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I keep on hearing that no-clean solder paste or flux is bad for analog or digital signals because it makes analog signals "noisy" and promotes cross talk on digital signals.

Like this claim:

Flux residues can add noise on analog-to-digital conversion circuits.

Source: Mike Jones - Expert Comment

or this:

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

Source: answer to "Why do we need to remove flux from circuit boards?" - Spehro Pefhany

I realize that it's probably not good to have anything on the surface of the PCB between traces. I realize that corrosion is bad. But if no-clean residue or RMA flux residue is left on a board, what happens? Most people say, it's bad don't do it.

My question is, through what mechanism does leaving a residue on a PCB affect performance? How does it affect performance Is it a parasitic resistance? Parasitic capacitance?

I could see a promoted parasitic capacitance with a residue and leakage current. How much?

Is there some other mechanism besides promoting cross talk that I'd have to worry about?

Has anyone ran into this problem directly?

Edit:

What is the mechanism for this noise? Can it be traced to some physical process?

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  • \$\begingroup\$ Addressing corrosion, not its electrical properties...Have seen crystalline honey-coloured rosin flux extending onto Cu trace where the Cu underneath is bright, while adjacent Cu trace exposed to environment is oxidized (dull brown). Impressed with flux protecting rather than corroding - seems its room-temperature activity is very low. \$\endgroup\$ – glen_geek Jan 10 '18 at 20:42
  • \$\begingroup\$ I once left water soluble flux on a board, the board failed and had poor conductivity on some through hole parts pins. I believe this was due to corrosion, the board worked fine for half a year and then broke. \$\endgroup\$ – Voltage Spike Jan 10 '18 at 21:20
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I have run into this problem with precision photodetection circuitry. After reworking a board any leftover flux threw off the instrument calibration due to increased surface leakage bypassing guard features on the PCB. This was on the order of 500pA to a few nA depending on how thick the flux layer was. Cleaning the amplifiers very carefully with IPA and a small amount of acetone as a drying agent fixed 'er right up. Generally I do not worry about no-clean flux residues outside of precision amplifiers, potentially out of my own ignorance. I have yet to attribute an observed problem with no-clean flux in non-precision or non-critical circuits. Case by case like most things I suppose.

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