CAF (conductive anodic filament) is a conductive filament that forms when metal ions migrate from anode to cathode under an electric field and humidity. Once it bridges the gap it causes a short circuit. It is one of the main failure modes of electronic assemblies under high humidity and bias.
Formation mechanism
Electrochemical migration: anodic metal dissolves into ions and migrates toward the cathode under the electric field.
Ion reduction and deposition: metal ions reaching the cathode are reduced and deposited, forming a conductive path.
Growth conditions: high humidity (ion migration needs water), ionic contamination (flux residue, electrolyte) and a bias between anode and cathode.
The time factor: growth takes time and is a classic cumulative failure, so short tests struggle to reproduce it.
High-risk applications
High-humidity environments: outdoor equipment, near bathrooms, the plum rain season in southern China.
DC bias: applications that stay energised for long periods without polarity reversal, such as battery-powered sensor nodes.
High ionic contamination: ionically contaminated flux, incomplete cleaning, fingerprint contamination.
Fine-pitch designs: between adjacent joints of fine-pitch devices such as BGA and LGA.
High voltage gradient areas: between connector pins, between transformer winding turns.
Detection methods
Visual and microscopic observation: locate the filament or corrosion products.
EDS composition analysis: confirm the elemental composition of the filament and identify the source metal.
Ion chromatography (IC): measure the ionic contamination level.
Ionic migration testing: a dedicated method that accelerates CAF growth under controlled temperature, humidity and bias.
X-ray: observe fine growth between inner-layer joints.
Design improvement directions
Reduce ionic contamination: use low-contamination cleaners and fluxes, and control ionic contamination in production.
Electric field design: increase creepage distance between energised parts and reduce voltage gradient concentration.
Remove the driving factor: avoid continuous DC bias where AC bias is possible.
Material selection: choose CAF-resistant laminate and solder systems.
Protective measures: apply conformal coating as a barrier, paying attention to coating quality.
Frequently asked questions
Are CAF and electrochemical migration the same problem?
They are different expressions of the same mechanism. Electrochemical migration (ECM) describes the process; CAF is the conductive filament product that forms under specific conditions. The industry often uses the two interchangeably, but distinguishing them clearly in a technical report makes analysis easier.
Why did my product pass damp heat testing without showing CAF?
Because damp heat testing verifies insulation performance, not CAF specifically. CAF needs particular bias conditions and time to accumulate. Targeted verification requires ionic migration testing or CAF accelerated testing, whose conditions differ from routine damp heat.
Service notes
Our laboratory holds both CMA and CNAS accreditation, covers about 10,000 m2 and runs more than 500 sets of equipment. All testing is carried out in our own laboratories — nothing is subcontracted. Plant visits and on-site witnessed testing are welcome.
Scheduling follows the actual project pipeline. You can state your preferred lead time when you submit samples; we will confirm a feasible plan against current capacity and keep you updated at each milestone. Once the report is issued we send the electronic copy; corrective-action advice can be arranged separately.
