Edwin,

The problem with tin whiskers has been known since the 1940 - 1950s, and was
solved over 30 years ago by prohibiting the use of pure tin plating on
component leads, component bodies, connectors, and mechanical components.
With the lead-free electronics initiative, the problems with tin whiskers
have returned.

Tin whiskers are electrically conductive, crystalline structures of tin that
sometimes grow as a result of mechanical stress in the tin crystal lattice,
from surfaces where tin (especially bright electroplated tin) is used as a
final finish.  Tin whiskers have been observed to grow to lengths of several
millimeters (mm) and in rare instances to lengths up to 10 mm.  Because they
are crystalline structures they are very strong, and capable of conducting
significant amounts of power before melting or vaporizing.

Whisker growth does not require moisture or ionic contamination as an
activator.  Growth rates from 0.03 to 0.9 mm/yr have been reported, and in
instances where component leads plated with pure tin (bright electroplate),
whisker growth was observed immediately following bending for PWB insertion.

Growth of whiskers has been shown to be controlled by tinning with tin-lead
(SnPb) solder, and as little as 2% Pb in the alloy or tin plating will stop
whisker growth.

Conformal coatings will not stop whisker development, and preliminary
results show that in many instances, resilient conformal coatings appear to
only slow whisker growth - with the probability of punch through increasing
as the coating thickness or hardness is decreased.

The NASA NEPP website is an excellent resource.

Bob Cooke
[log in to unmask]


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Douthit
Sent: Monday, July 26, 2004 9:47 AM
To: [log in to unmask]
Subject: Re: [TN] Tin Whiskers

Edwin,

NASA did some experiments and have pictures on their web site
(   http://nepp.nasa.gov/whisker/index.html  )  of whiskers "punching"
through
coatings. I don't know if the coating "inhibited" growth (time, length,
or amount).

David A. Douthit
Manager
LoCan LLC

Edwin Louis wrote:

>Does anyone know whether a Polyurethane conformal coating would inhibit Tin
>whisker growth or at least contain it?
>
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