Whatever you do, dont make the same mistake I did! I got a "Kester 2331-ZX" , an "organic acid water-soluble flux that is pH neutral " . doesnt sound so bad, i used this on my circuit board then washed it off with some water. The next day my circuit board was corded to hell, actual traces were corroded away. The stuff is also super conductive and shorted out everything. I was actually using one of those diy copper toner transfer boards, but still I wouldnt want to use this stuff on a professionally manufactured board either.
I tried the kester 951 pen and it worked great. The flux was not as effective as i would have liked but it left everything amazingly neat afterwards without any residue!
I also tried the kester 186 Flux Pen and that worked a little better than the 951. The only disadvantage is that it left coating of used flux on everything. The coating had no negative effects whatsoever, it was just kind ugly lookin. This is advertised as a "no clean" flux also so you can be assured the leftover flux is non conductive and non corrosive .