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January 2007

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Subject:
From:
David Duross <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, David Duross <[log in to unmask]>
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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