John Larkin said:
My real concern is leaving hygroscopic crud under the parts. If we
bake them dry, they'll pass test, but may well later suck moisture out
of the air and fail in the field.
We've never had any leakage problems with rosin flux. It's not
conductive whether you clean it or not.
John
John,
Here are some posts I collected from Bob Wilson on cleaning flux
from pcb's. His comment on activator salts leaving a white residue
being dynamite is absolutely correct. I ruined a perfectly good dvm
by trying to clean it after it started acting up in high humididy.
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Subject: Re: Flux cleaning ?
Date: 2001-10-09 16:16:48 PST
Have you had any problems with it attacking plastic parts?
Never mind fooling around with stuff whose solvency you don't
understand. Partial or incorrect removal of resin based flux is FAR
worse than doing nothing at all.
Resin fluxes require both polar and nonpolar solvents to remove all
components completely. Using a bit of solvent (even the correct
blend) and a brush, is REALLY bad news. What is mainly does is just
spread the flux around all over the place.
There are 2 main ingredients in resin fluxes. First, there is the
resin itself. The active ingredient in resin is Abetic Acid. Resin
requires a nonpolar sovent to remove it (such as trichloroethane, or
even toluene).
The other ingredient is the activator salts (typically chlorides and
fluorides). These are soluable in a polar solvent ONLY (such as
isopropanol, or even water). They are ABSOLUTELY UNAFFECTED by the
nonpolar solvents that will dissolve the resin component.
Attempting to use Disc Brake cleaner, or any other aggressive
NON-polar solvent will simply remove the protective resin (that
previously and harmlessly encapsulated the hygroscopic and
conductive activator salts), and expose these salts to the air. This
results in high impedance conductive paths all over the PCB, and
possible corrosion.
You cannot see the activator salts, in most cases, but in higher
concentrations they do appear as a slight white residue. This is
dynamite!
So before anyone uses whatever snake oil thay happen to find laying
around, it is wise to understand the chemistry that underlies this
operation.
For years an excellent flux remover was a 30% mixture of isopropyl
alcohol, with the remaining 70% being trichloroethane or freon.
Since both trichloroethane and freon are no longer easily available,
a good substitute is plain ordinary "lacquer thinner" (mainly
toluene). This mixture will remove ALL parts of the flux. Pure 99%
Isopropanol (aka "rubbing alcohol") WILL work as well, since it DOES
dissolve both polar and non polar residues, BUT it is extremely slow
to dissolve the non-polar stuff (resin). The above mixture acts much
faster,and still does not harm most components (polystyrene caps are
the exception, but they dissolve in nearly anything anyway).
The other alternative is to remove the resin based flux with a
proprietry Saponifier in a water solution (Kester makes one). This
requires very strong agitation, and hot water/saponifier solution.
Finally, one can just use water-soluable-flux solder. Some brands so
not have very good "fluxing" action, although I have had good luck
with the stuff made by Alpha Metals. Warm, HIGH PRESSURE water spray
(to get under ICs and so on) is a must to remove this. One good way
is to stick the PCBs in a dishwasher. Personally, I use water
soluable flux wherever possible. It works well and DOES remove
completely. One final point: you must ALWAYS clean water soluable
flux off the PCB. NEVER leave it on there. It is extremely
hygrosopic and will result is a malfunctioning PCB after it has
absorbed atmospheric water (1 to 4 weeks later).
Bpb.
You need a solvent blend that contains both ionic and non-ionic
solvents. A very good blend is 70% trichloroethane and 30%
isopropanol (isopropyl, or "rubbing" alcohol). The trichlorethane is
an aggrressive non-ionic solvent and the isopropanol attacks the
ionic salts.
Although isopropanol is a drugstore item, trichloroethane is not. If
you have a problem with this, xylene or toluene (or even ordinary
lacquer) can be used instead, although these are more aggressive
solvents and some parts may be affected (test first). Increasing the
proportions to 50-50 will reduce the aggressiveness of the blend.
Even straight isopropanol can be used, since it can dissolve bothe
ionic and non-ionic residues. However it is a very feeble non-ionic
solvent, so it will take a long time to attach the resin and other
non-ionic residues.
Be sure to immerse the PCB in a reasonable volume of solvent, and
rinse in a fresh bath. **Use lots!!** DO NOT JUST SWAB IT AROUND
WITH A COTTON SWAB!!! All this will do is to spread the flux all
over the place, and partially dissolve the rosin, exposing the
activator salts that were previously trapped harmlessly. EITHER
CLEAN THE PCB PROPERLY OR NOT AT ALL!
BOb.
Naptha would work well if the crud were purely non-ionic, however I
suggested the Full Monte cleaning because the true makup of the crud
was not known, and also because there will CERTAINLY be some small
amount of flux residue if the motherboard were made in the orient
(which most are).
You are correct that there is no rule that a solvent mix must be
used. Sequential cleaning (as long as the rosin is removed first to
expose the activators) is perfectly acceptable.
I agree with Bob. Most of the flux is no clean today. Why use an
extra processing step (washing) if you don't need to. Especially
when you have some non-washable parts on the board.
Now, I use a turpine based solvent that:
1) Smells great (it is orange peeling oil really)
2) Is predominantly non-toxic and non polluting.
3) Most importantly, does a great job in cleaning the boards.
The only drawback is that it takes a long time (when compared to
freon) to evaporate. If you use compressed air to get the majority
of it off, then it will dry in a few minutes. It does a great job
and if used in liberal quantities and some scrubbing, leaves no
residue. That cannot usually be said about acohols and freon based
solvents.
Chuck
I agree, that clean water wash is good if all the components are
sealed. Another method that does not require careful drying is to
use a turpine (orange/lemon peeling oil) based solvent. It smells
good, cleans good, takes a while to evaporate (I suggest using
compressed air) and leaves very little or no residue. It will eat
styrene but that is the only plastic that I have found that it
doesn't like. BTW never use any type of alcohol on poly carbonate
components. Like the plugs on the ends of phone cords.
Chuck
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Regards,
Mike Monett