Any small particle of dust, even smoke will kill many sectors of a hard drive (mainly because the gap between the head and the plate is less than any of those particles, about 1 micron). Replacing a board is fine, because its on the outside of the disk chamber. Using a clean room might be ok, as long as it is better than a class 10,000 room ( which means there is less than 10,000 ppm of any loose particles). Most companies that do a HD repair service are far cleaner than that, which is why they cost a fortune to repair. I dont do repairs, but I work in a clean room facility,I dont think i would try to attempt it, unless I dont care whats on the drive.
I do take them apart when they are dead though, just because the plates look cool =)