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Subject:	Tin additives - was Re: [LF] Back to square one
From:	Camille Good <[log in to unmask]>
Reply To:	(Leadfree Electronics Assembly Forum)
Date:	Mon, 22 May 2006 14:09:22 -0700
Content-Type:	text/plain
Parts/Attachments:	text/plain (91 lines)

Jean-Paul,

Regarding your question of whether any other elements besides lead have the potential of preventing tin whiskers, if you look through the forum archives there is quite a bit of discussion about this at various times.

Bismuth is the element I have seen most commonly cited as a tin-whisker suppressant; bismuth is often added to component finishes to reduce tin whiskering, although there is a lot of concern that this makes components not backwards compatible because the bismuth in a SnPb solder joint creates a SnPbBi alloy that has a melting point under 100 C.

There was a post on this forum from Robert J. Lang ([log in to unmask]) on May 2, 2006, titled "[LF] Could Tin Disease Be An Underlying Cause Of Tin Whiskers?", that talked about copper impurities in a tin joint possibly increasing the compressive forces in the joint, but a nickel barrier between a copper layer & the tin reduces the growth of tin whiskers. You may want to contact him for more information.

Hope that helps,
-Camille

[log in to unmask] wrote:
Hello Joe,
I am curious about the "evidence" that you are referring to, to the effect
that "lead-free" solder will make electronics less reliable.

Three questions:
1) which lead-free alloy(s) does this refer to?
2) which products have failed in the field and what was the failure mode?
As you know, there has also been SnPb failures in the field (Werner can tell
you more about that), and that did not make SnPb technology unreliable as a
whole.
3) where has the evidence been published?

In the same vein, I am also wondering about the SWATCH report that John
brought to the attention of this forum. John can probably enlighten us on this.
The pictures I saw were very dark and not too clear (originals are probably
sharper but I have not had the privilege of seeing them). The text highlights
tensile forces as being the driver behind the growth of tin whiskers. All
technical studies I have seen to date suggest that compressive (not tensile)
forces drive the growth of tin-whiskers. I am curious as to why tensile forces
would prevail. Any physical evidence or measurement that you, or others, can
share to that effect will be appreciated. Also, I was wondering if other
elements, other than exclusively Pb, have the potential of preventing
tin-whiskering. The SWATCH report seems to focus exclusively on SnPb alternatives.

I apologize for my ignorance here and I am looking forward to learning more
from this forum about tensile forces driving tin-whiskering, as well as
"lead-free" being less reliable than good old SnPb. By the way, my questions are
purely technical, which I hope remains acceptable to the forum.

With best regards,
Jean-Paul

In a message dated 5/22/2006 1:11:11 A.M. Eastern Standard Time,
[log in to unmask] writes:

In sum lead-free solder will not make the environment better, but it could
well make it worse. It will not use less energy, it will use more. It will
not
be cheaper, it will be more expensive. It will not make electronics more
reliable, evidence is that it will make them less so.

Quote of the month:

"Research is what I'm doing when I don't know what I'm doing", Wernher von
Braun.
_______________________________________________
Jean-Paul Clech

EPSI Inc., P.O. Box 1522, Montclair, NJ 07042, USA
tel.:+1 (973)746-3796, fax: +1 (973)655-0815

_http://www.jpclech.com_ (http://www.jpclech.com/)

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