Could someone please help me with a problem.
I have a PCB that fits in an ABS box and is to be completely potted, top and bottom.

I am investigating device failure modes, as I need to ensure that the outside surface temperature does not reach 135C.
I've worked out that one SOT23 device may reach a junction temperature of 150C, under fault condition. It sits on the top of the PCB,
althought the PCB sits quite low in the box.

How do estimate the overall thermal resistance of the potting compound (ignoring the ABS box for now) from the device case to the surface?
So i can work out the surface temperature.

I know that the thermal resistance is length /(area x thermal conductivity)

The device is very small source of heat, which will radiate in three dimensions.

I know the box dimensions, 100x65x35mm. The potting compound has thermal conductivity of 0.48W/mk. thermal resistance of SOT23 device is 250c/w.