Conductive Anodic Filament: Design Rules and Process Limits

A board that passes every inspection and every electrical test can still fail in the field from a mechanism that grows after it is installed. Conductive anodic filament is the name for that mechanism, and it is worth understanding because its symptoms look like contamination, moisture or a design error, while its cause is a combination of all three.

What the Filament Is

Conductive anodic filament is a conductive path that grows inside the laminate between two conductors at different potentials. It forms under an applied bias in the presence of moisture and ionic contamination, and it grows along the interface between the resin and the glass fibers rather than through the bulk resin.

The growth is electrochemical. The anode corrodes, metal ions migrate along the fiber surface toward the cathode, and a conductive salt deposits in a narrow track. The track is thin, often a few micrometres wide, and it penetrates the laminate in the direction of the fiber bundle.

Because the path is inside the board, nothing on the surface indicates its presence. The first symptom is usually a resistance that falls over weeks or a short that appears after a period in a humid environment.

Why the Glass Interface Is Vulnerable

Glass fibers are hydrophilic, and they are treated with a sizing that promotes bonding to the resin. Where the bond is imperfect, the space between the fiber and the resin is a pathway along which water and ions travel far more easily than through the resin itself.

The pathway becomes continuous where the laminate contains voids, where the resin does not fully wet the fiber, or where mechanical stress has created a separation. A drilled hole that is close to a glass bundle is a common initiation point, since drilling exposes fiber ends and creates a local defect.

Our laminate material properties notes describe how resin content and glass style influence the amount of fiber-resin interface available for such a path.

The Conditions That Drive Growth

Three conditions are required: a bias between conductors, moisture in the laminate, and ionic contamination that can dissolve and move. Remove any one of them and the mechanism stops, which is why the failure appears only in some installations and not in others.

Bias is present in any operating board, but the driving force is larger where the voltage difference is high, which is why the mechanism is associated with power and high voltage circuits more than with logic. A large plane-to-plane voltage with a thin dielectric is the worst case.

The temperature accelerates everything. Chemical mobility roughly doubles for every ten degrees, so a board that would survive twenty years at ambient may fail in a few years at an elevated operating temperature.

Design Features That Raise the Risk

The distance between conductors and the potential difference between them set the electric field that drives the migration. Small holes on a tight pitch, with a small dielectric distance between inner layers, concentrate the field and shorten the path.

Holes drilled through a stack where the fiber bundles run parallel to the hole-to-hole direction give the filament a straight path to follow. Our aspect ratio notes explain how stack geometry and drill diameter interact, which is the same geometry that sets the fiber exposure.

Resin starved laminates result from a press cycle that does not allow enough flow or from a glass style with insufficient resin content. The resulting voids are the voids that fill with moisture.

Process Sources of Contamination

Ionic residue can arrive from several places. Plating chemistry trapped in a hole, flux residue that was not fully cleaned, residues from the fabrication process left in a laminate void, and salts from handling with bare hands all contribute.

The cleaning step is therefore part of the reliability design rather than a cosmetic step. A board that is fluxed, soldered and left with residue is a board with an electrolyte already in place, waiting for moisture.

Assembly cleaning and the choice of a no-clean or water-wash process therefore change the risk profile, and the choice should be made with the operating environment in mind rather than on cost alone.

Stack-up Choices That Reduce Susceptibility

Increasing the dielectric thickness between the layers that carry the largest potential difference reduces the field strength, and the effect is substantial because the field is inversely proportional to the separation. This is often cheaper than any other mitigation.

Choosing a laminate with a higher resin content and better fiber wetting improves the interface and removes the void pathway. Where the application demands high voltage and long life, a material qualified for the environment exists and should be selected on the data rather than on the general purpose grade.

Avoiding unnecessary holes through high stress regions, and increasing the spacing between the holes that remain, breaks the straight fiber path that the filament prefers.

Detecting the Problem

The classical detection method is a temperature humidity bias test on a test coupon with a daisy chain of closely spaced vias, held at a voltage for hundreds or thousands of hours, with the insulation resistance monitored continuously. The resistance trace shows a gradual decline before the short, and the time to failure is the data the test produces.

On a failed product, the evidence is found by sectioning along the suspected path. The filament appears as a dark track along a fiber interface between two conductors, and its composition confirms the mechanism.

Our judging PCB quality notes describe where to look for the surface indications that accompany an internal failure, and the sectioning approach that follows.

Preventing It in Practice

Clean the boards properly and verify the result rather than assuming it. Ionic contamination testing of the finished assembly is a short procedure with a numeric result, and it converts a discussion about cleanliness into a measurement.

Control the laminate purchase specification so that resin content and void content are stated rather than implied. Our hole copper notes describe how plating quality interacts with the same interfacial region, since a rough barrel is another place where resin and plating meet imperfectly.

Finally, keep moisture out of the finished product. Conformal coating over the assembly blocks the external moisture path, potting fills the spaces around the conductors, and both are effective where the surface is clean before they are applied.

When to Test for It

Testing is warranted where the product will operate for years in a humid environment, where the voltage difference between adjacent conductors is large, or where the board is densely drilled with small holes on a fine pitch. Where all three apply, a humidity bias test belongs in the qualification plan.

For a consumer product with a short service life in a dry environment, the test is unlikely to be informative and the design effort is better spent elsewhere. The mechanism is real in every case but it is not equally probable in every case.

The honest position is that most boards never see a filament and that the ones which do are usually in exactly the conditions described above. Recognizing those conditions is the practical skill.

FAQ

Is conductive anodic filament the same as dendrite growth? The chemistry is similar, but a dendrite grows on a free surface while a filament grows within the laminate along a fiber interface. The countermeasures overlap but the inspection methods differ.

Does conformal coating prevent it? Coating blocks moisture arriving from outside the board and can also block surface dendrites. It cannot stop a path that has already been established inside the laminate.

What does gopcb provide where this risk is present? We provide laminate options with stated resin and void characteristics, humidity bias test coupons with continuous resistance monitoring, sections that identify the failure path, and process records for cleaning and ionic contamination. Where a design is constrained, we test the coupon rather than relying on the general reputation of the material.