What is this?
We know that vacuum soldering chips on carriers, substrates or boards
will eliminate most included vapours, but from where does the gas come?
And why can you see large voids inside the solder joint even after
vacuum soldering? - In order to study this closer we made some
experimenting. We placed plasma cleaned 80Au/20Sn preforms on clean
goldplated alumina substrates. These assemblies were then placed on a
polished-steel hotplate at 300 degrees Celcius. Hot and dry nitrogene
was flown to exclude air or air pollutions. The melting solder preform
was then studied in a microscope during the melting and photos were
taken after 10, 60 and 120 seconds. (The preform melted after a few
seconds only). One could see that microscopic "bubbles" occured in the
preform's surface after 10 seconds, and that the bubbling accelerated.
After 120 seconds the whole gold/tin preform was covered by thousands of
microscopic voids or bubbles. The whole process reminded of water, just
in the start before real boiling: lots of small bubbles on the bottom of
your pan. I must say that I was perplexed, because I expected the molten
gold/tin to be like a mercury surface: smooth and shiny. - My questions
are the two following:

a) Is it known if gold/tin solder has something built-in in the bulk
structure that can outgass?
b) Am I wrong from the beginning? Maybe the outgassing comes from the
goldplated alumina thinfilm substrate, and that the "bubbles" just
passes the gold/tin preform on their way to the freedom.?

The preforms, 5x7mms, were 50um thick and made by a wellknown american
company in the game, and are  made by punching. The manufacturer
guarantees that they are extremly clean and free from contaminations.

                      Regards / Ingemar Hernefjord
                      Ericsson Microwave Systems

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