andrewb
Well-known member
@andrewb, so with Kester 63/37 solder, cleaning the flux off is optional?
Technically yes, if it's Kester 44. But it's unsightly, and gets uglier as the board ages, because the flux will turn from yellow to brown.
Because I just like boards to look nice, I usually remove most if not all old flux from boards I restore, many having prior work and flux that is decades old. I have never found a board to be damaged from any of these flux residues.
Also, see the datasheet for Kester 44:

The only time I have seen 'damage' is when I left Kester 331 (water wash/organic) flux for about a year on a Quantum board I was assembling. When I eventually washed it off, the joints and PCB were hazy where the flux had been, like it had etched the surface very lightly. But even then it was very slight, and didn't affect operation of the board at all. It was more cosmetic if anything. But if you'd left it for 40 years, it might be different.


