1. PCB company products have encountered a [micro-short circuit phenomenon in the inner layer of the circuit board]. Upon investigation, this issue was identified as CAF (Conductive Anodic Filament), referred to in Chinese as “conductive anode filament” or “anode glass fiber filament leakage phenomenon.” However, in literal translation, few can grasp the meaning, right? CAF is essentially the occurrence of micro-short circuits within the inner layer of the circuit board or the solder resist green paint layer.

2. This problem has been a persistent concern for Shenzhen Honglijie. Over time, some courses on PCB manufacturing have been integrated, and discussions with various PCB manufacturers have led to a compilation of insights regarding CAF. I am sharing my experience here for your reference.

3. The formation principle of CAF can be illustrated with the top diagram. CAF occurs when DC voltage is applied to the printed circuit board while it is placed in a high-humidity environment. This can happen from Line to Line, Hole to Hole, or Hole to Line connections. In this environment, copper metal at the high-potential anode is first oxidized to Cu+ or Cu++ ions. These ions then migrate through the existing compromised pathways along the glass fiber yarn toward the cathode. Simultaneously, electrons from the cathode move toward the anode. During this process, copper ions encounter electrons and are reduced back to copper metal, gradually forming a copper film that spreads from the anode to the cathode, a phenomenon also known as “copper migration.”


Many people encounter CAF for the first time and are often troubled by its recurring behavior. Once CAF establishes a conductive path, it can be periodically damaged by high-resistance Joule heating. When using a multimeter to measure CAF, one might observe fluctuations in the readings, with values consistently drifting. Unless the specific conditions are resolved, the CAF issue will repeatedly manifest in the same area.

In summary, to create a CAF defect, five failure conditions must be simultaneously present. This means there must be a precise combination of time and location for CAF to occur. Therefore, these incidents are not mere accidents; they result from a series of errors:

1. Water vapor (unavoidable in atmospheric conditions)

2. Electrolyte (seemingly difficult to eliminate)

3. Copper exposure (as copper foil is used as the substrate in PCBs, it is unavoidable)

4. Bias voltage (an inevitable aspect of circuit design)

5. Channel (this parameter seems to be improvable)

Metal ions undergo electrochemical migration (ECM) in non-metallic media due to an electric field, creating a conductive channel between the anode and cathode, leading to circuit shorts.

Anode: Cu – Cu²⁺ + 2e⁻ (Copper dissolves at the anode)

H₂O – H⁺ + OH⁻

Cathode: 2H⁺ + 2e⁻ – H₂

Cu²⁺ + 2OH⁻ – Cu(OH)₂

Cu(OH)₂ – CuO + H₂O

CuO + H₂O – Cu(OH)₂ – Cu²⁺ + 2OH⁻

Cu²⁺ + 2e⁻ – Cu (Copper deposits at the cathode)

Ion migration is generally considered to occur in two stages. In the first stage, moisture causes the resin and reinforcing material to chemically hydrolyze via the silane coupling agent, creating a reversible leakage path along the epoxy resin/reinforcing material. In the second stage, under voltage or bias, the copper salt undergoes an electrochemical reaction, depositing conductive channels between circuit patterns, resulting in short circuits. This stage is irreversible.

How can we prevent or mitigate CAF occurrence?

To address or prevent CAF, we can focus on the five necessary conditions mentioned above. Eliminating just one of these conditions can prevent its occurrence.

1. Enhance the CAF resistance of circuit board materials.

Choosing the right circuit board materials is crucial for preventing CAF. However, higher protection typically comes with a cost, as substrates with strong CAF resistance often require special ordering. Recommendations for CAF-resistant substrates include:

– Reducing the content of impurities in the materials.

– Ensuring that glass fiber cloth is thoroughly impregnated with resin for optimal bonding.

– During PCB substrate production, multiple glass fiber bundles should be woven into fabric and soaked in resin, ensuring the resin fully penetrates the gaps. Poorly set parameters at this stage can lead to voids in the glass fiber bundles, providing openings for CAF to exploit.

– Using low-hygroscopic resin. For further reading, see articles on PCB structure and function.

2. PCB layout design to avoid bias voltage and spacing issues.

The design of through holes, circuit dimensions, and stacking structures can significantly influence CAF. As products shrink and circuit density increases, PCB manufacturing capabilities have limits. Closer proximity of adjacent lines under DC bias voltage raises the likelihood of CAF. Generally, higher bias voltage or smaller distances increase CAF probability. Current manufacturer guidelines recommend the following PCB size design values for CAF protection:

– Minimum distance from hole edge: 0.4mm

– Minimum hole-to-line distance (Drill to Metal): 12mil (0.3mm)

– Recommended aperture: 0.3mm

Additionally, based on practical experience, CAF gaps often align along the same glass fiber bundle. Arranging through holes or pads at a 45-degree angle can help reduce CAF incidence.

3. Wicking control in the PCB manufacturing process.

High temperatures generated during mechanical drilling or laser cutting can melt resin, forming slag that adheres to the inner copper edges and hole walls. Poor contact can occur during subsequent copper plating, necessitating a de-smear operation before plating. This process swells and relaxes glue residues to facilitate proper copper penetration. However, de-smearing can cause some bite corrosion to the through hole and potential wicking. To expedite the de-smearing, some manufacturers increase the fluffing tank temperature, which can excessively loosen the interface and lead to copper migration.

According to IPC-A-600, wicking acceptance standards are:

– Class 1: Copper infiltration should not exceed 125µm (4.291 mil)

– Class 2: Copper infiltration should not exceed 100µm (3.937 mil)

– Class 3: Copper infiltration should not exceed 80µm (3.15 mil)

Yet, as technology advances, even 0.1mm (100µm) copper infiltration may not meet actual needs. For example, from a 0.4mm hole to its edge, accounting for copper infiltration, only 0.2mm remains. Current manufacturing capabilities suggest controlling copper penetration below 50µm (2mil). In contrast, the layout distance from hole to hole in factory systems has decreased to 100µm (4mil), which poses a significant challenge for CAF prevention.

Moreover, during mechanical drilling, if feed speed is too high or if the milling cutter exceeds its lifespan, glass fibers may tear, creating gaps.

4. Waterproofing and moisture control during PCB processing.

In PCB assembly operations—such as solder paste printing, component placement, and high-temperature reflow—contaminants like solder, adhesive, dust, and moisture can remain on the circuit board. These substances are prone to electrolysis and may promote electrochemical migration. Using sealants to cover junctions that could create voids and lead to CAF can help prevent moisture infiltration.