[AR] Re: Tin whiskers
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Mon, 1 Aug 2016 15:40:58 -0400 (EDT)
On Sun, 31 Jul 2016, Norman Yarvin wrote:
...it only takes one or two percent of lead to suppress whiskering.
(And that in an already-thin tin plating; in a world where lead-acid car
batteries are made by the millions, banning lead in tin platings seems
pretty silly.)
It is pretty silly. The ill-advised attempt to eliminate lead from
electronics has caused grief out of all proportion to any likely benefit.
Unfortunately, although aerospace/hi-rel stuff is nominally exempt from
the no-lead rules, that doesn't help much if you're using COTS parts,
which are almost invariably tin-plated now. (Even if the part is also
available with solder plating, there is the interesting question of
whether the parts you get are the right variant -- it's by no means
unheard-of for distributors to get them mixed up. But most parts simply
aren't available with anything but tin now.)
Conformal coatings can help...
Unfortunately, they have their own problems. Doing them right is a little
tricky, they make debugging and/or repairs a bit of a nightmare, and they
can seal *in* corrosion problems if things aren't done just right.
They also help against whiskers that are floating around in the
environment (say, zinc whiskers from a nearby piece of galvanized
steel).
Generally better addressed by excluding such conductive debris, i.e. by
being more careful about materials choices and cleaning.
The only entirely satisfactory fix is to solder-dip all parts before use.
It's a costly pain but it does work, when you're making your own boards.
Conformal coatings, despite their headaches, may be the best fix for COTS
boards.
Henry
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