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June 2006

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Subject:
From:
"Dehoyos, Ramon" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Dehoyos, Ramon
Date:
Thu, 22 Jun 2006 11:03:43 -0400
Content-Type:
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        I knew there was some Indiana Jones in you, Richard. In my
barrios I had to jump over fences, dodge bullets and bite dogs back to
stay alive too. So when I joined the Army it was easy. Do not want to
relive them, I like peace.


 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Thursday, June 22, 2006 9:57 AM
To: [log in to unmask]
Subject: Re: [TN] Board material

Hi, Lee
Good enough. My experience with it began in 1977 on telephone network
circuit cards used in Saskatchewan, Canada. I was a telephone tech at
that time and ran into CAF when attempting to troubleshoot the boards
while straddling the pole. I eventually had to pull the entire network
cards out and replace them. Nobody could figure out why the boards were
"leaking" current from the power supply to the signal lines, disrupting
the service. They finally figured it out that with the extremely cold
mornings turning into extremely hot days, the condensation was actually
infiltrating the pwb along the power supply buss terminal (not conformal
coated), and over time CAF formed.
I ran into CAF again in 1979 at Honeywell, and this time I recognized
it. When the pwb supplier changed to the different pre-preg, no more
problems. 

Those were the good old days, straddling the pole and trying to debug
the network while simultaneously shooting the brown bears trying to
climb the pole to eat me for breakfast, all in a howling sandstorm on
the God-forsaken wide open sand prairies near Saskatoon. Nobody around
for several hundred miles, and sometimes I would not see another person
for three or four days while I worked. The last thing I needed was CAF.
Impossible to see or detect using standard troubleshooting tools of the
time. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee parker
Sent: Thursday, June 22, 2006 8:34 AM
To: [log in to unmask]
Subject: Re: [TN] Board material

Richard

Actually CAF was first identified by Dave Lando who was in the same Bell
Laboratories organization as Werner and myself. This was in 1974, just
after we opened the new and world's largest PCB shop in Richmond.
Probably the most important attribute of CAF is as the name suggest, the
filaments grow from the anode to the cathode which is the opposite of
what one would anticipate.

Best regards

Lee

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


----- Original Message -----
From: "Stadem, Richard D." <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, June 22, 2006 9:06 AM
Subject: Re: [TN] Board material


In about 1978, when CAF was realized as a problem in standard PTH power
supply boards, the strands were continuous through the pre-preg. It was
then that the fab shops started to used chopped fiberglass mat weave, in
order to provide shorter strand lengths so CAF formation was less likely
to happen. However, because of circuitry being so close together
sometimes with fairly high potential, CAF has become more of an issue
again.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
Sent: Thursday, June 22, 2006 7:36 AM
To: [log in to unmask]
Subject: Re: [TN] Board material

        Hi Werner, could you expand on "Having PCB materials without
continuous glass fibers makes them immune to internal CAFs"?  I was
under the impression that the continuous glass fibers were the strength
of the board.
        Regards,
        Ramon

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Wednesday, June 21, 2006 3:58 PM
To: [log in to unmask]
Subject: Re: [TN] Board material

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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