If you have a Water Soluble Flux Pen like the one shown here. Please throw it away immediately.

This type of flux flows under SMD chips and starts to eat away at vias and tracks.

What is really bad about this, is that it will work fine for a few months and then the vias will go open circuit.

The flux sticks around that long after rinsing it off? That's a bit scary! :/

Yep. Every thing looks nice and clean but it stays under the chips.

So far this Pen has cost me a FTDI chip, j722t module and damaged a coil.

When you use that water soluble flux, aren't you supposed to literally submerge and wash the board in hot water, rinse and dry? Did you literally throw it in the sink? Or just splash water over the surface?

Same goes for getting rid of normal flux, cept with solvents, not water. You still have to be liberal and drown the board/parts thoroughly in solvent to get them clean.

Sucks that you got bitten, but all fluxes are corrosive by their very nature, the trick is to get rid of them properly with thorough washing down in appropriate fluids.

Fred.