Yes, indeed.

Yes, correct.

Correct again.

5. ### Management Thread Starter Active Member

Sep 18, 2007
306
0
I swear last question.

When they say that the via plating is for example 10 um, does that mean that the inner diameter is 10 um or is the inner (empty diameter) the outer diameter minus twice the inner?

Example:
Outer Diameter: 200 um
Via Plating: 10 um
Inner Diameter: 180 um ????

Apr 27, 2009
97
1
Well, that's when gray areas start to show up. Normally, 10um plating means that the thickness of the plating is 10um. Therefore, you would subtract 2 times 10um from the outer circle diameter to get to the inner circle. The plating value goes on the fab drawing of your PCB design, but you have to make sure the PCB house understands what you mean by 10um plating.

Also, the drill table in your fab drawing shows finished holes, whether they are vias or through holes for components. The PCB manufacturer adjusts their drill dimensions based on the plating specifications. So they have to clearly understand what your specification means. Even so, when everything is understood, the tolerances for plating are quite high, so you will end up with large variations in plating thickness. No big deal for vias or component pads, but important for your exact calculations.

So, my advise is for you to talk to the PCB manuafacturer you use to understand the meaning of plating specification and the tolerances they consider acceptable.

7. ### Management Thread Starter Active Member

Sep 18, 2007
306
0

That's what I thought plating meant but your right doesn't matter what I think it means the manufacturer must get on the same page with the designer.

Industry wise, does "finished hole" mean pre or post plating?

Sorry I'm not the designer just the guy looking at specs and doing some modeling.