Conductive anodic filament (CAF) is also a possibility.

From:
http://www.emtonthenet.net/glossary/CAF.html
CAF
'Conductive Anodic Filamentation' (also referred to as CFF - Conductive Filament Formation) is a sub-surface failure mode of woven glass reinforced epoxy laminate PCB substrates.
In the presence of voltage and moisture, conductive filaments of copper salts can form. They will grow between copper vias, pads or tracks, from the anode (+V) to the cathode (-V) commonly along the route of glass fibres within the laminate. (Note that surface copper dendrites form by a different plating mechanism and grow in the reverse direction).
Exposure of the laminate to high temperatures increases the likelihood of CAF failure due to breakdown of the bond between the glass fibres and the epoxy. This effect has been shown as significant in the context of changing profiles for lead-free reflow.
Additionally the drive towards finer pitch components is increasing CAF susceptibility of product. Higher voltage gradients enhance CAF growth, but more significantly CAF growth becomes much more rapid as the gap between features decreases.
Laminate manufacturers have a good understanding of the CAF mechanism and CAF-Free laminate materials are available.
Mike Green
Electronic Product Design
http://epd.lmms.lmco.com/
Lockheed Martin Space Systems
1111 Lockheed Martin Way
Sunnyvale, CA 94089
office  408-743-1635
Synergy works at LMC.  "Show me the data."
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Bhaskar.C.S
Sent: Saturday, October 16, 2004 2:25 AM
To: [log in to unmask]
Subject: [TN] internal shorts in MLB'S

What are the various reasons for getting internal shorts in Multilayer Pcb's?


REGARDS

D.VENKATESWARLU

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