Conductive Anodic Filament (CAF) is an electrochemical failure mechanism in printed circuit boards where copper ions migrate along the glass fiber/epoxy interface within the laminate — creating conductive filaments that grow from anode to cathode between vias or traces on different layers, causing internal short circuits that are invisible from the board surface and extremely difficult to detect or repair, representing a critical reliability concern for high-density PCBs with fine via pitch.

What Is CAF?

• Definition: An electrochemical migration process where copper dissolves at the anode via, migrates as Cu²⁺ ions along the interface between glass fibers and epoxy resin inside the PCB laminate, and deposits as metallic copper at the cathode via — forming a conductive filament that shorts the two vias internally within the board.
• Glass Fiber Path: CAF exploits the weak interface between glass fibers and epoxy resin in FR-4 and similar laminates — moisture wicks along this interface by capillary action, creating an electrolyte pathway that enables copper ion transport deep inside the board.
• Internal Failure: Unlike surface dendritic growth which is visible, CAF occurs entirely within the PCB laminate — the conductive filament is hidden between layers, making it undetectable by visual inspection and difficult to find even with cross-sectioning.
• Via-to-Via Shorts: CAF most commonly occurs between adjacent vias on different nets — the glass fiber bundles that run between via drill holes provide the migration pathway, and the voltage difference between the vias provides the electrochemical driving force.

Why CAF Matters

• High-Density PCB Risk: As via pitch decreases (< 0.8 mm) and layer count increases, the number of closely-spaced via pairs increases dramatically — each pair is a potential CAF failure site, and the shorter migration distance makes failure more likely.
• Latent Failure: CAF failures typically occur after months to years of field operation — the filament grows slowly until it bridges the gap, then causes a sudden short circuit that may be intermittent (filament breaks and reforms).
• Difficult Diagnosis: CAF failures are internal to the PCB — they don't appear on the surface, can't be seen with X-ray (the filament is too thin), and require precise cross-sectioning through the failure site for confirmation.
• Automotive/Aerospace Concern: Long-lifetime applications (15-20 years for automotive, 20-30 years for aerospace) are most vulnerable — CAF has sufficient time to develop even at low humidity and moderate temperatures.

CAF Prevention

CAF is the hidden electrochemical threat inside printed circuit boards — growing conductive copper filaments along glass fiber interfaces to create internal short circuits between vias that are invisible from the surface and nearly impossible to repair, requiring careful laminate selection, via spacing rules, and glass weave management to prevent this insidious failure mechanism in high-density PCB designs.