I've read several times that solder flux left on a joint, be it wire-to-wire or wire-to-PCB, can eventually cause corrosion. With that in mind I bought a flux-remover pen, but I'm not really happy with the way it works -- it just can't get into all the nooks and crannies on my boards.
Is there a cheaper way of removing the flux? Or is it not necessary with the "multicore" tin/lead solder I'm using? I'm thinking, for instance, is there a fluid in which I can bathe the boards, using a soft paintbrush to wash them, that won't harm the components?
On the other hand, does lacquering the board after assembly prevent corrosion by removing all access to air and moisture?
Not enough information. What solder? What flux? What components?
Water soluble flux can be removed with warm water. Ultrasonic bath is very effective. Secondary rinse in IPA (iso-propyl alcohol), and blow dry.
Rosin based flux can be removed with IPA. Ultrasonic bath is very effective. Secondary rinse in IPA and blow dry.
Some components are ultrasonic sensitive, and some are solvent sensitive. If you have a mix of both, you need to select a solder system that allows you to remove flux (or not, if appropriate), in a manner that won't harm either. Like all engineering, it's a balancing act.
"Multi-Core" solder was originally maufactured by Ersin, and is considered an RMA (Rosin Mildly Activated), solder flux. It is removed with IPA in critical applications, or the flux residue is ignored in commercial (consumer) applications.
Board coatings (lacquering) of any kind (urethane, silicone, acrylic) are a great idea IF:
1) The board is absolutely free of flux and other surface ionic contaminates prior to coating
2) You don't have to repair the board afterwards (although some silicones and urethanes are easier in this regard)
3) Your board doesn't have EXTREMELY LOW leakage current needs. At times air is the only acceptable insulator.
So. If your design has no ultrasonic or IPA sensitive components, and is soldered with RMA flux, scrub it with iso-propyl alcohol and a toothbrush, or IPA in an ultrasonic cleaner. Rinse it with fresh IPA, and then blow it dry gently with clean compressed air. If it's that important, wear clean surgeon's gloves to keep your finger oils and salts off the boards during handling, though now we're talking about guard ring layouts and the like.
Dipping or spraying the clean board with something like Krylon acrylic may help, or may hurt, depending on the board and it's final application. As I say, not enough information. Tell us more.