In this video, we are going to have a look at stitching VIAs. These are the VIAs which are for example recommended when you are doing layout and when your signal is changing reference plane for example when you have multi-layer PCB and when your signal goes from the top to the bottom layer, then, it is recommended to use stitching VIAs. Here you can see a stitching VIA. Here are some stitching VIAs. These are ground VIAs. Okay? Here you can see some ground VIAs. Here are some VIAs close to the place where signals are changing reference planes. So, what I did, I created a very simple board where initially, I didn’t place any VIAs. And I wanted to see, what will happen with return currents. In one of the previous videos, we learned that the return currents for high frequencies, they flow under the signals. Okay? But you know when we change
these reference planes, when we go from top to the bottom layer, what is going to happen with return current? How it is going to flow? It cannot flow under the signal. So that’s why I simulated it. Okay? So, this is the simulation with no stitching VIA. And here you can see, this is the track on the top layer and this, under the track, this is the ground plane. And on the ground plane we can very nicely see the return current, the red color, this is the highest current density so most of the return current flows under this track. However, when the track is changing layers, suddenly, the return current it spreads around this ground plane. So, what do you think, how this picture is going to change, when we place a ground VIA close to this place where we are changing the layers? What do you think, how this is going to look when we simulate something like this? This, where we placed ground VIA close to this place where we are changing layer. Do you think it is exactly same situation or exactly same situation as this? Or do you think it’s going to look better or worse? And that’s what this video is going to be about. My name is Robert Feranec, I’m from FEDEVEL Academy and this is the board what we are going to simulate. On the top layer you can see there is just this simple track routed. On layer two, there is ground plane. On layer three, there is nothing. Layer four is ground plane. Five is nothing. Six is ground plane. Seven, ground plane. Eight nothing. Layer nine, ground plane. Layer ten, there is nothing, it’s empty. Layer eleven is big ground plane and layer twelve, this is the bottom layer, there is, this is the layer where our signal continues. Why so many ground planes? Because there are only two nets in this board. There is only this signal and the ground net. In reality, some of the ground planes would be for example power planes or you would place there power polygons. But for this simulation, I just use the ground planes inside of this PCB or some empty layers. Okay. Let’s have a look at the simulation. What I set up or how I set up this simulation and what we are simulating? Many people always ask, what kind of software I use for the simulation, so I’m using ADS from Keysight and the setup is very simple. You can see there are only two nets, the ground net and signal one net and there are only two components. J1, it is here, and J2 connector. The J1 connector is set as power supply and the J2 connector, this is basically sink. This is the place where we are going to sink the current. And options for the simulation are very simple. I just run the simulation in a few points, and I set this Include Resistive Losses in Ground and then I just run the simulation, and these are the results what we have got. We are looking at the results of AC analysis for current density. Okay? Here you can see the blue color, it means there are almost no currents or no currents flowing through these planes or through these places. Red color it means there are the, there is the highest current density so high currents are flowing through these places. This picture what we can see here it is for one megahertz and I set the fixed range so we can easily
compare the colors, we can easily compare the results between different simulations. When I zoom in, as I explained, here you can see, this is the top layer so you can very nicely see here the track, under the track on the ground plane on layer two there are the return currents flowing under the track, then on the other ground planes you can see also on this ground plane there are some return currents flowing and in the middle there are not really, there are some return currents flowing in the middle, but not so many or not so much as on these other ground planes. And then on the bottom you can see again the track is routed here on the bottom and return currents are here under the track. Okay? Now, the question is what is going to happen when we place a ground VIA close to this place where we are changing layers. So, what is going to happen when we use this ground VIA here? Let’s go back to our simulation. This is the simulation with no ground VIA and watch what is going to happen. Specially, watch here in these places. Watch the color, how it is going to change. So, I’m going to switch to the other simulation. Do you see the difference? So, this is before and this is after we placed the ground VIA close to this place where we are changing the layers. So, this is the situation when we place the
stitching VIA. This ground VIA it is called stitching VIA. And you can very nicely see then that the currents which before they spread all over the ground plane on the layer two, now it looks much better. Now these currents which spread over this plane, they are much lower, they are green and, in this situation, they are red or yellow. So, just placing this one small ground VIA close to this place, it improved the layout. We kind of defined or helped to define the return currents for this signal which we routed on our PCB. We defined the path for return currents. So, we told the return current to flow together with our signal. So, in this place where we are changing the layers for the signal, we are basically also changing layers for our return currents. And that’s why this situation improved so much. Because the return current it flows together with the signal. And the stitching VIA, this ground VIA it is just a standard simple ground VIA which is connecting the ground planes together. Okay? There is nothing special about this stitching VIA. But what I really wanted to see? I wanted to see the influence of the distance of the stitching VIA from this place where we are changing these layers. I really wanted to see that these return currents they really flow through this stitching VIA. So, what I did? I created another board, and I placed this stitching VIA here in the middle. Okay? So, I moved it from this place, I moved the stitching VIA, the ground VIA and placed it somewhere here in the middle. What do you think? How the return currents are going to look now? Let’s go back to our simulation. So, this is the simulation where the stitching
VIA is placed here and now, I’m going to switch to the new simulation. Wow! Can you see where the ground VIA is placed? Here. Yeah. So, you can very nicely see that the return currents, they flow under the signal here and then when we are changing the layers, the return current it just has to somehow find the path how to go on the bottom layer and it finds this ground VIA which is here so, it flows here and then it is changing the layers and goes down. Very, very interesting. By the way, when you are not changing the reference planes, for example when you are routing on top layer and then you continue on layer three, you are still using the layer two as the reference plane. In this case, you don’t have to use the stitching VIAs. If we simulate this, this is what you will see. Okay? So, on layer one, there is this track routed and you can see the return currents, they are under the track and here when we change the layer and when we continue routing the signal on layer three, then the ground plane on layer two it is still reference plane for this signal on layer three so the currents they just flow above and below the signal. You can see it very nicely here. And no stitching VIA is needed. The currents, they, they can very nicely follow the tracks. Okay? So, don’t forget about this. You don’t always need to use stitching VIAs when you are changing layers. What I found very interesting is, notice that the return currents, they don’t flow only directly under this track, they flow also here on this ground plane and yeah, that’s something what I was not really expecting, and it means, because the return currents they flow on this layer two and also on this layer four maybe stitching VIA here could also improve something, little bit. But, yeah, that’s something for comments. Okay? Leave comments, what do you think about this situation. Did you know that the return currents they can actually flow on multiple layers below the track? It can be because the currents what we are simulating are quite high but I’m pretty sure they, these return currents they always also return on multiple layers just maybe for some standard signals these return currents on the other layers can be very very low. So, we can maybe ignore them or something and that could be reason why it’s not really necessary to use stitching VIAs or so when you are not changing the reference
planes. I don’t know. What do you think? Leave comment. I have done two more simulations. In the first one, I was curious to see if placing more stitching VIAs around the place where we are changing layer, if this is going to help or how much it is going to help. So, this is what happened. Okay? Here you can see, here are multiple stitching VIAs. And when we compare it with the one VIA simulation, there is a little bit of difference but like
not really much. So, this is no stitching VIA, this is one stitching VIA and this is six stitching VIAs here in the place where we are changing layers. Why to be so shy? Why just place there six stitching VIAs? Why don’t place there like many many VIAs and that’s what I did in the last simulation. You can see, I placed ground tracks around
the signal track, and I placed there like a lot of ground VIAs. And these are the results from the simulation. Very nice! Like there is no, almost no different color in the other places of the ground planes. The return currents, they like almost fully flow under these tracks. Wow! So, this is definitely like the best from all the simulations. To make it simple to compare, I placed all the results on one place so you can very nicely see that this
one is the worst one. No! This is the worst one, okay? This is the best one. So, when you are doing PCB layout, you really don’t want the return currents to spread all over
you board. This is bad. For very important signals, you would like to control the return currents. You would like the return currents travel together with these important signals. So, this is the worst situation, this is the best one. In reality, why you don’t always need to use stitching VIAs is because there are for example lot of decoupling capacitors and every capacitor has ground VIAs so these VIAs are connecting ground planes together and basically, they will also provide the path for these return currents. But still, you need to be aware of this problematic and that’s why I created this simulation, so you can nicely visualize what is actually happening if you don’t have any stitching VIAs on your boards. And again, don’t forget that on your board there will be not only one signal. There will be fifty, hundreds, thousands of signals… And you really don’t want the currents of all these signals to mix together because that can make the quality
of the signal worse. And that’s the reason, why for very important signals you would like to control the return currents. Okay! I really hope you found this video useful. Don’t forget leave comments. If you like this video, then press like button. If you would like to see the future videos, don’t forget to subscribe. I would like to thank you very much for watching and see you next time. Bye! Leave comments! Don’t forget, leave comments!
