Jim,
This surprises me and I'd like to know more. In practice, in VP
soldering, I doubt whether there is any great significance. Firstly, the
bulk of the vapour is anaerobic, with just the top interface in contact
with the air. This air will be heated and therefore the RH, even in
humid climates, will become extremely small. Then PFCs are demonstrably
resistant to hydrolysis in the presence of free H and OH radicals (this
is why they have atmospheric residence times of thousands of years):
there is simply not enough energy available to break the F-C bonds.
Naturally, Mr Arrhenius will say that, at 200°C, the energy will be much
higher so there will be more reactivity, but it would require temps
exceeding 300°C for it to become significant. Notwithstanding, I believe
that we are talking about 1 molecule in 10^n where n is a very large
integer that will decompose at soldering temperature under practical
conditions in a given time, unless there is something new that I haven't
heard about (always possible, of course).
I just looked at Fluorinert data sheets on the 3M web site (requires
some searching!) and there is no mention of a hydrolytic reaction in
anything I could find.
Brian
"Marsico, James" wrote:
>
> 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 rnnge. 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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