[sdiy] PCB heat simulations?

[Mattias Rickardsson]

Hi all,

is there any good tool (or rules of thumb) when it comes to
understanding the passive cooling airflow around PCBs?

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 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?

Any pointers are welcome! :-)

/mr