Steve
1. To the best of current scientific knowledge, fluorocarbons are not
believed to cause any ozone depletion.
2. By definition, these fluids are organic and they are volatile,
therefore they are VOCs and accepted as such under European legislation,
if their vapour pressure exceeds a threshold. However, they do not
photochemically react with NOx to form tropospheric ozone, which is what
VOCs are really about. They are therefore exempted from VOC regulation
in the USA.
3. Whether you consider a perfluoroether is a PFC or not is semantic.
Perfluoro- is a prefix meaning the all carbon-hydrogen bonds in a
molecule have been replaced by carbon-fluorine bonds. -carbon means that
the central atom or atoms in the molecule are carbon. If you consider
that methyl ether is a hydrocarbon, then these are perfluorocarbons. But
ether is often described as an oxygenated HC, so these could be
described as oxygenated PFCs...
4. Let me give you a little background knowledge about fluorocarbons. An
F-C bond requires a lot of energy to break, much more than, say, H-C,
Cl-C or Br-C. This is why fluorocarbons are so stable. PTFE, for
example, is a linear PFC polymer which does not break down at
temperatures up to over 250°C, whereas a linear HC polymer, such as PE
starts to melt at under 70°C and to decompose at 150°C. The UV
photolytic energy at ozone layer altitudes is insufficient to knock away
the F atom, like it does Cl and Br atoms, the main cause of ozone layer
depletion. This is why an FC is not an ozone depleter. A CFC is, because
the chlorine atoms are knocked off by the UV radiation. It is also the
strong FC bond which renders PFCs practically non-toxic: there is no
metabolic breakdown in the body (this is why it can be used as a
short-term blood substitute, combined with a characteristic of strongly
absorbing oxygen. I believe that it is not used in normal open-heart
surgery but only in cases where a total blood change is required, e.g.,
in cases of a genetic alteration of blood cells.). It is also why the
atmospheric residence times are so long, because FCs are chemically
inert: there is neither hydrolytic nor photolytic decomposition in the
atmosphere. In truth, no one has a clue what the breakdown mechanisms
are likely to be. It is surmised that breakdown may occur when the FC
bonded molecules diffuse to the ionosphere, where bombardment from
ionising radiation will be sufficient to knock the fluorine atoms for
six. What the atmospheric chemistry will be, then, is anybody's guess.
It is conceivable that some free F atoms may diffuse back down to the
ozone layer, where they could cause some minor ozone depletion but worse
still may happen at higher altitudes. The use of such long-lived
molecules (including also SF6) is therefore playing the sorcerer's
apprentice. We may be starting something which we'll never be able to
stop.
5. This is why I stated the last sentence of my previous message that
PFCs should be avoided wherever possible and, if not possible, emissions
should be minimised. They do have a saving grace in this respect:
they're so darned expensive, no one can afford to be too profligate in
their use.
6. The Ausimont statement is more or less correct as far as it goes, but
it does not go far enough. It's as if the tobacco companies published a
statement that smoking cigarettes will not cause gout. It's true, but
they don't make any statement that it may cause cancer. If enough PFCs
and HFCs (or their various derivatives) were emitted, it could cause
enough global warming to fry us all up and make our planet look like
Venus. A Dutch government paper, published about 10 or 15 years ago,
warned that HFCs used to replace CFCs could contribute to 30% (I think)
of global warming gases sometime in this decade, if appropriate measures
were not taken to prevent the problem happening. As it is, I think that
the PFC loading in the atmosphere contributes to only 2 - 3% of the
man-made increase of global warming gases (but that quantity will hardly
diminish 50 centuries from now).
7. For the anecdote. Do you know the greatest source of PFCs in the
atmosphere? It comes from aluminium smelting. Bauxite and fluorite are
added to carbon crucibles and electrolysed. Under some conditions, the
carbon and the fluorite may react to form perfluoromethane or carbon
tetrafluoride, an extremely stable gas. Fortunately, this problem is now
recognised and it happens mainly under inefficient smelting conditions,
so smelters now take measures to minimise the production of CTF,
although it still occurs. Probably half of the atmospheric PFC loading
comes from this source.
8. I academically like the vapour phase soldering process because it is
asymptotic and inert. I do not recommend it, though.
Brian
"Marsico, James" wrote:
>
> Having used the vapor phase process in the past (1983 - 1995), I can only
> say that nothing is more convenient than forced air convection reflow.
> True, V.P. was a forgiving process... nothing generally went wrong, except
> the equipment. Let's not forget the cost of the Flourenert (spelling?). I
> didn't think anyone was still marketing V.P. equipment.
>
> 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: Craig Hillman [SMTP:[log in to unmask]]
> Sent: Wednesday, June 27, 2001 10:28 AM
> To: [log in to unmask]
> Subject: [TN] How do you reflow?
>
> Werner brings up an interesting point. Does anyone have an opinion
> on the
> trends in solder reflow in the contract assembly industry? Are we
> seeing a
> resurgence in vapor phase? Has flowing nitrogen during infrared
> fallen to
> the wayside? Interested in seeing your responses.
>
> Best Regards,
> Craig
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Werner Engelmaier
> Sent: Thursday, May 31, 2001 1:19 PM
> To: [log in to unmask]
> Subject: Re: [TN] AW: [TN] Pad in Via for BGA's - Part II
>
> Hi Rush,
> Vapor phase soldering was and is thew most elegant engineering
> solution to
> soldering--no over-temperatures, no profiles, absolute temperature
> uniformity. What killed this approach in the past was inadequate
> equipment--thhis is no loger a problem.
>
> Werner Engelmaier
>
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