[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 21:13:44 -0400
John,
I can't speak for NASA but I do read and hear what they say and they say AVOID
pure tin at all costs. While the solder alloys are not pure tin, the soldering
process does not wet the entire surface of the parts that have become pure tin
plated. Furthermore, I'm told that no matter what the solder alloy is, if
there is pure tin under it, the pure tin still has a propensity to whisker and
push up through the alloy.
From http://nepp.nasa.gov/WHISKER/background/index.htm
Suggestions for Reducing Risk of Tin Whisker Induced Failures:
The uncertainties associated with tin whisker growth make it extremely
difficult to predict if/when tin whiskers may appear. The following list
provides some suggestions for reducing the risk of tin whisker induced failures.
1.
Avoid the use of PURE TIN plated components if possible. Utilization of
procurement specifications that have clear restrictions against the use of pure
tin plating is highly recommended. Most (but not all) of the commonly used
military specifications currently have prohibitions against pure tin plating.
Studies have shown that alloying tin with a second metal reduces the propensity
for whisker growth. Alloys of tin and lead are generally considered to be
acceptable where the alloy contains a minimum of 3% lead by weight. Although
some experimenters have reported whisker growth from tin-lead alloys, such
whiskers have also been reported to be dramatically smaller than those from
pure tin plated surfaces and are believed to sufficiently small so as not to
pose a significant risk for the geometries of today's microelectronics.
2.
Post Procurement It can be dangerous to rely on the part manufacturer's
certification that pure tin plating was not used in the production of the
product supplied. NASA GSFC is aware of several instances where the procurement
specification required "No Pure Tin", but the product supplied was later
determined to be pure tin. In some of these instances, tin whisker growths
were also discovered. Users are advised to analyze the plating composition of
the products received as an independent verification.
When simple avoidance of pure tin plating is not a viable option (such as in
cases where its use is discovered late in system integration/test), then the
following approaches may also be considered to reduce risk.
1.
Solder Dip the plated surfaces sufficiently using a tin-lead solder to
completely reflow and alloy the tin plating. Obviously, special precautions are
required to prevent thermal shock induced damage, to prevent loss of
hermeticity and to avoid thermal degradation. This approach may have limited
success since it may be difficult to ensure that the entire surface is properly
reflowed. See the April 2004 Photo of the Month for one example of the
limitations associated with this particular mitigation strategy.
2.
Replate the whisker prone areas. Some manufacturers may be willing to
strip the pure tin plate from finished products and re-plate using a suitable
alternate plating material such as tin/lead or Nickel. Caution is advised if
considering use of an external plated finish (e.g., Sn-Pb or Cu) on top of an
existing pure tin deposit. There is some evidence that whiskers may still form
from the pure tin layer and protrude through the thin external deposit.
3.
Conformal Coat or foam encapsulation over the whisker prone surface
appears to be beneficial but the limitations are not completely understood.
NASA GSFC experiments suggest that use of Uralane 5750 conformal can provide
some benefit by reducing the growth rate, but tin whiskers can grow through
conformal coating and once exposed can then short to other tin whiskers or
other exposed surfaces. See also a technical paper (~11 Mbytes) and
presentation (~19 Mbytes) by Dr. Tom Woodrow (Boeing) from 2005 in which he
evaluated 6 different conformal coat materials for purposes of inhibiting
whisker formation and subsequent shorting hazards.
It has also been demonstrated experimentally that conformal coating can
restrict the availability of tin sufficiently to minimize the risk of plasma
formation. However, such factors as the minimum thickness of coating necessary
to prevent whisker growth or plasma formation have not been determined.
Similarly, it has been shown that foam can prevent sustained arcing but the
effects of foam type, foam density, pore size etc. have not been evaluated.
Additional studies and evaluations are underway to try to answer the critical
open questions in order to provide more detailed suggestions in the future.
4.
Evaluate Application Specific Risks. A variety of application specific
considerations may be used to assess the risk of whisker induced failures and
assist in making "use as-is" or "repair/replace" decisions. These factors
include circuit geometries that are sufficiently large to preclude the risk of
a tin whisker short, mission criticality, mission duration, collateral risk of
rework, schedule and cost. To assist in evaluating application specific risks,
David Pinsky (Raytheon) has developed a tin whisker risk assessment algorithm
which can be reviewed (note: reference to this algorithm herein does not imply
endorsement by NASA).
In 2002, Dr. Mike Osterman (CALCE Center at the University of Maryland)
published a white paper outlining pros and cons of assorted strategies for
mitigating risks associated with tin whiskers.
Responsible NASA Officials:
Michael Sampson/NASA GSFC Code 306
Dr. Henning Leidecker/NASA GSFC Code 562 Additional Researchers:
Jong Kadesch/Orbital Sciences Corp.
Jay Brusse/Perot Systems
Last Updated:
February 22, 2007
DISCLAIMER:
This website provides information about tin whiskers and related research. The
independent research performed during the past 50+ years is so vast that it is
impractical to cover all aspects of tin whiskers in this one resource.
Therefore, the absence of information in this website about a particular aspect
of tin whiskers should NOT be construed as evidence of absence.
-----Original Message-----
From: tinwhiskers-bounce@xxxxxxxxxxxxx
[mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of Wayt, John
Sent: Thursday, March 13, 2008 9:05 PM
To: tinwhiskers@xxxxxxxxxxxxx
Subject: [tinwhiskers] Re: Conformal Coating ? When Reliability Goes Astray
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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