Best way to heat sink many resistors

Question

I have a PCB design with a long row of 1206 power resistors. Although only a few of them will be active at a time, they can dissipate up to 1 watt each, so I would like to heat sink them. If they get too hot they may overheat the components they're connected to, or just fail sooner.

Picture for reference:

For electrical and space reasons I can't attach either pad of the resistors to a large plane. I've placed thermal pads under the resistors, but they cannot be electrically connected to it, only thermally.

My original plan was to "pour" thermal epoxy over the resistors after reflow assembly, this would spread the heat out from the active resistors, and also couple it into the nearby thermal pads. I tested that, and it works, but the issue is that the epoxy is much thicker than I anticipated (like frosting). This makes smearing it onto the board without covering adjacent LEDs quite difficult and time consuming, not really suitable for production.

I also considered placing a dab of thermal compound (not epoxy) under each resistor before reflow, but the issue with that is that the thermal compound must be able to withstand reflow temperatures. I found one (boron nitride), but it's a very watery consistency and it dries to a crust in seconds, so that also proved to be impractical.

Does anyone know of an elegant way to heatsink these resistors?

Answer 1

One cleaner way to go is to use a thermal pad. Those come into sheets that you can cut to the appropriate size, you can then screw an aluminum heatsink to it. There is a long thin heatsink that would fit there.

The advantage of this method is that you don't have to do anything special for the SMT process and can be mounted afterward. It also avoids the mess of using epoxy / thermal compound, can be disassembled easily.

These pads come in different thicknesses from less than a millimeter to 5mm, so it "squeezes" when compressed, allowing for small height differences between components.

If you cannot afford screws, 3M manufactures a thermal pad that is sticky on both sides. I've been using that for years and yet one has come apart. It also comes in sheets that you can cut.

Answer 2

Think outside the box.

If you're shedding that much power in the dropping resistors that you're exceeding the rating, consider a separate, lower VDD rail for the LEDs, made with a DC-DC step-down converter. Then you can optimize the voltage so that the resistor values can be lower, and therefore shed less heat for the given current.

Efficiency will be better and heat will be less. Win-win overall.

Also, do your LEDs need cooling? I'm guessing they do. Consider a thermal solution on the back of your board using thermal vias and a heatsink. Maybe mount the whole board to the sink with a thermal pad in between. If you have no backside components that would be easy to do.