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A new approach to investigate conductive anodic filament (CAF) formation

Ling Chunxian Zou (Engineering and Process Control Division, National Physical Laboratory, Teddington, UK)
Chris Hunt (Engineering and Process Control Division, National Physical Laboratory, Teddington, UK)

Soldering & Surface Mount Technology

ISSN: 0954-0911

Article publication date: 2 February 2015




This paper aims to describe the development of an approach that uses a flexible substrate to investigate the mechanism of conductive anodic filament (CAF) growth and effect of different material and manufacturing variables.


A new approach using a simulated test vehicle (STV) has been developed to study the CAF phenomena. The STV can be easily built under controlled conditions in the laboratory using different glass fibres and resin powder to investigate the effect of different variables separately on CAF. The advantage of the STV is that CAF can be formed in relatively short period in a controlled way, and CAF growth can be easily identified using a back-lighting under a microscope due to the thin flex material used as the test sample.


STV has been used to investigate a number of effects on CAF formation: different glass fibres, reflow process, acid contamination in drilled holes, desmear process and glass bundle size. The results demonstrate that for finished fibres acid contamination (plating solution) at the electrode was necessary for CAF formation. However, for unfinished glass fibres (loom state and heat cleaned) CAF can be formed without acid contamination. The reflow process significantly increases CAF formation. Running an aggressive desmear process and using large glass fibre bundle also increased CAF formation.


This new approach will be of benefit for printed circuit board (PCB) supplier to evaluate CAF performance on different resin systems and glass fibres to provide high CAF resistance quality PCBs. The test period (168 hours) would be much shorter than the traditional CAF testing (1,000 hours).



This paper has been produced in a Joint Industry Project, part of the Materials Measurement Programme sponsored by the National Measurement System unit of the UK’s Department for Innovation Universities and Skills.

The authors wish to acknowledge the finical support and assistance to the project provided by the following companies: Aero Engine Controls, European Space Agency, GEN3 System, Graphic PLC, IBM Corporation, Invotec Group Limited, Isola Group – Europe, MBDA Missiles, Robert Bosch GmbH, Rolls-Royce, Texas Instruments, TRW Automotive.

The authors would also like to acknowledge the many helpful discussions with Martin Wickham, and note the help from Anthony Jackson, Paul Comer and Alun Morgan.


Zou, L.C. and Hunt , C. (2015), "A new approach to investigate conductive anodic filament (CAF) formation", Soldering & Surface Mount Technology, Vol. 27 No. 1, pp. 22-30.



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