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Copper required to cool LED when conformal coating used

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LED being used: https://www.mouser.com/ds/2/311/GW JDSRS1.EC - DURIS E 5 (EnglishDeutsch)-520766.pdf

This LED has an exposed thermal pad, if I'm interpreting the data sheet correctly θJ to pad is 26°C/Watt.

If I use the maximum current rating of the LED of 180ma, then Pd=3 * 0.18 = 0.54W

Using the TI app. note below, rule of thumb of 2.37 in² per Watt, I would need 1.2 in² for 40° rise, this assumes double sided copper.

I read the following to determine how much PCB copper is required https://www.ti.com/lit/an/snva419c/snva419c.pdf

Hopefully this is correct so far?

Now what if the board has conformal coating?
 

The thermal pad is supposed to be soldered to the heatsink so conformal coating would have to be removed before mounting it. You might get away with a dot of thermal paste between the pad and heat sink if you derate it slightly but you would have to monitor the temperature to find the safe limit. If you do that, I would still suggest you remove all conformal coating from under the LED as it probably isn't a good heat conductor.

Brian.
 
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betwixt said:
The thermal pad is supposed to be soldered to the heatsink so conformal coating would have to be removed before mounting it. You might get away with a dot of thermal paste between the pad and heat sink if you derate it slightly but you would have to monitor the temperature to find the safe limit. If you do that, I would still suggest you remove all conformal coating from under the LED as it probably isn't a good heat conductor.

Brian.
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The conformal coating would be applied to the entire board after it is populated (outdoor application). My concern is the reduced cooling effect of the copper when its covered in conformal coating.
 

There would be some insulation effect from the conformal coating but I would expect it to be quite small as the layer is very thin. The coating manufacturer may be able to provide information on its thermal properties.

More important though - check the coating and solvents in it do not attack the LED. Some experimentation may be needed, perhaps dropping a spare LED into liquid coating fluid for a while to make sure there is no chemical reaction. Many bright LEDs have a silicone window that might react or detach in the presence of solvents.

Brian.
 
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    Jester

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The role of the copper is to remove the *heat* from the place where it is produced. Then it is distributed over a larger area. Finally it is removed by radiation and (party by convection).

The conformal coating will not affect radiation cooling much (the change will be very small) but the convection will be affected. If the temp rise is ~10C or less, convection can be neglected.

But some power LED arrays have their own heatsinks and some have fans too (I think they are more like the PC CPU fans) and they may have far more increase in temperature. For 0.5W, you can safely ignore the effect of the conformal coating. A part of the copper heatsink will be covered by the LED module and this you need to exclude from the calculations.
 

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