Great post.
A fellow employee at Hughes Aircraft, albeit a different division and someone I
didn't know, developed the water soluble flux ("lemon juice") that became a
standard. Hughes held the patent but sold the manufacturing rights so that
anyone could purchase the flux.
On 9/11/2015 12:23 AM, Fred Townsend wrote:
Was KSYN3A mod problem
David, you polar comment doesn't make sense to me so let's talk about the
real issue: Cleaning PCBs.
Why do we do it? Besides esthetics there are two main reasons. 1} To get rid
of flux which may be reactive with RF and reflow where it isn't wanted...
like into contacts, mechanical devices, and shaft encoders. 2} To get rid of
ionic compounds which may lead to circuit leakage and insulation breakdown.
For many years one of the better ways to clean boards involved Freon vapor
degreasers. They used a compound known as Freon TE which contained Freon and
ethanol. Then, as Wes mentioned, the EPA banded Freon. Without Freon, which
acted like the fire extinguisher Halon, ethanol became explosive so
degreasers were banded.
There are many types of fluxes. Most of the industry switched its machine
soldering to water soluble fluxes so only soap and water were needed for
board cleanup. Water was in some ways superior to ethanol because ethanol is
a non-polar solvent meaning it does not capture ions, the types of atoms
that support electron flow (see 2} above). However the most popular cored
solders used in hand soldering are still rosin flux based so alcohol is
still very viable for hand spot cleaning of PCBs. Ethanol and isopropanol
are both good flux cleaners but perhaps are not as good as some other
solvents as degreasers and of course they do not remove ions as water does.
It should be mentioned that both pure ethanol and isopropanol are flammable
meaning they burn with a dull blue flame. They don't burn like gasoline but
many gasolines do contain ethanol.
I have heard it said here to use only pure alcohol. That's fine if you are
drinking the stuff otherwise the reasoning for this proclamation is somewhat
nebulous. If you are letting alcohol or any other solvent evaporate on the
PCB you are NOT REMOVING FLUX. You are just moving the flux around. After
absorbing the flux in alcohol you need to remove it wet (i.e. as a liquid).
There are a number of ways for doing this but perhaps the easiest is to blot
with a paper towel. The alcohol with flux will be pulled in and trapped in
the paper fibers. I like compressed air too. Now back to purity issues.
Using a 91/09 or 70/30 isopropanol, commonly referred to as rubbing alcohol,
adds water to the process. The water slows the evaporation rate, collects
ions, and makes it difficult or impossible to burn. However 50/50 dilutes
the alcohol so much that is no longer effective so it is not recommended. So
a little water is a good thing; too much you drown. Don't forget your board
has seen lots of water during the manufacturing cycle. Don't be afraid of a
little more water but of course don't operate anything without thoroughly
drying.
73,
Fred, AE6QL
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