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September 2004

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Fri, 10 Sep 2004 18:03:35 +0300
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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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