A fairly large number of Japanese IC companies had Sn-Bi as an approved lead
finish with approximately 2-3 wt % Bi. More Bi appeared to result in finish
cracking during lead forming. It was claimed that the Bi greatly deterred
whiskering. Another benefit may be avoidance of the damaging allotropic
phase transition that occurs in tin at < 13 C ('tin pest'). Several sources
have stated that only several tenths of a percent of Sb, Bi, or Pb in
solution with Sn will prevent this destructive phase transition. So far,
almost all the focus of concern on matte Sn lead finishes has been on
whiskering, and long term solderability. Since Green legislation is driving
Sb2O3 from IC component mold compounds, there may be unreasonable resistance
to putting Sb into matte tin. Continuing concerns with Pb contamination of
SAC solders in mixed system assembly, and the possibility of concentrating
the 96 C ternary SnBiPb eutectic in critical sites due to a zone-refining
mechanism during cool down from reflow, may disallow Bi in matte Sn. I
wonder if the allowed 1000 ppm of Pb 'contamination' would be sufficient to
prevent tin pest..... Some are equally concerned that SAC solders could
suffer tin pest, although one report indicates this should not be a concern
due to the very low solubility of Ag in the Sn. This could be an
interesting discussion, and one that is long overdue.
Best regards,
Leo
Director of Applications Engineering
ASAT, Inc.
3755 Capital of Texas Highway, Suite 100
Austin, Texas 78704
ph 512-383-4593
fx 512-383-1590
[log in to unmask]
www.asat.com
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-----Original Message-----
From: David Suraski [mailto:[log in to unmask]]
Sent: Friday, January 07, 2005 7:20 AM
To: [log in to unmask]
Subject: Re: The use of Bismuth
Maybe we should look at it from the other side. What is the ADVANTAGE of
adding bismuth to lead finishes? We read below that some people are for
it, some against it, and some indifferent about it. So why should we
bother? If it has to do with minimizing whiskering, I've yet to see
anything definitively proven about the advantages that adding Bi brings.
Regards,
David Suraski
AIM
-----Original Message-----
From: Mike Fenner [mailto:[log in to unmask]]
Sent: Friday, January 07, 2005 8:06 AM
To: [log in to unmask]
Subject: Re: [LF] The use of Bismuth
I think we need to keep a sense of perspective in this area.
Saying that Bi is to be avoided at all costs is a bit like saying
drinking a glass of coke will be fatal. How so? It's obvious - coke
contains phosphoric acid, which is highly corrosive.
1) As Bev points below out the presence of Bismuth does not
automatically mean that solder joints will melt at 96C. Search back for
references on this as this has been aired a number of times on this
forum.
2) The availability of Bi will not be affected by the ending of lead in
soldering. The laws of supply and demand would ensure that anyway, but
even if Bi could only be obtained by first smelting lead it wouldn't
matter. This is because total lead usage will reduce something like
about 0.5% maximum overall as a result of our efforts in lead-free.
3) Metals have a volume change on phase change. Usually it's a
contraction, but as it happens pure bismuth expands on freezing, as do
some of its alloys, but not all. For expansion you need over 50% Bi
content.
4) If the EU even looks as though it wants to ban Bismuth, then I will
for one will give up and start digging a cave.
Regards
Mike
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Thursday, January 06, 2005 7:10 PM
To: [log in to unmask]
Subject: Re: [LF] The use of Bismuth
Ladies and Gentlemen,
There is NO actual or proposed EU legislation to ban the use of bismuth.
Period. Yes, Bi is a byproduct of lead mining but more and more of it
now is coming as a byproduct of mining other metals (tungsten). Check
out the US Geological Survey web page for bismuth.
http://minerals.usgs.gov/minerals/pubs/commodity/bismuth/bismumcs04.pdf
And as Pratap points out, it is not SnBi by itself that is bad, it is
the SnBiPb alloy that is the problem. Mixing SnBi with SnPb, as shown
by Boeing is REALLY bad. However, Tom Woodward (Boeing) has also shown
that smaller amounts of Bi is actually not a problem, at least not to
the same extent. I do not want to put words in Tom's mouth that
everything is hunky dory about lesser amounts than in the SnBi eutectic,
but things do not look as bad as first thought for alloys containing
single digit amounts or Bi.
regards,
Bev Christian
Research in Motion
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]]On Behalf Of Richard Schumacher
Sent: January 6, 2005 12:03 PM
To: [log in to unmask]
Subject: Re: [LF] The use of Bismuth
Dewey, what finish alloys use up to 3% Bi?
Genny, what indications have you seen that the EU may eventually want to
ban Bi?
> -----Original Message-----
> From: Leadfree [mailto:[log in to unmask]] On Behalf Of Whittaker, Dewey
> (AZ75)
> Sent: Thursday, January 06, 2005 10:30 AM
> To: [log in to unmask]
> Subject: Re: [LF] The use of Bismuth
>
>
> We are seeing components with this finish. There is data to support
> the acceptability of a final lead finish containing up to 3% Bismuth.
> Dewey
>
> -----Original Message-----
> From: Leadfree [mailto:[log in to unmask]]On Behalf Of Genny Gibbard
> Sent: Thursday, January 06, 2005 9:15 AM
> To: [log in to unmask]
> Subject: [LF] The use of Bismuth
>
>
> 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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