Is a fan really needed? We have run tons of FPGA in our research lab and never put a fan. I suspect a simple heatsink might be enough, if needed at all.
I understand that if the case has limited airflow due to small venting slots, then the fan might become necessary. But only when a case is used.
I 3D printed the NegSol case v5.2 from thingiverse, and I think the slots are very small... not much airflow will be allowed. I am not using the case yet. I might use it in the future. Due to the small venting slots, a fan might be necessary then.
Here's my 3d printed case:
So far, I have been enjoying my de10-nano with Mister without a case, fan or heatsink for over a week now, mostly playing with the Minimig core. I had no problems from any overheating. The FPGA gets warm but not hot. I am planning to put a heatsink on the FPGA soon. I'd rather avoid having a fan because I hate that fan noise. Part of the enjoyment I have is the silence of the whole thing, compared to my PC and RPi which both sound like rattling jet engines when the fans are spun up.
I think the bare circuit board with the blinking lights, see through plexiglass top, and brass feet that came with the official DE10 nano kit looks cooler than any case.
EDIT: Ok I managed to put my Mister in the case. It wasn't easy and required a bit of force. I also added a heatsink to the FPGA, and ordered one of those ultraquiet fans and a fan pcb from http://www.misteraddons.com
Here's the pic:
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