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Hi Jeff,
The heat capacity of your board will be determined by the amount of
localized heating that occurs on it, by the conductive paths that heat
will be carried away from the board (ground planes, heatsinks,
chassis, etc.), by the convective parameters (fan cooling, laminar or
turbulent flow), by the temperature rating/derating of your
components, as well as the ambient temperature conditions the product
will see.
Typically, if you have FEA available, you can run a model which
captures the areas where most of the power is being dumped in to the
board. Then add this to your delta T at ambient, and add this to your
THETAjc calculations for targeted die. Then compare if you are within
the allowable range for your IC's. Also make sure that the board is
not reaching levels close to or above the Tg of the FR4 material (this
can range from 120C to 170C typically).
I have used an equivalent thermal conductivity 0.39 (W/in-C) for a PCB
0.062" thick, 6 layers with 3 ground planes and 3 signal layers. I
have run some modeling and experimentation to verify the the k.
Though colleagues have also used .47 (W/in-C) for the same setup.
Frank Hinojos
Watkins-Johnson Co.
______________________________ Reply Separator _________________________________
Subject: DES : Question About Power Density Capabilities of PCB's
Author: "Jeff Fries" <[log in to unmask]> at INTERNET
Date: 7/16/97 9:57 AM
Hello -
I am designing a 10 layer (2 planes, 8 signal layers) printed circuit board
that will have surface mount components on both sides and through hole
components on one side. The board is about 8.25" x 10.5" x .062". It is a
power I/O board that will have 8 isolated inputs and 8 isolated outputs. The
board will be extremely dense, and could at times generate some respectable
heat (each output will put out about 3W @ 50Volts). To give myself a better
feeling about pursuing the manufacturing of this PCB, I was wondering if
anyone knew of any guidelines or rules about the power density capabilities
of a PCB. I don't know if it is specified as such a beast, or whether it is
in terms of heat/area or heat/volume, but I just worry about the response of
the PCB over time and component life. If anyone knows of anything that
would help me with my design I would appreciate your input.
Thank you for your time and response.
Jeff Fries
Hardware Design Engineer
Harmon Industries
[log in to unmask]
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<html><head></head><BODY bgcolor="#D8D0C8"><p><font size=2 color="#000000"
face="Arial">Hello -<br><br>I am designing a 10 layer (2 planes, 8 signal
layers) printed circuit board that will have surface mount components on both
sides and through hole components on one side. The board is about
8.25" x 10.5" x .062". It is a power I/O board that will
have 8 isolated inputs and 8 isolated outputs. The board will be extremely
dense, and could at times generate some respectable heat (each output will put
out about 3W @ 50Volts). To give myself a better feeling about pursuing
the manufacturing of this PCB, I was wondering if anyone knew of any guidelines
or rules about the power density capabilities of a PCB. I don't know if it
is specified as such a beast, or whether it is in terms of heat/area or
heat/volume, but I just worry about the response of the PCB over time and
component life. If anyone knows of anything that would help me with my
design I would appreciatee your input. <br><br>Thank you for your time and
response.<br><br>Jeff Fries <br>Hardware Design Engineer<br>Harmon
Industries<br>[log in to unmask]</p>
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