Re: PCB Thermal Analysis

On Jan 31, 12:33 pm, "MK" <nos...@xxxxxxxxxx> wrote:
"rickman" <gnu...@xxxxxxxxx> wrote in message

news:f96574f1-923f-4079-ae08-8ac1956a19c2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



I have a very small board that I would like to model for thermal
analysis. I am pretty confident that I can get by with a first order
analysis since I only want to know if I have a prayer of a chance of
dissipating the heat from this board, rather than a detailed analysis
of the range of temperatures across the board. I was thinking about
drawing a simple spice circuit with current to represent heat,
resistors to represent thermal resistance and voltage to represent
temperature.

Sounds pretty simple, right? But even after doing some web searching,
I have no idea where to find details on the thermal conductivity of
PCB material and the values to use for transmitting heat from the
board to the air. I am surprised that this is not commonly used, but
it appears that most IC data just *assumes* that your board is large
enough that it is a good heatsink. I did not find a single reference
with info on the PCB.

Am I not looking hard enough? Can anyone point me to the raw data on
PCB thermal characteristics I need for this analysis?

Hello,

I think you are going about this the wrong way - my experience of thermal
modelling is that you need fancy finite element software, good accurate
models and good material info. The first is expensive and the second two
very expensive.

If this is your tiny amplifier problem I suggest you design a test board
that you can measure temperatures on and get it made. It will cost you about
three days. Another trick that I have used is to measure a similar board,
putting heat in by wiring directly to resistors on any handy footprints.

Our thermal models were never very close to the real thing but the
measurements on mockups were.

Yes, this is the small line driver board. I am surprised that you say
you found thermal modeling to be inaccurate. I understand that finite
analysis software can be expensive... I don't know of any CAD software
that is cheap. But the model seems rather simple to me if you know
the values to use. I have been searching some more and I think I have
a paper that describes the thermal conductivity of a multilayer PCB
pretty well. I don't see anything in it to describe thermal vias, but
I do have some data on calculating the thermal conductivity using
material parameters, so I guess I could attempt to model the vias
myself.

I have considered building a trial board. I could even do that with
board stock I have around. But I don't have a good way to model the
thermal resistance of the chip to board connection. Of course, even
this would be better than *no* information. I may give that a try.

.

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