- Hey everybody, thanks for tuning in. I'm Zach Peterson. I'm a technical consultant with Altium. And today, we're gonna
talk about mounting holes. Should you ground them? Should you not ground them? Should they even be plated? These are all good questions. And like most questions in PCB design and in engineering in general, the answer is it depends. Let's look at PCB mounting holes, what you should actually do with them, where you should even put them, and how you can put them in your layout using your design software. Let's get into it. (upbeat music) There's a number of things to address with PCB mounting holes. Should they be plated? If they are plated,
should they be grounded? Should they be grounded on your board? Or should they be grounded
on your enclosure? Should you even ground your enclosure? Again, it all depends. And it depends what problems
you're trying to solve. Generally with grounding and grounding your mounting holes, and, you know, do you
ground them on the board versus the enclosure, it really depends on what
problem you're trying to solve. There's also an element of safety here. Generally with a high voltage and high current systems, you don't wanna be grounding a current carrying
conductor back to your case because now you risk creating a current through the user back to ground, which then of course
presents a safety concern. First, the most obvious question, where should you even put them? If I just have a plain old board, this is a pretty poorly drawn square but you get the idea. Typically, they'll just
be in the four corners. If you look on something more complex, like a laptop motherboard let's say, you'll generally have a couple that are scattered in between somewhere. The larger the board gets, the more mounting holes you tend to see. And there's actually a
good reason for that. The reason for that is you
wanna tamp down any vibrations on the board. So if this board, let's say
it's a very large board, doesn't have any mounting
holes on the sensor, and maybe you have a very heavy component like a large fan or a heat
sink somewhere in the middle, if this ever got bumped,
there's the possibility that this can start vibrating a little bit and you wanna tamp down on
vibration on larger boards. So as the board gets larger, you generally need more mounting holes to affix to the enclosure. That'll keep the things steady and sturdy and will prevent any vibration. And at minimum, what it does is it actually pushes
the vibration frequency required to start moving the board around and start doing any kind
of mechanical damage up to much higher frequencies. Okay, so that's pretty simple, right? Generally, put them at the corners unless there's some reason for the enclosure or the mounting method that prevents you from
putting them at the corners. So the next question is
should they be plated or should they not be plated? Well, there's really nothing requiring you to plate your mounting holes or to not plate your mounting holes. You can leave them unplated if you want. Typically, the reason you
would plate a mounting hole is because you want to actually
bridge that hole to ground either by bridging the
ground point on the board to the chassis directly, or you just wanna have more protection on the surface layer of the board. By having that exposed copper, you've now got a metal screw
pushing against the copper, and it's not pushing
against the solder mask. These are a couple of different reasons that you might wanna leave the copper exposed on those pads and plate those holes so that there's uniform
copper all the way through. There's uniform copper
that's touching the screw and you're using a metal screw back to a metal enclosure. You're now creating an electrical short between that mounting
hole and the enclosure. I'm just gonna draw kind of a basic board
here from the side view. And let's just say for
a moment, on this board, I've got my power regulator here. Let's just say for a moment that I have a enclosure, it is all the way around this bad guy. And I know I need to mount this board into my enclosure somehow. And so I put some mounting
holes on the board. Let's just say they're here at the edges. This is a metal enclosure. These are plated holes. And so the question becomes
how do I arrange my grounds? Do I even need to have a ground on the metal enclosure versus the board? Well, the answer is yes. You should have the
metal enclosure grounded. The reason for that is
because you wanna make sure that you're not creating
unintentional radiator by placing this metal enclosure here. This metal enclosure can actually act like a big antenna so it can radiate, but it can also receive
external radiation. And in general, if I have
a ground plane in my PCB, it may not be at the same potential as the metal enclosure. The metal enclosure could
be at a different potential with respect to Earth
than my PCB ground plane. And so the result is that
you have capacitive coupling between the two, simply because they're very close together inside the enclosure. And when you have capacitive
coupling between the two, you could actually have radiation
from the metal enclosure. So failing to ground a
big chunky piece of metal in the board or near the board is actually a source of
high-frequency radiation. Typically, for very
large metal enclosures, it could be anywhere from, you know, kilohertz to low megahertz. And it just all depends
on the noise sources that are around. So in this case where we
have some mounting holes and we have our metal enclosure, you might then ask, "Okay,
we know that we have these big pieces of metal floating around. I only have this thing set to ground because this is my ground plane. What do I do with all
the rest of this metal?" So it really depends on how you actually have the
device hooked up to power. How you have the device hooked up to power is gonna determine how you're
able to actually define ground in this system. So for example, are you using a battery? A battery is basically a
two-terminal connection, right? You have positive. You have negative and that's it. So if you have a battery, let's say it's a little coin cell battery. What do I have to do here to make sure that everything is set to
the same negative potential? Well, ideally all you could really do is just bridge this. So bridge your ground plane
and your mounting hole and the negative terminal on the battery. And then, bridge this. Bridge it back to the enclosure. So now, I have my enclosure, my plane, and my mounting holes all set to ground. This is really the simplest case. In general, you don't wanna have your metal enclosure
set to the same ground as your PCB ground plane that's carrying any return current. Because if this is carrying
some return current, this can also carry some return current. Where can it carry the
return current from? Well, it could carry the return current from this mounting hole if I bridge this back to ground. So here, I've got my battery. I've got my regulator. And then, let's say I've got some other components over here. And then, I've got my
connection on the surface layer. So current is gonna come this way. If you then have this dumping
into ground right here from this mounting hole, the return current could
follow through the enclosure back to the battery, or it could follow
through the internal plane back to the battery. So in general, you don't
want your enclosure to be a current-carrying conductor. This is what causes some
poorly grounded systems to actually give you a little shock when you touch them. It's because they are poorly grounded. With battery-powered systems, generally you don't worry about it. These tend to be smaller devices. They're carrying low current anyways, so they're not a safety issue. And you can also insulate this enclosure. So you might not actually be
touching the metal directly. This could be encased
in plastic, let's say. So this arrangement is nice because I've basically
created the big Faraday cage around my device. So I've got some
electromagnetic shielding here. So this is a decent arrangement when you have a battery-powered device. If you have a simple power supply, it's just got a two-wire output, it's basically doing the
exact same thing as a battery. It's just supplying a
positive and negative lead. That negative lead will
set your ground potential to the low potential. And that will provide your
path for return current. So because of this problem with putting a current on the enclosure going back to the battery, generally what is recommended is if this is plated on this side, what you do is you just don't connect it to the ground plane. Instead, it's just
connected to the chassis. So now current can't
actually go through here and go through the enclosure. It has to go into the ground plane, and then go back to the battery. There is something that can happen at high current or at high voltage which is capacitive coupling. So remember, capacitive coupling, we basically have a
big piece of metal here separated from another big piece of metal and they're separated some dielectric. So you basically created
a big capacitor here. So this can create a path
for high-frequency noise traveling along the ground plane and traveling along the enclosure. And so here I have what's
called common mode noise. So I wanna take a quick detour
and show you what I mean when I say common mode noise can happen in this type of system when
you have capacitive coupling. So here in this type of diagram
that you'll typically see on the guides about common mode noise, there will be some
capacitive coupling here in between our ground and
our grounded enclosure. So here, this capacitive
coupling can allow current to flow through the enclosure and back into the input
side of the system. So normally, you'll have some current flowing here from our
power over to our load. And then, you would
have some current moving this way along the ground plane. Makes sense, right? So this would be your input current and this would be your return current. However, if there's
capacitive coupling here and then there's some other
kinds of noise source, which we normally represent with just an AC current symbol here. Here, we've got a nice closed loop and we've got another
nice closed loop here that goes from the power into our load, and then back through
here through the enclosure and through the ground plane. And so what you will actually have happen is you can have current that is not just being
injected from power, but is also being coupled
here along the enclosure and through the ground plane. So you'll actually have
current here in red that is flowing in same
direction and same magnitude through the ground plane
and then through the signal. And so it's the same direction current flowing through the ground region, and then through the signal layer that is interpreted as common mode noise. Here, the path that both
of these noises complete is capacitive coupling
back to the enclosure. And then, instead of
flowing this direction, it flows the other direction, and goes through the enclosure, and then kind of completes this big loop. And so you can see here
how this noise source can induce some current
traveling along this loop but also traveling along this loop. And so that's facilitated
by this capacitive coupling between different portions of
the PCB back to the enclosure. So one important point to note
here in this type of diagram, because you'll see this
often on the internet, sometimes what you'll see is this AC source appears like literally inside this current loop, this is not meant to state that there's literally an AC source here. This is just supposed to
represent any source of noise that could induce a current in either of these loops and
create common mode noise. So that could be, say the magnetic field from a fast switching component
somewhere else on the board. It could be just some
external field that comes in, really any external noise source that could possibly induce noise. So just keep that in mind if you're ever looking on the internet for guides on common mode noise, and just keep that in mind because this is not meant to state that there is literally an AC source anywhere in the system. Okay, everybody, so just to recap, we looked at what are mounting holes, where some good places are to
ground your mounting holes, some different methods for
grounding mounting holes, when you should connect them to the plan and when you shouldn't, and I know that this is a complex topic, we'll be revisiting it again. There's a lot of advice online. Some of it is conflicting. Just remember you need to think about what specific problem
you need to solve in your board in order to properly
connect mounting holes. If you're looking for a great tool that has all of the
CAD tools that you need to place mounting holes,
pads, vias, traces, whatever you need in your board, go get CircuitMaker. It's a free tool. You can download it and
start using it immediately, super easy to learn and use. And it's a great tool to learn the process and the practice of PCB design before you move on to
a more advanced program like Altium Designer. Thanks everybody, have a great day and don't forget to call your fabricator. (dramatic logo whooshing)
