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?
Best Answer
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.