What is CAF?

Conductive Anodic Filament (CAF) is a phenomenon that can occur in printed circuit boards (PCBs) where electromigration by means of a copper salt filament takes place between two points (anode to cathode) inside the pcb and is related to the failure of the board due to the formation of conductive traces between different copper features, such as traces and planes, through the insulating material (resin) that separates them.

Several factors, including moisture, humidity, and the presence of certain ionic contaminants in the environment, can trigger this reaction.

CAF has been a factor since 1976 when it was first identified. Over the years, the increased pcb density requirements have also increased the risk of CAF influencing pcb functionality and reliability.

It is well known in the industry that for CAF to occur within a laminate, certain conditions need to be in place. These include high humidity, high bias voltage under test, high moisture content, surface and resin ionic impurities, glass-to-resin bond weakness, and exposure to high assembly temperatures (lead-free applications). There are four known ways that CAF forms at the surface and within the laminate: 1) hole-to-hole, 2) hole-to-trace, 3) trace-to-trace, and 4) layer-to-layer.

Here’s a step-by-step explanation of how Conductive Anodic Filament occurs:

1. Environmental factors:

PCBs are often exposed to different environmental conditions, including moisture and humidity. These external factors can penetrate the PCB’s insulating material over time.

2. Presence of ionic contaminants:

The insulating material in PCBs can contain impurities or ionic contaminants. These contaminants can act as conductive paths if they come into contact with copper features on the board.

3. Electrochemical reaction:

When the PCB is exposed to moisture or humidity, it creates a conductive medium. This conductive medium facilitates the migration of copper ions from one copper feature (e.g., a trace or plane) to another through the insulating material.

4. Formation of conductive paths:

As the copper ions move through the insulating material, they form microscopic conductive traces called “conductive anodic filaments.” These filaments act as unintended electrical connections between the copper features, which are meant to be electrically isolated.

5. Failure and short circuits:

If these conductive anodic filaments grow large enough over time, they can lead to electrical shorts between different parts of the PCB. This can result in the malfunctioning of the circuit or even catastrophic failure.

Causes of CAF

The following can be identified as potential causes for the occurrence of CAF:

• Drilling that leaves glass fibers and/or laminate fractures.
• Delamination of inner layers.
• Plated through holes too close to each other.
• Plated through hole location relative to the glass weave.
• Separation of glass and epoxy in the laminate.
• Hollow glass fibers.
• Residual stress relief in the glass fabric.
• Decomposition of the laminate.
• Resin starvation in the laminate.

Preventing Conductive Anodic Filament:
To prevent Conductive Anodic Filament, PCB manufacturers and designers take several measures:

1. Material selection:

Choosing high-quality, low-ionic-residue materials for the insulating layers can reduce the risk of CAF.

2. Increasing the via-to-via gap can reduce the chances of CAF.

3. Staggered or offset vias, normally 45° to each other.

4. Conformal coating:

Applying a conformal coating over the PCB can provide an additional layer of protection against moisture and contaminants.

5. Environmental control:

Minimizing exposure to moisture and humidity during storage and operation can help prevent CAF.

6. Design considerations

Designing PCBs with appropriate spacing between copper features, especially high-voltage or high-current traces, can reduce the likelihood of CAF.

7. Testing and qualification:

Performing reliability tests, such as Highly Accelerated Stress Testing (HAST) and Temperature Humidity Bias (THB) testing, can help identify potential CAF issues before the PCBs go into production.

Overall, understanding and mitigating the risk of Conductive Anodic Filament is crucial in ensuring printed circuit boards’ long-term reliability and performance.