- Hey everybody, thanks for
tuning in to Altium Academy. I'm Zach Peterson, I'm a
technical consultant with Altium. And today we're gonna talk
about the copper thickness or the copper weight that
you should use in your PCB. So what value of copper
weight should you use? Should you just go with your manufacturer stackup recommendation? Should you try and figure
something out yourself? Should you start looking
through material data sheets and seeing what's available? We're gonna try and answer
all those different questions, 'cause there's a few different trade-offs you should think about when you're selecting your copper weight. Let's go ahead and get started. (upbeat music) - Get started with copper weight. Basically, if you have a PCB and you have your copper on either layer, let's just say we're on the top layer. When you look at the copper
thickness in your board, we actually don't specify
it as a thickness, we specify it as a weight,
so we call it copper weight. Typical values for most vanilla PCBs that you'll find out there
are one ounce per square foot. Pretty weird unit system
to use, but that's okay. This is kind of a standard
value that you'll find. Another really common value
is 0.5 ounces per square foot. And usually when people
talk about copper weight, they don't say the square foot, they'll just say one ounce of copper. This is what they really mean, is one ounce per square foot of copper. That's what the copper thickness is, and this is normally specified as being on both sides of the board. If you build out a typical stackup, like let's say your
standard four-layer stackup where you've got your
first internal plane layer and then your second internal layer, and you look at the copper weight, generally a standard
stackup that you'll see from a lot of manufacturers, will just apply whatever value, whether it's half ounce or one ounce, to all of these layers. It makes it really easy
to build a stackup. Most laminates are gonna come with one of these two options as a standard option. And they're just gonna press that together in their process, and bam,
you've got your copper weight. This is gonna be pretty
standard in every stackup. Depending on your application,
you might be wondering, should I go with a different
value of copper weight? What are the different
values of copper weights? There are some manufacturers
that do something called heavy copper, and this
isn't a super common term, but heavy copper essentially
refers to any value bigger than one ounce per
square foot copper weight in your PCB stackup. That could be two. It could be three, could be four, could be all the way up to like 10. You can actually find
some really big values for copper weight on PCB
laminates that are used in PCB stackups. If heavy copper is gonna be used, where should you use it? Should it be an internal layer? Should it be external layers? For heavy copper being
used in a plane layer, you could technically do it anywhere as long as the manufacturer
can get the different layers to bond to each other in their heating and pressing process when
they build out the stackup. So if it's just a plane layer
it's essentially just gonna be uniform copper everywhere. And if it's not gonna be etched, then you get a nice uniform
bond between this layer and the copper or between
this layer and the copper and so on and so forth
throughout the layer stack. Same thing on the top layer. Top layer, you can do
heavy copper if you want, or you can just go with
one of the standard values. Now there comes a little bit of a problem if you're gonna do heavy copper
on like an internal layer. And let's say, you're gonna
go to two or three or four on an internal layer. If you actually look at what
happens on an internal layer with heavy copper, once the
pressing process starts, what they'll try and do is, they will try and press this
top layer down over this trace. So if you have an etched
trace with heavy copper, let's say it's a, you know,
two ounces per square foot or larger, then what'll
happen is this could leave a little bit of room around
this trace, this top layer. And this gap that you get
here is gonna be determined by the resin content in
each of these layers. When the stackup is being created and the layers are being pressed down onto each other and heated, the resin content that is in
this layer will start to flow. And ideally it will just
fill in all of these cracks and fill in all these gaps. And there will be no space leftover. So this can happen in your PCB if you have heavy copper
traces that are etched on an internal layer. So if you have a higher resin content, so if your resin content goes up, then it's more likely to
fill in these little gaps. So that's one of the reasons
you shouldn't use heavy copper on an internal layer, is that it can actually
decrease the peel strength of this laminated stackup. So once it's laminated, it can be peeled, although it takes some force, but if you don't have a good
bond here at the copper, and then at this interface
around these traces, it can reduce the peel strength. So basically the more traces
you have on the internal layer, the more of these gaps you're gonna have, and then the more you're
gonna have to worry about the peel strength going down. So that's what happens
on the internal layer. On the external layer, go ahead and etch your heavy copper traces all day. Just make sure that if you
do want to use heavy copper or, you know, high copper weight, that the manufacturer or the fabricator that you're gonna use
can actually work with it and that they have a
process that will work with these heavy laminates. Make sure to call them
if you are gonna do that and get a standard stackup
just as a starting point, then you can start to work
out if you need to use a different layer count,
if you need to maybe go to two layers and just do
that, it really depends. We've talked about some of the
manufacturing aspects as far as what happens when you
go to heavier copper. But what about current? How much current can you
put through heavier copper? What about impedance? Let's say you want to do a
controlled impedance line. Should you do controlled impedance with heavier or lighter copper? Well, technically you
certainly can do heavier copper and do an impedance controlled line. You just need to know
how the heavier copper affects the impedance. It is true that, you
know, let's say I have just for the moment, this trace, its impedance Z sub zero, is
a function of three things, this width, and then this height, we'll just draw it all the way down, this height between the planes and then also the copper thickness. So we'll just call this T. So remember the copper
thickness is just a proxy for whatever the copper weight is and you can actually convert copper weight into a thickness value. So there's actually a table in one of the articles that's
linked in the description. Go check out that article. It has a handy table that
you can use to convert between copper weight value
and then an actual thickness. Your impedance depends on the width, the distance between the planes and the thickness of your copper. The thickness is actually very, has a very weak influence
on the impedance. You can actually like take
the thickness and double it, go to 2T, and it's only
gonna decrease Z sub zero by like, I don't know, a few ohms. Let's call it three ohms or so. That's just an estimate. The point is that it's
a very small change. So going from, you know,
half ounce to one ounce is not gonna have a huge effect. Now flip this around. If you were to take W,
let's say the width, and go to 2W, so double the width, you could actually get a really
big drop in the impedance. So that's because when
you double the width, you decrease the inductance and by decreasing the inductance, you then decrease the impedance. Go back and review some of our videos on characteristic impedance
if you want to see how the width and the inductance, and then the capacitance of
the trace are all related. And that'll tell you pretty
much how you can calculate how the impedance changes
if you do this type of transformation from, you know, the width to double the width. And then I'll let you guys work out what happens when you take the thickness and then double the layer thickness. So this is essentially what
happens when you're dealing with, you know, a switch from T to 2T, you do have a slight
decrease in impedance, it's not very big, you can
basically not worry about it. And even if you were worried
about like a two or three, you know, ohm drop in impedance, you can do an adjustment on
W and it'll make up for it. And it's not gonna be
a very big adjustment. So you can still hit your
controlled impedance. So generally, if you are working
with controlled impedance and you're wondering, Hey,
should I use heavier copper? The typical practice is to just go with the default copper weight. If you're dealing with a board
that needs heavier copper, it's usually not gonna be
a board that also needs controlled impedance. It's usually a board that's
operating at high temperature or at high current, and in some boards where you're working with like a high power electronics, those two things kind of go together. So you could be operating at high current just because you need to
supply current somewhere. But you could also be
operating at high temperature because you're working with
components that have some, you know, they're gonna
have some resistance drop and that's going to cause
some of that energy that is being carried by your
currents to then get converted into heat, causes your board to heat up and so that's another
reason that you might like to use heavier copper. So there's actually a
table in the IPC standards that you can use to figure
out what is the trace width and thickness, and
rather the copper weight that you would need in
order to ensure that your trace's temperature rise does not exceed too large of a value. So basically the way this works is, you pick your current target
that you know you're gonna use, and then you pick a temperature rise limit that you can tolerate. And then you figure out
what the copper weight is and you figure out the width. So I'm gonna show you all
how to do that right now. Okay, everybody, so what
we're looking at right here is an example from the IPC standard. So this is the IPC 2152 standard
nomograph that you can use to relate the conductor width,
they have it here in inches, to a cross-sectional area if you want, but really what matters
here is the copper weight, which you can see in each of these lines. And then you have an
allowed temperature rise that you'd like to hit and the current that the wire carries. So you can use this to
figure out the width of a conductor that you need. So I have these lines drawn on here, just kind of as two examples, and so I'll walk through those. So if I just zoom in here
to the conductor width. Here, let's just say we
have a 0.15 inch wide trace. So 150 mils. Okay. So if we trace over here
to our copper weight of one ounce per square foot, and then trace up here
to 10 degrees Celsius, and then trace over here to the left, we can see that we would have about, call it about 2.75 amps that this particular
trace could carry, okay? This is a pretty conservative number, but basically what this is telling you is if you take this trace
and run it at 2.75 amps, it should rise, or its
temperature should rise by about 10 degrees C, okay? So you notice we started over
here with the conductor width and then got back to a current. So if we like, we could
have said, well, you know, maybe we want to go up to six amps, okay. So if I start on this same track, go over here to my one ounce
per square foot copper. And then I go all the way up
here to the line with six amps, you'll see it intersects with
this curve that corresponds to 45 degrees Celsius, okay. So in that particular case, we would expect if we run
this same 150 mil wide trace at six amps, it's temperature would rise by 45 degrees C with
respect to ambience, okay. So these are temperature rise values, not absolute temperature values. You could also go the other way. And that's what this
other orange line shows. So here we just started with a current, and then we said, okay,
with our one amp of current, I don't want to have the
temperature rise any more than 30 degrees C. So that's what this, this
middle curve shows, okay. And if I have, in that case, half ounce per square foot copper, then I would need to have this trace be, this is about 40 or so mils wide, so that's what these different lines mean. So you can really just trace
this out for anything you like. Now these are a bit conservative. They only apply to one
specific material system, and they don't say anything about whether or not you have traces above or below. And also you really don't need to worry about this cross
sectional thing, you know, nobody really uses it. People just use the width of the conductor and then the copper weight, okay. So what you can immediately see, or you should be able to immediately see is that for a given conductor width, if I go over to a higher copper weight, and then I trace up to a
given temperature rise, so here at 10, I could
carry a larger current, just like what I would expect, okay. So this is kind of our justification, or at least our experimental justification for using a heavier copper
to carry higher current. And that all makes sense. So the article that we'll
link in the description actually has this
particular graphic in it, you can use that as
your own starting point for estimating the current
that a conductor can carry. Just note that, like if
you're in Altium designer, one thing that you can do is you can use the PDN analyzer extension. That extension will, you can
use to determine the power loss in a conductor as it's
operating with a given current. And from that, you
could actually determine what the temperature rise might be. So that's a really useful
tool you could use. Now, if you're inside Altium designer and you create a brand new PCB, what you can do is you can open
up the layer stack manager. And once you get inside
the layer stack manager, you'll be able to actually
set the copper weight that you want on each layer. So right now, just by default,
it comes up with one ounce, they mean one ounce per square foot. Here, you click on this drop down menu and you've got a whole lot of options. So from here, you can edit your stackup. Now, if you're creating your own stackup, obviously send it over to your fabricator and just check with them, ask them, Hey, what's your standard stackup. Or, you know, can you create this stackup? And they'll be able to verify it for you. But from here, you know, just set whatever copper weight you like or that you've been given
from your fabricator. If you're a circuit
maker user, same thing, there's a layer stack manager, you'll see a window that
looks just like this, it'll just be on a white
background or a gray background, and you'll be able to
do the exact same thing. Just click the drop down menu and select your copper weight
that you want on your board. That particular chart
from the IPC 2152 standard is frankly not very accurate. I've actually been in conversations with someone who was part of
the original working group that worked on developing those charts and the people who
worked on it at the time, and I think anyone who can, you know, think about how material properties affect thermal performance, they knew that that particular chart was only applicable to one material system and one particular type of stackup. If you start placing
copper above and below, depending on the extent of the copper, it's gonna modify what the
allowed temperature rise is gonna be for a given
width and copper weight. And so all of those factors
need to be taken into account. So you can look at that chart as kind of like the least
bad tool that we have to start sizing out conductors for a particular current level that you need to carry in your traces. So I say least bad because it's
a little over conservative. I think a lot of people would
agree that it's a bit over conservative and it doesn't
apply to every material system. So if you're gonna use that
table, that's perfectly fine. Just go ahead and make sure
that you keep that in mind. If you go to a different material system, such as one that has a
higher thermal conductivity, you can actually use a
little bit larger current and vice versa. If you have to settle
with the material system that has lower thermal conductivity, you have to then settle
for somewhat lower current. The other option is you have
to make your traces larger in order to accommodate a
certain current in your board. So that's really important
in power electronics, because if you're doing power electronics and you need to route some
conductor that carries current, you need to make sure
that it's sized properly so that it doesn't heat up too much. Now that current needs to be delivered to all your other components. You need to make sure
that it's sized properly and it doesn't take up too
much space on your board. So that's the other challenge. Some of the stuff that we've
been talking about so far with the IPC 2152 nomograph, that particular table
was developed for traces. So what about planes? So we'll come back and
we'll talk about planes and how much current they can carry and how to work with power
planes in an upcoming video. So stay tuned. All right, everybody. So again, go check out the
blog link in the description. There's a nice little table in there that can be used convert between
your copper weight values and the actual thickness
of the copper film. If you don't have it
memorized, that's okay. There's a table in there that you can use. So hit that like button,
hit the subscribe button, as always leave your questions
in the comment section. And I will do my best to get to them. I know I was slackin'
the last couple of weeks, but I'll get to them
as fast as I can folks. All right, thanks everybody. And definitely, on this stuff, don't forget to call your fabricator. (crash of heavy metal)
