Conductive anodic filament, also called CAF, is a metallic filament that forms from an
electrochemical migration Electrochemical migration (ECM) is the dissolution and movement of metal ions in presence of electric potential, which results in the growth of dendritic structures between anode and cathode. The process is most commonly observed in printed circui ...
process and is known to cause
printed circuit board (PCB) failures.
Mechanism
CAF formation is a process involving the transport of conductive chemistries across a nonmetallic substrate under the influence of an applied electric field.
[ IPC TM-650 2.6.25 Conductive Anodic Filament (CAF) Resistance Test: X-Y Axis https://www.ipc.org/4.0_Knowledge/4.1_Standards/test/2-6-25.pdf] CAF is influenced by
electric field strength
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field f ...
,
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
(including
soldering
Soldering (; ) is a process in which two or more items are joined by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involv ...
temperatures),
humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
, laminate material, and the presence of manufacturing defects. The occurrence of CAF failures has been primarily driven by the electronics industry pushing for higher density circuit boards and the use of electronics in harsher environments for high reliability applications.
Failure modes and detection
CAF commonly occurs between adjacent
vias
The Vias GmbH (stylized VIAS) is a rail service company based in Frankfurt (Germany). The name of the company was taken from the Latin word via for ''way'' and the letter ''S'' for service. It operates rail services in the states of Hesse, Rhine ...
(i.e. plated
through holes
A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...
) inside a PCB, as the copper migrates along the glass/resin interface from
anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
to
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
. CAF failures can manifest as current leakage, intermittent
electrical shorts, and even
dielectric breakdown
Electrical breakdown or dielectric breakdown is a process that occurs when an electrical insulating material, subjected to a high enough voltage, suddenly becomes an electrical conductor and electric current flows through it. All insulating mate ...
between conductors in printed circuit boards. This often makes CAF very difficult to detect, especially when it occurs as an intermittent issue. There are a few things that can be done to isolate the fault location and confirm CAF as a root cause of a failure. If the issue is intermittent then putting the sample of interest under combined temperature-humidity-bias (THB) may help recreate the failure mode. In addition, techniques such as
cross sectioning or
superconducting quantum interference device (SQUID) can be used to identify the failure.
Considerations and mitigation
There are several design considerations and mitigation techniques that can be used to reduce the susceptibility to CAF. Certain material selection (i.e. laminate) and design rules (i.e. via spacing) can help reduce CAF risk. Poor adhesion between the resin and glass fibers in the PCB can create a path for CAF to occur. This may depend on parameters of the silane finish applied to the glass fibers, which is used to promote adhesion to the resin. There are also testing standards that can be performed to assess CAF risk. IPC TM-650 2.6.25 provides a test method to assess CAF susceptibility.
Additionally, IPC TM-650 2.6.16 provides a pressure vessel test method to rapidly evaluate glass epoxy laminate integrity.
[ IPC TM-650 2.6.16 Pressure Vessel Method for Glass Epoxy Laminate. https://www.ipc.org/TM/2.6.16.pdf] This is helpful but it may often be better to use design rules and proper material selection to proactively mitigate the issue.
See also
*
Whisker (metallurgy)
Metal whiskering is a phenomenon which occurs in electrical devices when metals form long whisker-like projections over time. Tin whiskers were noticed and documented in the vacuum tube era of electronics early in the 20th century in equipment t ...
External links
Material & Process Influences on CAFConductive Anodic Filament (CAF) Formation
References
{{Reflist
Electrochemistry
Semiconductor device defects