You can always buy a heat sink for your Raspberry Pi, but what fun is that? Here's how to recycle a heat sink from a desktop PC, creating a very large passive cooling solution!
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Step 1: Cut Heat Sink to Size
Pretty easy to find, but take apart an old PC to see what kind of heat sink is available. Take it off, then cut as needed with a saw, mill, etc. You may want to clean up the edges with a bandsaw etc.
Step 2: Thermal Tape and Insulate
Mark where you want the heat sink to rest on your Pi's processor with a marker, then apply thermal tape to that area on the heat sink as shown. I used this kind [Amazon affiliate], though others should work as well.
As the heat sink is likely conductive, if there is any overhang, I recommend coating it with electrical tape in order to prevent any shorts as seen in the second image.
Step 3: Apply
Remove covering from thermal tape and firmly apply heat sink to processor.
Step 4: Test!
You may want to take a baseline reading before applying the heat sink. Log on via PUTTY or other terminal program and input vcgencmd measure_temp to get the temperature in °C.
As shown here, it's running at nearly 59 degrees or ~137°F. Seems a bit hot for just sitting there. After application of the heat sink it was significantly cooler. Sketch illustrates just how much more cooling surface area is available with this new heat sink ~130X!
So far it seems to be performing well on my Raspberry Pi NAS setup without any sort of fan. Either way, I think it looks cool, and fun to reuse something that would otherwise end up in the trash!