It doesn't. Which is why lead-free tin finishes are not specified in high-reliability electronic assemblies such as for space or avionics, at least not by anyone who knows what they are doing.
Don't choke on your tongue there, buddy.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Fenner
Sent: Wednesday, June 12, 2013 9:07 AM
To: [log in to unmask]
Subject: Re: [TN] minimum thickness of Type UR Conformal coat and tin whiskers
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.
________________________________
This message and any attachments are solely for the use of the addressee and may contain L-3 proprietary information that may also be defined as USG export controlled technical data. If you are not the intended recipient, any disclosure, use or distribution of its content is prohibited. Please notify the sender by reply e-mail and immediately delete this message and any attachments.
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
|
