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
PCB Copper Thickness Versus Rth - is this graph correct?
Started by ●January 9, 2014
Reply by ●January 9, 20142014-01-09
On Thu, 09 Jan 2014 03:55:55 -0700, Klaus Kragelund <klauskvik@hotmail.com> wrote:>> ...snip to keep Aioe happy > 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 transfer 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 > > KlausRule 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.
Reply by ●January 9, 20142014-01-09
On Thu, 9 Jan 2014 02:55:55 -0800 (PST), Klaus Kragelund <klauskvik@hotmail.com> wrote:>Hi > >I have a PCB, 50mm x 50mm, and I need to optimize cooling of the PCB. A number of dissipating components are spread out on the PCB to produce an uniform temperature across the PCB. > >Currently I am using 0.5 Oz PCB thickness, but it is possible to increase that 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 increase 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-dissipate-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 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 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 transfer 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 > >KlausIt'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 Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●January 9, 20142014-01-09
On Thursday, January 9, 2014 5:55:55 AM UTC-5, Klaus Kragelund wrote:> Hi >=20 >=20 >=20 > I have a PCB, 50mm x 50mm, and I need to optimize cooling of the PCB. A n=umber of dissipating components are spread out on the PCB to produce an uni= form temperature across the PCB.>=20 >=20 >=20 > Currently I am using 0.5 Oz PCB thickness, but it is possible to increase=that to 1 Oz.>=20 >=20 >=20 > 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 in= crease of the copper thickness would be insignificant with respect to the R= th.>=20 >=20 >=20 > Found this graph, figure 3 on page 2: >=20 >=20 >=20 > http://www.iaasr.com/wp-content/uploads/2013/10/Using-PCB-copper-area-to-=dissipate-the-heat-produced-by-surface-mount-components.-Rev1.pdf>=20 >=20 >=20 > Increasing the copper from 0.5oz to 1oz would reduce the thermal resistan=ce from 260K/W to 180K/W>=20 >=20 >=20 > But, is this valid. If we take the example of a single hotspot device in =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.>=20 >=20 >=20 > 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 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.>=20 >=20 >=20 > So, for the actual design, evenly spaced components would not benefit fro=m thicker copper thickness. Is this a valid assumption?>=20Hi 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. =20 George H.>=20 > Cheers >=20 >=20 >=20 > Klaus
Reply by ●January 9, 20142014-01-09
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 Website: http://seventransistorlabs.com "Klaus Kragelund" <klauskvik@hotmail.com> wrote in message news:90916480-16ca-49bb-b266-96950e9e05ff@googlegroups.com... Hi I have a PCB, 50mm x 50mm, and I need to optimize cooling of the PCB. A number of dissipating components are spread out on the PCB to produce an uniform temperature across the PCB. Currently I am using 0.5 Oz PCB thickness, but it is possible to increase that 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 increase 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-dissipate-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 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 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 transfer 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
Reply by ●January 10, 20142014-01-10
On Thu, 9 Jan 2014 17:23:31 -0600, "Tim Williams" <tmoranwms@charter.net> wrote:>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.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 Precision electronic instrumentation
Reply by ●January 10, 20142014-01-10
On Thursday, January 9, 2014 4:24:51 PM UTC+1, John Larkin wrote:> On Thu, 9 Jan 2014 02:55:55 -0800 (PST), Klaus Kragelund <klauskvik@hotma=il.com>>=20 > wrote: >=20 >=20 >=20 > >Hi >=20 > > >=20 > >I have a PCB, 50mm x 50mm, and I need to optimize cooling of the PCB. A =number of dissipating components are spread out on the PCB to produce an un= iform temperature across the PCB.>=20 > > >=20 > >Currently I am using 0.5 Oz PCB thickness, but it is possible to increas=e that to 1 Oz.>=20 > > >=20 > >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 i= ncrease of the copper thickness would be insignificant with respect to the = Rth.>=20 > > >=20 > >Found this graph, figure 3 on page 2: >=20 > > >=20 > >http://www.iaasr.com/wp-content/uploads/2013/10/Using-PCB-copper-area-to=-dissipate-the-heat-produced-by-surface-mount-components.-Rev1.pdf>=20 > > >=20 > >Increasing the copper from 0.5oz to 1oz would reduce the thermal resista=nce from 260K/W to 180K/W>=20 > > >=20 > >But, is this valid. If we take the example of a single hotspot device in=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.>=20 > > >=20 > >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 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.>=20 > > >=20 > >So, for the actual design, evenly spaced components would not benefit fr=om thicker copper thickness. Is this a valid assumption?>=20 > > >=20 > >Cheers >=20 > > >=20 > >Klaus >=20 >=20 >=20 > It's complex. >=20 >=20 >=20 > The surface area of the board is the convective path to the air.=20 >=20 >=20 >=20 > Copper pours and planes spread the heat out from a component. Spreading t=hermal>=20 > resistance is usually important. A surface-mount resistor or transistor c=an get>=20 > very hot if it can't spread the heat laterally into the board surface. >=20 >=20 >=20 > The thicker the copper, and the more un-interrupted planes, the better th=e>=20 > lateral heat spreading.=20 >=20 >=20 >=20 > 1 oz copper has a sheet thermal resistance of about 70 K/watt. That's the=theta>=20 > from opposite of a square of copper foil of any size. >=20 >=20 >=20 > Example: a 1206 resistor with normal pads and traces. >=20 >=20 >=20 > https://dl.dropboxusercontent.com/u/53724080/Thermal/V220_1206/1206.txt >=20 >=20 >=20 > https://dl.dropboxusercontent.com/u/53724080/Thermal/V220_1206/DSC06287.J=PG>=20 >=20 >=20 > https://dl.dropboxusercontent.com/u/53724080/Thermal/V220_1206/IR_0056.jp=g>=20 >=20 >=20 > The resistor is a hot spot, because the heat doesn't spread laterally ver=y well.>=20 > Theta would be much lower if the pads were bigger, or if there were therm=al vias>=20 > to other-layer copper pours or planes. >=20 >=20 >=20 > So just physically spreading out parts doesn't solve the hot-spot problem=. Lots>=20 > of copper is the best lateral heat spreading mechanism on a PC board. >=20I 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
Reply by ●January 10, 20142014-01-10
On Thursday, January 9, 2014 4:46:46 PM UTC+1, George Herold wrote:> On Thursday, January 9, 2014 5:55:55 AM UTC-5, Klaus Kragelund wrote: >=20 > > Hi >=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > I have a PCB, 50mm x 50mm, and I need to optimize cooling of the PCB. A=number of dissipating components are spread out on the PCB to produce an u= niform temperature across the PCB.>=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > Currently I am using 0.5 Oz PCB thickness, but it is possible to increa=se that to 1 Oz.>=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > So I was looking for a graph of the thermal resistance on a certain are=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.>=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > Found this graph, figure 3 on page 2: >=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > http://www.iaasr.com/wp-content/uploads/2013/10/Using-PCB-copper-area-t=o-dissipate-the-heat-produced-by-surface-mount-components.-Rev1.pdf>=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > Increasing the copper from 0.5oz to 1oz would reduce the thermal resist=ance from 260K/W to 180K/W>=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > But, is this valid. If we take the example of a single hotspot device i=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.>=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > If on the other hand, with a PCB with decreased copper thickness, I hav=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.>=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > So, for the actual design, evenly spaced components would not benefit f=rom thicker copper thickness. Is this a valid assumption?>=20 > >=20 >=20 > Hi Klaus, To my mind what's important is how the heat is being removed f=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. =20>=20I has only limited contact to the enclosure, regretfully Cheers Klaus
Reply by ●January 10, 20142014-01-10
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message news:afsuc9p4usebvl10l8hjpil2v26738ck6e@4ax.com...> 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.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 Website: http://seventransistorlabs.com
Reply by ●January 10, 20142014-01-10
On Fri, 10 Jan 2014 00:27:59 -0800 (PST), Klaus Kragelund <klauskvik@hotmail.com> wrote:>On Thursday, January 9, 2014 4:24:51 PM UTC+1, John Larkin wrote: >> On Thu, 9 Jan 2014 02:55:55 -0800 (PST), Klaus Kragelund <klauskvik@hotmail.com> >> >> wrote: >> >> >> >> >Hi >> >> > >> >> >I have a PCB, 50mm x 50mm, and I need to optimize cooling of the PCB. A number of dissipating components are spread out on the PCB to produce an uniform temperature across the PCB. >> >> > >> >> >Currently I am using 0.5 Oz PCB thickness, but it is possible to increase that 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 increase 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-dissipate-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 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 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 transfer 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 >> >> >> >> 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. >> > >I have the resistors spread out and all components have as much copper as possible to provide lateral heat spreading: > >https://www.dropbox.com/s/y55jw3urqgr5329/900mW into 12x 1206.pdf > >Cheers > >KlausThat 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 Precision electronic instrumentation
