PCB CAF Failure: Preserve the Evidence Before Changing Material

Author: Vivi Peng | Published for icallin.com | Updated: October 15, 2026

A board that loses insulation resistance during humid operation needs a failure location before it needs a new laminate. Conductive anodic filament formation, or CAF, creates a conductive path inside reinforced PCB insulation. A surface leakage problem can produce a similar electrical symptom while requiring a different corrective action. The useful starting point is therefore to connect the suspect conductor pair, its physical path and its manufacturing history.

Identify the path behind the resistance drop

CAF develops within the laminate, commonly along the interface between glass reinforcement and resin. Moisture, electrical bias and a susceptible internal path allow copper-containing conductive deposits to extend between conductors. Isola's CAF investigation describes interface degradation followed by electrochemical growth, and examines how material and processing conditions influence the result.

Electrochemical migration includes growth across a surface or through insulation. CAF is its internal, reinforcement-associated form. Use the observed path to choose between surface and internal investigations.

The distinction matters commercially as well. Replacing the laminate may leave assembly contamination untouched. Changing a cleaning process may leave an internal glass-resin path untouched. Approve the corrective action against the identified failure mechanism, rather than against the appearance of the electrical symptom alone.

Preserve useful evidence before destructive analysis

Record the affected nets, their voltage relationship, the conditions preceding the event and the board revision. Keep failed samples linked to laminate lots, fabrication panels and assembly history. Photograph the surface before cleaning or cutting, so later inspection can be compared with the original condition.

Initial observation	Useful next investigation
Visible residue or growth between exposed conductors	Localize surface leakage and examine contamination
Suspect path between buried conductors or plated holes	Review the internal geometry and select a sectioning location
Resistance changes after drying	Record the recovery without treating it as proof of a repaired filament
Failures cluster by panel or process history	Compare the corresponding material and fabrication records

Use these observations to select the next investigation and confirm the diagnosis.

IPC's CAF resistance test method warns that preparation and electrical testing can disrupt tiny filaments. Agree the localization, sample allocation and sectioning plan before the laboratory consumes the evidence.

That plan should distinguish a suspected internal path from damage created during preparation. A resistance trace establishes when leakage occurred. Physical analysis addresses where it occurred. Keep both records together when deciding whether the evidence supports CAF, another migration mechanism or a different insulation defect.

Internal CAF and surface migration require different localization paths. The illustration is conceptual and is not a micrograph or a measured failure result.

Review the laminate and the fabricated board together

Evaluate a CAF-resistant material as part of the finished insulation system. Ask which resin, reinforcement, construction and processing conditions were represented in its qualification. Compare those conditions with the production board, and identify which differences need further evidence before approving the material.

IPC's method identifies drilling, hole preparation, construction and thermal history as relevant records. Build a comparison around the suspect region: which process revision, panel position or material lot changes between accepted and affected boards? Ask the fabricator for the records that distinguish those groups, then select the next investigation from that evidence.

Isola's experiments also show why a resistance decrease deserves careful interpretation: some investigated changes recovered after drying, without confirmed filament growth. Its findings support evaluating resin and glass-finish combinations, while preserving the distinction between insulation resistance and physically confirmed CAF. A higher glass-transition temperature alone does not close that evidence gap.

Make qualification results comparable

Set the requested method revision, coupon construction, exposure conditions and decision criteria before samples are built. Assign responsibility for agreeing deviations and reviewing their effect. A supplier can then respond to a defined evaluation task rather than an open-ended request for “better CAF resistance.”

IPC-TM-650 2.6.25C specifies conditioning, biased exposure and treatment of assignable failures. Read the reported result against that procedure.

Put two reports into a comparison sheet. Can engineering identify the same conductor geometry and acceptance basis in both? Can purchasing identify which production construction each report supports? Close a mismatch before treating the results as equivalent qualification evidence. Keep any service-life correlation as a separate engineering decision.

Release a prevention package that procurement can preserve

Combine the approved material construction, controlled fabrication process, relevant geometry and qualification evidence in one release package. Name the changes that require review: a laminate substitution, a reinforcement change, another fabrication facility or a revised thermal process may alter the evidence supporting the board.

Engineering controls the susceptible geometry and qualification basis. Manufacturing controls the process that produces it. Procurement preserves the approved specification and routes deviations for review. Record these responsibilities in the purchase and release documents, with a clear owner for each decision.

Frequently Asked Questions

Does a clean board surface rule out CAF?

A clean surface directs attention toward possible internal paths when leakage remains unexplained. Confirm the failure location through electrical and physical evidence before deciding on the material or process change.

Is a larger via gap enough to prevent CAF?

Choose spacing together with the intended bias, moisture exposure, construction and process. Ask whether the qualification evidence represents that combination, and review the affected geometry when any of those inputs changes.

Written by Vivi Peng for icallin.com.