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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