[sdiy] PCB heat simulations?

[ASSI]

On Wednesday 13 November 2013, 11:08:01, Mattias Rickardsson wrote:
> is there any good tool (or rules of thumb) when it comes to
> understanding the passive cooling airflow around PCBs?

There are a number of commercial CFD simulators that specifically target PCB 
design, but aside from the shocking sticker price they come with a steep 
learning curve.  The rules of thumb for passive air cooling are twofold: 1) 
avoid recirculation of hot air and 2) heat transfer into a laminar air flow 
is very inefficient, so you need to maximize the transfer area.
 
> If I have a horizontally mounted PCB with some components that easily
> reach far above ambient temperature, how can I know if there is a
> convection going on above those components?
[…]

If there is any temperature difference, there is convection.  Hot air is 
less dense, so it will be replaced by denser cooler air one way or the 
other.  The temperature of your heat source in quasi-static operation will 
in turn be determined by the volume of air that is moved.

> If I knew that, I could
> choose to design the surrounding construction in a suitable way for
> the airflow - or if I knew that there is no convection I could skip
> those thoughts altogether and focus on other means of heat transfer.
> 
> I googled a bit but never found a simple visualisation tool or other
> simple rule of thumb of these *very* un-simple physical phenomena.
> Given how complicated they are, I expect that a few advanced experts
> are the only ones that can come close to understanding them, but
> theory should have trickled down to some handy design rules after all
> these years. For instance, if one of my 1 cm * 1 cm components are 50
> degrees C and the rest of the PCB is at room temp, what happens with
> the air above it? What temperature difference (or power draw) is
> typically needed to give convection?

If that component is at 50°C and stays that way, then you have enough 
airflow to keep it at that temperature.  :-)  More airflow would mean less 
temperature difference to ambient and less airflow would increase the 
difference of course.  The datasheet Rth values usually assume free 
convection.  Passive heatsinking tapers out at around 10K/W.  For typical 
"hot" components on a PCB the majority of the heat transfer is actually 
through the pins into the PCB, so if you can arrange for this heat to flow 
into a few large copper planes this will often be more efficient than 
anything related to airflow improvement.


Regards,
Achim.
-- 
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