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Subject: Re: [geda-user] H-bridge footprint help
From: Bob Paddock <bob DOT paddock AT gmail DOT com>
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> As for standing the part up, there are heatsinks which are mounted to
> the PCB, and the tab is screwed to the heatsink.  What do you think
> about them w.r.t. the shock/vibe issue you raise?

I've seen  heatsinks like that for TO-220's held to the board with #6 screws.
The screws were sheared off right along with the 220 leads.

>  I think you have
> expertise in design for extremely harsh environments

Coal Mining equipment where you have the worse of everything.
Water, caustic explosive dust, heat in the cases, 30 knot wind from
the barn sized ventilation fans.
The machines were literally beating themselves against rocks at times.
The hardest thing encountered are petrified tree stumps.
For reasons I do not know, they are called Kelian (sp?) Bottles.
Run into those, you blast as nothing cuts them.
They are well on there way to turning into diamonds but the pressure
in the area was not high enough to get it done.

> The one problem with laying the part down is that in general I don't
> know how to calculate the heat dissipation of a large PCB pad.

http://www.ti.com/adc/docs/midlevel.tsp?contentId=76735 Thermal Calculator.
Also note the reference material on the far right of the screen.

Sections of IPC-2152 address the thermal rise.

 http://blog.ipc.org/2009/10/09/new-ipc-2152-current-carrying-capacity-standard-solves-50-year-old-mystery/

http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/
and Via calculator are a help at times.

>Maybe there is some commerical thermal analysis program
> out there which can compute the thermal dissipation of an aribtrary
> pad?

I know of Anasys because they are near by, have not used them:
http://www.ansys.com/Products/Simulation+Technology/Electromagnetics

There was a IPC-7351 calcualtor, but I think it was bought by Mentor.
http://landpatterns.ipc.org/default.asp