Conductive anodic filament (CAF) formation is a well-studied phenomenon that is driven by chemical, humidity, voltage, and mechanical means. It is characterized by a sudden loss of surface insulation resistance. CAF can form between adjacent lines on a given layer of a printed circuit board (PCB), between plated through holes (PTH), or between a plated through hole and a line on the PCB.  Flux chemistry, damage from multiple soldering steps, and excessive voltages (beyond designed voltages) accelerate the onset of CAF.  The mechanism of CAF is an electro-chemical transport of ions across an electrical potential between anode and cathode.

Discovered by Bell Labs engineers in the late 1970’s, (CAF) arises from a two-step process1. The first step involves the degradation of the fiber/epoxy interface of a PCB. Once the interface has degraded, delamination of the copper traces or a void in the PCB will occur. This is the first step to CAF formation.  For a CAF to create a short circuit, the presence of a halide ion and a metal is required. Chloride ions were present in all of the CAFs found.  However, other ions such as bromides have been found in the conductive path along with chlorides. The sources of halides in research studies have been associated with flux, but product line contamination, or the PCB itself cannot be ruled out.

Epichlorohydrin (C3H5OCl) is commonly used in the manufacturing of epoxy resin. Although most of the chlorine is driven out in processing, a trace amount of free chloride exists in the finished PCB. This is usually about 100ppm.

The Mean Time Before Failure (MTBF) for a CAF to create a short circuit is dependent on humidity, applied voltage, anode-cathode spacing, and the available halide source. The reaction will not take place without a void, or without a hollow glass fiber in the matrix to provide the pathway for electro-chemical migration. Many studies have found that CAFs follow the Arrhenius equation with primary acceleration factors of humidity, temperature, and applied voltage. Figure 4-1 shows a CAF that formed between a plated through hole and the power plane of a multi-layer board.

Additionally, the thermal gradients associated with component reflow increases the onset of debonding of the copper from the PCB and hence can decrease the time needed to produce a CAF by a factor of two2. Coefficients of thermal expansion (CTE) mismatches are a primary driver of degradation of the epoxy metal interface. However, butter coating of the surface layers of multi layer PCBs, with additional epoxy before stack up, has improved resistance to CAF.  This butter coating process, adopted by many board makers, is routinely used today to enhance
reliability of power PCBs.

Humidity pays a key role driving CAF formation, however, material/lot dependence for any given PCB on the MTBF at constant voltage, and physical spacing was found by Welsher2 et al. His experiments also showed that by mixing triazine with fiberglass, a PCB with significantly better (20-30 times the standard) potential for reducing CAF formation was created.

As stated earlier, flux chemistry plays a role in CAF formation.
Water soluble fluxes (WSF) that are poly glycol based (polyethylene, polypropylene) and contain a chlorine activator increase the likelihood of CAF. Even bromine-activated fluxes can create CAF but the mechanism for bromine traced CAF requires 15% bromine in the flux to be found chemically3.

For many years, the use of the
J-STD-004 method for measurement of surface insulation resistance (SIR) could not identify a CAF. The method contained a reversing of the anode and cathode during testing, and
continuous monitoring was not specified. This allowed CAFs that formed on the surface of a comb structure to be blown out by the test method. Clearly, a better method needed to quantify the phenomena that was observed in real use but could not be seen in SIR testing.

Ready and Turbini4 developed a better method. They utilized a linear circuit containing an op amp that reduces the current flow as the CAF grows and reaches the short circuit stage.  They fabricated a PCB structure that contained the circuit and used a comb structure with tight spacing to generate CAFs.  With this circuit, Ready and Turbini created and recorded many CAFs successfully.  Figure 4-2 is a schematic diagram of the circuit they used. Figure 4-3 is the comb structure they used.  In figure 4-4, a dendrite that formed on the test structure was preserved by the action of the linear circuit.
 
In cases where incidence of CAF must be reduced or eliminated, there are a few options at our disposal:

1) Use triazine based PCB construction methods
2) Avoid use of water soluble fluxes based on medium to high
     molecular weight polyglycol chemistries containing high
    concentrations of halide activators
3) Avoid multiple reflows, or hand soldering rework
4) Use generous line and space rules (>5mil space minimum)
5) Butter coat the B staged PCBs before stack up and curing
6) Use laser drilling rather than mechanical drilling, as
    mechanical drilling can pull voids when a dull bit is used

By taking the above steps, the onset of CAF formation can be slowed down to extend the useful lifetime of the product.

References

     1 Welsher, T.L.,J.P. Mitchell, and D.J. Lando, 18th Annual
        Proc. Reliability Physics (1980):235-237.
     2 Welsher, T.L.,J.P. Mitchell, and D.J. Lando, Annual
        Report of the Conf. on Electrical Insulation and Dielectric
        Phenomena (1980):234-238.
     3 Ready, W.J.3 Master of Science in Metallurgical Engineering
        Thesis, Georgia Institute of Technology, (1997).
     4 Ready,W.J.4, L.J. Turbini, R. Nickel, and J. Fisher, J.
       Electronic Materials. 28(1999):1158.