Why do circuit boards have to be assembled in a germ-free environment?

Answer 1

Circuit boards do not have to be assembled in a germ free environment.

You may be thinking of the manufacture of integrated circuits, which must be processed in a "clean room". In general "clean rooms" have only had to be dust free, not germ free. However as feature size keeps reducing on integrated circuits eventually the feature size will get smaller than the size of germs, at which point "clean rooms" will need to be germ free.

I once heard a story about a special type of vacuum tube that a company was manufacturing many years ago. After the old employee that was in charge of that production line retired. suddenly they could not make these tubes work anymore and the line shutdown. So they brought the employee back (figuring something was not properly documented in the procedure) and had him show them what he did to make one of these tubes and that tube worked perfectly. For a while they were baffled as he had done nothing different than what the procedure said, until they realized he chewed tobacco and always had residue on his hands. Examining the tubes he made and the tubes others made they found tobacco mosaic virus inside the tubes he made. When they started adding a tiny amount of tobacco mosaic virus to each tube before sealing it up, every tube they made worked perfectly from then on!

There has been some speculation that someday genetically engineered viruses will be used to "grow" the features on integrated circuits when they get too small for current methods to work anymore.