CAF susceptibility of PCA substrates – an NPL project

Citation

(2001), "CAF susceptibility of PCA substrates – an NPL project", Circuit World, Vol. 27 No. 4. https://doi.org/10.1108/cw.2001.21727dab.005

CAF susceptibility of PCA substrates – an NPL project

CAF susceptibility of PCA substrates – an NPL project

Keyword: Conductive filaments

An NPL, National Physical Laboratory, project has been launched to evaluate the factors affecting CAF formation. A significant amount of DTI sponsored funding is available for partners willing to participate in this project, including PCB manufacturers and users.

Conductive anodic filamentation (CAF) is a well-known phenomenon seen within the weave of epoxy-glass PCA substrates. Field failures due to CAF are caused because the copper filament reaches the anode, creating a short, or the copper dissolved from the anode is enough to destroy the anodes integrity.

The decreasing gaps between vias and the increasing number of layers seen in modern PCAs are resulting in mounting concern for many large communications, microelectronics and automotive companies. CAF susceptibility is set to increase even further as electronics are exposed to harsher environments.

Variation in CAF susceptibility has been documented from different suppliers of bare board FR4, and issues of quality control are being highlighted. Indeed, some Japanese and US companies are already marketing CAF-free substrate materials, and most will provide some CAF data in their data sheets.

The project will determine the design limits for multilayer boards in terms of CAF susceptibility, to help PCB suppliers and users, by producing a test method to qualify PCB substrate CAF performance.

The factors to be evaluated are:

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  substrate material and Tg;

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  environmental conditions (temperature/humidity);

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  via diameter and proximity (parallel and staggered);

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  voltage gradient;

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  via hole wall finish (laser or drill);

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  reflow temperature – lead free profiles.

For more information contact: Alan Brewin at NPL. Tel: +44 (0)20 8943 6805; E-mail: alan.brewin@npl.co.uk