[SI-LIST] Re: Thermal Profile WITHIN a trace
- From: Curt McNamara <curt.mcnamara@xxxxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 1 Aug 2012 17:07:57 -0500
Maybe another way of looking at this:
If you had a "narrow" temp source against a wide expanse of copper (a plane)
then yes there would be a definite gradient. Taking this towards the extreme,
the edge of the copper plate could be near room temperature while the area next
to the heat source could be very hot. This would be particularly true if the
heat source was "high frequency" i.e. it switched rapidly between hot and cool.
This seems more like a thought experiment than a practical one though. Heat
sources don't seem to work that way in real devices. In particular, if you look
at thermal characteristics of FET junctions under load, it may not be valid to
use duty cycle as a way to predict overall temperature. See IR datasheets for
an example.
With regard to IR measurement: the color of the surface makes a big difference
in the reading, and the size of the IR detector determines if you can get an
accurate reading on a small device. The newer Fluke instruments do an excellent
job of this, and are useful for debug as well as design verification.
Recommended reading:
Hot Air Rises and Heat Sinks by Tony Kordyban.
And of course thermal modeling is the way to really see what is happening. We
use Algor thermal analysis, and I am sure there are other good packages.
Curt
Curt McNamara, M. Eng., P.E. // Principal Electrical Engineer
LOGICPD
Logic Product Development
411 Washington Ave. N. Suite 400
Minneapolis, MN 55401
T // 612.436.5178
F // 612.672.9489
curt.mcnamara@xxxxxxxxxxx
www.logicpd.com
/ / / / / / / / / / / / / / / / / / / / / / / / /
This message (including any attachments) contains confidential information
intended for a specific individual and purpose, and is protected by law. If you
are not the intended recipient, you should delete this message and are hereby
notified that any disclosure, copying, or distribution of this message, or the
taking of any action based on it, is strictly prohibited.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of steve weir
Sent: Wednesday, August 01, 2012 4:40 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Thermal Profile WITHIN a trace
Doug, in order to break the isotherm approximation, one has to have an
extremely low thermal resistance on one side of the copper, very high thermal
resistance on the other, and thick copper. So if we had a direct freon spray
on the circuit board, we would see a substantial gradient through the Cu.
Short of that, we won't.
For example take an extreme case where we have a 2oz plane that is in direct
contact with a 0.25" thick Al heat sink with infinite cooling on the far side
of the heat sink on one side, and an infinitely thick epoxy board on the other:
All heat flows through the Cu, and then through the Al heat sink to our
infinite sink.
Thermal resistance is:
Cu: 1.65E-7 C/W/square
Al: 2.49E-5 C/W/square
Taking the entire heat source as on one side of the copper: Copper %
temperature rise one side to the other as a ratio of the total temperature rise
= 1.65 / 250.65 = 0.66%. However, that is pessimistic as the heat source
actually distributes through the copper thickness.
This works to equalize the temperature in the Cu as all of the heat goes
through the Cu closest to the heat sink and almost none goes through the Cu
furthest from the heat sink. If we solve the integral, we get about 0.25%
temperature rise from one side of the Cu to the other.
Steve.
On 8/1/2012 7:53 AM, Doug Brooks wrote:
> I realize that we usually do that. But what about these two considerations:?
> 1. If the boundary conditions are different on the top and the bottom
> of the trace, can the distribution be uniform within the trace?
> 2. What about my second question re the relative agreement in
> measurement methods?
>
> Doug
>
>
>
>
> At 12:24 AM 8/1/2012, steve weir wrote:
>> Doug, In most cases the thermal resistance of the path between the
>> copper and the sink in any direction is much greater than through the
>> copper owing to the facts that: 1) the copper is never more than a
>> few mils thick for even 3oz Cu, and 2) Cu has good thermal
>> conductivity ( about 400W/(m*K) ). So you can reasonably approximate
>> the copper through its thickness as an isotherm.
>>
>> Steve
>>
>> On 7/31/2012 2:59 PM, dbrooks9@xxxxxxxxxxxxxxx wrote:
>>> Assume a typical trace carrying moderate current. It has still
>> ai= r on one
>>> side and board material on the other side. It will reach an equili=
>>> brium temperature above the ambient based on the i^2*R heating and
>>> the conv= ectioncooling. I am wondering about the thermal profile
>>> WITHIN the trace. = We would normally assume that copper is such a
>>> good thermal
>> conductor that =
>>> the thermal profile would be uniform. But we would assume that
>> the conducti=
>>> vity is so good that the current density would be uniform, too,
>> and we know=
>>> that there are exceptions to that (e.g. skin effect.) Since the
>>> boundary c= onditions would be different at the still air and at the
>>> board
>> material, I =
>>> believe there must be a thermal profile within the trace.
>>> How much variation might there be in point temperatures within the = trace?
>>> Carrying this argument one step further, as= sume we measure the
>> temperature
>>> of the trace using two methods:1= . Measuring the resistance
>>> (therefore measuring the average temperature of = all points within
>>> the trace)2. Using an IR microscope (therefore= measuring the
>>> temperature at one point on the surface of the trace.)= Assuming the
>>> measurements are correctly made, how much difference migh= t there
>>> be in the measurements based on internal thermal profiles?
>>> ------------------------------------------------------------------
>>> To unsubscribe from si-list:
>>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
>>> field
>>>
>>> or to administer your membership from a web page, go to:
>>> //www.freelists.org/webpage/si-list
>>>
>>> For help:
>>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>>
>>>
>>> List forum is accessible at:
>>> http://tech.groups.yahoo.com/group/si-list
>>>
>>> List archives are viewable at:
>>> //www.freelists.org/archives/si-list
>>>
>>> Old (prior to June 6, 2001) list archives are viewable at:
>>> http://www.qsl.net/wb6tpu
>>>
>>>
>>>
>>
>> --
>> Steve Weir
>> IPBLOX, LLC
>> 150 N. Center St. #211
>> Reno, NV 89501
>> www.ipblox.com
>>
>> (775) 299-4236 Business
>> (866) 675-4630 Toll-free
>> (707) 780-1951 Fax
>>
>> All contents Copyright (c)2012 IPBLOX, LLC. All Rights Reserved.
>> This e-mail may contain confidential material.
>> If you are not the intended recipient, please destroy all records and
>> notify the sender.
>>
>> ------------------------------------------------------------------
>> To unsubscribe from si-list:
>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>
>> or to administer your membership from a web page, go to:
>> //www.freelists.org/webpage/si-list
>>
>> For help:
>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>
>>
>> List forum is accessible at:
>> http://tech.groups.yahoo.com/group/si-list
>>
>> List archives are viewable at:
>> //www.freelists.org/archives/si-list
>>
>> Old (prior to June 6, 2001) list archives are viewable at:
>> http://www.qsl.net/wb6tpu
>>
> Check out our resources at http://www.ultracad.com
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List forum is accessible at:
> http://tech.groups.yahoo.com/group/si-list
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
All contents Copyright (c)2012 IPBLOX, LLC. All Rights Reserved.
This e-mail may contain confidential material.
If you are not the intended recipient, please destroy all records and notify
the sender.
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
