Beats me. Tastes good, though.

I think I read it will become corrosive if left on the board.

Depends on the type of solder you're using, and what the flux contains.

Some flux is corrosive, and over time, will do damage.

Some flux is sticky, and can attract moisture and create shorts between 2 adjacent traces.

I've worked on 20 year old boards that have had flux on them for who knows how long - and it hasn't seemed to cause any issues.

To me, it's like making dinner - when you're done, you wipe the stove top off, you don't leave the little bit of spaghetti sauce that fell out of the pan there. Just part of cleaning up after the job.

i believe its corrosive over time.

Its also unsightly. You don't have to clean up excess paint on your floorboards when you paint your house, but i would say you should

They make no-clean flux if you have a need for it.

Flux can be hygroscopic (attracting water), can arc at high voltage, can continue to "clean" (corrode), and just looks bad.

Clean up your work, and work like a pro.

If it causes no other problems, one good reason to clean it off is so that if/when you need to re-solder that joint the flux gunk won't prevent the bond from occurring. You want clean, bare metal to metal contact. Flux residue gets in the way of that.

So no one else eats it around here? YUMMY!

Every now and again i melt a solder joint that has flux that smells like christmas.

LOL, it's this stuff:

http://www.parts-express.com/pe/showdetl.cfm?partnumber=341-222

I dont know if this is right, but Ive been doing this for a long time and never had any issues.

I usually end up cutting wires on coils, etc, instead of de-soldering and re-soldering. Gives me a new, clean wire that I know I stripped myself. Could just be my OCD kicking in there. I then strip the wires, dip them in my flux up to the rubber coating, and then touch my hot soldering iron to each wire to burn it off, which cleans off any crap from the wires. I then dip my iron in the actual flux, and then wipe it on the sponge after it sizzles for a second or two. I then tin both my wires with a little solder, and then commence the soldering to the coil, or whatever.

Board soldering is was different. I dont use too much flux expect on my iron, and if I see some nasty crap at the point I am trying to solder. Other then that, pretty much just solder and test the leads.

Might be wrong, not sure, like I said, works great for me.

Lionel trains?

Generally, the flux residue will start to turn brown after a couple years, and can be corrosive to a degree... depending on what kind of flux it was. More modern stuff is much less of an issue than it used to be, and many of the kester liquid fluxes say that it can safely be left on a board. Though with RA and RMA fluxes, I'd still remove them anyway, just in case. I use Kester 245 solder for all my repair work, which has a no-clean flux core, so that I don't have to worry about it. Though I do use a lot of liquid flux for de-soldering, particularly if the old solder looks like it's heavily oxidized.

Some of the organic water-soluble fluxes can be highly corrosive if not cleaned off, such as Kester 331. Always best to read the datasheets to confirm that kind of stuff.

-Hans