Been assembling a lot of USB PCBs lately and it got me thinking about the best way to clean them after assembly. "Dirty" flux was used in the assembly work,I don't have to clean it off, but if I don't it leaves an ugly sticky mess. Unfortunately the black soldermask on the vintage USB boards seems to accentuate the flux residue more than the other colors. I've also recently started using a gel in a syringe instead of a flux pen; it's more viscous and leaves more residue. So a good clean is more important than ever.
In the past, my standard cleaning method was a plain cotton swab with 99% isopropyl alcohol. It can dissolve flux residue, but this swab is prone to tripping over sharp corners of parts; crumbles quickly, and leaves tiny cotton fibers everywhere. Sponge swabs and cloth-tipped swabs are used later, and these swabs are basically designed for industrial use. Cotton is packed and wound tighter than regular cotton swabs anyway, and they are "low-lint" while exhibiting superior abrasion resistance and cleanability for repeated wipes. It helps reduce shreds and stray cotton fibers but does not completely eliminate them,
As you can see here, isopropyl alcohol can also leave behind an unattractive looking residue. Its visibility depends a lot on the angle of the light, I deliberately chose the worst angle for the photo. I'm not sure if this residue is actually from the alcohol, or if it's a residue from dissolving in the alcohol, but whatever is leaving streaks on the PCB when it dries. I find that I need to wait until it dries before using another thousand cotton swabs to wipe the dry spots and get rid of the streaks. The end result is still not perfect although it is very good. But the entire cleaning process can be time-consuming, taking several minutes per board.
