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April 2003

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Subject:
From:
Paul Signorelli <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 17 Apr 2003 11:15:50 -0500
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Joe
You comments are correct...
Whiskers are very hard to see visually, maybe no one is really looking for them.
They are also very fragile, They can break off and the casuse is gone.
i suspect they are right behind dendrites on the Prateo chart.
i was involved in a Space Failure caused by a tin whisker on a Terminal.
i have seen lots of them...
Paul Signorelli
Reliability Engineer

>>> [log in to unmask] 04/17/03 10:08AM >>>
Some of our component suppliers have notified us that they will be
changing some lead finishes to pure tin.  We suspect that this may
be the beginning of a landslide, and as a high-rel supplier, we have to
consider the risk of shorting from tin whiskers.

Until about a decade ago, we used a lot of parts with tin finish, and
never saw a whisker, at least that we know of.  Whiskers appear to be rare,
but the risk does not seem to be well understood, or at least quantifiable.
Maybe they're lost within the noise of normal component failures.

Technical literature has some anecdotes and a few interesting pictures,
and there are suggestions on possible causes (stresses within the plating
or from lead forming), and mitigations (e.g. matte tin is better than bright

acid tin, conformal coat is good).  What we don't seem to have is
anything that a reliability engineer could use to allow comparison of risks
and benefits.

One easy reaction would be to hot-solder dip the leads, but this will
be expensive, and there is a finite probability of damage from the extra
handling, ESD, moisture exposure, and the heat cycle.

Maybe we could anneal the parts after plating or lead forming, or even
the whole assembly after soldering.  Don't know what temperatures or
bake times would be needed to anneal pure tin.

I'm posting here instead of a lead-free forum because because this is
going to affect us all, whether we use tin/lead solder or not.  And even
with the dwindling membership, the jokes are better.

Joe Kane
BAE SYSTEMS Platform Solutions
Johnson City, NY

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