Werner,

I agree with the point that you make that at this stage in the evolution of
the technology we cannot predict with absolute confidence the reliability of a
lead-free joint (connection).    The very interesting discussion in this forum
over the past few weeks has highlighted the fact that the basic metallurgy of
the lead-free alloys is very different from that of the tin-lead system they
are replacing and that is inevitably going to have implications for the way
these alloys behave under long term mechanical and thermal stress.

I have to admit that when I used the word "proven" I was using a more limited
definition than the one that you might use.    What I mean is that the
companies who have been using the alloys I mentioned undertook their own accelerated
test program before their adoption and have monitored real time field
performance since then to determine whether the confidence established on the basis
of their preliminary (and in your terms possibly rather primitive) ALT has been
justified FOR THEIR PRODUCT by actual experience in service.     With
something like 200 million boards of a wide variety of types produced with these
solders since 1999 and with a substantial number having now been in service for
nearly five years there have been, as far as we know, no failures in the solder
alloy.   There were and still are some failures in connections due to "cold
joints", which is the same defect that occurs with tin-lead solder when the
joint surfaces do not get to the temperature necessary to form a metallurgical
bond with the substrate, but that is a process control issue rather than a
reflection of some fundamental weakness in the alloy itself.

I think that to be realistic, Werner, this is the way that the industry will
have to gradually build up its confidence in the reliability of lead-free
solders- do enough "pragmatic" ALT to get sufficient confidence to use the alloy
in production and then see how that squares up with field experience- and if an
unanticipated failure occurs go back and adjust the ALT to test for that
failure mode.

That is the way the industry built up confidence in tin-lead solder.
Tin-lead solder was adopted in the early days of the electronics industry when
there was even less reliability data available than there is now for the lead-free
solders.    There were some unexpected failures with tin-lead (e.g. due to
gold embrittlement) but the industry learned from these experiences and adjusted
practice accordingly so that overall a high level of reliability was
achieved.     There is no reason to believe that the same thing will not happen with
lead-free solders.

In regard to the current anxiety in the industry that we are moving into
totally unknown territory, Dr Jenny Hwang made an interesting and in a way
reassuring observation in some discussions during Internepcon Tokyo a few weeks ago.
 She said that so far the behaviour of the lead-free alloys has been entirely
consistent with known metallurgical science.    Because metallurgy is only
incidental to its business of making functioning circuitry the electronics
industry tends to have a fairly narrow view of the very large subject of
metallurgical science.   While there are around the world many very good metallurgists
working in the field of soldering technology the knowledge of most people in
the practical side of the industry is limited to the metallurgy of the tin-lead
system.  People like Dr Hwang, however, have experience in the wider field of
metallurgy science and can therefore put the issues associated with lead-free
solder into context.    The fact that the behaviour of lead-free solders seems
to be consistent with the general understanding of the behaviour of metals
does not mean that there will be no unanticipated problems but if the
electronics industry draws on all the resources of metallurgical knowhow that are
available it can approach this change with a little more confidence than it seems to
have at the moment.

By the way, Werner, I enjoyed your workshop in Brussels last year in which
you made some similar points about lead-free joint reliability and look forward
to the possibility of hearing you again in Australia later this year.

Keith Sweatman
Nihon Superior Co., Ltd.

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