Conductive anodic filament growth

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

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. 

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. 

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. 

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. 

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. 

In electrochemical migration, failure can occur due to dendrite growth, open circuit short, or conductive anodic filament (CAP) formation. Dendrites can form on the surface due to con-taminahon left by the solder flux (paste) or other residues. Under a bias voltage, the metal at the anode goes into solution, migrates toward, and plates-out at the cathode. The nature of the dendrite will depend upon the surface metallization. The following oxidation reactions can occur at the anode ... 

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