Conductive Anodic Filament: CAF Failure Explained
What CAF Is
Conductive anodic filament is a copper-containing path that grows inside the laminate along the resin and glass interface, from an anode toward a cathode, while the board is under bias in a humid environment. It is not a surface phenomenon and it is not the same as electromigration on a surface, although both end in a short. The filament is electrochemical: copper at the anode dissolves, migrates through the resin-glass bond under the applied field, and deposits as a conductive salt that eventually bridges two conductors. Because it grows inside the material, it is invisible to inspection and appears as a sudden, often intermittent, short after months of service.
Why It Takes Three Conditions
CAF needs all three of the classic ingredients at once: moisture, bias and an electrochemical path. Remove any one and the mechanism stops. That is why the failure is associated with humid climates, with boards that see condensation, and with assemblies that are powered continuously rather than cycled. The presence of ionic contamination, from flux residue, from plating chemistry or from the environment, makes the electrolyte more conductive and accelerates the growth. The field strength between the two conductors sets the driving force, and the distance between them sets how far the filament has to travel before it bridges.
The Glass Weave Is the Road
The filament follows the interface between the resin and the glass fibres, and it tends to travel along the yarn bundle rather than straight across. That is why the same drill-to-drill spacing can behave differently on two laminates, and why a board that passes on one fabric can fail on another with a different weave style. Where a drill hole lands in the gap between two yarn bundles, the resin-rich region is thicker and the path is longer; where it lands on a bundle, the interface gives the filament a direct route. The consequence is that CAF performance is a property of the material system and the drilling, not only of the spacing on the drawing.
Design Limits That Prevent It
The most effective lever is the distance between biased conductors, and the number to use is the hole wall to hole wall distance, not the centre to centre pitch. Large plated holes with a wide drill have thin walls between them even at a respectable pitch, so the design rule is normally expressed as a minimum wall thickness between the two barrels. Higher voltage demands more distance, and the spacing should be set from the worst-case voltage the board will see rather than the nominal rail. Where the spacing cannot be increased, the alternatives are a laminate with better CAF resistance, a thicker board, or a change in the layer assignment so that the two nets are not adjacent through the same wall.
Process Factors
Several process steps decide whether the theoretical margin survives manufacturing. Drilling with a worn bit or with the wrong feed and speed leaves a rough, resin-starved wall and can crack the resin between yarns, creating the path. Desmear that is too aggressive etches the resin back from the glass and opens the interface; desmear that is too gentle leaves smear that later absorbs moisture. The lamination cycle matters as well, because incomplete cure leaves the resin more permeable. A plating bath with organic contamination adds the ionic species the mechanism needs.
How CAF Is Tested
The standard test builds a coupon with a pattern of closely spaced plated holes or parallel conductors, conditions it in a humid chamber, applies a bias and monitors the insulation resistance. Typical conditions run at 85 C and 85 percent relative humidity, with the time to failure recorded, and the result is a ranking rather than a pass or fail number on its own. Faster screening uses a higher bias and a shorter coupon. What the test really shows is where a material system and a hole spacing combination sits relative to another, which is why it is used to qualify laminates rather than to certify a design.
Field Conditions and Diagnostics
In the field, CAF often shows as a resistance drop that appears and disappears, because the filament is thin and partly destroyed by the fault current. That behaviour makes it easy to misdiagnose as a connector or a firmware problem. Confirming the mechanism requires a cross section at the failing location, and the evidence is a copper-rich, hollow or tree-like track running through the resin between two holes, usually starting from the anode. Once the mechanism is confirmed, the design answer is spacing or material, and the process answer is drilling quality and cleanliness.
Moisture, Assembly and Coating
The environment the finished assembly sees is part of the CAF equation. A board that is assembled with a water-soluble flux and then cleaned incompletely carries a hygroscopic residue that holds moisture exactly where the filament wants it, and a no-clean process that leaves ionic contamination does the same over a longer period. Conformal coating slows the ingress of moisture and is one of the few effective field mitigations, but it works on the surface and cannot stop a filament that is already growing between two barrels inside the laminate. When a product will operate in humid or condensing conditions, the sequence that works is a design with generous wall thickness, a laminate qualified by test, a clean process with verified ionic contamination and a coating over the finished assembly.
FAQ
What is conductive anodic filament? A copper-containing conductive path that grows inside the laminate along the resin and glass interface between biased conductors in a humid environment.
Is CAF the same as electromigration? No. Electromigration is a surface and interface phenomenon in the presence of flux and moisture, while CAF grows through the bulk laminate.
Which spacing matters? The wall thickness between the two barrels, not the centre to centre pitch, since that is the distance the filament has to cross.
Does higher voltage make it worse? Yes. A larger field accelerates the growth, so higher voltage nets need more spacing or a better material.
Can CAF be found by inspection? No. The filament is inside the material, so it is confirmed by electrical test and by cross section rather than by visual or optical inspection.
Conclusion
CAF is a design and materials problem that only becomes visible as a field failure, so the defence is margin rather than screening. Set the wall thickness between biased barrels from the worst-case voltage, choose a laminate with measured CAF resistance, and keep drilling, desmear and lamination under control. Confirm the mechanism by cross section when it appears, and fix the geometry rather than the test limit. Material and spacing qualification belongs with PCB capabilities, the drilling and lamination steps to PCB manufacturing, and the reliability checks are part of quality management. Boards built for humid or high-voltage service are normally qualified during a prototype PCB assembly build in 2026.
