[tinwhiskers] Re: Conformal Coating ? When Reliability Goes Astray
- From: "Kane, Joseph E \(US SSA\)" <joseph.kane@xxxxxxxxxxxxxx>
- To: <tinwhiskers@xxxxxxxxxxxxx>
- Date: Thu, 13 Mar 2008 18:14:23 -0400
This could be a good forum. Not sure who's signed on yet, but
I'll throw this out there.
In order to cause a failure, whiskers must grow straight enough and
long enough to contact an adjacent conductor. They have to contact
the adjacent surface with enough force to make electrical contact, which
in some cases means penetrating through a second layer of conformal
coating on the other surface. And they have to do this in the time
frame
that the product is in service.
Looking at the available literature, it seems like the following things
are
at least partly true, depending on circumstances. Of course, your
mileage
may vary.
1. In some cases, conformal coat can delay the onset of whiskers (longer
incubation period).
2. In some cases, it can prevent whisker growth altogether.
3. In most cases, conformal coat slows down the growth rate.
4. For whiskers that grow through coating, most are gnarled, kinked, or
otherwise unlikely to cause a direct short. It's possible that the
coating
causes this, e.g. maybe the stress of penetration affects the
morphology.
5. Whiskers may grow through coating, but may have a harder time
penetrating the coating on an adjacent conductor without buckling.
Any one of these effects may be pretty good mitigation.
As far as statistical models go, it seems like there are still too many
unknowns to make meaningful predictions. But since the entire field
seems to
thrive on example and anecdote, I think it's worth asking this:
Does anyone know of a failure of a fine-pitch electronic component that
has
been conformal coated?
Furthermore, has anyone seen a picture of a long, straight whisker
that's
grown through a coating?
With all of the pure tin terminations that are out there, we should have
some
examples by now.
Joe Kane
BAE Systems
Johnson City, NY
-----Original Message-----
From: tinwhiskers-bounce@xxxxxxxxxxxxx
[mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of Ray
Sent: Thursday, March 13, 2008 5:30 PM
To: tinwhiskers@xxxxxxxxxxxxx
Subject: [tinwhiskers] Re: Conformal Coating ? When Reliability Goes
Astray
Bob,
The harder the conformal coat the more it will slow done the propagation
of
the tin whiskers, slow down is NOT the same as stop. It is the
un-contented
nature for tin to want to be in a crystal string, not in a ball. It
will
penetrate, just like a trees roots will penetrate through concrete. In
eutectic solder there is enough lead to keep the tin contented and keep
it
from forming tin whiskers. Therefore, a hard conformal coat (urethane),
only
slows down the growth of the tin-whiskers. The better the adhesion, the
more
it will mitigate the growth.
In regards to adhesion of the conformal coat, any trace of silicon on
the
surface, which may not be detectable with a 30x power microscope, is the
worst contaminate for either urethane or acrylic conformal coat
materials.
This is in addition to other residues.
Respectfully,
Raymond Bennett
President
RNB Enterprises, Inc.
602-889-3461 Direct
602-978-0248 FAX
-----Original Message-----
From: tinwhiskers-bounce@xxxxxxxxxxxxx
[mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of Bob Landman
Sent: Thursday, March 13, 2008 1:38 PM
To: tinwhiskers@xxxxxxxxxxxxx
Subject: [tinwhiskers] Conformal Coating ? When Reliability Goes Astray
We are told that conformal coatings are a successful tin whiskers
mitigation
strategy. Correct me if I'm wrong, but I don't believe it's been
demonstrated to any significant degree that any conformal coating on the
market today will "prevent" tin whiskers from punching through the
coating.
I note that it's popular to use the word "mitigate" and that's a word
that
is not as strong a word as "avoid" or "prevent". I await someone who
can do
the math on how statistically significantly conformal coatings
"mitigate"
tin whiskers.
A dictionary states that the word means "To moderate (a quality or
condition) in force or intensity; alleviate."
By how much? The word itself gives us no clue.
If a whisker can grow from one pin on an IC package, then certainly, it
can
also grow from adjacent pins as well and then don't we have the perfect
opportunity for shorts?
I just read the article below on reliability of conformal coatings that
I
thought worth sharing if we are going to have to count on such a
coating to
save our lives.
-Bob Landman/H&L Instruments,LLC
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