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My manager at work told me that the liquid flux we use when reworking SMD circuit boards is electrically conductive,and that is why he INSISTS that we clean every board we work on.Is he right,or is he pulling my leg?

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    \$\begingroup\$ Fun fact: some cheap jfet based microphone amplifier circuits rely on flux leftovers to create gigaohmish bias resistors. \$\endgroup\$ – PlasmaHH May 17 '15 at 20:25
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If your boards contain high-impedance analog circuits, then the conductivity of the flux is a real concern. Leakage current through flux is a common source of error in high-gain high-impedance analog circuits.

For other types of circuits, reliability is a bigger concern. Fluxes are reactive chemicals, and if left on the board they can cause corrosion and lead to circuit failures in the field. There are "no clean" fluxes that are meant to minimize this issue, but even these might not be appropriate for high-value circuits with high reliability requirements.

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It depends on the flux. RMA/RA fluxes are pretty benign, though some recommend cleaning. Water soluble fluxes should be cleaned. No-clean fluxes should be thrown into the garbage (or perhaps disposed of in an environmentally responsible manner).

My favorite SnPb solder flux is Kester 44 (RA flux), and the datasheet says:

Cleaning: Kester 44 possesses excellent fluxing ability, the flux residue is non-corrosive and non-conductive under normal conditions of use. When exposed to an elevated temperature and humidity environment (38°C, 94% RH) for 72 hours, there is no evidence of corrosion caused by the flux residue. Throughout its many years of wide usage, 44 Rosin Flux has produced many billions of soldered connections. In all these billions of solder joints, involving the most delicate and critical of electrical and electronic components, there has never been an authentic instance of corrosion by the flux residue under normal conditions of use. This mild property of the residue permits leaving the flux on the assembly for many applications.

I think cleaning is more important for inspection and cosmetic reasons with Kester 44, but if a part or two is added by hand soldering after the cleaning operation (perhaps parts that cannot be washed) I don't think there is any reason to remove the flux for most applications.

Sometimes it's easier to have a rule (such as always wear an ESD strap, even when handling resistors) rather than worry about people making the wrong decision.

'No-clean' fluxes have been found to be electrically conductive and are not easy to clean (requiring strong solvents and scrubbing).

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In my experience, he is correct. I've had some circuit boards behave erratically until I cleaned off the Flux.

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