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August 2004

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Subject:
From:
"David D. Hillman" <[log in to unmask]>
Reply To:
(Leadfree Electronics Assembly Forum)
Date:
Thu, 5 Aug 2004 09:56:40 -0500
Content-Type:
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text/plain (121 lines)
Hi Mike! Your fingers are moving much faster than mine as you beat me to
the punch! The tin/lead/bismuth phase diagram shows that the
tin/lead/bismuth phase is thermodynamically possible but the real question
should be "are the soldering kinetics such that the tin/lead/bismuth phase
will be formed?". There are a number of parameters which need to be in
alignment for the tin/lead/bismuth phase formation (as you pointed out).
The folks at  NIST (e.g  Carol Handwerker and crew) have published a paper
on the tin/lead/bismuth phase formation question and Rockwell Collins
published a paper at the 2001 IPC EXPO Conference documenting that having
bismuth in the solder alloy doesn't necessarily mean the tin/lead/bismuth
phase will form during a soldering operation. Many soldering technologists
(e.g. military electronics specifically) have made a conscious decision to
avoid the tin/lead/bismuth phase formation question altogether by just not
allowing the use of any bismuth containing solder alloys on products.

Dave



                      Mike Fenner
                      <[log in to unmask]        To:       [log in to unmask]
                      om>                      cc:
                      Sent by: Leadfree        Subject:  Re: [LF] backwards compatibility of Sn/Bi plated components with Sn/Pb
                      <[log in to unmask]         solder paste
                      >


                      08/05/2004 08:18
                      AM
                      Please respond to
                      mfenner






Can I just go a little forensic on this? (and I am happy to be further
nitpicked).

Certainly if Bi is present in the surface finish there is the theoretical
possibility of the ternary 46Bi/34Sn/20Pb eutectic MP 96C being formed in
the joint. Certainly there have been studies which demonstrate that under
ALT testing the strength of solder joints can be reduced.

But it is possible that with only 4% Bi in otherwise Sn on the component,
that there is not enough Bi present in the total joint for this to form.

I am just mindful that in real life most solders are not 100%% pure -
typically solders are in the  99.9X% to 99.99% range. So in practice quite
a
few "known hazardous" items could at least be measurably present although
so
diluted as to have no practical effect.

So possibly this level of Bi would be too diluted to do anything, that
presumably is the contention. In addition  2-3% Bi was - maybe still is -
customarily added to wave solder as a dulling agent for inspection purposes
for years with no reported failures. This was in consumer goods though so
maybe they never got stressed enough for it to matter. Depending on the
application this could still be the case even with Bi above 0.0X% levels.

Have the component manufacturers given out any data, or has anybody seen
data, which support their claims for compatibility?



Regards

Mike Fenner
Indium Corporation

T: + 44 1908 580 400
M: + 44 7810 526 317
F: + 44 1908 580 411
E: [log in to unmask]
W: www.indium.com
Pb-free: www.Pb-Free.com



Its not so much a question of reliability, as it is melting temp.
When Bismuth is added to Lead, an alloy forms with a melting temp of less
than 100 deg C.
This can cause many issues, especially when trying to double side reflow!

Regards,
Danielle

Danielle L. Casha
Process Technician
EPM
905-479-6203 ext 272


Hi,

I have gotten conflicting information on the backwards compatibility of
using Sn/Bi plated components with Sn/Pb solder paste.

As I understood the situation, under no circumstances could Bismuth be
mixed with Pb.

However, chip suppliers are stating that if the bismuth is maintained to
below 4% of the plating, the reliability of the solder joint is not
compromised.

Can someone shine some light on this specific backwards compatibility
issue.

Thanks,
Haim Halpert

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