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Hi Nigel - One of my mentors always cautioned me to "consider the source
of the data and why potential conflicts of interest may be present" when
digesting industry published test results or data analysis. My
recommendation is to read the specific references that are discussed in
the CEL ppt file and determine/interpret those results yourself. Rockwell
Collins/Iowa State University did extensive thermal cycle testing of SnBi
surface finishes for both tin/lead and leadfree solder joints (published
at the SMTAI Conference in 2007). Our test results were in agreement with
the NIST (Dr. Carol Handwerker), Celestica (Dr. Polina Snugkovsky) and NPL
(Dr. Chris Hunt) published data. Also look at the JCAA/JGPP test report
which demonstrated that tin/lead components used with a leadfree solder
alloy that contains bismuth can result in significant solder joint
integrity degradation. The current industry data set supports the use of a
SnBi surface finish in a tin/lead soldering process as long as the overall
solder joint Bismuth content remains below approximately 6%. The ability
of SnBi surface finish to eliminate tin whisker - well, that is a whole
different discussion that requires lots of beverages.
Dave Hillman
Rockwell Collins
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Nigel Burtt <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
08/13/2010 02:34 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Nigel Burtt <[log in to unmask]>
To
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Subject
[TN] SnBi finish
The received wisdom on having Bismuth involved in any lead to lead-free
assembly transition is that it should be avoided because of the
possibility of
forming a low-melt Sn-Pb-Bi phase within the solder joint, possibily at
grain
boundaries and risks realiability problems. But how likely is it that for
example,
SMT AE caps RoHS-compliant from Panasonic using a SnBi termination finish
will provide the right Sn/Pb/Bi contributions to create the troublesome
alloy?
I found a presentation from NEC which explains their choice of SnBi finish
and
explains research they commissioned that found the low-melt phase will not
occur.
<http://www.alpha.cooksonelectronics.com/sacxdatalibrary/pdfs/NECSMT.PDF
>
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