I'm using this brand and type paste flux, and it works very well... except for one small problem. After assembling 5 prototypes, most had bizarre issues, which were resolved when the flux was removed via 91% isopropyl alcohol bath, soak, scrub, and dry.
Now this flux claims to be "no-clean" and "non-conductive", however the behavior was such that it was at least partially conductive. Some of the signal paths are very high impedance (10M ohms) but ironically, most of the problems centered around SOIC-28 and 100+ pin LGA components at less than 5 volts.
What's your experience with no-clean paste flux? Do they become at least slightly conductive after heating?
Answer
While flux may be pristine before it's used, its job is to clean the surfaces of the metals the solder is supposed to join, physically and electrically. So where does all the junk the flux dissolves go? Into the flux, of course, and if there's enough of it and it's the right kind of junk, it could well provide a conductive path through the cooled flux which will be removed when the board is cleaned.
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