Hi Wayne - yes, granted the NASA study isn't on a printed circuit board
with various component configurations which would be really nice to have
as a data set but still, the testing is one of the longest running data
sets looking at long term tin whisker growth kinetics. Tin Whisker testing
can be very complicated in terms of having controls which behave and are
representative of product situations which is one reason the industry
struggles with understanding tin whiskers. Lots of very good work
currently in progress and most of that work will produce only small
incremental gains but they will still be gains. I know its frustrating
but the industry will be able to better understand the tin whisker
phenomenon as we gain more knowledge and then develop better
mitigation/resolution solutions. There will always be various failure
modes we fight on assemblies and tin whiskers are just the next one in
line to be tackled.
Dave
From: Wayne Thayer <[log in to unmask]>
To: <[log in to unmask]>
Date: 06/17/2013 11:56 AM
Subject: Re: [TN] minimum thickness of Type UR Conformal coat and
tin whiskers
Sent by: TechNet <[log in to unmask]>
Hi Dave-
If the NASA study referred to isn't a bunch more than the previously noted
urethane delamination from tin plating "as a feature" study, then it
doesn't deserve mention in the context of selecting a conformal coat
thickness: That reference is only about tin whisker growth on tin plated
brass, with no conductors to short out and no dielectric, soldermask, or
flux residue to alter the adhesion equation.
Dave, this is what was referred to before:
http://nepp.nasa.gov/whisker/reference/tech_papers/2010-Panashchenko-IPC-Tin-Whisker.pdf
If you have some NASA paper with more appropriate context to actual
printed circuit boards, could you please reference them?
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Monday, June 17, 2013 11:02 AM
To: [log in to unmask]
Subject: Re: [TN] minimum thickness of Type UR Conformal coat and tin
whiskers
Hi gang - sorry for the late reply but as Doug detailed, I was enjoying
the SE US whitewater for the last week. As for tin whiskers and conformal
coating mitigation, a conformal coating material captures and contains tin
whiskers but does not eliminate them. There currently is some
investigative work in progress under the SERDP organization contracts by
Celestica/BAE and Rockwell Collins which will provide some insight on how
conformal coating can alter the tin surface interface reactions thus
impact tin whisker initiation/growth. The 4 mil thickness value that Phil
mentioned is from the IPC JSTD 001E Space Addendum criteria and is based
on a 12+ year ongoing investigation by NASA Goddard with a urathane
conformal coating material. There is no consensus on what is the minimum
thickness necessary for tin whisker risk mitigation by a conformal coating
material yet - although the published data does show thicker is better.
There is also an IPC JSTD 001 task group working on a "State of the
Industry" conformal coating assessment effort that is ongoing right now
which should provide the industry a baseline of typical coverage/thickness
for various conformal coating materials types/application methods. This
baseline could be used in an effort to develop what conformal coating
minimum thickness would be adequate for tin whisker risk mitigation. So
the short answer for Phil's question is there is a flurry of industry
activity trying to provide an answer to his question. Good information
takes time.
Dave Hillman
Rockwell Collins
[log in to unmask]
From: Douglas Pauls <[log in to unmask]>
To: <[log in to unmask]>
Date: 06/12/2013 08:12 AM
Subject: Re: [TN] minimum thickness of Type UR Conformal coat and
tin whiskers
Sent by: TechNet <[log in to unmask]>
Phil,
While this is an answer I "should" know, I don't. Dave Hillman regularly
attends and presents at the CALCE yearly conference on whiskers and so he
keeps up on all of that. At present, my esteemed colleague is bumping his
head on rocks, kayaking upside down, on some white water in North
Carolina. He should be back in the office on Monday and will no doubt
answer then.
From our discussions, the general rule is still "no conformal coating
prevents whiskers". A thicker coating may cause the whisker to expend
more energy punching through and yet more energy to punch through an
adjacent coating on a lead (usually resulting in buckling), but I have yet
to hear about some magic thickness of any kind of coating that completely
mitigates whiskers. But I could be wrong.
Dave?
Doug Pauls
From: Phil Bavaro <[log in to unmask]>
To: <[log in to unmask]>
Date: 06/11/2013 02:26 PM
Subject: [TN] minimum thickness of Type UR Conformal coat and tin
whiskers
Sent by: TechNet <[log in to unmask]>
Doug et al,
Is there a disagreement in the industry as to what minimum thickness of
urethane is required in order to mitigate tin whisker concerns?
I am hearing that the .003+/-.002" does not provide enough of a minimum
thickness and that the number is as high as .004". I can understand
wanting the minimum being raised to .002" but higher than that would seem
to make the process much more difficult to control.
I have a potential customer asking if we measure the thickness on the
individual component leads which is another can of worms it seems. We
always used flat samples to document our thicknesses.
I did not get to attend this years APEX so I might have missed the latest
data.
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