Does anyone have any rules of thumb about heat dissipation ratings of components once they have been potted in epoxy or silicone type rubber compounds?
For example is a 1/4 w resistor (in air) still capable of dissipating 1/4 w once it has been potted.
Just comparing the thermal conductivity specs of air and epoxy/silicone doesn't tell the whole story because air has conduction and convection whereas a solid only has conduction.
Air also has radiation... Most organic substances are opaque to far IR (heat) radiation so you lose this
mode of dissipation in a potting compound even if its transparent. Perhaps pot a resistor and a thermistor
side-by-side and make some measurements?
Heat conduction is fairly predictable if you know the geometry and the thermal conductivity of the
comound used. This sort of thing is often simulated to predict thermal behaviour and max temperature
rises.
Are you using thermally conductive epoxy?
http://www.digikey.com/product-search/en/fans-thermal-management/thermal-adhesives-epoxies-greases-pastes/1179754?k=thermal%20epoxy
Are your components dissipating their full wattage?
Do things really need to be potted?
What ambient temperatures will things be expected to be in?
Rule of thumb ??? , make up 2 or 3, burn them in to see how hot things get (maybe add some temperature sensors internally for the test).
http://www.pottingsolutions.com/my%20site/Technology/potting_hints.htm
They don't come out and say it directly but they imply that the potting compound is preferable to no compound with respect to thermal dissipation.
LarryD:
Are your components dissipating their full wattage?
Do things really need to be potted?
What ambient temperatures will things be expected to be in?
200mW RS485 through hole termination resistor in a liquid level sensor. Potting is the only guaranteed way to keep water out long term (10+ years). Max ambient 60 deg C.
The potting compound will probably be Raytech silicone gel
http://docs-asia.electrocomponents.com/webdocs/0e0b/0900766b80e0bed4.pdf
with 2/3 silica sand as a filler.
I don't have the instrumentation to do mm scale temperature measurements so I've derated the resistor by a factor of 5.
Low wattage and having it submersed, will help.
Assumption, the compound is safe for energized electronic equipment.
.