Forming gas is a complicated topic, so I will provide some preliminary background in this section, then get into the soldering part next time.
Don't you mean "formic"?
Forming gas is a mixture of hydrogen (H2) and an inert gas (usually nitrogen, N2) that is used to reduce oxides on metal surfaces to water. Please don’t confuse this with formic acid (HCO2H), which I hope to touch on in another posting later this year.
Safety
The reason for the dilution of hydrogen by the inert gas is to keep the hydrogen below 5.7% (by volume), as this is the point above which the hydrogen can spontaneously combust. Gas companies such as Linde and Air Products consider forming gas at less than this level to be an inert mixture, so the fittings used for gas cylinder attach are the standard CGA580 type used for nitrogen, argon, helium and so on. Depending on the gas supplier, they may allow a maximum of either 5.0% or 4.0% hydrogen, to ensure they are within safety margins.
All this notwithstanding, 100% hydrogen furnaces are used around the world in a variety of different processes, and I have also seen soldering processes around the world where 10% and even 20% hydrogen/nitrogen forming gas is in use. I am not saying that >5% H2/N2 can not be safely used, but you have to be careful when using it.
Gas Supply
There are three ways of supplying gas for forming gas-based soldering processes:
1/ Mixing hydrogen and nitrogen in a special panel. Sometimes this may also incorporate a catalytic reactor that reacts ppm traces of oxygen, with hydrogen to form water: the water is then removed by adsorption. This process makes a very "clean" forming gas that will have optimal reducing properties. Usually, the nitrogen source is from vaporised cryogenic N2, and the hydrogen is from a cylinder or "tube"-based sources.
2/ Cylinder supply. A single cylinder, or a manifolded bank of cylinders may be used to provide the gas as a mixture. Usually, this is used as-received without being cleaned up.
3/ Ammonia cracking. Basically, NH3 -> 3N2 + H2. This is feasible, but results in a fixed 3:1 ratio of N2 to H2, and is never used (to my knowledge) in soldering. It is also massively inefficient in terms of costs and power usage to make the ammonia, plus the ammonia usually has a much higher moisture content than a nitrogen plus hydrogen gas mixture.
What does it do in soldering? I’ll get into that next time: I'll be talking thermodynamics and kinetics, and there WILL be a test.
Cheers! Andy
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Rainer: Interesting! Is the temperature high enough that the use of argon is required (avoiding the formation of nitrogen reaction products)? As long as the oxygen level is low, this could help, but without more data, it's difficult to say. I am way behind on my "Part II", but please stay tuned.
ATIQ: Nitrogen is 79% of the atmosphere and comes out of the top of a the "nitrogen distillation tower" in high volume and with low oxygen levels (typically less than 5ppm). Argon is only 0.9% of the air, and is a "heavy", so comes out of the bottom of the tower along with oxygen. Nitrogen is by far the cheaper option for low temperature (say around
Can you tell me more about peak reflow temperature?
Thanks,
Raffy
I am design the foming gas for ON SEMICONDUCTOR in Vietnam. Can you give me the foming gas drawing to reference?
Thanks,
Nguyen Van Duyet
Nguyen: Not sure how to answer this, as forming gas is a gas (khí). The best people to contact would be your gas supplier, who can advise on best safe usage. High H2 concentration becomes more dangerous, of course.