Electronic – Mounting a heat sink on a S202S02 SSR

Yes, thermal resistance/conductance is a two-way street, for passive conductive cooling devices, with equal speed limits posted for both directions.

OK, now for the caveats: If your heat sink is rated for forced air flow, and you have no forced air flow, then the heat flow through the heat sink will be different. Your heat sink is rated for certain conditions at both ends. If you meet those conditions (at both ends), except for reversal of temperature conditions, then you can expect equal but opposite heat flow.

Say that your 10C/W heat sink is rated 10C/W for conduction of heat from a 1W source to still air, with the contact area with air being fins. Now, you put those fins INSIDE your enclosure, in contact with still air, and you place the outside end of your heat sink in contact with a device (say, a cold plate) that will keep that end of the heat sink 10C cooler than the inside air. In that case you will get 1W of energy flow from the fins of the heat sink to the cooling device (cold plate).

You would want to pay attention to such things as: Warm air rises and cooler air falls. Air fins are most effective when hot air can rise from them and allow cooler air to come in contact with the fins. Cooling fins, on the other hand, would be more efficient when placed such that cooled air can fall away from them.

To directly answer your first question: No, not even close. A little more copper around a surface mount part isn't going to get rid of 11W of heat. No way.

One answer may be to parallel multiple FETs. Not only does that cut the total dissipation by the number of parts, but the dissipation on each FET is reduced by the square of the number of parts. So if one FET dissipates 10W, then two parallel FETs would dissipate 5W total, and each FET would dissipate only 2.5W.

That's in theory. In practise they won't share the load exactly equally, so you have to design for a bit worse than that per FET. The good thing about paralleling FETs is that they have a positive temperature coefficient. The Rdson goes up with temperature. This helps them ballance somewhat and prevents runaway of a single part, like could be the case with bipolar transistors.

Ultimately you have to decide what you really want. Switching 40A is going to make some heat. One way or the other you're going to have to deal with that. You can tell us you're space contrained all you want, but ultimately the physics will dictate a certain amount of space, surface area, forced cooling, or whatever. It may not be possible to meet all the constraints. Not all combinations of small size, high current, and low cost are possible.