The effectiveness as a reducing agent is directly related to how far below the Equilibrium Gas Ratio the environment is. For example, starting with a closed, pure hydrogen atmosphere at 300C the tin will be reduced, and the hydrogen will oxidize until the partial vapor pressure ratio reaches 0.2828. The rate of metal oxide reduction will slow steadily as the driving force reduces.

Any outside source of oxygen will use up the driving force prematurely. Forming gas with a higher percentage provides a larger surplus of hydrogen to help keep the Gas Ratio below the limit. Replacing the used up hydrogen with a flow of fresh hydrogen can also allow an effective equilibrium to be maintained below the reduction limit.

Also, just for fun, it turns out CO, which is more exothermic, is a slightly better reducing agent than H₂ at temperatures below about 800C.

Andy Magee
Flex Guru
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David D Hillman wrote:

Hi Andy! A quick question - what is the minimum temperature needed in the
reflow profile for Hydrogen to be effective in reducing Sn oxide? I have
heard 300C which would mean that hydrogen would not be useful. Does a 5%
hydrogen mixture provide some Sn oxide prevention instead of a full
reduction?

Dave

Andy Magee <[log in to unmask]> on 08/17/98 05:35:47 PM

Please respond to [log in to unmask]

To:   [log in to unmask]
cc:
Subject:  [TN] Nitrogen/Hydrogen

Dennis,

Commercial forming gas is typically 5% hydrogen. It's used to
prevent/reduce light oxidation, but I never found it to be very
effective because of the inevitable mixing during the purge cycle. If I
remember right the explosive mixture limit is 8% hydrogen. There's also
an upper limit that I forget the value of where you're safe again, and
that's where hydrogen furnaces operate.

We used to crack ammonia to create a "pure" hydrogen atmosphere in the
hot zone for making hermetic glass seals. The conveyor portals were
blanketed with nitrogen. We had a few interruptions in the nitrogen that
automatically cut off the hydrogen, but you still get quite a boom out
of the residuals.

Andy Magee
Flex Guru
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