PCB Copper Thickness Versus Rth - is this graph correct?

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
Hi

I have a PCB, 50mm x 50mm, and I need to optimize cooling of the PCB. A num
ber of dissipating components are spread out on the PCB to produce an unifo
rm temperature across the PCB.

Currently I am using 0.5 Oz PCB thickness, but it is possible to increase t
hat to 1 Oz.

So I was looking for a graph of the thermal resistance on a certain area of
 PCB versus the copper thickness. My initial feeling would be that the incr
ease of the copper thickness would be insignificant with respect to the Rth
.

Found this graph, figure 3 on page 2:

http://www.iaasr.com/wp-content/uploads/2013/10/Using-PCB-copper-area-to-di
ssipate-the-heat-produced-by-surface-mount-components.-Rev1.pdf

Increasing the copper from 0.5oz to 1oz would reduce the thermal resistance
 from 260K/W to 180K/W

But, is this valid. If we take the example of a single hotspot device in th
e center of the board, the increased thickness would reduce the thermal res
istance from the device to the rest of the board, so the temperature would  
be close to uniform.

If on the other hand, with a PCB with decreased copper thickness, I have a  
number of devices spread evenly on the PCB and dissipating individually the
 same amount of power, the heat would then also be uniform. But the transfe
r of the heat to the surroundings are convection and conduction, and these  
should not be affected by the thickness of the copper layer.

So, for the actual design, evenly spaced components would not benefit from  
thicker copper thickness. Is this a valid assumption?

Cheers

Klaus

Re: PCB Copper Thickness Versus Rth - is this graph correct?
On Thu, 09 Jan 2014 03:55:55 -0700, Klaus Kragelund  

Quoted text here. Click to load it

Rule of thumb for heat dissipation of free standing surface, no fan is
1 C rise per watt over 100 sq in area.

That kind of implies that thicker copper, which is in series with your  
copper/PCB to air transfer, doesn't make a lot of difference.

But gut feel is that thicker copper also gives you some thermal mass,  
which might save a marginal part during a 'spike' of dissipation.

Re: PCB Copper Thickness Versus Rth - is this graph correct?
On Thu, 09 Jan 2014 07:08:19 -0700, RobertMacy

Quoted text here. Click to load it

This generalization gives an average temperature rise. Thicker copper
gives a reduction in hotspots, or improvement in uniformity of
temperatures across the surface.

For a uniform surface dissipation, your ballpark gives a 60%
overestimate of rise, which from my experience is closer to  
(delta)t = 1degC per milliwat per cm^2 (+/-20%)  

- another easily remembered relationship, without multipliers (if you
ignore milli and centi...).

I suspect that another ballpark constant is available, giving a
hotspot (delta tH/ delta tAv) estimate vs radius/thermal conductivity.
RL

Re: PCB Copper Thickness Versus Rth - is this graph correct?
wrote:

Quoted text here. Click to load it

It's complex.

The surface area of the board is the convective path to the air.  

Copper pours and planes spread the heat out from a component. Spreading thermal
resistance is usually important. A surface-mount resistor or transistor can get
very hot if it can't spread the heat laterally into the board surface.

The thicker the copper, and the more un-interrupted planes, the better the
lateral heat spreading.  

1 oz copper has a sheet thermal resistance of about 70 K/watt. That's the theta
from opposite of a square of copper foil of any size.

Example: a 1206 resistor with normal pads and traces.

https://dl.dropboxusercontent.com/u/53724080/Thermal/V220_1206/1206.txt

https://dl.dropboxusercontent.com/u/53724080/Thermal/V220_1206/DSC06287.JPG

https://dl.dropboxusercontent.com/u/53724080/Thermal/V220_1206/IR_0056.jpg

The resistor is a hot spot, because the heat doesn't spread laterally very well.
Theta would be much lower if the pads were bigger, or if there were thermal vias
to other-layer copper pours or planes.

So just physically spreading out parts doesn't solve the hot-spot problem. Lots
of copper is the best lateral heat spreading mechanism on a PC board.




--  

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

We've slightly trimmed the long signature. Click to see the full one.
Re: PCB Copper Thickness Versus Rth - is this graph correct?
On Thursday, January 9, 2014 4:24:51 PM UTC+1, John Larkin wrote:
Quoted text here. Click to load it
il.com>
Quoted text here. Click to load it
number of dissipating components are spread out on the PCB to produce an un
iform temperature across the PCB.
Quoted text here. Click to load it
e that to 1 Oz.
Quoted text here. Click to load it
 of PCB versus the copper thickness. My initial feeling would be that the i
ncrease of the copper thickness would be insignificant with respect to the  
Rth.
Quoted text here. Click to load it
-dissipate-the-heat-produced-by-surface-mount-components.-Rev1.pdf
Quoted text here. Click to load it
nce from 260K/W to 180K/W
Quoted text here. Click to load it
 the center of the board, the increased thickness would reduce the thermal  
resistance from the device to the rest of the board, so the temperature wou
ld be close to uniform.
Quoted text here. Click to load it
 a number of devices spread evenly on the PCB and dissipating individually  
the same amount of power, the heat would then also be uniform. But the tran
sfer of the heat to the surroundings are convection and conduction, and the
se should not be affected by the thickness of the copper layer.
Quoted text here. Click to load it
om thicker copper thickness. Is this a valid assumption?
Quoted text here. Click to load it
hermal
an get
e
 theta
Quoted text here. Click to load it
PG
g
y well.
al vias
. Lots

I have the resistors spread out and all components have as much copper as p
ossible to provide lateral heat spreading:

https://www.dropbox.com/s/y55jw3urqgr5329/900mW into 12x 1206.pdf

Cheers

Klaus

Re: PCB Copper Thickness Versus Rth - is this graph correct?
On Fri, 10 Jan 2014 00:27:59 -0800 (PST), Klaus Kragelund

Quoted text here. Click to load it

That link doesn't work for me.

We've found that resistors from 0603 to 1206 can all dissipate a half watt or so
if their end caps are soldered to big copper pours. The central hot-spot
temperatures are the same.


--  

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

We've slightly trimmed the long signature. Click to see the full one.
Re: PCB Copper Thickness Versus Rth - is this graph correct?
Quoted text here. Click to load it

That URL has spaces.  Try one or more of the following:

<https://www.dropbox.com/s/y55jw3urqgr5329/900mW into 12x 1206.pdf>
https://www.dropbox.com/s/y55jw3urqgr5329/900mW%20into%2012x%201206.pdf
<https://www.dropbox.com/s/y55jw3urqgr5329/900mW%20into%2012x%201206.pdf
http://goo.gl/YKzqoI

Matt Roberds


Re: PCB Copper Thickness Versus Rth - is this graph correct?
On Friday, January 10, 2014 5:47:45 PM UTC+1, John Larkin wrote:
Quoted text here. Click to load it
tmail.com>
Quoted text here. Click to load it
 A number of dissipating components are spread out on the PCB to produce an
 uniform temperature across the PCB.
Quoted text here. Click to load it
ease that to 1 Oz.
Quoted text here. Click to load it
rea of PCB versus the copper thickness. My initial feeling would be that th
e increase of the copper thickness would be insignificant with respect to t
he Rth.
Quoted text here. Click to load it
-to-dissipate-the-heat-produced-by-surface-mount-components.-Rev1.pdf
Quoted text here. Click to load it
stance from 260K/W to 180K/W
Quoted text here. Click to load it
 in the center of the board, the increased thickness would reduce the therm
al resistance from the device to the rest of the board, so the temperature  
would be close to uniform.
Quoted text here. Click to load it
ave a number of devices spread evenly on the PCB and dissipating individual
ly the same amount of power, the heat would then also be uniform. But the t
ransfer of the heat to the surroundings are convection and conduction, and  
these should not be affected by the thickness of the copper layer.
Quoted text here. Click to load it
 from thicker copper thickness. Is this a valid assumption?
Quoted text here. Click to load it
g thermal
r can get
Quoted text here. Click to load it
 the
Quoted text here. Click to load it
the theta
t
7.JPG
.jpg
very well.
ermal vias
lem. Lots
s possible to provide lateral heat spreading:
Quoted text here. Click to load it

This should work (no white spaces):

http://www.electronicsdesign.dk/tmp/900mW_into_12x_1206.pdf


Quoted text here. Click to load it
t or so
Quoted text here. Click to load it

I have 900mW for 12 resistors, but that's because I have a maximum 15 degre
es delta hotspot temperature requirement  

Cheers

Klaus

Re: PCB Copper Thickness Versus Rth - is this graph correct?
On Thursday, January 9, 2014 5:55:55 AM UTC-5, Klaus Kragelund wrote:
Quoted text here. Click to load it
umber of dissipating components are spread out on the PCB to produce an uni
form temperature across the PCB.
Quoted text here. Click to load it
 that to 1 Oz.
Quoted text here. Click to load it
of PCB versus the copper thickness. My initial feeling would be that the in
crease of the copper thickness would be insignificant with respect to the R
th.
Quoted text here. Click to load it
dissipate-the-heat-produced-by-surface-mount-components.-Rev1.pdf
Quoted text here. Click to load it
ce from 260K/W to 180K/W
Quoted text here. Click to load it
the center of the board, the increased thickness would reduce the thermal r
esistance from the device to the rest of the board, so the temperature woul
d be close to uniform.
Quoted text here. Click to load it
a number of devices spread evenly on the PCB and dissipating individually t
he same amount of power, the heat would then also be uniform. But the trans
fer of the heat to the surroundings are convection and conduction, and thes
e should not be affected by the thickness of the copper layer.
Quoted text here. Click to load it
m thicker copper thickness. Is this a valid assumption?
Quoted text here. Click to load it
Hi Klaus,  To my mind what's important is how the heat is being removed fro
m the pcb.  Is there some thermal connection to the outside world? (like br
ass standoffs.)  Or is it just cooled by air conduction/convection?  In the
 former the thickness of the copper would help... where if it's just air co
oling, and approximately uniform temperature across the pcb already, then t
hicker copper won't do much.    

George H.
Quoted text here. Click to load it


Re: PCB Copper Thickness Versus Rth - is this graph correct?
On Thursday, January 9, 2014 4:46:46 PM UTC+1, George Herold wrote:
Quoted text here. Click to load it
 number of dissipating components are spread out on the PCB to produce an u
niform temperature across the PCB.
Quoted text here. Click to load it
se that to 1 Oz.
Quoted text here. Click to load it
a of PCB versus the copper thickness. My initial feeling would be that the  
increase of the copper thickness would be insignificant with respect to the
 Rth.
Quoted text here. Click to load it
o-dissipate-the-heat-produced-by-surface-mount-components.-Rev1.pdf
Quoted text here. Click to load it
ance from 260K/W to 180K/W
Quoted text here. Click to load it
n the center of the board, the increased thickness would reduce the thermal
 resistance from the device to the rest of the board, so the temperature wo
uld be close to uniform.
Quoted text here. Click to load it
e a number of devices spread evenly on the PCB and dissipating individually
 the same amount of power, the heat would then also be uniform. But the tra
nsfer of the heat to the surroundings are convection and conduction, and th
ese should not be affected by the thickness of the copper layer.
Quoted text here. Click to load it
rom thicker copper thickness. Is this a valid assumption?
Quoted text here. Click to load it
rom the pcb.  Is there some thermal connection to the outside world? (like  
brass standoffs.)  Or is it just cooled by air conduction/convection?  In t
he former the thickness of the copper would help... where if it's just air  
cooling, and approximately uniform temperature across the pcb already, then
 thicker copper won't do much.    
Quoted text here. Click to load it

I has only limited contact to the enclosure, regretfully

Cheers

Klaus

Re: PCB Copper Thickness Versus Rth - is this graph correct?
On Fri, 10 Jan 2014 00:29:10 -0800 (PST), Klaus Kragelund

Quoted text here. Click to load it

You can get serious PCB cooling with a Bergquist-type thermal pad between a PCB
and a metal (or even plastic) enclosure.

Here's a couple of pages from a thermal study I did...

https://dl.dropboxusercontent.com/u/53724080/Thermal/ESM_HX_6-7.pdf

We're about to order a batch of custom die-cut silicone-free pads from
Bergquist, roughly $12 each.

The board has a lot of internal copper, and has "thermal antenna" pours on the
bottom side to extract heat from critical parts, into the Bergquist pad.

https://dl.dropboxusercontent.com/u/53724080/PCBs/ESM_rev_B.jpg





--  

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

We've slightly trimmed the long signature. Click to see the full one.
Re: PCB Copper Thickness Versus Rth - is this graph correct?
The defining quantity is the spacing of said components relative to the  
lateral diffusivity (i.e., how far sideways along the board the heat will  
spread out).

I believe it's around 3cm for 2oz copper (ah, such wonderful juxtaposition  
of units :) ), so putting equal-dissipating components on a grid of around  
6cm center-to-center (note a triangular mesh allows maximal packing) will  
be about optimal between copper/board thickness and utilization.  Such  
spacing will allow about 2W per component.

Use proportionally smaller spacings for thinner material.

Not necessarily smaller for thinner foil only, but let's see.  FR-4 is  
0.81 W m^-1 K^-1 while copper is 400; the average board is 1600um thick.  
0.5oz copper is 17um, or say 34um total (double sided).  The conductivity  
per square of copper is 0.0136 W K^-1, and of FR-4, 0.0013 W K^-1.  So  
even for thin plating, it's still true that copper dominates the lateral  
conductivity.

Tim

--  
Seven Transistor Labs
Electrical Engineering Consultation
We've slightly trimmed the long signature. Click to see the full one.
Re: PCB Copper Thickness Versus Rth - is this graph correct?

Quoted text here. Click to load it

Theta depends on the part size, too. If you dump heat into, say, a circular
patch on an infinite metal sheet, theta depends on the patch area. Theta goes to
infinity as the contact area goes to zero. Getting the heat out locally, close
to the part, is often the bottleneck.


--  

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

We've slightly trimmed the long signature. Click to see the full one.
Re: PCB Copper Thickness Versus Rth - is this graph correct?
Quoted text here. Click to load it

Yes.  Or, since you "can't do equations", ;-)

For surfaces with no surface heat dissipation (lateral heat spreading  
only), the thermal resistance between concentric cylindrical surfaces is:
Rth = ln(r2 / r1) / (2 pi sigma_th)

Which of course diverges for r1 --> 0.

When the surfaces dissipate heat linearly with temp difference (true of  
solid conductors, but a poor approximation of actual convection or  
radiation), solutions take the form of the complex Bessel function (i.e.,  
T(r) = c1 * J_0(i*c2*r)).  A closed form solution (albeit in terms of the  
Bessel function) is left as an exercise for the student. ;-)

Tim

--  
Seven Transistor Labs
Electrical Engineering Consultation
We've slightly trimmed the long signature. Click to see the full one.
Re: PCB Copper Thickness Versus Rth - is this graph correct?
wrote:

Quoted text here. Click to load it

Which translates to "You can't do the equations either."

There is a reason that people use Spice and thermal FEM software. And
measurements.


--  

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

We've slightly trimmed the long signature. Click to see the full one.

Site Timeline