Hi George,
CAF stands for 'Conductive Anodic Filament'—CAFs are capillary paths, 
typically from PTH to PTH, along the glass fibers of the PCB glass fiber 
reinforcements. The get started by damage done during PTH drilling with a dull drill, 
propagate further depending on internal vapor pressures, and on application of a 
potential difference transport metallic ions forming a conductive path. Now, 
this conductive path does not give you a dead open, but lowers insulation 
resistance by couple of orders of magnitude. 
Having PCB materials without continuous glass fibers makes tham imune to 
internal CAFs.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com

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