Ed,
     Hydrogen reducing gas atmospheres are pretty common in the hybrid and
hermetic packaging industries. I used to work with this sort of stuff with
a previous employer. My experience is on hermetic ceramic semiconductor
packaging. The seal on these packages was made with 80/20% Au/Sn alloy. The
sealing surface on the package was 60 micro inch gold over nickel over
Tungsten. During the high temperature die attach process prior to seal
there would be Nickel diffusion through the gold and oxidation of the
nickel on the surface. The H2 scrub reduced the Nickel oxides back to
Nickel and gave a surface solderable without flux in an N2 atmosphere. This
worked well provided the delay between scrub and solder seal didn't exceed
a couple of days (time for the Nickel to oxidise again).  I did quite a bit
of work with a University using secondary ion mass spectrometry to get a
good process. You need around 78% N2, 12% H2 by volume in the atmosphere
Peak temperatures of 30 to 350C work well with times at peak of 1 to 2
minutes. If you are trying to high temperature solder a system that isn't
sealed then you can carry out the soldering in the reducing atmosphere
without flux. If you are soldering a sealed hermetic package then the
soldering should be done in an inert atmosphere after the reducing scrub to
keep the moisture content in the sealed package to reasonable levels.

     The fun part of this process is the safety aspect. You are putting H2
into a belt furnace with open ends. Air is only prevented from getting in
by the gas curtains at each end of the belt. You had better make sure that
the gas curtains and the furnace atmospheres are balanced. We had an
explosion (before my time there) when a design fault on one furnace, where
a relay failure allowed H2 and O2 solonoid valves to open simultaneously
into a 350C furnace. You need flame offs over the exhaust ports and H2
detectors around the furnace are useful. BTU used to make this sort of
equipment. No doubt others do too.

Hope this helps

Jeremy Drake
Celestica Ltd

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