Brian is correct when he says that perfluoroethers (and other
perfluorocarbons) are extremely inert (the Russians used them as
artificial blood for Mujahadin casualties way back when) and have a
fairly high global warming potential.
Vapour phase is still around, but has undergone something of a
renaissance in recent times. An alternative approach, known as
condensation soldering has been devised to address the environmental
concerns and issues such as tombstoning/popcorning, which plagued some
of the early processes.
Essentially, condensation soldering is performed in a sealed chamber.
The unit(s) to be soldered enter the chamber. A partial vacuum is drawn
through the chamber to ensure the unit is sealed effectively and to
remove any potential contaminants. The heat transfer fluid is then
pumped onto the hot-plate in the chamber.
The fluid then enters the vapour phase and condenses onto the unit(s)
thus effecting heating. The amount of material pumped onto the hotplate
controls the rate of heating, and by using discrete HTF pumping stages,
just about any profile can be created and controlled very tightly.
When the alloy has reached liquidus, the chamber is then evacuated,
again under partial vacuum to remove all the valuable heat transfer
fluid from the chamber. This also has the effect of solidifying the
alloy. When the chamber is emptied of HTF, the chamber door can then be
opened and the unit exits via a cooling fan to enable handling.
Condensation soldering is very efficient and tombstoning and popcorning
are things of the past. Issues such as the large losses of HTF and
Health and Safety issues associated with vapour phase soldering have
been eliminated. In addition, with lead-free processes in particular,
the temperature delta across the board is generally less than 5ºC, which
can be of great significance.
I hope of some interest.
Phil Kinner
Chief Chemist
Concoat Ltd
2C Albany Park, Frimley Road,
Camberley, Surrey, GU16 7PH
Tel: +44 (0) 1276 691100
Fax: +44 (0) 1276 691227
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: 17 October 2001 07:38
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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