I have a board that has fairly high working voltages up to 100V with small clearances down to 12 mil between the high voltage and ground or low voltage nets. This is unavoidable because the whole board is tiny and in some cases the HV and low voltage are adjacent pads on the same component.
This is currently assembled using a water wash cleaning step.
Is it possible to assemble this using no-clean? What are the possible problems with having flux residue on a high voltage board like this? Is it better to clean (with the possible ionic contamination from the detergents) or not clean (with flux residue)?
Also, could I do conformal coating over flux residue? Some references about this: http://www.circuitinsight.com/programs/51472.html
https://aimsolder.com/technical-articles/conformal-coating-over-no-clean
Note: In this case, signal integrity (as on high speed boards) is not much of a concern; failure modes involving a high voltage to low voltage net short are. 1mA leakage from 100V to the microcontroller on the board would be bad; even 0.1mA would still result in very abnormal operation, although the board would survive. The board may be exposed to high humidity environments, so there may be condensation, although there wouldn't (normally) be direct water immersion.
Best Answer
To the best of my knowledge conformal coating requires very clean surfaces for proper adhesion (see doi: 10.1088/1741-2560/10/3/031002 ). At least in our lab all boards are cleaned using a complex cleaning procedure which has a final "rinsing with deionized water including conductivity monitoring" step (doi: 10.1109/EMBC.2016.7591102 ). For 100V a track clearance of at least 0.2 mm is recommended according to my information so 12 mil = 0.3048 mm should be fine ( http://www.creepage.com/ ). From my experience even the "no clean" (I typically use SMD291 and SMD291AX) can leave some serious amount of residue depending on the used amount which can impact electrical signal quality. At least I observed signal degeneration in HF applications with 0.4 mm BGA ICs. After additional cleaning steps with deflux etc the signals typically normalize.
residuals after using SMD291 and only sloppy mechanical cleaning: