Bob
This is the insoluble problem! Flux wicking under insulation, especially
with stranded wires, is a definite risk. One way I handled this was to
cut the wirw a couple of cm longer than I needed but to strip it to the
right length. I mixed some basic fuschin (an intense red dye) into a
thinnish RMA liquid flux (the dilution you can find by trial and error).
I then dipped the tip of the wire into the flux and then into a solder
fountain, slowly advancing the wire until the red flux residues advanced
to about 0.5 mm before the insulation started. I then cut the excess
tinned wire off, formed it around the pretinned post and fused the two
together with a soldering iron and a tiny bit of solid wire, where
necessary. Sufficient flux remained to ensure a reliable joint and
sectioning revealed no flux wicking under the insulation. No cleaning
was done, or necessary, after this operation because the minute amount
of flux was restricted to the joint where the temperatures involved in
the tinning + fusing operations ensured volatilisation of all the halide
activators and no flux reached the PCB. The dye ensured you could see
what was happening. No failures were reported after umpteen years of
service in tropical climates. Laborious but effective.
Do NOT do local cleaning. All you will do is spread potentially harmful
residues over a larger area where they will cause more harm than good.
Just because they can no longer be seen does not mean they are harmless.
Much better to keep a minimal quantity of relatively harmless flux
residues in a confined space where they can do no harm. Furthermore, if
you try to clean, the flux-contaminated solvent, be it water or organic,
will, most certainly, wick up the insulation, when you will be back to
square one. Catch 21.5!
Hope this helps
Brian
bbarr wrote:
>
> Since OA flux residues are corrosive if not removed, how does one handle
> soldering rework wires to boards? Won't the flux wick under the insulation
> and cause a problem? Assume I do not have the option of using a no clean
> flux. Is my only choice an RMA chemistry with local cleaning (which is an
> option I have available if needed)? Any other ideas?
>
> Thanks.
>
> Bob
>
> Robert Barr
> Manufacturing Engineering
> Formation, Inc.
>
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