Re: Lead Solder

"Steve Richardson" <oldnoccer@xxxxxxxxxxxxxx> wrote in message
news:U4udnfW8WYMRsOfZnZ2dnUVZ8t2dnZ2d@xxxxxxxxx
Many are concerned that high reliability applications are just not going
to
be able to get leaded parts so catastrophic failures are inevitable,

Greg

While I think some of these rulings are part of a daft growing nanny
state,
I am interested to know about the link between lack of 'leaded parts' and
'catastrophic failure being inevitable'. Surely they have a substitute
solder - is it not suitable in some way ?

Although lead/tin solder was invented before they could possibly understand
such things, it is in fact a superb piece of engineering and the best
alternatives we have today are very much inferior. The alternatives are
basically solid tin with very small amounts of other metals, most popular is
copper and silver, this results in a solder that is stiff and brittle so
fails due to fatigue in a high vibration environment, lead makes solder
flexible. Also pure tin grows 'tin whiskers', a general name for at least
two failure mechanisms involving the growth of very fine conductive threads
outwards from a solder joint, either across a surface or along the strands
of glass in a PCB. These mechanisms are still being investigated and are not
fully understood, but the consequence if a random failure of the circuit due
to shorts, lead is the only known metal to prevent this.

Another problem is that all lead free alloys have a higher melting point so
the circuitry has to be subject to a higher temperatures which reduce it's
reliability, especially problematic for surface mount components since the
entire component has to be heated not just the pins. These higher
temperatures also cause more oxidising and so require more aggressive fluxes
which can themselves lead to corrosion and failure if not completely
consumed or cleaned off, again particularly difficult with surface mount as
there are very small crevices between the PCB and component. PCB's used to
be effectively cleaned in an ultrasonic bath of Arclone but all such
solvents are now banned and the cleaning processes available are inferior.

Lead also helped slow down oxidation of component leads and PCB finishes
during storage, the lead free solders oxidise much faster even when dry
packed and many components and boards are now being quoted as having only a
3 month shelf life, barely manageable even in a medium volume manufacturer
like ours and almost unworkable for small companies. This again results in a
need for a more aggressive flux with all the problems that brings, and where
do the fumes go?, up a chimney straight into the environment!. Some experts
have calculated that the lead ban will actually increase overall pollution
not decrease it.

Lead free electronics is in it's infancy and much of the needed research is
only now being done, far too late, Europe has rushed this matter without any
real understanding or concern for the consequences, driven by certain
members who are totally paranoid.

In short we're going to see a significant reduction in the reliability of
electronics, bad enough in consumer products but imagine the consequences of
failure of a critical system in a chemical or a nuclear plant or in aircraft
systems.

Greg


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