Brian,
I'm getting conflicting comments from someone from 3M who is claiming that
Fluorinert will decompose at temperatures of 200C and above and this is not
a good thing in the presence of water (usually available as moisture in the
air). Any comments?
Jim Marsico
Senior Engineer
Production Engineering
EDO Electronics Systems Group
[log in to unmask] <mailto:[log in to unmask]>
631-595-5879
-----Original Message-----
From: Brian Ellis [SMTP:[log in to unmask]]
Sent: Wednesday, October 17, 2001 2:38 AM
To: [log in to unmask]
Subject: Re: [TN] Vapor Phase Soldering material
Back in the days? Vapour-phase soldering is still done! The great
advantages of perfluorocarbons over other halocarbons are that a)
they
are chemically extremely stable because the C-F covalent bond
requires
much energy to break them; b) they are virtually non-toxic (for the
same
reason: they do not break down in the body). In fact they can be
used to
replace blood in the body for short periods as they do allow oxygen
to
be dissolved and some "sportsmen" have even injected PFCs into the
bloodstream to improve their endurance performance by a higher
oxygenation; c) they are very inert chemically; d) they withstand
high
temperatures; e) they are NOT ozone-depleting. Teflon is a solid
PFC.
The disadvantages are a) that they are very expensive; b) they are
EXTREMELY global-warming (typically 1 kg of PFC is equivalent to 10
tonnes of carbon dioxide which is roughly equivalent to half the
emissions of a medium car during its lifetime); c) their stability
(~10,000 years atmospheric residence time) is such that end-of-life
disposal is very difficult and costly.
The fluids, be they pure PFCs or PFEs, used for soldering are
sufficiently stable at 210 - 260°C that there is little significant
decomposition. Water hardly enters into the equation because it is
boiled off instantaneously, even if there be some condensation on
the
cooling coils. Theoretically, there may be some hydrogen fluoride
generated (no hydrofluoric acid), but I believe that the quantities
would be really negligible, in practice. With the presence of other
organics from the flux residues, I would personally be more
concerned
with trifluoroacetic acid, another nasty, but I have not heard of
any
cases. Carbonyl fluoride is also not to be excluded, also very
toxic.
There is another fluoro-compound, whose name escapes me for the
moment,
which did cause some concern in the early days, which is toxic in
the
ppb range. However, in practice, I have never heard of any of these
toxic substances being present in sufficient quantity to be of
practical
concern.
The real crunch comes when there is a secondary blanket, used in the
early days to reduce losses of the primary fluid. This was initially
always a pure CFC-113 (e.g., Freon TF, Arklone P etc.). Later,
because
of the ozone depletion potential of CFC-113 being as high as 0.8, a
lower BP PFC was also proposed. CFC-113 does decompose at the vapour
temperature of the primary fluid, so that the interface zone between
the
two vapours was always a hotbed of chemical reactions. This
certainly
produced, especially in the presence of water, whose BP was higher
than
that of the secondary fluid, a whole panoply of acids and other
toxic
substances. For this reason, these machines had both a molecular
sieve
to de-water the condensed vapours, but also a filter to remove the
acids, otherwise the stainless steel tank corroded into holes in a
matter of months. It is possibly this that you are thinking of. The
most
important decomposition product was hydrogen chloride, which becomes
hydrochloric acid in the presence of water, but there was also
phosgene,
hydrogen fluoride, trichloroacetic acid and goodness knows what
else. I
feel reasonably certain (abstraction of any environmental effects),
if
vapour phase soldering with a CFC-113 secondary blanket were
invented
for the first time today, it would never be allowed into a workshop
because of the H&S aspects.
Hope this makes it clear.
Brian
"Marsico, James" wrote:
>
> Back in the days of vapor phase soldering, I seem to recall that
> hydrofluoric acid was a bi-product of the fluorinert mixed with
water, or
> was it the Freon mixed with the fluorinert? Could anyone
elaborate?
>
> Jim Marsico
> Senior Engineer
> Production Engineering
> EDO Electronics Systems Group
> [log in to unmask] <mailto:[log in to unmask]>
> 631-595-5879
>
>
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