[tinwhiskers] Re: Conformal Coating ? When Reliability Goes Astray
- From: "Wayt, John" <John.Wayt@xxxxxxxxxx>
- To: <tinwhiskers@xxxxxxxxxxxxx>
- Date: Thu, 13 Mar 2008 21:04:40 -0400
Hello everyone,
I was going to ask the question to the group at some point anyway, but
in light of the NASA comment I thought I'd ask now. Since they see it
necessary to re-plate their parts with tin/lead, does this mean that
there has not been adequate research into the use of trinary or
quaternary lead-free solder alloys for whisker mitigation or are they
not comfortable with the research that has been published?
John Wayt
Electronic Product Engineer
Sensus Metering Systems
450 North Gallatin Avenue
Uniontown, PA 15401
Phone - 724-425-7617
Cell - 724-331-4305
Fax - 724-439-7861
john.wayt@xxxxxxxxxx
-----Original Message-----
From: tinwhiskers-bounce@xxxxxxxxxxxxx
[mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of Bob Landman
Sent: Thursday, March 13, 2008 8:19 PM
To: tinwhiskers@xxxxxxxxxxxxx
Subject: [tinwhiskers] RE: [tinwhiskers] Re: Conformal Coating ? When
Reliability Goes Astray
Joe,
Someone two weeks ago (maybe three?) commented on the tin whiskers
teleconference that a new AMD part was found to have whiskers on all the
leads (I think it was a TQFP package). I don't know if it was
conformally coated or not but what struck me as a significant data point
was the fact that the part was "new" since we tend to think of whiskers
on parts that have some age on them. The person has not further
commented. I hope he's on this forum now (he didn't have the details
with him at the time he commented) or that someone else here heard the
comment, got the name of the person (I'm new to the telecom so didn't
catch it).
As for a whisker having to penetrate another pin of the package, I don't
believe that's necessary. Whiskers at different potentials, according
to Jay Brusse at NASA Goddard, will attract each other.
If conformal coatings were sufficient then why is NASA spending so much
money to have parts dipped in hot tin/lead solder at Corfin Industries?
(ONR ManTech research project)
Project Report for ManTech Research Project S1057:
Tin Whisker Mitigation - The Use of Robotic Solder Dipping to Replace
Electronic Part Surfaces Finishes of Pure Tin With a Tin-Lead Finish
http://www.bmpcoe.org/
From
http://nepp.nasa.gov/WHISKER/reference/tech_papers/kadesch2000-article-e
ffect-of-conformal-coat-on-tin-whisker.pdf
"Although NASA prohibits the use of pure tin plating, there is still a
possibility that some devices
may still contain pure tin plated surfaces (i.e., Commercial
Off-The-Shelf Components (COTS), hybrids, etc.). The presence of
conformal coating is often used to mitigate the whisker concern. In
these situations, projects need to understand the risks of continued use
and potential benefits of using a protective coating over the tin
surface."
"Some general observations from our experiment to date include:
. Even though complete penetration of whiskers through coating is not
yet observed, it is definite
that the coating is slowing down the whisker growth."
from
http://nepp.nasa.gov/WHISKER/reference/tech_papers/2006-Woodrow-Conforma
l-Coating-PartII.pdf
"CONCLUSIONS
During 401 days of exposure to ambient conditions, all of the conformal
coatings tested suppressed the formation of tin whiskers when compared
to the uncoated controls.
During subsequent exposure to high humidity, the controls all grew large
amounts of whiskers that were long enough to penetrate the coatings in
test. The coating that best suppressed the formation of growths under
the coating was Coating D. The worst coating for suppressing growths was
the acrylic (Coating C) which was penetrated by numerous OSE's and
whiskers. All of the other coatings fell somewhere in between Coating D
and the acrylic in their ability to suppress nodule, OSE and whisker
formation. All of the coatings (both thick and thin) were eventually
penetrated by whiskers which
indicates that these coatings can not be depended on as a foolproof
mitigation strategy."
As I began this discussion, I'd like to underscore what concerns me
about conformal coatings. The word "mitigate" is a very misleading word.
I respectfully suggest that when it comes to hi-rel systems such as
military, aerospace, medical as well critical utility industry systems
(which includes the electric grid we all depend on daily as Phil just
indicated) that we need to do more than "mitigate" the problem.
This is a problem that never existed until manufacturers were seduced by
the EU to eliminate tin/lead plated parts from their production runs.
My company does not want them; we are exempt from having to use them as
we supply the electric industry. We industrial manufacturers (as I'm
sure is the case in the medical industry) do not have NASA's budget to
hot solder dip parts. Furthermore, even if we did, we have a serious
concern as do others who have studied this matter for a long time, that
the additional temperature cycle stress of hot tin/lead dipping is
reducing component reliability. I cannot begin to imagine the cost to
hot dip every resistor, capacitor, inductor, connector and semiconductor
part on our dense boards.
And if you are an environmentally minded person, let's not forget the
additional energy being wasted to do it (or to solder lead-free SAC
alloys at a 30C higher temperature).
Tests have been done on hot dipping that indicate there is no damage
(the ManTech draft report at www.bmpcoe.opg) but more tests have to be
done.
And I have to say that it just strikes me as a bizzare process we never
should have had to bother with in the first place!
Sincerely,
Bob Landman
H&L Instruments,LLC
-----Original Message-----
From: tinwhiskers-bounce@xxxxxxxxxxxxx
[mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of Kane, Joseph E
(US SSA)
Sent: Thursday, March 13, 2008 6:14 PM
To: tinwhiskers@xxxxxxxxxxxxx
Subject: [tinwhiskers] Re: Conformal Coating ? When Reliability Goes
Astray
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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