John, do a Google search on the words: tin pest 

You'll get a flock of hits to explain this. Not related to tin whiskers.

Jack 


Jack Crawford, IOM
IPC Director Certification and Assembly Technology 
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-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of John R. Sieber
Sent: Sunday, October 09, 2005 4:46 PM
To: [log in to unmask]
Subject: Re: [LF] Lead: Intentionally added but below 1000 ppm?

Keith,

What is tin pest?  Are you talking about whiskers?

It should be very easy to validate test results for Pb in tin.  Tin that
is purified to Grade A per ASTM International B 339-00 has 500 ppm Pb or
less.  'Grade A for tinplate' has 100 ppm or less.  NIST SRM 1727
corresponds to Grade A for tinplate and contains Pb certified at 33 ppm.
There is one grade of higher purity listed in B 339:  Ultrapure Grade
with 10 ppm Pb or less.

John Sieber
NIST

At 04:33 AM 10/9/2005, you wrote:
>Gordon,
>
>One answer to your question "For what purpose is lead intentionally 
>being added at such a low level ..." is, "to reduce the possibility of 
>tin pest".
>
>In a study that I reported at the IPC/JEDEC lead-free conference in San

>Jose last April Nihon Superior found that of all the elements added to 
>high purity tin at the 100ppm level the only one that was able to stop
tin pest completely
>was lead.    In fact it works at levels less than 100ppm and anecdotal
>evidence suggests that as long as it is above about 50ppm the tin (or 
>the high-tin lead-free solder) will not undergo that allotropic 
>transformation.
>
>On the other hand high purity tin with a lead content of only 14ppm did

>undergo the allotropic transformation spontaneously when exposed to a 
>temperature
>of -45C.   And lead-free solders made from that high purity tin by
adding
>silver and/or copper also transformed albeit at a slower rate.
>
>However, it should not be necessary to deliberately add lead to protect
a
>"lead-free" solder against tin pest.   All that you need to do is not
take out
>more of the naturally occurring lead than necessary.    The naturally
>occurring
>ores from which tin is extracted are seldom completely pure so that the
tin
>smelted from them contains numerous impurities including lead.   There
are
>procedures for removing as much as possible of those impurities but the

>cost of
>doing so rises steeply if you try to go beyond 99.9% purity.    At
99.9%
>purity
>there is usually enough lead left to prevent tin pest.
>
>The message is, therefore, if you want to eliminate the risk of tin 
>pest in a "lead-free" solder make it with standard purity tin (99.9%)
rather than high
>purity tin (99.99%) (and save yourself some money).    If somehow you
have
>ended up with solder that has been made with such high purity tin and 
>it is going to be used in an application where it might be exposed to 
>sub-zero temperatures then it would be worthwhile adding a little lead 
>to bring the level up to
>50 - 100ppm.     If there is any uncertainty about whether that is in
>breach of
>the RoHS or ELV directives then make sure you do not need to add lead 
>by selecting a solder that was made with tin no purer than 99.9%.
>
>Keith Sweatman
>Nihon Superior Co., Ltd
>
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