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How long can flux be left on a PCB?

I am working on one of my PCBs and I have put flux on it to solder. Can I leave it overnight. Is that ok? Or how long can flux be on the board?

update: 715 water solderble. My question is can any type of flux live it over night?

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    \$\begingroup\$ What do the instructions for the flux say? \$\endgroup\$ – Leon Heller Nov 7 '15 at 19:06
  • \$\begingroup\$ Practically speaking, a flux intended for electronic (vs plumbing/mechanical/jewelry) usage is probably mild enough that you can leave it overnight (if not substantially longer), at least on a prototype where you goal is to validate a design, not build a long-life product. Additionally, there fluxes which are designated no-clean, in that they are intended to be left on the circuit permanently. \$\endgroup\$ – Chris Stratton Nov 7 '15 at 19:46
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    \$\begingroup\$ Flux labeled "no-clean" does not imply that you never have to clean it off... see: electronics.stackexchange.com/questions/159492/… \$\endgroup\$ – rdtsc Nov 19 '15 at 0:31
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Water-soluble Flux such as Kester AZ2331 MUST be removed from the PCB as quickly as possible after soldering.

The manufacturer of your Flux will have their recommendations in the data sheet.

Our experience is that Kester Water-soluble flux left on the PCB for more than several hours results in the beginning of corrosion.

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