Do I really need to get rid of flux?

[xorophone]

How important is it to clean flux off of finished circuit boards? I've heard people say it can cause corrosion, which in the long run can lead to shorts. Some people even claim the flux itself is conductive, but I doubt that's true. I always spray my boards with one of those laquer sprays that are supposed to prevent corrosion, so I guess the flux underneath can't cause any problems. Is that true? On my latest builds I've skipped getting rid of the flux and just sprayed on top of it. Hopefully it won't cause any problems.

Thank you!

[EBK]

I nearly never get rid of excess flux.  So, if that's bad, we'll both find out eventually.  I'll find out sooner, perhaps, because I don't spray them with anything afterward either.  Maybe if all my pedals are prone to corrosion after years of service, it will make that one that somehow stands the test of time worth a fortune.   

[LiLFX]

Firstly, it depends on the chemistry of the flux. If it's not a "no clean" flux then you should clean it. However, even if it is a "no clean" flux IPC says it depends on the solder process and design. More specifically, imagine a high density design (lots of tiny fine pitch parts packed tightly). Contaminants in the flux that got there in the assembly process can lead to reliability concerns. A thru-hole PCB for a stompbox is probably going to be fine. A cell phone? A radio that is in a satellite being shot into space? Probably want to clean it.

[amptramp]

It was our standard practice in military and spacecraft electronics to clean off the flux with trichloroethane and stiff brush about a half inch wide.  The idea was that flux could be embedded with contaminants and flux made it difficult to inspect solder joints.  For commercial work that is not SMD, this should not be necessary.

[Mark Hammer]

It's not critical to remove it, but it sure helps when the circuit doesn't work and you need to firgue out if there is a short or crack somewhere.