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June 2013

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Wed, 12 Jun 2013 09:26:25 -0500
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There have been several papers published at APEX on the topic. But whether they address the specific issues raised today I leave to wiser heads than mine.

APEX 2007: 

Effect of Conformal Coating on Tin Whisker Growth, Vijay Kumar and Linda Woody, Lockheed Martin

Parylene as a Suppressant for Tin Whiskers Growth on Printed Circuit Boards, Rakesh Kumar
Specialty Coating Systems

Whisker Penetration into Conformal Coating, Stephen McKeown, Joseph Kane, Dr. Stephan Meschter
BAE Systems, Johnson City, NY


APEX 2008: 

Effects of Tin Mitigation Processes on Whisker Growth and Solder Joint Reliability
for Chip and Small-Outline Package Components, Tom Lesniewski and Tom Higley, Northrop Grumman Network Communications, San Diego, CA 92128

APEX 2010:  

Conformal Coatings for Tin Whisker Risk Management, William Fox and Linda Woody
Lockheed Martin Missiles and Fire Control, Ocala, Florida


And Chris Hunt did publish also but the APEX papers of his do not talk about conformal coating. (He may have published somewhere else on whiskers/coatings. Dave who published on tin coated braided wire may want to weigh in after he returns.)

Having done the file search on the above whiskers/coatings APEX papers and read their summaries the answer to "Does it help?" seems (to me) to be somewhere between "yes/no/maybe/it depends." (i.e. it's "above my pay grade.")

Greg Munie
IPC Director of Design Programs


>  -------Original Message-------
>  From: Mike Fenner <[log in to unmask]>
>  To: [log in to unmask]
>  Subject: Re: [TN] minimum thickness of Type UR Conformal coat and tin whiskers
>  Sent: 12 Jun '13 09:07
>  
>  Richard, slightly tongue in cheek I ask: How does soldering with tin/lead
>  prevent tin finish on PCB whiskering?
>  
>  Phil I recall Chris Hunt of NPL in UK did some work on this. You can
>  probably download a copy of paper from their WWW.
>  
>  Best Wishes
>  
>  
>  
>  Mike
>  
>  
>  
>  -----Original Message-----
>  From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
>  Sent: Wednesday, June 12, 2013 2:22 PM
>  To: [log in to unmask]
>  Subject: Re: [TN] minimum thickness of Type UR Conformal coat and tin
>  whiskers
>  
>  Not only can the whiskers grow through the conformal coating, they can grow
>  under it like weeds in a stream. NASA has some good pictures of this on
>  their website.
>  The best tin whisker mitigation scheme is called tin/lead solder.
>  
>  -----Original Message-----
>  From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
>  Sent: Wednesday, June 12, 2013 8:10 AM
>  To: [log in to unmask]
>  Subject: Re: [TN] minimum thickness of Type UR Conformal coat and tin
>  whiskers
>  
>  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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