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November 2013

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From:
Wayne Thayer <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wayne Thayer <[log in to unmask]>
Date:
Wed, 6 Nov 2013 12:16:05 +0000
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Hi Phil-

I don't believe the "wicking" is DIRECTLY responsible for any failures that aren't direct shorts after plating.  

CAF is voltage gradient related, but not to the extent a spark is--the radius of curvature of the initiation point is likely a second, or even third, order effect.  However, the overall reduction in distance between voltage terminals caused by "wicking" is a primary factor in CAF formation. 

"Wicking" is mostly a process indicator.  It indicates that the adhesion between the resin and fibers in the laminate has been damaged.  If that damage is severe enough, it will enable the chemical reaction (ion movement) which is directly responsible for CAF.

The threshold for "wicking" has been set at a level which IPC believes shows good process controls are being employed and the resin-to-fiber bond is not likely to have been damaged by the drilling and other processes used during PCB manufacture.  This threshold appears to be achievable by most high end vendors with good yields and there have been few or no complaints from end users that products are failing due to this standard not being strict enough.  As Joyce has suggested, stepping outside of it is dangerous because we don't have the data to defend doing that, and generating/analyzing that data would be time consuming and expensive.

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Tuesday, November 05, 2013 7:03 PM
To: [log in to unmask]
Subject: [TN] IPC-6012 Copper Wicking Defect in PTH

I know what the spec limits are for wicking and I have a coupon that just barely violates the maximum requirement for class 3A.

I was asked what kind of failure this would lead to if it were used and I could not answer the question.

I researched the Technet archives and found an email from Paul Reid where he stated:

"The worst-case condition is a short between holes drilled on a tight grid.  The most insidious condition that wicking is a site for CAF (conductive anodic filament) growth."

There is no tight grid on this board so the short between holes is not going to happen.  CAF is always a concern but I would have thought that a failure would be related to some kind of stress point mechanism of failure based on how rough the hole looks.  I know a picture would make this a lot easier but I am not at liberty to release it.  The copper in the hole is very thick but the entire hole looks like it had a bad drill and overly agressive desmear operation.  There is wicking all up and down the wall of the hole but only excessive in length in one spot.

So the question is:  How does excessive wicking cause a failure?  This was the kind of question that I used to send to Werner to get further education on.

Any and all comments are appreciated.

Phil

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