Conductive Anodic Filament (CAF) in PCBs: Formation, Testing and Prevention

This Gold Phoenix PCB Knowledge Center article explains what conductive anodic filament (CAF) is, how it forms inside multilayer PCB laminates, how it is detected by accelerated testing, and how design, material and process choices prevent it.

CAF is an electrochemical failure mechanism in which copper dissolves at an anodic hole wall or conductor and migrates through the glass-fiber/resin interface until it forms a conductive path to a nearby cathode. It is one of the few PCB failure modes that is completely invisible: the board passes bare-board electrical test, works normally in the field for months, and only then develops leakage current, signal corruption or a hard short that cannot be repaired. Typical victims are high-density multilayer boards with fine hole-to-hole spacing, high voltage bias between adjacent nets, and long service lives in warm or humid environments — telecom line cards and base stations, automotive engine and battery-management electronics, industrial motor drives, medical instruments and aerospace control units.

What Is Conductive Anodic Filament (CAF)?

Conductive anodic filament — usually shortened to CAF — is an internal short-circuit mechanism that develops inside the laminate of a multilayer printed circuit board. Under a sustained DC voltage bias and moisture, copper at an anodic feature (typically a plated through hole wall or an inner-layer conductor) dissolves ionically. The copper ions travel along microscopic pathways at the interface between the glass fiber bundles and the surrounding resin, and deposit as a copper-rich filament toward the nearest cathodic feature — usually a neighboring hole wall, pad or conductor. Once the filament bridges the anode and the cathode, insulation resistance collapses and the board fails electrically.

Because the filament is buried inside the laminate, no visual inspection, automated optical inspection or standard e-test will reveal it. This is why CAF is treated as a qualification and reliability issue rather than a production defect, and why it deserves dedicated attention in any high-reliability PCB program.

CAF vs. Surface Electrochemical Migration

Designers sometimes confuse CAF with surface electrochemical migration (ECM), but the two are different problems:

• Location: ECM occurs on the external surface of the board, between exposed conductors, across solder mask or under flux residue. CAF is entirely internal, along the glass-to-resin interface inside the laminate.

• Driving mechanism: Both need moisture and voltage bias, but ECM needs a continuous electrolyte film on the surface, while CAF needs only absorbed moisture within the laminate and a weak interface pathway.

• Appearance: ECM produces visible dendrites or mossy copper deposits. A CAF filament is buried in the laminate and is normally discovered only through cross-sectioning after failure or through dedicated accelerated testing.

• Prevention: ECM is controlled by cleanliness, conformal coating, surface spacing and mask design. CAF is controlled by material choice, drilling quality, lamination and hole spacing rules.

Understanding the distinction matters during root-cause analysis: a leakage path measured between two plated holes with intact solder mask and clean surfaces is almost certainly CAF, not surface migration.

How a CAF Filament Forms

The CAF process is generally described in four stages:

Several manufacturing defects feed directly into stage three: resin smear left after drilling that later cracks away from the wall, resin recession exposing bare glass bundles at the hole wall, inner-layer registration errors that reduce hole-wall-to-copper distance, and lamination voids or delamination at the glass interface. This is why CAF is considered jointly a design, material and process problem.

What Accelerates CAF Risk?

The main risk drivers are:

• Voltage bias: The higher the DC potential between adjacent features, the faster the anodic dissolution. High-voltage sections of power supplies, battery-management boards and industrial controls are the highest-risk areas of a design.

• Hole-to-hole and hole-to-copper spacing: Small spacing creates a steep voltage gradient and short migration distance. Conductor pairs routed diagonally along the warp direction of the glass cloth are particularly vulnerable when fiber bundles directly link the two features.

• Moisture and temperature: Warm, humid environments raise both moisture absorption and ion mobility. Uncoated boards in non-enclosed assemblies are at higher risk.

• Glass style and resin adhesion: Coarse glass weaves with exposed bundles, or laminates with poor glass wetting, provide natural migration channels.

• Drilling and lamination quality: Rough hole walls, smeared or receded resin, and inner-layer delamination create the physical pathways CAF needs.

• Thermal cycling: Repeated CTE mismatch stress between copper, glass and resin opens micro-channels along the interface over time.

How CAF Is Tested

CAF cannot be assessed by ordinary bare-board continuity testing. The industry standard approach is accelerated stress testing per IPC-TM-650 method 2.6.25 (with IPC-9691 providing the test guidance). A specially designed test coupon — such as the widely used “comb” or “H-pattern” coupons with many biased hole pairs at graded spacings — is subjected to elevated temperature and humidity (commonly 65 °C / 85 % RH, or more aggressive highly accelerated stress conditions) while a DC bias of around 50–100 V is applied between the anodic and cathodic features. Insulation resistance is monitored continuously; a sharp, progressive drop indicates filament growth.

Key points about CAF testing:

• Coupon testing, not production testing: The coupon must use the same laminate, glass style, drill parameters and lamination cycle as the production boards to be meaningful.

• Comparable results: Results are reported as time-to-failure at given spacing and bias, allowing comparison between laminate grades and process windows.

• Qualification, not a lifetime guarantee: Passing a 500-hour or 1000-hour test does not mean zero lifetime risk; it validates the material and process combination against the chosen stress profile. Design margins still matter.

• Definitive confirmation: Cross-sectioning a failed device remains the final proof — a copper-rich filament along the glass interface between anode and cathode is the signature of CAF.

Design Rules to Prevent CAF

• Maximize hole-to-hole and hole-to-copper spacing. As a practical floor, keep at least 0.5–0.75 mm (20–30 mil) between adjacent plated hole walls in voltage-biased areas, and more where the working voltage is high. When dense fanouts make this impossible, break the direct glass path by staggering holes rather than placing them on a straight line along the cloth warp direction.

• Keep high-voltage nets physically apart. Group high-bias conductors into dedicated regions of the board with extra clearance, and avoid routing them as tight parallel pairs over long distances.

• Control annular rings and registration. Adequate annular ring compensates for drill and registration tolerance, keeping the copper of one net as far as possible from the hole wall of another.

• Balance the stackup and copper. Balanced construction serves both warpage control and CAF prevention, since it limits thermally induced microcracks at the glass interface.

Material Selection for CAF Resistance

• Choose CAF-resistant laminates. Modern high-Tg and mid-Tg FR-4 grades are qualified against CAF test profiles; ask the fabricator for the CAF test report of the exact laminate construction being quoted, including the glass style.

• Prefer flat, tightly woven glass styles. Weaves such as 1080, 2116 or spread-glass variants expose fewer continuous fiber bundles at hole walls than coarse, open weaves.

• Match Tg and CTE to the duty cycle. Low-CTE, high-Tg materials survive thermal cycling with less interface cracking, keeping migration pathways closed.

• Qualify halogen-free and high-resin-content systems carefully. Some of them absorb more moisture or wet glass less effectively and must be verified against CAF before use in biased, humid applications.

Fabrication Process Control

• Drill with sharp, well-maintained tooling to produce clean hole walls; excessive smear, roughness or resin recession at the hole wall is a direct CAF enabler.

• Optimize desmear and lamination. Complete smear removal, proper oxide or alternative bonding treatment on inner layers, and a lamination profile with adequate pressure and dwell time ensure intimate glass-to-resin contact with no voids along the interface.

• Maintain cleanliness before coating or potting to remove ionic residues that would otherwise combine with moisture to raise leakage.

• Bake and pack boards dry, and use conformal coating or enclosure design to limit moisture ingress in humid installations.

Summary

Conductive anodic filament is a slow, internal, electrochemical failure that no visual inspection or standard e-test will catch. It grows where three conditions meet: a DC voltage gradient, moisture, and a weak pathway along the glass-fiber/resin interface. Because the mechanism is buried inside the laminate, prevention is the only practical strategy.

Design engineers can keep CAF risk low by maximizing spacing between biased plated holes and conductors, using CAF-qualified laminates with tightly woven glass, and demanding qualified drilling and lamination processes from the fabricator. For designs that operate at elevated voltage, temperature or humidity, adding a dedicated CAF coupon to the qualification program gives real evidence that the stackup and process window are safe — long before field failures have a chance to appear. In short, CAF control is a combination of spacing discipline, material qualification and process capability, and it deserves the same attention as impedance control or warpage control in any high-reliability PCB program.