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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