Bogert,

You will need to send this photo to a thrid party (Steve Gregory:
[log in to unmask] or Daan Terstegge: [log in to unmask])
and have them post it on their site.

David A. Douthit
Manager
LoCan LLC

- 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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