Inge
So many questions! First of all let me be quite clear that PFCs have no
effect on the ozone layer. Their problem is purely one of radiative
forcing, potentially causing climate change. This is because their
absorption spectrum very closely matches the radiation spectrum from the
earth's surface. Of course, this is exacerbated by their high stability
which is why they have an atmospheric residence time of the order of
10,000 years.
When any vapour or gas is released into the atmosphere, there are a
number of effects which can take place. Locally, if the machine is
installed in a closed room without draughts, most of the emitted vapour
which is not bound to the assembly will fall to the floor, because of
its high gaseous density. If there were enough, it could actually form a
layer but anyone moving around would kick it about. However, any
ventilation in the room would tend to extract it, at least over a period
of time. The important point to remember is that one of the principal
mechanisms is molecular diffusion due to thermal agitation, analogous to
Brownian movement in liquids. If the room were left undisturbed for
several days, this would cause the composition of the air to be
homogeneous. Exactly the same happens at the level of the external
atmosphere, of course helped by weather conditions. As a rough guide,
any molecules emitted at a given place would circumnavigate the earth in
a homogeneous band at the latitude of the emission within about 10 to 15
days, at levels under the tropopause. Within one year, they would be
evenly distributed throughout the whole of the hemisphere. Within 15
years they would be evenly distributed throughout the whole of the
atmosphere in both hemispheres, including stratospheric altitudes up to
at least 80 km. This is why CFCs such as you were using to clean printed
circuits 15 to 40 years ago are causing problems in the Antarctic today,
or at least between August and October of each year.
The problem with perfluorinated compounds is that the bond between the
fluorine and carbon atoms is so strong that almost nothing can destroy
it, except thermal decomposition at temperatures exceeding well over
250° C. This does not happen in nature. Chlorine-carbon and
bromine-carbon bonds are weaker and will decompose under the action of
ultraviolet radiation as stratospheric levels, hence the major cause of
ozone-depletion. This does not happen with fluorine-carbon bonds.
Hydrogen-carbon bonds are very weak and are often destroyed in the
troposphere by the action of free hydroxyl radicals. This is why HCFCs
have a lower ozone-depleting potential than CFCs. The more atoms there
are with strong bonds in the molecule, as a general rule, the longer the
residence time. As a perfluorinated compound, by definition, has only
fluorine-carbon bonds, it's residence time is very long. This is a
simplistic explanation of very complex atmospheric chemistry. This is
not just theoretical but has been confirmed by practical tests with many
different compounds. From this, models have been developed which allow
extrapolation. Whether this is valid for thousands of years, of course,
cannot be confirmed and if a given PFC is stated to have a lifetime of ,
say, 10,000 years, this means that it is very likely to be between 5000
years and 20,000 years. We do have reasonable confirmation of the rate
of decay of substances with lifetimes of several centuries. This has
been possible because we can track the decay rate of each of the
substances which are controlled under the Montreal Protocol, as we know
that each one is being phased out with fairly strict reporting in
different countries. These include halons with a lifetime of about 300
years. We have begun tracking HFCs with lifetimes of 500 to 2000 years
and it is confirmed that these seem to conform to the mathematical
models that were developed beforehand.
One thing which I do not know is the actual decomposition mechanism of
the fluorine-carbon covalent bond. I speculate that this may be broken
by ionising radiation at very high altitudes, possibly in the
ionosphere. However, I would have difficulty in declaring this as a
major cause of the reduction of concentration of PFCs in the atmosphere
because I would imagine that it would take millions of years for all the
molecules to have reached those altitudes and come down again! I think
are much more likely explanation would be the one in a trillion chance
of a high energy particle hitting a PFC molecule and knocking the
bejabers out of it.
Over the last decade or so, atmospheric chemists have studied not only
the decomposition mechanisms of halocarbon molecules, but also the
effect that the resultant new molecules may have. One example of this is
with the nPB. This solvent has a fairly short atmospheric residence
time, a matter of two or three weeks, it are being decomposed by
hydrolysis in the atmosphere. If it reached the ozone layer, it would
cause catastrophic ozone-depletion. If it were emitted in the tropics,
it would reach the ozone layer because of the uplift of storms in the
Intertropical Convergence Zone. If it were emitted in mid-latitudes,
very little would reach the ozone layer. However, some of them
bromine-containing decomposition products are more stable than nPB
itself and can reach the ozone layer sufficiently to cause damage. The
only reason that nPB has not been added to the list of controlled
substances in the Montreal Protocol is because the atmospheric
scientists have not been able to determine accurately the
ozone-depleting potential of the substance, as the series of reactions
is quite complex and very much latitude-dependent. Between you and me, I
feel it should be included as a controlled substance simply because the
range of ozone-depleting potential in the areas where it is commonly
used is known to be at least similar to that of HCFCs which are
controlled substances. Such a precautionary principle could be applied
equally because of its suspected neuropathic effects and possibly
reproductive toxicity.
Another problem with vapour-phase soldering is the fact that the vapour
is not dissipated immediately after the process is finished. Obviously,
some may be trapped under components but this could be removed by forced
ventilation. Vapour may also be trapped within components,
non-impregnated wound components being particularly bad. For example, a
ferrite pot with a winding of 0.1 mm wire may retain measurable
quantities of vapour for several days. More insidious, FR-4 laminate,
soldered at temperatures above its glass transition temperature becomes
a virtual sponge with open pores into which PFC molecules can be
trapped. These may also slowly escape, particularly as the temperature
increases in service. The quantities thus released may be small but are
not negligible.
I hope this explanation will be of help to you to understand a little
bit of the factors which govern the use of PFCs.
Brian
Hernefjord Ingemar wrote:
> How does the VPS emitted molecules behave? Do they mix and spread or dothey keep together like clouds? And what are their mass compared to air?To reach the stratosphere, they need be a bit lighter than oxygen. And what time does it take for these molecules to reach the altitude where they build a layer of their own? Do they pass the ozone layer or stay below?And how do we know about the time it takes to get rid of them? Do we calculate only or is this process measured in real tests? And how do we knowthe emissions are not decomposited to something else (caused by ionic irradiation, UV, gamma etc)?
> Inge
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
> Sent: onsdag 28 januari 2009 15:35
> To: [log in to unmask]
> Subject: Re: [TN] Vapor Phase Soldering
>
> Werner
>
> And when the Brussels greens (sprouts?) decide to restrict perfluorocarbons? I think this may happen sooner or later. It would be almost impossible to devise a VPS machine with zero emissions and a practicable throughput. Depending on the components used, it can take days for an assembly to lower its emissions to undetectable levels after soldering.
>
> Brian
>
> Werner engelmaier wrote:
>> Hi Brian,
>> I fully agree with you--on both counts.
>> Unfortunately, you need to lay the blame for the need for VPS squarelyat the feet at the misguided 'greens' in Brussel.
>>
>> Werner
>>
>>
>>
>>
>>
>>
>>
>> -----Original Message-----
>> From: Brian Ellis <[log in to unmask]>
>> To: TechNet E-Mail Forum <[log in to unmask]>; Werner engelmaier
>> <[log in to unmask]>
>> Sent: Wed, 28 Jan 2009 5:56 am
>> Subject: Re: [TN] Vapor Phase Soldering
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> Yes, some do, at least as far as the practical aspects of soldering
>> are concerned. However, very few even cast a thought to the
>> environmental aspects, which are horrendous. The radiative forcing due
>> to halocarbons, mostly fluorinated) has increased from near-zero to an
>> average of 0.34 W/m² in the last 50 years; this is about 21% of the
>> total net anthropogenic climate change effect (IPCC 2007 Fig SPM2).
>> The perfluorinated compounds used in VPS are the most pernicious of
>> these because their long atmospheric residence time (>10,000 years)
>> make them continue to have their effect over many tens of generations
>> to come. Â
>>
>> As an engineer, VPS is the best thing since sliced bread. As an
>> environmentalist, it is the worst thing since Noah was told to build
>> his ark. Â
>>
>> BrianÂ
>> Â
>>
>> Werner engelmaier wrote:Â
>>
>>>
>>>
>>>
>>>
>>>
>>> Hi,
>>> Does any one know of an assembly house that has vapor phase
>>> capability and knows what they are doing?Â
>>> WernerÂ
>>>
>>> ----------------------------------------------
>> -----Â
>>
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