Conductive Anodic Filament (CAF) is a failure mode in printed circuit boards (PCBs), and it is a growing concern in the electronics manufacturing field. The conductive copper salt filament can form within the boards, the formation process is a type of electrochemical migration that grows along the epoxy or glass interface from the anode to the cathode sub-surface.
CAF Formation Conditions
This is an electrochemical process, which requires the following to occur:
- Electrolyte – Water
- Voltage bias – Force that drives the reaction
- “Pathway” – A way for the ions to move from the anode to the cathode
Many other factors are believed to accelerate the formation process, including, high temperatures, high humidity, repeated thermal cycles, high voltage gradient between anode and cathode, some soldering flux ingredients and more. Other problems, such as component failures and exceeding maximum operating temperatures might also contribute to a CAF-related failure.
How to Avoid?
There are many different measures you can take to minimize the risk of CAF failure. Research into ways avoid this problem are ongoing, but avoiding the conditions enabling CAF formation will help prevent it. Here are some of the factors to consider.
- Moisture and Humidity: Because it requires an electrolyte, a higher water content increases the chance of such failure. Increased humidity leads to higher moisture content, which decreases CAF performance.
- Processes Bring Acid Contamination: Processes used during PCB manufacturing and assembly can introduce acid contaminations, which increases the likelihood of CAF formation. The use of some soldering fluxes and the introduction of acid residues during the plating process are examples of this.
- Bias and Voltage: Since bias voltage is the force that drives the reaction, a high-voltage bias will significantly lower the chances of filament formation. Higher voltage decreases CAF performance as well.
- Pre-Existing Defects: Pre-existing defects, such as fracturing, voids, wicking, contamination and mis-registration can also create pathways for problematic filaments. You need to be careful when drilling holes so as not to cause damage to the board. Such damage can create these pathways by causing cracks, wicking and other defects. Drill speed, feed rate and other factors influence how likely these issues are to occur. Partial defects such as incomplete bridging between features can contribute as well. High temperature like those from environmental temperature, repetitive thermal cycling and reflow with a high peak temperature create more stress on a board and increase the likelihood of damage as well CAF formation.
- Materials: Another influential CAF failure factor is the materials used to make the board, flex- and rigid-flex PCB. Using CAF resistant materials, as MADPCB does, is one of the most effective ways to prevent CAF formation and failure. Of course, using CAF resistant CCL and FCCL laminate materials will increase PCB cost.