Flux is magic, but you’ve got to clean it up. Recently I was working on a instrumentation amplifier circuit and it just wasn’t working. Check the PCB. Check the schematic. Look for solder bridges. What’s going on? I was getting crazy results. Desolder the op-amp and put it on a little breakout board (SOIC) and plug it into a copy of the same circuit on a breadboard and it works fine. Try two different chips and it works fine! Put the chip back onto the PCB and it doesn’t work!
Anyhow, this was a protptype PCB with no soldermask and as a result, there was a lot of trace area in contact with the flux. I was using Kester 2331-ZX flux and it’s water soluble and VERY conductive. The resistance between two freshly fluxed pads on the breakout board below is about 100 KOhm. Without a solder mask, the resistance between adjacent pads can get really low. That’s just unacceptable for a precision circuit. Cleaning the board with isopropyl alcohol solved my problems and turned a non-functional heap of junk into a working instrument.
Finally, flux can be corrosive. The Kester 2331-ZX water soluble flux is great stuff, but besides being conductive, it’s also pretty corrosive. Check this out: I fluxed a few pads on this breakout board and let it sit for about a month. Now they are corroded and green.
Water soluble flux is nice, but you gotta clean your board! Some of the “no-clean” fluxes may not be as corrosive or conductive, but I’ve learned my lesson and clean every board.
Clean your boards!