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Date: | Thu, 11 Jan 2007 21:44:35 -0600 |
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I don't claim to be an expert but I do not believe it is CAF. The following is
some information that helps define CAF for some of the members that are not
familiar with the topic.
What is CAF? CAF is an acronym for Conductive Anodonic Filament growth.
Bell Labs first identified CAF in 1976. Field failures were later identified in 1980.
CAF is formed as a result of several events. As an epoxy resin system is
heated there is the possibility that the resin shall separate from the fiberglass
reinforcement. The separation forms an open channel at the resin to fiberglass
interface along the length of the fiberglass strand. This separation is random
and next to impossible to find by X-section. When an assembled board is
exposed to high humidity the CAF channel fills with water. The copper
corrodes and an electrochemical pathway develops. The water acts as an
electrolyte, the copper circuitry becomes the anode and cathode and the
operating voltages act as the driving potential. Basically a filament containing
copper grows along the channel formed at the epoxy resin and glass interface.
The growing filament ultimately forms a short between two unlike conductors
resulting in a field failure.
I don't see how CAF can form in a freshly assembled board. From the
information I have read on the subject it seems to take time (months not
days) for the copper filament to grow. However, nothing is impossible.
I would like to know what the laminate system is. FR4 is too generic a term
now. Nelco -6, Isola FR406, IS410... The reason why I ask is that the phenolic
resin systems have a lower bonding strength when compared to traditional
dicey-cured resin systems, IE FR4. Mechanical force can cause damage to the
phenolic resin systems.
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