Brian,
Thanks for stating the facts so clearly. It's such a long time since vapour
phase was popular and it was misunderstood then.
The toxic thermal degradation product, by the way, is PFIB,
perfluoroisobutylene. I heard horror stories of this chemical being produced
during the early days of machining PTFE when the perpetrator was smoking as
well. A sliver lands on the lighted end, he inhales and Goodnight Vienna.
Was this an urban myth?
Regards
Eric Dawson
> -----Original Message-----
> From: Brian Ellis [SMTP:[log in to unmask]]
> Sent: Wednesday, October 17, 2001 7:38 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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