David

First of all I worked for Bell Laboratories for 27 years most of it in the 
Interconnection Laboratory that identified CAF in the first place. We never 
found a for sure CAF field failure, only CAF like failures.

Based on this limited evidence, however, a large fraction of the PCB product 
made in Richmond was converted to BT to make sure the problem would not 
occur. Later in the middle 80's other members of the same Laboratory did 
more work to understand the issue. They demonstrated that CAF boards in 
service were highly unlikely to have CAF since the surface humidity of an 
installed working PCB is all but zero because the temperature is elevated by 
the heat generated by the conductors and components. At this point, the 
product using BT was converted back to FR4 and CAF became essentially a 
closed issue.

Identifying CAF is very difficult. First one must have evidence that the 
growth began at the Cathode and not the Anode which is normally the case. In 
the work done by Lando who discovered CAF the coupons were biased. So far as 
I am aware this is still the case.

I would be very reluctant to call any field failure CAF without overwhelming 
evidence.

Best regards

Lee

J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389
Best regards

Lee


----- Original Message ----- 
From: "David Duross" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, January 11, 2007 10:44 PM
Subject: Re: [TN] Is this CAF?


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.

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to 
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to 
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 
for additional information, or contact Keach Sasamori at [log in to unmask] or 
847-615-7100 ext.2815
----------------------------------------------------- 

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------