FEDEVEL
Platform forum

gyuunyuu1989 , 07-10-2023, 02:46 PM

I saw the "What is The Best VIA Placement for Decoupling Capacitors?" of Robert Feranec. This video showed that having decoupling capacitor connected directly to an IC's VCC and GND pin is better than connecting it through PCB vias. PCB vias and PCB copper tracks both have inductance. But why should the vias be worst than tracks in this subject of decoupling capacitor connections?

qdrives , 07-11-2023, 01:53 PM

Good question.
First of all, most vias are about 1.6mm long -- the thickness of the board. If you take a simple rule of thumb and apply 1nH per mm, you get 1.5nH for a via.
Vias also cause impedance discontinuity that affect power delivery in some frequencies.

gyuunyuu1989 , 07-13-2023, 07:10 AM

Is via worse than track simply because of the impedance discontinuity?

qdrives , 07-13-2023, 02:57 PM

If the via is to take a signal from a layer that has a different reference layer, you also need to add a return via. You can watch multiple videos of Robert about this.

But in part, you would need that simulator that Robert used to verify every detail.

gyuunyuu1989 , 07-13-2023, 05:39 PM

Yes, I am aware that for (high speed) signals, we need to have a ground via nearby although I have not come aross detail of how far this ground via can be. This is simply because we need a path for the return currents and a single via on its own does not have a reference so we need to put grounded vias near to it.

Now when we come to decoupling capacitors, the capacitor is not connected to copper carrying signal. It is connected to copper that is connected to power or ground. So now with regards to vias used with decoupling capacitors, which is worse: 1.6mm via or 1.6mm copper track?

qdrives , 07-14-2023, 01:39 PM

What are you designing?
A multi GHz FPGA running <= 1.1V?
Or a board with <= 100MHz microcontroller?

For those last type designs, I never encountered any problem without ever taking SI or PI into consideration. On 4 layer boards and only 1 Gnd plane

For answers to some of your questions you would need to use such a simulator. One thing I can say is that the length of via/track is just a small fraction in the difference between via and track.

gyuunyuu1989 , 07-16-2023, 01:40 PM

I am only trying to understand the subject of PI. Lets assume all frequency range, few MHz upto a 10 GHz. How does a 1.6mm via differ from 1.6mm copper track across this frequency range? I mean both have inductance but the geometry of the via must imply some ways in which it is different from a mere PCB stripline or microstrip.

qdrives , 07-17-2023, 01:08 PM

Just take a look at the 3D shape of a via and of a track above a plane. Are they similar?
What is the wave length of 10MHz and what about 10GHz?
If you take the length of the track/via and compare that with the wave length (or 1/2, 1/4, etc)...
But take a look at this video https://www.youtube.com/watch?v=tF9FGZ_5vAE to see what can influence things in a via.

gyuunyuu1989 , 07-19-2023, 09:18 AM

Yes the shape of via is different from track, more imporantly, a via does not have a reference plane. But that is what the question is about. How does high frequency behaviour of a via differ from a track of same length, with and without grounding vias nearby it. I am asking from perspective of power integrity which is not the same thing as signal integrity.

qdrives , 07-19-2023, 03:16 PM

Well, the power has to come from somewhere. The capacitive coupling between a track and return plane is higher than one via to the next via. That 'plane' capacitance is also used.

Perhaps I am struggling with your question as I feel you have already answered it: "...a via does not have a reference plane." A via is like a vertical track. It needs a vertical return structure. As the return via is at a greater distance compared (most) tracks to a return plane, the impedance is higher.

robertferanec , 07-23-2023, 04:15 PM

It may require simulations - but as @qdrives explained it will be all together. Structure of VIA connection is different from a direct connection with a wide track and that's what makes the difference.

gyuunyuu1989 , 09-25-2023, 06:08 AM

vias are as ubiquitous as microstrip and striplines. I thought it would be simple way to get details of their characteristic impedance.

qdrives , 09-25-2023, 03:27 PM

Nothing in electronics is simple if you want to get high quality details.