Any electronic assembly can have multiple failure modes, ranging from mechanical to electrical. One class of failure modes that is not so obvious is an electrochemical failure mode, known as conductive anodic filamentation, or CAF. This failure mode arises through an electrochemical reaction between copper and the resin system in the PCB substrate in the presence of water. This means your PCBA has the potential to act like an electrochemical cell when it is in operation under the right conditions.
Can CAF be prevented? There are some simple steps that can be taken to suppress CAF growth in PCB substrate materials. First, we’ll look at the major causes of CAF and the ingredients required for it to occur. At that point, it becomes much easier to see how this can be prevented.
What is CAF?
According to Turbini and Ready, conductive anodic filamentation is an “electrochemical failure mode of electronic substrates involves the growth of a copper containing filament subsurface along the epoxy-glass interface, from anode to cathode” (Source: L. J. Turbini and W. J. “Ready. Conductive Anodic Filament Failure: A Materials Perspective.” Schools of Materials Science & Engineering, Georgia Institute of Technology.)
As an electrochemical failure mode, it is caused by an electrically-driven reaction between the following materials in the PCB substrate:
- Copper that makes up the traces in the PCB
- Leftover etchant, water, and other volatile processing chemicals adsorbed in the PCB substrate
- Voids in the system near the anode, possibly due to lack of flow in fine weaves or due to decomposition of the resin system
The chemical reaction between these materials is driven by an applied voltage and leads to growth of cuprous salts and copper oxide. Ionic copper salts are conductive materials that grow outward from the copper trace in the PCB. CAF growth tends to occur along the glass weave in the substrate, as shown below. If there is voiding in the PCB material, such as in cases where there is low resin content in very tight weaves or when NFPs are present in vias, CAF growth can occur easily.
Conceptual drawing showing growth direction of CAF along the glass weave in a PCB substrate. [Source: C. Navarro. "Development of a standard test method for evaluating conductive anodic filament (CAF) growth failure in PCBs." Circuit World 28, no. 2 (2002): 14-18.]
How CAF Growth Causes Failure
If CAF is extreme and leads to extensive cuprous salts, the potential problem is two conductors are bridged and a short circuit is created, as shown in the conceptual image above. This is most probable in dense layouts or in high-voltage designs. The typical pathway by which CAF growth begins is as follows:
- The PCB is exposed to water, which adsorbs in the substrate material
- Water accumulation causes local breakdown of the resin system, producing a low pH environment
- Electrochemical reactions leading to cuprous salts growth begins once a voltage is applied
- Over time, further electromigration and growth is enabled by further deposition of salts
Like most other aspects of testing and qualification of electronic assemblies, there is a standardized test method for evaluating CAF growth with a test coupon. The industry standard test method is specified as IPC-TM-650, Method 2.6.25A. The test method uses standard test coupon designs such as those specified in IPC-9253 to 9256. If CAF is believed to be the cause of failure in a PCB, it can be identified using any of the standard micro-materials characterization techniques. The most effective tool for investigating this failure is with scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy from a microsection.
SEM image showing identification of CAF growth along glass filaments in a PCB substrate. [Source: A. Caputo. “Conductive Anodic Filament (CAF) Formation.” Materials Science, 18 Jan 2012]
While the action of CAF may seem unavoidable and inevitable, this potential failure mode can be suppressed with some simple design decisions. Remember the four ingredients needed for growth AF: water, copper, voltage, and an acidic environment near the copper anode. If you can prevent formation of this environment, then you can suppress CAF growth. Some of the main strategies include:
- Include a desiccant in the PCBA packaging/enclosure to adsorb any humidity in the package
- Use an IP6x-rated enclosure and connectors to prevent moisture ingress
- Make sure your manufacturer uses cleaning and bake-out processes during fabrication to remove moisture and excess volatiles from the board materials
- Use a material with a higher resin content (looser glass weave) as there will be lower chances of void formation
- Remove non-functional pads on vias, especially when the separating dielectric is thinner
- Use phenolic-cured materials to build the bare PCB
The final point is important as it can limit material selection, although the range of phenolic materials has steadily expanded to include advanced low-loss phenolic-cured laminates. Phenolic-cured PCB core/prepreg materials are RoHS-compatible but they may impact processing temperatures in reflow, depending on the solder being used. PCB materials were traditionally composed of an epoxy resin with a dicyandiamide (dicy) cure agent. Phenolic-cured FR4 materials are needed in order to be compatible with Pb-free solders as they have higher decomposition temperature and can withstand higher solder process temperatures.
If you plan to specify materials for manufacturing, keep these points in mind and make sure you know how to pair up substrate materials with solder in your PCB. Typically, if you specify a specific material, your fabrication house should know what is the appropriate solder to use with the material so that the material system remains at the appropriate process temperature during reflow.
When you need to decide material requirements, assembly specifications, and packaging specifications to prevent conductive anodic filamentation, make sure you have the complete set of system analysis tools from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.