Electronics > Projects, Designs, and Technical Stuff
Vapour phase Soldering
helius:
lower viscosity - better heat transfer to the center of large FBGAs
jeremy:
--- Quote from: mikeselectricstuff on January 26, 2015, 10:49:20 am ---
--- Quote from: jeremy on January 26, 2015, 08:05:46 am ---Ok, but to play devils advocate here:
Can anyone shed any light on why fluid viscosity really matters in a batch oven? I can't think of anything.
--- End quote ---
Lower viscosity - better run-off from the PCB?
--- End quote ---
Yes, this is about the only sensible thing that I could come up with. But I think this is again less of a problem when you are doing single boards in a slow batch oven. Inline systems sure; of course you don't want the fluid to be taken away by conveyer. I had a look, and both fluids have almost identical surface tension, so given how inert this stuff is (it doesn't really dissove) I'm sure it will be just as easy to pour off either one.
--- Quote from: helius on January 26, 2015, 11:08:28 am ---lower viscosity - better heat transfer to the center of large FBGAs
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I'm not sure this is correct; we're talking about the viscosity of the fluid being heated, not the viscosity of the vapour. Although they probably will be related. As a reference, the HT series, which is more viscous, still has a kinematic viscosity of 10 times less than olive oil. And ~3 times more than tap water at room temperature. Of course this will decrease with heating.
helius:
heat is transferred to the parts by a liquid phase, not a gas. the vapor condenses on the surface, so the board is entirely immersed in liquid, less than 1mm thick. when the peak temperature is reached, the vapor stops turning to liquid. so vapor itself does not do anything to the components.
see the presentation: (the slide decks on this channel are extremely detailed)
especially the point on "low surface tension". Viscosity and surface tension are related, although the relationship is different for different types of materials.
jeremy:
--- Quote from: helius on January 26, 2015, 11:51:43 am ---heat is transferred to the parts by a liquid phase, not a gas. the vapor condenses on the surface, so the board is entirely immersed in liquid, less than 1mm thick. when the peak temperature is reached, the vapor stops turning to liquid. so vapor itself does not do anything to the components.
see the presentation: (the slide decks on this channel are extremely detailed)
...
especially the point on "low surface tension". Viscosity and surface tension are related, although the relationship is different for different types of materials.
--- End quote ---
I think you have perhaps misunderstood my point, because although the video mentions that surface tension is important, the measured surface tension of both the LS and HT fluids basically are identical. So regardless of how important that particular quality is, there is no difference between LS and HT. In my understanding (which could be totally wrong! my chemistry is a bit rusty), with high surface tension the condensed fluid would be more likely to stick in droplets on various hot parts of the PCB in the same way that you see water "beading" on hydrophobic surfaces. These droplets would absorb some of the energy of the phase transition in their own thermal mass, thus making the transfer of heat into the board slower. You want the low surface tension stuff so that the fluid will be more likely to drip off and flow rather than stick together, leaving a higher thermal gradient to encourage the phase transition.
The viscosity of the HT fluid is likely much lower than water at ~230C, and washing boards in water does not ever seem to be a problem in terms of its viscosity.
I also don't think it is as simple as saying that the fluid does all the work. The vapour carries the potential energy via its latent heat of vapourisation. When the vapour comes into contact with the cold board, the board will absorb the heat as a counterpart to the phenomena that allows the vapour to transition to a liquid phase (in a pretty yin-yang kind of way). So it is really neither the vapour nor the liquid that does the work, but the action of transitioning between the two which releases the heat. This transition is prevented when the board is the same temperature as the liquid as the vapour cannot deposit it's energy anywhere in order to return to the liquid phase. I think heating merely from the liquid being hot would be much less significant, and could almost to the point of being negligable.
If you look at the lovely wikipedia picture I found (attached, taken from http://en.wikipedia.org/wiki/File:Energy_thru_phase_changes.png) you can see that for water, it takes only 8kJ to reach the phase transition zone, but then it needs a whopping 40kJ to actually turn into steam. So if we were using water as the vapour medium, even if the condensed liquid cooled all the way to 0C, it would only deposit 8kJ of energy (ignoring the phase transition energy). However, if the water vapour used the thermal gradient of a cold board to condense, it would heat the board 5 times faster, even if the water only cooled by 0.1C before dripping off the board. So it is the transition itself which is important
Thank you for sharing the video, it was very interesting. I will have a dig through his channel later.
And just to reiterate, I'm not arguing that HT is better in performance, I'm just arguing that it is perhaps better in performance/$ for a small batch oven. It is double the price after all.
helius:
I think you are focused too much on the cost of the fluid. it could cost 10 times as much and it would still be less than the cost of engineering a proper heat loop, cooling system, mechanical elevator, vapor detector, control system, user interface, and chassis.
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