I ordered a passive heatsink for system-on-chip of the Raspberry Pi 3 model B. Since it fits well I'll share the details:
Fischer Elektronik ICK S 14 X 14 X 10 heatsink (Element 14 catalogue 1850054, AUD3.70).
Fischer Elektronik WLFT 404 23X23 thermally conductive foil, adhesive (Element 14 catalogue 1211707, AUD2.42 ).
To install you need these parts: two lint-free isopropyl alcohol swabs; and these tools: sharp craft knife, a anti-static wrist strap.
Prepare the heatsink: Swab the base of the heatsink. Wait for it to dry. Remove the firm clear plastic from the thermal foil, taking care not to get fingerprints in the centre of the exposed sticky side. Put the foil on the bench, sticky side up. Plonk the heatsink base onto the sticky side, rolling slightly to avoid air bubbles and then pressing hard. Trim around the edges of the heatsink with the craft knife.
Prepare the Raspberry Pi 3 system-on-chip: Unlug everything from the RPi3, turn off the power, wait a bit, plug the USB power lead back in but don't reapply power (this gives us a ground reference). If the RPi3 is in a case, just remove the lid. Attach wrist strap and clamp to ethernet port surround or some other convenient ground. Swab the largest of the chips on the board, ensuring no lint remains.
Attach heat sink: Remove the plastic protection from the thermal foil, exposing the other sticky side. Do not touch the sticky side. With care place the heatsink squarely and snuggly on the chip. Press down firmly with finger of grounded hand for a few seconds. Don't press too hard: we're just ensuring the glue binds.
Is it worth it?
This little passive heatsink won't stop the RPi3 from throttling under sustained full load, despite this being one of the more effective passive heatsinks on the market. You'll need a fan blowing air across the heatsink to prevent that happening, and you might well need a heatsink on the RAM too.
But the days of CPUs being able to run at full rate continuously are numbered. Throttling the CPU performance under load is common in phones and tablets, and is not rare in laptops.
What the heatsink allows is for a delay to the moment of throttling. So a peaky load can have more chance of not causing throttling. Since we're only talking AUD7.12 in parts a passive heatsink is worth it if you are going to use the RPi3 for serious purposes.
Of course the heatsink is also a more effective radiator. When running cpuburn-a53 the CPU core temperature stabilises at 80C with a CPU clock of 700MHz (out of 1200MHz). It's plain that 80C is the target core temperature for this version of the RPi3's firmware. That's some 400MHz higher than without the heatsink. But if your task needs sustained raw CPU performance then you are much better off with even the cheapest of desktops, let alone a server.