Cathodic Anionic Filiments and how to prevent them in our PCB Designs

[Ex-Silicon Valley]

Does anyone know where I can get information about Cathodic Anionic Filiments (CAF)? These are the little whiskers that can form possibly creating a contact/short.
Thanks,

[Tom]

IPC-T-50 Terms and Definitions.

Cathodic Cleaning:
Electrolytic cleaning in which the work is the cathode.

Electrolytic cleaning:
Cleaning in which a current is passed through an alkaline solution with the part to be cleaned being one of the electrodes.

So I guess this process removes those little whiskers.

[Skip Yutkus]

I believe PC-FAB magazine, (now Printed circuit Design and Manufacture), had several articles on this subject, you may be able to find them in their archives.

Skip

I found this one and I believe there is at least 1 other.

CAF Resistance of Non-Dicy FR-4," Erik J. Bergum, September 2002

[robert Tarzwell]

hi
Cadthodic Anionic filiments are small groths of metalic ions that form wiskers, they are quite conductive so should be avoided. Sorry Tom but what you refer to is cleaning using reverse palting and not anything to do with wiskers.
The Cathodic Anionic fliliments grow because of a few well defined properties , first you need a fairly pure metal, silver , tin and some times copper all can grow conductive filiments, you also need moisture or humidity to create a small plating cell with in the circuit board, and you need a DC current/voltage from past work around 5 to 15 volts is best. to help the growth the board would have a contaniation of acid or caustic surface, typicaly boards have a acidic surafce.
the voltage causes the pure metal to plate just like a plating tank the filiments can grow as far as .2 inch, any conbining of metals seems to stop the growth if you add even the smallest amount of lead to a tin bath it will not create filiments, which is why we always add a small lead anode to our tin baths.
hope this helps Robert Tarzwell megadawn.com

[Ex-Silicon Valley]

Thanks everyone for the CAF information!
I am a PCB Developement Engineer for a well known component manufacturer located in Southern California. My companies Reliability labs use HALT (Highly Accelerated Life Test) tests on our components (diodes, IC, etc.) that we design and manufacture. These components under test are soldered to my PC boards and then put thru these various HALT testing procedures. This concept of C.A.F. is undefined but we seem to have had some board failures due to CAF. Our test PC boards are sometimes put in an autoclave and subjected to a high moisture environment, then placed in high temperature ovens for further high temperature cycling, etc. This is when CAF seems to be forming in our test boards, sometimes causing test board failures. When I design these test boards, I try to keep the (anode and cathode) current traces from running in parallel or on top of each other so no whiskers can form and shoot/grow up to contact another trace. I can't seem to find any good, solid information about how to design a PC board that nullifies these CAF formations. I understand alot of CAF is just theory and no hard facts/evidence has been issued. Any information you have concerning how to design a "CAF free" PC board in these types of HALT testing environments would be appreciated.

[robert Tarzwell]

Hi there are many ways how to design printed circuit boards to eliminate CAF, I am a consultant and could help you, please e-mail me direct to discuss your problems
robert Tarzwell rtarzwell@megadawn.com