[sdiy] tin whiskers

[Tony Clark]

> So - does this mean all the new gear people are buying because it's
> supposed to be modern and maintainable is actually going to fail as
> often as the old stuff? HA! Too much to worry about...
>
> -----Original Message-----
> Good story in the news today:
>
> http://news.yahoo.com/s/ap/20071006/ap_on_hi_te/tin_whiskers

   Not really.  The article is about 2 years behind on the current state
of electronics.  I've been doing lead-free for over a year and a half
and there have been no whisker issues, and my first run boards still
look as good today as they did then.
   The article is mis-leading in that it makes it sound like the cause of
the whiskers is unknown.  It _IS_ known.  Tin forms crystal growth (the
whisker) when it is subjected to micro-strain.  The micro-strain occurs
because the pure tin is usually plated on top of a dissimilar metal
such as steel or copper and isn't properly annealed afterwards (not
surprising).  In lead/tin solder, the lead is more ductile and
alleviates the micro-strain in the metal.  No micro-strain, no
whiskers.
   Modern lead-free solder alloys are using alternative metals to help
with the ductility and/or inhibit tin crystal growth.

   It doesn't mean that tin whiskers are no longer a problem, but it is
extremely minor compared with the other problems associated with
lead-free.  The biggest problem with lead-free is the higher reflow
temperatures.  This causes all sorts of fun stuff, especially if you
are running mixed component boards with large thermal masses.  But
again, these problems can be worked around.

   So far I have been using SAC-305 lead-free and it is an decent
lead-free solder.
   Nihon SN100C is the best stuff out there and I prefer this for HASL
finishes on my circuit boards (if you can find a house that uses it) as
this stuff has super good shelf-life (over 6 months).

   Although I much prefer leaded assembly (and think the government
entities did make a ridiculous mistake), at the end of the day, there's
really nothing wrong with lead-free and reliability is not an issue
(_if_ the manufacturer has done everything properly).

   Tony

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