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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