September 12, 2004
We also think overhang is the problem. Poor copper plating control allowed excessive copper buildup. After tin/lead plating and abrasion using scotch brite, the tin/lead/copper flaked off into slivers which were trapped under the solder mask. Ionic contamination test failed to detect the slivers and I did not expect it to. See attached MIL-P-55110 coupon for details.
----- Original Message -----
From: Jeffrey Bush<mailto:[log in to unmask]>
To: [log in to unmask]<mailto:[log in to unmask]>
Sent: Friday, September 10, 2004 11:10 AM
Subject: Re: [TN] PWB Conductive Contamination Under Solder Mask; What Can Cause it?
Sounds like an overhang that has dislodged from surface prep prior to
masking. Overhang can be caused from excessive plating, image
resolution problems and poor develop. If the condition is present when
potentials are not close, then I doubt the cause is dendritic or any
type of electro migration - which occurs over time in operation. A
microsection of a line will provide the answers you seek. I suggest
sending a board to Susan.
Jeffrey Bush
Director, Quality Assurance and Technical Support
76 Technology Drive - POB 1890
Brattleboro, Vermont 05302-1890
Tel. 802.257.4571.21 Fax. 802.257.0011
[log in to unmask]<mailto:[log in to unmask]>
http://www.vtcircuits.com<http://www.vtcircuits.com/>
-----Original Message-----
From: Brian Ellis [mailto:[log in to unmask]]
Sent: Friday, September 10, 2004 11:04 AM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [TN] PWB Conductive Contamination Under Solder Mask; What
Can Cause it?
The original poster sent photos to me.
It would seem probable that the problem may be due to fine fibres or
hairs trapped under the solder mask, causing vesication, water
absorption, leakage and electrochemical migration.
These fibres occur also in places where they do not short-circuit
conductors. They are all shapes and sizes.
Brian
- Bogert wrote:
> September 8, 2004
>
> A technical challenge for all you printed wiring board (PWB)
> manufacturing experts. Please help.
>
> The attached photographs show conductive contamination trapped under
> LPI solder mask covered over by Type UR conformal coating (.003 in
> nominal thickness).
>
> The PWB is a two sided PTH (no SMT parts) board of FR-4 (thickness
> range is .062 to 0.125 inch depending on the PWB part number) with
> solder mask applied over bare copper. The PWB is used in a military
> application. The PTH are tin-lead. The same type contamination has
> also been found on multi-layer PWB'S.
>
> The photos show what look like tin whiskers (I do not think they are
> since SEM analysis shows the material as a combination of tin, lead
> and copper and normally tin whiskers grow only on virgin plating such
> as pure tin, silver, zinc, etc.).
>
> I have two questions as follows and request your expert opinions in
> answering the questions:
>
> 1) What process or material problem could have caused the conductive
> filaments to form, and how to prevent it from happening again? My
> best guess is contamination residue from the etch-back process.
>
> 2) If the PWB passes shorts testing and end-item functional test, is
> there any concern with continued use in a humid environment given that
> the contamination is trapped under a layer of solder mask as well as a
> layer of Type UR conformal coating.
>
> Any info available on how long solder mask and conformal coating can
> retard moisture entry? I know that neither material will prevent
> eventual moisture entry. Is there any data that shows how long these
> materials will retard moisture from causing dendrite growth problems?
>
>
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> Any concern for dendrite growth or other no-no's?
>
>
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