sorry, I hit the 'send' before I was ready. Furthermore, I'm fascinated by all the matters about the atmosphere. Obviously, our existence is much dependent of what happens up there. However, I don't use to accept everything without a little sceptical reflexion, and so is the case even here, namely, the famous ozone layer. All this investigation and writing and fear...are we sure about the role ozone plays, I mean, is the thickness, variation and existence of the ozone layer really that critical? Ozone alone can't be the only catcher of cosmic radiation, can it? How can this tiny layer of O3 molecules retard or even stop the enormous velocity of those space particles? Well, you can see how ignorant I am. But as such, I also am willing to learn.
Thank you for taking you precious time to give answer.
Inge
-----Original Message-----
From: Hernefjord Ingemar
Sent: måndag 2 februari 2009 09:08
To: 'Brian Ellis'; TechNet E-Mail Forum
Subject: RE: [TN] Vapor Phase Soldering
Brian,
Thanks. You are extremly well equipped above the neck, so as to say. Now, I have a comment on...
...can destroy it, except thermal decomposition at temperatures exceeding well over 250° C....If so, why not simply let the vapour evacuation pass a torch flame or an electric heater >250 C? Wouldn't that 'kill' the PFCs?
-----Original Message-----
From: Brian Ellis [mailto:[log in to unmask]]
Sent: fredag 30 januari 2009 12:50
To: TechNet E-Mail Forum; Hernefjord Ingemar
Subject: Re: [TN] Vapor Phase Soldering
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 do they 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 know the 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 squarely at 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? WernerÂ
>>>
>>> ----------------------------------------------
>> -----Â
>>
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