Conductive Anodic Filament Growth In PCB Laminate
Conductive anodic filament, usually shortened to CAF, is a failure in which a conductive path grows through the laminate between two conductors held at different potentials. The path follows the interface between the resin and the glass fibre bundle, and once it bridges the conductors the resistance falls and the board fails.
CAF is a slow, moisture driven process. It appears in boards that have been in service for years, and it is strongly associated with fine hole to hole spacing, with thin laminate and with humid environments where condensation is possible.
What CAF Actually Is
Under an applied voltage and in the presence of moisture, copper can dissolve at the anode and migrate towards the cathode. When that migration happens along a glass fibre bundle inside the laminate rather than on the surface, the resulting conductive path is a conductive anodic filament.
The failure is progressive. Insulation resistance falls over time as the filament grows, and it finally drops to a short when the path is complete. Because the process is slow, a board can pass every test at the factory and fail in the field several years later.
The Role Of The Glass Fibre Interface
The bond between the resin and the glass fibre is the weak point. If the resin does not wet the fibre completely, a thin gap remains along the bundle, and that gap becomes a channel that moisture and dissolved copper can follow. The fibre then acts as a pipe through the laminate.
Poor resin wetting is caused by insufficient coupling agent on the fabric, by contamination, or by a lamination cycle that does not allow the resin to flow into the bundle. The result is not visible on the finished board, which is why material and process control matter more here than inspection.
Hole Wall Spacing And Design
CAF needs two conductors, a path and a voltage. The path most often runs between two plated hole walls, because drilling damages the fibre bundles locally and the plating provides the copper electrodes. Hole to hole spacing is therefore the primary design lever.
A spacing of 0.5 mm between hole walls is comfortable for most materials, while 0.3 mm is aggressive and 0.25 mm requires a material specified for CAF resistance. The spacing should be measured between the finished hole walls rather than between the drill centres. The related spacing rules are described under design guidelines for manufacturability.
Humidity And Bias Voltage
Moisture is essential to the mechanism, so the environment decides whether a marginal design fails. A board operating in a dry enclosure may never show CAF, while the same board in a humid climate can fail within a few years. Condensation is worse than a high ambient humidity.
Bias voltage provides the driving force. Higher voltages accelerate the process, and an alternating field is generally less damaging than a direct one because the direction of migration reverses. The combination of a high DC bias and a humid environment is the worst case for any design. Protecting the finished assembly from moisture is described under conformal coating and board protection.
Test Methods
The standard test uses a specially designed coupon with a comb of holes or conductors at a defined spacing, which is biased and held in a humid chamber for a fixed period. Insulation resistance is monitored throughout, and a fall below a threshold is taken as a failure.
Typical conditions are 85 C and 85 percent relative humidity with a bias of 50 to 100 volts, run for 500 to 1000 hours. The test is long, so it is used to qualify a material or a process change rather than to screen production lots.
Prevention And Material Choice
Prevention works on three fronts: better bonding between resin and fibre, generous hole to hole spacing, and a drilling process that produces a clean hole wall without excessive damage. A fabric with spread or mechanically finished yarn wets more evenly than a standard weave.
Where the design cannot relax the spacing, a laminate with a higher glass transition temperature and better moisture resistance is the answer, and it should be qualified by the test described above. How materials are chosen for a build is described under PCB design and fabrication.
Process Control and Verification
Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
Checks Before Release
The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.
A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.
FAQ
Is CAF the same as electromigration? No. Electromigration happens on the surface between conductors, while CAF grows through the laminate along the glass fibres. The two have different causes and different cures.
Which materials resist CAF best? Laminates with a well bonded resin system and fabrics made from spread or flattened yarn generally perform better. The specific material should be chosen using measured test data rather than a general claim.
Can a board with CAF be repaired? In practice no. The conductive path is inside the laminate and cannot be removed, so the assembly has to be replaced. Prevention at the design and material stage is the only real control.
