Conductive Anodic Filaments

What is it?

Conductive anodic filament (CAF) is of conductive filaments along the interface between the epoxy dielectric and the glass fibres of the circuit board laminate.

Why does it happen?

The filaments grow from an anodic area to a cathodic area and typically bridge two oppositely biased copper conductors. Its occurrence is exacerbated by exposure of boards to humidity either during use or storage. CAF formation is also enhanced by certain processing chemicals, increased thermal cycling and high voltage gradients. The move to Lead-Free electronics has lead to increased concerns about CAF formation, especially since the main Lead-Free solders require higher processing temperatures.

Some other factors known to influence the formation of CAF include;

• The circuit board design. The presence of two closely spaced holes or vias along the axis of the glass fibres can provide a high field gradient and a direct path along which the filaments can grow.
  
• The PCB manufacturing process. The quality of the manufactured board is important as rough drilling, inner layer contamination, poor lamination and metal plating can all encourage CAF formation.

What problems can it cause?

CAF formation occurs in four main areas; from a through-hole to a through-hole, from track to track, from through hole to track, and from layer to layer, the most common failure mode being hole to hole. Failure occurs in two distinct stages. The first involves the degradation of the resin-glass interface the second is an electrochemical migration process which allows the filament growth. The first step could be reversible and the laminate's insulation resistance may be restored by baking and drying. The second stage, the actual CAF growth, is believed to be irreversible. Failure is a function of voltage bias, relative humidity, hole to hole and track to track spacing, temperature and the type of resin system.

How can it be prevented?

Careful selection of laminate type can reduce the likelihood of CAF formation. Conventional FR4 laminates are typically cured using dicyandiamide and this material has been associated with CAF formation. Laminates such as those based on 'BT' resin systems, which do not use dicyandiamide as a curing agent, tend to have better resistance to CAF. Unfortunately, BT resin based laminates are significantly more expensive than their FR4 type counterparts.