1958 film about NASA Soldering Techniques which has some fascinating details about how things were done during the Space Race, and presumably, continue to be done. The overall message about cleanliness couldn’t really be any clearer if they tried — it’s so critical it looks like those chaps in the film spend far more time brushing and cleaning than actually wielding those super clean soldering irons.

https://hackaday.com/2022/04/13/nasa-hardware-techniques-soldering-space-electronics-like-its-1958/