Jason

Just goes to show how opinions can vary!

VP reflow is ideal in the sense that it is asymptotic, i.e. it is physically impossible to exceed a temperature fixed by the fluid's BP at the reigning atmospheric pressure (yes, the temperature is not constant but will vary +/- a degree or two, but this is not important). It is also totally anaerobic, so that the flux in the paste is used to best advantage.
The method is ideal for reflowing complex flex circuits where it is not possible to avoid shadowing of some parts by other parts in IR processes. The criterion of good use is that the parts must remain in the vapour phase until all condensation has ceased, i.e. they must reach *throughout their mass* the quasi-asymptotic temperature. Only in this way can one
be sure of not withdrawing some PFC liquid with the assemblies. As it is, you will withdraw some vapour trapped under components, but this may be minimised by instituting a 2 minute dwell period in the freeboard zone of the machine i.e. above the vapour level  but below the lip. This will allow much of it to slowly roll out from under the components back into
the machine.

Depending on the machine design adequate preheating and postheating/cooling cycles are entirely feasible.

Subsequent flux removal is usually extremely easy, compared to other reflow methods.

The negative sides already mentioned are the cost of the fluid. Very few of the existing machines pay more than lip attention to reducing emissions of the fluid. At a price, it would be possible to design a machine where the losses could be kept down to very low levels, say 1/100th of those from existing machines.

However, the ***BIG*** crunch has not been mentioned by my illustrious colleagues. The PFCs are amongst the worst "global warming" products known to man. Climate Change is currently occupying atmospheric scientists around the world, as is well known. The measure of a substance to provoke it is known as the Global Warming Potential (GWP) which is an index
relative to the effect of carbon dioxide, the best-known culprit. The GWP of these PFCs and derivatives with BPs in the range of 215°-265°C is typically about 10 000, yes, ten thousand. That means that 1 kg of Fluorinert or Galden or whatever has the same atmospheric effect, over a period of time, as 10 tonnes of CO2 or, say, 15-20 tonnes of liquid fuels. As
such, they are specifically mentioned in the Kyoto Protocol as being in the list of targetted substances. Now, unlike the Montreal Protocol, this does not mean that they will be phased out in a given period by international agreement. Each nation is free to manage their emissions, whether they be from fossil fuel or PFCs, as they see fit. Two scenarios are
possible, with perhaps some variants. Let us imagine that country A imports 100 tonnes per year of PFCs for VP reflow, leak inspection, safety blankets for flammable solvents, cooling fluids, plasma etching, cleaning inertial navigational instruments etc. The government may say that global warming emissions have to be reduced by the agreed amount. This may
include a choice between alienating the dozen or so voters using 100 tonnes of PFCs or alienating the motorist-voters consuming 20 million tonnes of petrol (gasoline). Which would you choose if you were the Minister of Energy or whatever? Scenario B is a country, also importing 100 tonnes of PFCs, and the Minister says that banning the use of such a quantity
is peanuts and may be harmful to the manufacturing economy and reducing the consumption of 20 Mtonnes of petrol can only be beneficial to public health, anyway.

However, there is a second, minor, but moral crunch. The residence time in the atmosphere of PFCs is counted in thousands of years, compared with c. 100 years for CFCs. We all know, now, a little of the decomposition chemistry of CFCs and their effect on the ozone layer. Quite frankly, atmospheric scientists do not have a clue as to what will happen to PFC
vapours emitted today in 1 000, 5 000 or 10 000 years. What kind of legacy is this to leave our great-to-the-power-of-twenty-grandchildren (assuming that some stupid joker doesn't blow up the planet in the meanwhile)? Will they thank us for this gift (and it may turn out to be 'gift' in German as well as in English - pun intended)? What is certain is that some
of these molecules will reach the exosphere, where anything can happen: there is no modelling for this.

I therefore say categorically that, if we must use these chemicals, let us do so, but let us make sure that we don't emit a single gram more than is absolutely and strictly necessary, even if we have to make 100% emission-free machines to do so.

Brian

Jason Gregory wrote:

> Hello techies!
>
> I am looking for anyone who has performed any qualifications on the advantages/disadvantages of vapor phase reflow. Particularly dealing with higher temp alloys. Is it the way to go? Is convection better? Who are the major manufacturers? Are silicon castments to keep the bottomside components from falling off mandatory? Any replies are GREATLY appreciated.
>
> Jason Gregory
> Production Manager
> Electrospec
> (713)784-4900
> (713)784-1194 fax
>
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