[tinwhiskers] RE: [tinwhiskers] Re: Conformal Coating ? When Reliability Goes Astray
- From: "Bob Landman" <rlandman@xxxxxxxxxxxxxxxxx>
- To: "tinwhiskers@xxxxxxxxxxxxx" <tinwhiskers@xxxxxxxxxxxxx>
- Date: Thu, 13 Mar 2008 20:19:01 -0400
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-effect-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-Conformal-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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