Hi Genny! The issue with having Bi in a solder joint borders on a Doug
Paul's philosophy " it depends" (Doug and I have included the Bi topic in
our Pbfree tutorial). If there is Pb in a solder alloy (either
intentionally with using Sn63 or left over from Pbfree solder reworking a
component that had some Pb in the lead finish) then the potential of an
alloy of Sn/Pb/Bi with a melting point of 98C can form. The Sn/Pb/Bi phase
diagram shows that thermodynamically the alloy formation is possible -
however, depending on the process and just how much Pb is available in the
solder joint, soldering kinetics may or may not result in alloy formation.
This is the reason why some folks are not worried about Bi (e.g. I've never
had an issue) or some folks won't touch it with a 10 foot pole (e.g. the
possibility exists therefore if I don't allow Bi I can avoid the issue).
Carol Handwerker and the folks at NIST have published a couple of papers on
the formation of the Sn/Pb/Bi potential that are very good. Another rarely
remembered fact is that Bi has the wonderful ability to increase volume
upon solidification (just like when liquid water changes to ice). Some Bi
containing alloys will expand 0.2%-0.5% after solidification which can lead
to solder joint integrity problems depending on the use environment thermal
excursion range.  Dr. Tom Woodrow of Boeing published a paper which shows
very "fluffy" Bi containing solder joints which underwent multiple
expansion/contraction as they were thermal cycled up to 125C (the Bi solder
joints didn't melt but the temperature was high enough to cause very small
expansion/contraction action). Many investigators have shown this phenomena
when thermal cycling  the 42Sn/58Bi solder alloy.  Due to these  Bi
physical phenomena (really cool to us material engineers) many people have
decide to just not deal with Sn/Bi finishes on components. Hope this helps.

Dave Hillman
Rockwell Collins
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             Genny Gibbard
             <Genny.Gibbard@VC
             OM.COM>                                                    To
             Sent by: Leadfree         [log in to unmask]
             <[log in to unmask]                                          cc
             >
                                                                   Subject
                                       [LF] The use of Bismuth
             01/06/2005 10:15
             AM


             Please respond to
                "(Leadfree
                Electronics
             Assembly Forum)"
             <[log in to unmask]
             >; Please respond
                    to
               Genny Gibbard
             <Genny.Gibbard@VC
                  OM.COM>






So I am curious, what are people thinking about the use of bismuth in lead
free termination finishes.  I am not talking about solder containing Bi.
The only supplier I have come across so far specifically mentioning they
use
Bi is NEC.  They have a doc on their website, called "Japan's best kept
secret" http://www.cel.com/pdf/marcomnews/japan_best.pdf        that claims
that SnBi is recommended by JEITA, as opposed to the recommendation of
JEDEC
for Matte Sn.  They make it sound as if a lot of companies are using this
plating, but I haven't come across any other components so far that use it.
The main concern I have is that I keep getting the impression that the next
metal that the EU will be gunning for eliminating is Bi, so it doesn't
sound
like a smart direction to go.

Thoughts?

Genny Gibbard (mailto:[log in to unmask])

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