Conductive anodic filament

BS butadiene styrene rubber CAFG conductive anodic filament growth... 

The ability of a material to resist water absorption, either from the air or when immersed in water, is important for printed circuit rehabUity. Besides the obvious concerns of moisture causing defects when a material is subjected to thermal excursions, absorbed moisture also affects the ability of a material to resist conductive anodic filament (CAF) formation when a... 

The density of IC packaging technologies such as ball grid array (BGA) and chip scale packages continues to increase. In turn, this requires greater intercoimection density in the printed circuits onto which they are assembled. The need for increased density in the PCB impacts each of the components of the base material as well as the way in which they are manufactured. To achieve high levels of interconnection, component pitch densities result in smaller and more closely spaced plated through holes and circuit features. As the space between these holes and features decreases, the potential for conductive anodic filament (CAE) failures increases substantially. 

Conductive anodic filament (CAF) formation is a term used to describe an electrochemical reaction in which conductive paths are formed within a dielectric material due to transport of metal or metal salts through the dielectric. These paths may form between two circuit traces, between two vias, or between a trace and a via, as illustrated in Fig. 9.12. CAF formation between a hole and a plane inside the PCB is also possible, and is similar in concept to hole-to-trace CAF formation. By definition, as circuit density increases, the space between these features decreases. With shorter paths between features, CAF growth becomes a more critical reliability consideration. 

Conductive anodic filament (CAP) resistance test X-Y axis ... 

Since the coating is intended to be waterproof it is imperative that all moisture be baked out before the coating is apphed, especially for hydroscopic substrates such as GI polyimide. If moisture is left in place, it can cause corrosion of traces and/or parts and will promote dendritic growth between conductors, as well as enabling the growth of conductive anodic filaments (CAF) along the glass fibers in the weave of the PWB. A bake of 93°C for 4 hours will be sufficient to drive out moisture from the CCA. 

Mitchell, J. P., and. Welsher,T. L., Conductive Anodic Filament Growth in Printed Circnit Materials, paper prepared for the Circuit World Convention II, Mnnich, pnblished as IPC Technical Report WC-2A-5, June 1981. 

Conductive anodic filament (CAF) As the base material system degrades, the resistance to CAF formation decreases. 

Board cleanliness is needed to ensure sufficient removal of contaminants that could affect functionality at present or in the future. Some contaminants can actually promote growth of undesirable substances on the PCBA that can cause shorting or corrosion that would affect the PCBA s electrical functional integrity and dielectric properties over a period of time. These contaminants can be in the form of surface dendrites, internal conductive anodic filaments, and so on. 

D. J. Lando et al. first used the term conductive anodic filament (CAF) to describe this faUtrre in 1979. They defined the mechanism of CAF formation as a two-step process degradation of the epoxy/glass interface, followed by the electrochemical reaction (see Eq. 56.2). 

Lando,D.X, Mitchell, IP., andWelsher,T. L., Conductive Anodic Filaments in Reinforced Polymeric Dielectrics Formation and Prevention, IEEE Reliability Physics Symposium Proceedings, Vol. 17, 1979, p. 51-63. 

Ready, W. I, and Turbini, L. I, The Effect of Flux Chemistry, Applied Voltage, Conductor Spacing, and Temperature on Conductive Anodic Filament Formation, Journal of Electronic Materiab, Vol. 31, No. 11,2002, pp. 1208-1224. 

Ready, W. 1.,Factors Which Enhance Conductive Anodic Filament (CAF) Formation, Master Thesis in Materials Science and Engineering, Georgia Institute of Technology, 1997. 

Ready, W.J.,Turbini, L.X, Stock, S. R., and Smith,B.A., Conductive Anodic Filament Enhancement in the Presence of a Polyglycol-Containing Flux, IEEE International Reliability Physics Symposium Proceedings, Dallas, 1996, pp. 267-272. 

Augis, X A., DeNure, D. G., LuValle, M. X, Mitchell,X P., Pinnel, M. R., and Welsher,T. L., A Humidity Threshold for Conductive Anodic Filaments in Epoxy Glass Printed Wiring Board, Proceedings of 3rd International SAMPE Electronics Conference, 1989, pp. 1023-1030. 

IPC-TM-2.6.25 (IPC Test Method), Conductive Anodic Filament (CAF) Resistance. ... 

It is significantly accelerated by the presence of hydrolyzable ionic contaminants (for example, hahdes and acids from flux residues or extracted from polymers). Delaminations or voids that promote the accumulation of moisture or contaminants can promote dendritic growth. Conductive anodic filament growth (discussed later) is a special case of dendritic growth. Time to failure is inversely proportional to spacing squared and voltage. The failure mechanisms in accelerated tests have been reviewed. ... 

Resin material can affect fiber/resin delamination, one of the prerequisites for conductive anodic filament growth. Meashng occurs at about 260°C for FR-4, but may occur at lower temperatures for boards with more hygroscopic resins. 

Identify probable failure modes (e.g., solder joint fatigue, conductive anodic filament growth). Accelerated rehabihty tests are based on the premise that the frequency and/or severity of the environmental exposure can be increased to accelerate the incidence of the failure that... 

Temperature, Humidity, Bias. These tests are designed to promote corrosion on the PCB surface and conductive anodic filament growth, either of which can cause insulation resistance failures. 

D. J. Lando, J. P. Mitchell, and T. L. Welsher, Conductive Anodic Filaments in Reinforced Polymeric Dieletrics Formation and Prevention, 17th Annual Proceedings of IEEE Reliability Physics Symposium, San Irandsco, April 24—26,1979, pp. 51-63. 

CAF conductive anodic filament CRH critical relative humidity... 

While this degradation process was discovered by accelerated-life-testing and discussed in the literature almost 20 years ago (Lahti et al., 1979 Lando et al., 1979), the authors have never observed or been told of a field failure due to conductive anodic filaments. This testing served to alert designers to the need to ... 

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