vapor phase soldering - alternate liquids

[jonroger]

The standard way to do vapor phase soldering is to use a liquid like Galden HT230.   But this is quite expensive.  Are there other liquids that would work?   For example 95% glycerol/glycerin and 5% water?  Or 75% glycerol and 25% propylene glycol?

If usable, what about safety?  I understand that these things are deliberately inhaled (in e-cigarettes).   I have no intention of inhaling any of it, but I'd prefer not to die if I get trace amounts.

[MagicSmoker]

Galden is a perfluorocarbon so is highly inert; anything ending in -ol has one or more hydroxyl groups which are anything but inert, especially at high temperature and in the presence of either acids or alkalies. So, no, you probably can't replace Galden with polyethylene glycol, etc.

Off hand I don't know of an acceptable substitute that would be easy to obtain (then again, I Am Not A Chemist) and cost less than Galden. Maybe a silicone oil, but you'd have to do your own research.

[jogri]

Silicone oil wouldn't work, that stuff isn't anywhere near its boiling point at 200°C (i don't know when or if it would boil, i never managed to heat it that high). And it would be an absolute PITA to get it off your board after you cooled it down.

You probably won't find any perfluoro compound that costs significantly less than Galden because you don't pay for the polypropylene part of it, you pay for the fluorination: No one wants to work with fluorine (that stuff is really nasty) and the ones who do charge exorbitant prices.

[wraper]

You can use another grade of Galden such as LS230 but otherwise there is no alternative. And the main point is that it boils at exactly 230^oC. Anything with significantly different boiling temperature simply won't work. Mixing things with different boiling temperature is pretty much pointless.

[jonroger]

I can get a mixture of glycerol and propylene glycol to have the right boiling point. So it's less inert - what does this mean practically?   It will interfere with the flux?    I doubt it will damage anything else.

IMO, radiant or convective heating of a PCB is a really lousy way to get a mixture of things with different reflectivity and mass to a single fixed temperature.

[wraper]

I can get a mixture of glycerol and propylene glycol to have the right boiling point.

You won't have one boiling point. That's why distillation works. And you will get PCB covered in junk.

[coppercone2]

unless it makes a eutectic solution

[wraper]

unless it makes a eutectic solution

It does not apply to boiling point. Constant boiling point mixture is called azeotrope. But you cannot adjust mixture proportions to get right temperature, there is only one particular sweet spot with those.

[jonroger]

Pure propylene glycol boiling (188C) is technically above the melting point of 63/37 solder (183C).

Another option is to change the pressure so that a single substance would have the right boiling point.    Certainly less convenient, but would allow use of a cheaper liquid.   

Or maybe I just need to put more effort into finding a US source of less than 7kg of Galden HT230.  But evidently it too is a mixture (alternative spelling prevented me from finding this).  So I need LS230.

https://www.eevblog.com/forum/projects/vapour-phase-soldering/100/

[ejeffrey]

I wonder if you really need an azeotrope.

Boiling is not exactly like melting where non eutectic mixtures have a slushy region between the separate liquidus and solidus points.  A non azeotropic mixture has one boiling point.  The problem is that the more volatile component preferentially vaporizes and the relative concentrations in the vapor and liquid phases are different.  As the liquid composition changes the boiling point changes.

If you have a closed system with a reasonably large liquid reservoir and the re-condensed vapor is efficiently returned to the liquid you should be able to keep the ratio of the liquid phase and therefore the boiling point change within a narrow enough bound not to matter.  I think you don't really care about the concentration in the vapor phase.  Maybe I am missing something and I don't know how hard that would be, but it doesn't seem crazy.

None of that changes the fact that I wouldn't want a PCB covered in propylene glycol or glycerol.  Also note that you would be heating both of these liquid above their flashpoint, so safety is an important issue.

[wraper]

Or maybe I just need to put more effort into finding a US source of less than 7kg of Galden HT230.  But evidently it too is a mixture (alternative spelling prevented me from finding this).

It's not a mixture in a sense it gets mixed. It's specially separated perfluoropolyethers with varying molecular size but with similar boiling temperature. Consider it similar to oil rectification to get different fractions.

I think you don't really care about the concentration in the vapor phase.

It's the thing you should care about the most. Once separation occurs in vapor phase, soldering temperature is screwed.

[wraper]

In EU you can buy 500ml/ 900g of Galden. Dunno about air shipping to US though. https://eleshop.eu/vapour-phase-galden-ls-230-liquid-500ml.html
Maybe just try making group buy.

[jogri]

Less inert means that you will probably get some ungodly new metal-organic compounds (and those are usually rather toxic) when your glycol starts to react with your solder. If you are especially lucky, it will also start reacting with the various plastics.

The only "alternative" that comes to my mind would be Dowtherm A given that it is not as reactive as your glycol mixtures, but it could also react with with your solder.

Changing the pressure is a bad idea since you need special equipment for that: If you want to pressurize it, you need to have a good filtration (it will leak into your room) and if you want to decrease the pressure you need a chemical vacuum pump (usually a diaphragm pump) because it will probably react with the hot parts in the pump. Also, you still need an inert liquid (no glycol etc).

[ejeffrey]

I think you don't really care about the concentration in the vapor phase.

It's the thing you should care about the most. Once separation occurs in vapor phase, soldering temperature is screwed.

I was thinking that since you are heating the liquid the temperature would be set by the boiling point of the liquid phase, and as long as you can keep that from changing too much the soldering temperature wouldn't change either.  But I guess I am not clear what happens in the vapor phase. 

[coppercone2]

unless it makes a eutectic solution

It does not apply to boiling point. Constant boiling point mixture is called azeotrope. But you cannot adjust mixture proportions to get right temperature, there is only one particular sweet spot with those.

mis used my word