From: "Tom Faloon"
Subject: Re: Thermal effects of potting compound?
Date: Wed, 16 Oct 2002 14:16:35 +0100
NNTP-Posting-Date: Wed, 16 Oct 2002 13:10:39 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
If you have a fan forcing sufficient cool air over both sides of the board,
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 will
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 material,
which is not a very good conductor. Cooling will be very inefficient
compared with having the board 'right way up'.
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 is
no simple formula. At the end of the day, you need to do some measurements!
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.
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.