From: "Graham Holloway"
Subject: Re: Thermal effects of potting compound?
Date: Wed, 16 Oct 2002 14:34:11 +0100
NNTP-Posting-Date: Wed, 16 Oct 2002 13:38:38 +0000 (UTC)
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"Tom Faloon" wrote in message
> Hi George
> If you have a fan forcing sufficient cool air over both sides of the
> then turning the board upside down probably won't be a problem.
> If you don't have forced air cooling, then turning the board upside down
> will probably degrade the cooling performance considerably, because you
> discourage convection. Some heat will be lost by convection off the upper
> side of the board, but the heat will have to pass through the PCB
> which is not a very good conductor. Cooling will be very inefficient
> compared with having the board 'right way up'.
If most of the heat is from SMD parts, eg. DPAK soldered to the PCB, and
there is no forced cooling, then the surface that faces uppermost doesn't
really matter as the bulk of the heat is being dumped into the PCB. The PCB
will conduct heat much better than air.
> Whether 'upside down' cooling is good enough depends very much on your
> application - On factors such as, how much heat you are dissipating, case
> and PCB dimensions, case material and ventilation, and PCB layout. There
> no simple formula. At the end of the day, you need to do some
> Re coating the board in 5mm of potting compound. This is sure to degrade
> cooling effectiveness. Talk to potting compound manufacturers, they may be
> able to suggest figures.
The effectiveness is only reduced by the reduction in surface area and to a
small extent by the drop in temperature due to the heat passing through the
potting. Again, the thermal resistance of this is much lower than air.
> You didn't say how much power you were dissipating in the D-PAK MOSFET. I
> would think twice before potting that, unless the power is very low! Apart
> from the effect on cooling, you will need a potting compound which can
> tolerate the surface temperature of the D PAK, and which won't crack, or
> disintegrate, with repeated heating & cooling.
There are many potting compounds available that are ideal for this purpose.
And compound over the D-PAK will help extract heat from the topside of the
Please note that potting will really only let you manage the heat more
effectively. The total heat lost will still be the same, but you might
benefit from the spreading of hot spots.
Once you've potted the PCB it might be worth attaching it to a heatsink or
part of a metal chassis
> Tom Faloon