From: firstname.lastname@example.org (Bob Wilson)
Subject: Re: Washing PCB's after soldering...with what?
Date: Thu, 21 Nov 2002 04:34:18 -0000
Organization: Your Organization
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In article ,
>We're hand-soldering (small series) our PCBs and are
>wondering what would be the best chemical for washing
>the boards afterwards.
>We've tried Acetone and Ethanol, the first is way too strong as
>it melts the caps and other plastic, the second (Ethanol - alcohol)
>leaves the PCB looking very dry and ugly with white powder
>where the soldering paste used to be.
>Does anyone here have experience with this kind of stuff?
The white powder you see is NOT just ugly! It is the corrosive activator
salts! What you have done is remove the harmless resin, and exposed the
corrosive activator salts. Not too clever!
You need to understand something of the mechanism of flux removal. Typical
resin flux is composed of a non-polar resin, with polar salts (so-called
"activators") in dispersion.
The resin is soluable in a nonpolar solvent, but unless you also include a
polar solvent in the mix (or as a second rinse), the result will be worse
than nothing at all. The PCB will APPEAR clean, but all the hygroscopic,
somewhat corrosive salts will remain. Generally these will be
almost invisibble, but will make their presence known by causing leakage
during humid conditions, or even slow corrosion. In your case, they are
readily visible, and in a few months will guarantee your PCB will be a mess.
Acetone is essentially useless as a flux solvent (which is why no commercial
flux removers contsin it). Alcohols are both a polar and a nonpolar solvent,
but their non-polar action is weak, meaning that they are slow to dissolve
the resin (resulting is a sticky surface, and long washing times).
Commercially available flux solvents are a mix of polar and nonpolar
solvents. One very popular one in industry was a 70:30 blend of
trichloroethane and isopropanol. The TCE was an aggressive nonpolar solvent
that rapidly dissolved the resin component, and the isopropanol (or
isoproply alcohol) dissolved the activator salts that were now exposed. TCE
is unavailable now due to its being a chlorinated solvent (ozone depletion).
A more readily available substitute for TCE is ordinary "lacquer thinner"
which is primarily toluene or xylene. Yes, this (as well as TCE) WILL attack
cheap plastics such as polystyrene, but almost all components (except
polystyrene caps, which aren't available anymore, anyway) will not be
significantly attacked. The addition of the 30% isopropanol substantially
reduces the aggressiveness of the lacquer thinner to plasitics.
There is another option, namely to use a saponifier. Kester sells one that
you mix 5% or 10% with hot water. The stuff is excellent. The resin is
saponified and put into water suspension, and the water itself (being the
most polar solvent going) readily dissolves the salts. Some companies get an
old dishwasher, and throw the boards in there with it.
So there ya go. Whatever you do, DO NOT just wash the PCB with various
solvents without understanding the principles of what the hell you are
doing. I have heard some REALLY inane suggestions for cleaning PCBs on this
newsgroup over the years by people who haven't a clue what they are doing,
and were just trying to take a stab in the dark. Examples are to use brake
cleaner (whatever the hell that is!), or brake fluid (brilliant!), or
various other solvents around the house that make little sense. The basic
principles are very simple, once you understand them.