Lately I’ve been assembling a lot of Retro USB boards, and that’s got me thinking about the best way to clean them after assembly. I use a “no clean” flux for my assembly work, so in theory I don’t have to have to clean it off, but if I don’t then it will leave an ugly sticky mess. Unfortunately the black soldermask of Retro USB boards seems to highlight flux residues more than other colors. I’ve also recently started using a gel flux in a syringe instead of a flux pen, and it’s stickier and leaves more residue. So good cleaning is more important now than ever.
In the past, my standard cleaning method was regular cotton swabs (Q-tips) with 99% isopropyl alcohol. It dissolves the flux residue, but the swabs snag easily on the sharp corners of parts, quickly become shredded, and leave tiny cotton fibers everywhere. I’ve now started using Chemtronics cotton tips, which are basically fancy Q-tips designed for industrial use. The cotton is packed and wound more tightly than Q-tips, and they’re “low lint”, whatever that means. It helps reduce shredding and stray cotton fibers, but doesn’t completely eliminate them.
The isopropyl alcohol also leaves a residue that’s visually unattractive, as you can see here. Its visibility depends greatly on the angle of the light, and I’ve intentionally chosen the worst angle for the photo. I’m uncertain if this residue is really from the alcohol, or whether it’s the remnants of the flux dissolved in the alcohol, but whatever it is leaves streaks on the PCB when it dries. I’ve found I need to wait until it dries, then use another dry cotton swab to buff the dried areas and remove the streaks. The end result still isn’t perfect, though it’s pretty good. But the whole cleaning process can be very time consuming, requiring several minutes per board.
It’s my understanding that commercial PCB assemblers wash the finished boards in hot deionized water and some kind of solvent. It’s essentially a special dishwasher for electronics. I don’t have that kind of equipment though, and I wouldn’t be excited about washing PCBs where my kitchen dishware and utensils go. In the past I’ve tried hand-washing finished boards with ordinary hot water and dish soap, and it worked OK but still wasn’t squeaky clean. Washed boards also require a special dryer, or a long period of air-drying to ensure all the water is out before powering the board.
Do you have a favorite method of board cleaning? Leave a note in the comments.