Filament: Preparation, Placement and Process Control

Conductive anodic filament growth is a failure that forms inside the laminate rather than on its surface. Copper migrates along the glass fibres under a voltage bias in the presence of moisture, and the filament that results grows until two conductors are connected. It is a slow, hidden and irreversible failure.

What Drives the Failure

Three conditions have to be present at once: a voltage difference between two conductors, moisture in the resin, and a path along the glass bundles. Remove any one of them and the filament does not form, which is why the control measures are grouped along those three lines.

The bias is usually present by design and the moisture is present in service, so the practical control is the path. Our laminate properties notes describe how the resin system affects how easily that path is formed.

The Path Along the Glass

The glass bundles in a laminate are coated with a coupling agent, and the bond between the glass and the resin is where the filament travels. A poor bond leaves a gap that moisture can enter and that copper can follow.

That is why the failure is associated with the glass weave rather than with the resin alone. A resin that wets the glass thoroughly is more resistant than one that leaves a continuous interface. Our layer assignment notes describe how the stack affects the distance the filament has to travel.

Hole Wall Condition and Its Contribution

Drilling damages the resin around the hole, and the damage creates a region where the glass and the resin are separated. A hole with a rough wall, with resin smear or with a cracked barrel is a starting point for a filament.

Desmear that is too aggressive opens the same path from the other direction, by removing resin from between the glass bundles. The window between insufficient and excessive desmear is narrow on a design with a small conductor spacing.

Spacing and the Electric Field

The filament grows faster where the field is high, so the failure appears at the small spacings and at the points where the geometry concentrates the field. Two holes with a thin wall of laminate between them are the classic case.

Increasing the spacing is the most effective single measure, and it is also the most expensive because it consumes area. Where the spacing cannot be increased, the measures are taken on the material and the process instead.

Material Choice

Laminates differ greatly in their resistance to filament growth, and the difference is measured by a standard test that applies a bias across a pattern at elevated temperature and humidity. A material with a high rating is chosen where the design is dense and the environment is humid.

The rating is not a guarantee, because it is measured on a defined pattern under defined conditions. It is a comparison between materials rather than a prediction of the service life of a particular design. Our laminate guide notes describe the families that are available.

Process Choices That Reduce the Risk

The process measures are a clean hole, a controlled desmear and a lamination cycle that fully wets the glass. Baking before lamination removes the water that would otherwise be trapped, and a well controlled press cycle produces a laminate with fewer voids.

Voids are the other path, because a void fills with moisture and connects two places that should be separated. Reducing voids is the same discipline that controls delamination, and the two are usually addressed together.

Testing for Resistance

The standard test uses a pattern of holes with a defined wall thickness, a bias applied between them, and a period at elevated temperature and humidity. The time to failure is the result, and a design can be qualified against it where the application requires it.

The test takes time, so it is a qualification activity rather than a routine one. Where a design is dense and the environment is aggressive, it is the only way to compare two candidate materials. Our board quality notes describe how the process data is controlled around it.

Appearance in the Field

A filament failure appears as a leakage path that grows with time, so the symptom is a resistance that falls rather than an open circuit. It is often found after a board has been in service for months, and it is frequently attributed to contamination until a section is taken.

Sectioning shows the filament along the glass, which is the evidence that distinguishes it from surface contamination or from a metallic migration on the surface. Our cleanliness measurement notes describe how the surface case is excluded.

Process Control and Verification

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 electrochemical migration? No. Electrochemical migration grows on the surface, and a filament grows inside the laminate along the glass.

Does a conformal coating prevent it? Not reliably, because the filament is inside the board rather than on it. A coating protects the surface path instead.

What does gopcb provide for CAF resistance? We provide material selection against a measured rating, hole and desmear control that avoids leaving an open glass path, bake and lamination control that reduces voids, spacing review at the design stage, and qualification testing where the application requires it.