PCB conductive anodic filament failure occurs when copper-containing conductive material grows through the PCB dielectric under moisture, electrical bias, and a vulnerable internal pathway, eventually causing leakage current or a short circuit between conductors.
CAF failures are most likely when opposite-polarity vias, holes, traces, or planes are closely spaced and the PCB operates in high temperature, high humidity, or condensing conditions.
Increasing spacing helps, but spacing alone is not enough. Reliable prevention also requires CAF-resistant laminate, strong resin-to-glass bonding, controlled drilling, clean fabrication, appropriate reflow exposure, representative CAF testing, and protection from moisture during operation.
What Is PCB Conductive Anodic Filament?
A conductive anodic filament, commonly shortened to CAF, is an internal electrochemical migration path that develops through a nonconductive PCB material.
The filament normally contains copper compounds and grows between conductors at different electrical potentials. As the conductive path develops, insulation resistance decreases.
The PCB may initially show only small leakage-current changes. Continued exposure can produce:
- Intermittent Electrical Leakage
- Reduced Insulation Resistance
- Unexpected Current Consumption
- Signal Instability
- High-Voltage Arcing
- Functional Reset
- Permanent Short Circuit
- Field Failure
IPC identifies IPC-TM-650 Method 2.6.25C as the conductive anodic filament resistance test for the X-Y axis of a printed board. IPC-TM-650 Test Methods
CAF is particularly important for dense multilayer PCBs because decreasing hole pitch and conductor spacing increase the electrical field across a shorter dielectric path.
How Does A Conductive Anodic Filament Form?
CAF formation requires several conditions to exist at the same time:
- An Applied Voltage Difference
- Moisture
- Mobile Ionic Material
- Copper Or Another Conductive Source
- A Susceptible Path Through The Dielectric
- Sufficient Exposure Time
The failure generally develops in two connected stages.
Stage 1: A Migration Path Develops
Moisture, thermal stress, drilling damage, weak resin-to-glass adhesion, or chemical contamination creates a pathway within the laminate.
The path frequently follows the interface between glass fibers and cured resin because this interface may be more vulnerable than the surrounding bulk resin.
Stage 2: Electrochemical Migration Occurs
With voltage applied, copper at the anode participates in electrochemical reactions. Ionic material moves through the moisture-containing pathway.
A conductive filament develops from the anode toward the cathode. As the filament approaches the opposite conductor, insulation resistance falls.
Isola’s CAF research describes this two-stage mechanism as degradation of the resin-glass interface followed by electrochemical migration under voltage and humidity. Material And Process Influences On CAF
What Conditions Are Required For PCB CAF Failure?
Removing or controlling one condition can slow the mechanism, but a reliable product should control the complete system.
| CAF Condition | Why It Matters |
|---|---|
| Moisture | Provides A Medium For Ionic Transport |
| Electrical Bias | Drives Electrochemical Migration |
| Copper Source | Supplies Material For Filament Growth |
| Ionic Contamination | Increases Conductivity And Reaction Activity |
| Internal Path | Allows Migration Through The Dielectric |
| Temperature | Accelerates Moisture Diffusion And Chemical Reactions |
| Time | Allows The Filament To Develop |
| Small Spacing | Increases Electrical Field And Shortens The Path |
A PCB stored in high humidity without electrical power may absorb moisture but not immediately develop a CAF short.
The same PCB may fail after voltage is applied because the electric field begins driving electrochemical migration.
Where Does CAF Grow Inside A PCB?
CAF can develop between several conductor combinations.
Plated Through Hole To Plated Through Hole
Hole-to-hole CAF is one of the most recognized configurations. The filament may follow glass bundles between two drilled and plated holes.
Via To Copper Plane
A plated via may be positioned too close to an internal plane connected to a different potential.
The actual dielectric separation must account for drill position, hole plating, internal-layer registration, and etching tolerances.
Via To Trace
A via barrel can create a high electric-field region near an internal conductor. The risk increases when the conductor-to-hole distance is small.
Trace To Trace
CAF can also develop between internal conductors, especially when both traces align with a vulnerable glass-resin interface.
Pad To Pad
Closely spaced through-hole pads or nonfunctional pads on internal layers may reduce the effective dielectric distance.
Hole To Board Edge
An exposed or contaminated routed edge can provide a moisture-entry pathway when an internal conductor is close to the edge.
PCB CAF Vs Surface Dendritic Growth
CAF and dendritic growth are both forms of electrochemical migration, but they normally occur in different locations.
| Failure Mode | Typical Location | Common Path |
| CAF | Inside The PCB Laminate | Along Glass-Resin Interfaces |
| Dendritic Growth | PCB Surface | Across Solder Mask, Flux Residue, Or Contamination |
| Ionic Leakage | Surface Or Internal | Moisture And Ionic Contamination |
| Conductive Tracking | Insulating Surface | Carbonized Or Contaminated Surface Path |
| Metal Whiskers | Metal Finish Or Component Termination | Mechanically Driven Metal Growth |
Surface dendrites may be visible under magnification. CAF is often hidden inside the PCB until destructive analysis is performed.
Cleaning an assembled PCB can remove surface contamination and reduce dendritic-growth risk. It cannot remove contamination or repair damage already sealed inside a multilayer PCB.
Our PCB assembly cleaning article explains the difference between visible residue removal and validated electrical cleanliness.
Is CAF The Same As PCB Delamination?
No. CAF and delamination are different failure mechanisms.
PCB delamination is the physical separation of bonded materials. CAF is electrochemical growth through an internal dielectric pathway.
However, both mechanisms may involve:
- Moisture
- Weak Resin-To-Glass Interfaces
- Thermal Stress
- Material Damage
- Poor Process Control
Delamination, measling, crazing, or microcracking can create moisture pathways that may influence insulation reliability. Their presence does not prove that a CAF filament exists.
The root cause must be confirmed through electrical data and physical analysis.
Why Is CAF Becoming More Important?
Modern PCBs place more electrical functions in a smaller area.
CAF risk becomes more important because of:
- Smaller Via-To-Via Pitch
- Reduced Hole-To-Copper Spacing
- Higher Layer Counts
- Fine-Pitch Connectors
- High Operating Voltage
- Higher Voltage Gradients
- Multiple Lamination Cycles
- Lead-Free Reflow Temperatures
- Outdoor And Automotive Use
- Longer Product Lifetimes
- Higher Humidity Exposure
- Smaller Finished Holes
A board can meet basic fabrication requirements and still have insufficient CAF reliability for its actual environment.
Products expected to operate for many years require more evidence than a short room-temperature electrical test.
What PCB Designs Have The Highest CAF Risk?
| PCB Design | Main CAF Concern |
| Dense Via Field | Short Hole-To-Hole Distance |
| High-Voltage PCB | Strong Electrical Field |
| Thick Multilayer PCB | Drilling Stress And Moisture Path Length |
| HDI PCB | Small Spacing And Multiple Lamination Cycles |
| Automotive Control Module | Humidity, Temperature Cycling, And Long Life |
| Outdoor Electronics | Moisture And Condensation |
| Telecom Backplane | Dense Connectors And Continuous Electrical Bias |
| Industrial Power PCB | High Voltage And Contaminated Environments |
| Medical Equipment | High Reliability And Long Service Requirements |
| Server PCB | High Layer Count And Dense Via Structures |
| EV Charging Equipment | High Voltage, Heat, And Humidity |
| Marine Electronics | Moisture And Salt Exposure |
CAF testing is not required for every low-risk circuit. It becomes more valuable when high voltage, close spacing, humidity, long service life, or severe environmental exposure are combined.
How Does Via-To-Via Spacing Affect CAF?
Smaller spacing shortens the migration distance and can increase the electric-field strength between conductors.
However, the finished-hole drawing distance is not the only value that matters.
The manufacturer must consider:
- Nominal Drill Diameter
- Finished Hole Diameter
- Copper Plating Thickness
- Drill Wander
- Layer Registration
- Pad Diameter
- Nonfunctional Pads
- Etching Tolerance
- Glass-Weave Direction
- Actual Voltage Difference
Two vias with the same center-to-center pitch may have different CAF risk if their finished diameters, pad structures, voltage potentials, or laminate constructions are different.
The PCB annular ring design should therefore be reviewed with conductor spacing and registration capability.
Is There A Minimum Via Spacing That Prevents CAF?
There is no universal via spacing that guarantees CAF resistance for every material, voltage, environment, and product life.
A safe distance depends on:
- Maximum Continuous Voltage
- Transient Voltage
- Temperature
- Relative Humidity
- Condensation Risk
- Laminate CAF Performance
- Hole Diameter
- Drill Quality
- Glass Style
- Product Lifetime
- Required Failure Probability
- Manufacturing Capability
A spacing that works for a dry indoor consumer product may be unsuitable for automotive, marine, outdoor, or industrial electronics.
Buyers should avoid copying one generic hole-to-hole spacing value into every design without environmental and voltage analysis.
Why Creepage And Clearance Rules Do Not Fully Prevent CAF
Clearance normally describes the shortest distance through air. Creepage describes the shortest distance along an insulating surface.
CAF develops inside the PCB laminate. Therefore, a design can satisfy external creepage and clearance requirements while still having insufficient internal conductor separation.
High-voltage design should evaluate:
- External Clearance
- Surface Creepage
- Internal Layer Spacing
- Hole-To-Hole Distance
- Hole-To-Plane Distance
- Hole-To-Trace Distance
- Routed Slots
- Board Edges
- Material Insulation Performance
- CAF Resistance
External safety spacing and internal CAF reliability are connected but separate design tasks.
Does A Higher Operating Voltage Increase CAF Risk?
Higher voltage can increase the electric-field strength that drives ion movement, but voltage must be evaluated with spacing.
The relationship is commonly considered as voltage gradient:
Voltage Gradient = Voltage Difference ÷ Dielectric Distance
A 100 V difference across a large separation may create less stress than a lower voltage across an extremely small internal distance.
Polarity also matters. The filament mechanism is directional, and failure time may change when conductor polarity or geometry changes.
The design review should identify actual electrical potentials instead of assuming every neighboring via experiences the same stress.
How Does Humidity Affect PCB CAF?
Moisture enables ionic transport through vulnerable laminate interfaces.
High relative humidity increases risk, especially when combined with:
- Elevated Temperature
- Electrical Bias
- Ionic Contamination
- Edge Exposure
- Condensation
- Thermal Cycling
- Pressure Changes
- Damaged Coatings
A product can experience internal moisture even when liquid water is never visible.
Moisture can enter through:
- Routed Board Edges
- Plated Holes
- Microcracks
- Connector Openings
- Unsealed Enclosures
- Damaged Conformal Coating
- Laminate Diffusion
- Repeated Temperature Cycling
The real operating environment should be defined before selecting the CAF qualification conditions.
How Does Temperature Affect CAF Formation?
Higher temperature can accelerate moisture diffusion and electrochemical reactions.
Temperature also changes:
- Resin Properties
- Moisture Absorption
- Ion Mobility
- Copper Reaction Rates
- Coating Performance
- Dimensional Stress
A product exposed to alternating hot and cold conditions may experience moisture pumping or condensation inside its enclosure.
CAF evaluation should therefore consider storage temperature, operating temperature, shutdown cycles, humidity, voltage, and expected lifetime together.
How Does PCB Material Affect CAF Resistance?
CAF resistance depends strongly on the laminate’s resin system, glass reinforcement, adhesion, moisture behavior, and processing quality.
Important properties include:
- Resin-To-Glass Adhesion
- Moisture Absorption
- Hydrolytic Stability
- Ionic Purity
- Glass Surface Treatment
- Resin Cure
- Thermal Stability
- Z-Axis Expansion
- Resistance To Reflow Damage
A laminate advertised as CAF resistant may reduce risk, but it does not guarantee the reliability of every finished PCB construction.
Isola identifies controlled glass reinforcement and resin chemistry as contributors to the CAF resistance of its 370HR laminate system.
Material performance should be verified using representative coupons and production processes.
Does High Tg Mean High CAF Resistance?
No. Tg and CAF resistance describe different properties.
Tg identifies a change in polymer mechanical behavior. CAF resistance depends more directly on moisture, resin-glass adhesion, ionic purity, internal damage, conductor spacing, and voltage bias.
A high-Tg laminate can still have poor CAF performance if:
- The Resin-Glass Interface Is Weak
- Material Moisture Absorption Is High
- Drilling Creates Microcracks
- The Lamination Process Is Incorrect
- Ionic Contamination Is Present
- Conductor Spacing Is Too Small
- The Actual Construction Was Not Tested
Material selection should evaluate thermal and electrochemical reliability separately.
Is CTI The Same As CAF Resistance?
No. Comparative Tracking Index, or CTI, evaluates tracking behavior on the surface of an insulating material under defined test conditions.
CAF develops internally through the PCB laminate.
A material with a suitable CTI rating does not automatically provide adequate CAF resistance. Likewise, strong CAF performance does not replace creepage and surface-tracking analysis.
Both properties may be important in a high-voltage product, but they address different failure paths.
How Does Glass Weave Affect CAF?
CAF often follows the interface between glass fibers and resin. The glass style and bundle orientation can therefore affect the available migration path.
Potentially relevant factors include:
- Glass Bundle Size
- Filament Distribution
- Warp And Fill Direction
- Spread-Glass Construction
- Resin Coverage
- Glass Surface Treatment
- Via Alignment With Bundles
Closely spaced holes aligned with the same glass bundle may create a more direct pathway than staggered features.
Designers normally do not control the exact position of every fiber. However, layout, glass style, laminate construction, and coupon orientation can be reviewed for high-risk products.
Can Staggering Vias Reduce CAF Risk?
Staggering vias may reduce the likelihood of two opposite-polarity holes sharing one direct glass-resin pathway.
Possible layout actions include:
- Increasing Center-To-Center Pitch
- Staggering Adjacent Via Rows
- Separating Opposite-Polarity Nets
- Moving High-Voltage Vias To Different Regions
- Avoiding Long Aligned Via Arrays
- Rotating Repetitive Patterns
- Reviewing Internal Plane Clearances
Staggering does not replace appropriate spacing, material selection, or qualification testing.
The benefit also depends on glass-weave orientation and the actual production stackup.
How Can Drilling Cause CAF Susceptibility?
Mechanical drilling can damage the laminate surrounding a hole.
Potential drilling-related problems include:
- Glass-Fiber Fracture
- Resin Smear
- Microcracking
- Excessive Hole-Wall Roughness
- Local Heating
- Resin-Glass Separation
- Drill Wander
- Closely Spaced Damaged Zones
Risk increases with:
- Worn Drill Bits
- Incorrect Feed Rate
- Incorrect Spindle Speed
- Excessive Hit Count
- Poor Entry Or Backup Material
- Thick PCB Construction
- Small Drill Diameter
- Unstable Stack Height
Two holes may meet dimensional requirements but still have overlapping damaged regions inside the laminate.
Can Desmear And Hole Preparation Affect CAF?
Yes. Desmear removes resin residue and prepares the hole wall for metallization.
Insufficient desmear may leave poor plating surfaces. Excessive chemical or plasma treatment may attack resin-glass interfaces and enlarge existing damage.
The process should control:
- Chemistry Concentration
- Bath Temperature
- Exposure Time
- Swell
- Permanganate Or Plasma Parameters
- Neutralization
- Rinsing
- Hole-Wall Condition
The complete plated through hole process must balance plating adhesion, hole-wall quality, and laminate integrity.
Does Lead-Free Reflow Increase CAF Risk?
Reflow does not create a CAF filament immediately because CAF normally requires moisture, electrical bias, and time.
However, excessive or repeated thermal exposure can damage the resin-glass interface and make the PCB more susceptible during later biased humidity exposure.
Possible effects include:
- Resin-Glass Separation
- Microcracking
- Increased Moisture Pathways
- Plated-Hole Stress
- Local Delamination
- Reduced Bond Strength
CAF tests may include thermal preconditioning when the objective is to represent actual assembly exposure.
The number and severity of preconditioning cycles should match the qualification plan rather than being selected arbitrarily.
Can PCB Delamination Lead To CAF?
Delamination can create a moisture pathway, but delamination does not automatically cause CAF.
CAF may also occur in a board without visible delamination or measling.
The mechanisms should be investigated separately:
- Delamination Requires Evidence Of Layer Separation
- CAF Requires Evidence Of Electrochemical Migration
- Both May Share Moisture And Interface Damage
- Either Defect Can Exist Without The Other
IPC-hosted research has reported CAF failures in samples without measling, showing why visual measling alone is not a reliable CAF predictor. Do Measles Cause CAF?
Can Conformal Coating Prevent CAF?
Conformal coating can reduce moisture and contamination reaching the PCB surface, but it does not make the internal laminate immune to CAF.
Its effectiveness depends on:
- Coating Material
- Cleanliness Before Coating
- Coating Thickness
- Coverage
- Adhesion
- Pinholes
- Connector Keep-Out Areas
- Board Edges
- Thermal Cycling
- Mechanical Damage
Moisture may enter through uncoated edges, mounting holes, connectors, coating defects, or diffusion through the coating.
The PCB conformal coating process should be treated as one environmental-control layer, not a substitute for CAF-resistant PCB design.
Can Solder Mask Stop CAF?
Solder mask primarily protects PCB surfaces. Internal CAF usually develops beneath the mask within the glass-reinforced laminate.
Solder mask may help reduce surface contamination and dendritic growth, but it cannot correct:
- Insufficient Internal Spacing
- Weak Resin-Glass Adhesion
- Drilling Damage
- Internal Ionic Contamination
- Poor Lamination
- Moisture Entering From Board Edges
A visually complete solder-mask coating does not prove internal insulation reliability.
How Is PCB CAF Tested?
CAF testing applies environmental stress and electrical bias to representative conductor patterns while insulation resistance or leakage current is monitored.
A test normally includes:
- Coupon Selection
- Initial Electrical Measurement
- Thermal Or Assembly Preconditioning
- Humidity Conditioning
- Applied Electrical Bias
- Continuous Or Periodic Resistance Monitoring
- Failure-Time Recording
- Physical Failure Analysis
IPC-TM-650 Method 2.6.25C addresses X-Y axis CAF resistance testing. IPC-9691B provides guidance on coupon design, materials, processing, test implementation, and result interpretation. IPC-9691B CAF User Guide
Test temperature, humidity, voltage, duration, threshold, and preconditioning should be defined by the applicable qualification plan.
Why Is 85°C And 85% RH Commonly Used?
A temperature of 85°C and relative humidity of 85% is frequently used in accelerated electronics reliability studies because it combines elevated heat and moisture.
However, 85/85 is not automatically the correct condition for every CAF requirement.
The test plan should define:
- Temperature
- Relative Humidity
- Bias Voltage
- Test Duration
- Resistance Threshold
- Coupon Geometry
- Preconditioning
- Measurement Frequency
- Failure Confirmation
A more severe test is not always more representative. Condensation, fixture leakage, or surface contamination can create results unrelated to internal CAF.
What Should A CAF Test Coupon Represent?
A useful coupon should represent the production risk rather than only the laminate name.
Coupon variables may include:
- Finished Hole Diameter
- Drill Diameter
- Hole Pitch
- Pad Diameter
- Glass Style
- Resin Content
- Laminate Lot
- Layer Construction
- Hole Plating
- Drilling Parameters
- Desmear Process
- Reflow Preconditioning
- Conductor Orientation
- Voltage Polarity
A coupon made with a different glass style, drill process, or spacing may not predict the final PCB accurately.
Where possible, coupons should be manufactured in the same production panel and process flow as the functional boards.
What CAF Test Patterns Are Common?
CAF coupons can evaluate several conductor relationships.
| Test Pattern | What It Evaluates |
| Hole To Hole | Migration Between Drilled And Plated Holes |
| Hole To Plane | Migration Between A Hole And Internal Copper Plane |
| Hole To Trace | Migration Between A Hole Barrel And Conductor |
| Trace To Trace | Migration Between Internal Conductors |
| Opposite Orientation Patterns | Effect Of Glass-Weave Direction |
| Multiple Spacing Groups | Effect Of Conductor Separation |
Using only one hole spacing may show whether that geometry passes but provide limited information about process capability and design margin.
Multiple spacing groups can help identify the relationship between geometry and time to failure.
How Is CAF Failure Identified?
A CAF event usually produces a significant insulation-resistance reduction or leakage-current increase under the defined test conditions.
A valid result should include:
- Initial Resistance
- Resistance Over Time
- Temperature And Humidity
- Applied Voltage
- Coupon Identification
- Channel Identification
- Failure Time
- Test Interruptions
- Final Resistance
- Physical Confirmation
A single final resistance measurement can miss intermittent behavior or the time at which degradation began.
Trend data is more useful for comparing materials, processes, and spacing.
Can Surface Leakage Produce A False CAF Result?
Yes. A resistance drop may come from surface contamination, condensation, fixtures, cables, connectors, or chamber leakage rather than an internal conductive filament.
Potential controls include:
- Clean Coupon Surfaces
- Guarding And Suitable Fixtures
- Controlled Chamber Ramp
- Dry Insulating Supports
- Blank Channels
- Surface-Protective Features
- Post-Test Cleaning Comparison
- Physical Cross-Section Confirmation
A failed electrical channel should not automatically be reported as CAF without confirming the failure location.
How Is A CAF Failure Physically Confirmed?
Failure analysis may use:
- Resistance Localization
- Optical Inspection
- Microsectioning
- Scanning Electron Microscopy
- Energy-Dispersive X-Ray Spectroscopy
- Chemical Analysis
- Dye Techniques
- High-Resolution X-Ray
- Acoustic Inspection
SEM and elemental analysis can help identify copper-containing material along the suspected path.
Microsection location is critical because a CAF filament can be extremely small. A section cut slightly away from the path may miss it entirely.
Our PCB microsection analysis explains how targeted sectioning improves defect identification.
Why Standard PCB Electrical Testing Cannot Prove CAF Resistance
Bare-board electrical testing is normally performed under controlled factory conditions for a relatively short time.
CAF may require:
- Moisture Exposure
- Elevated Temperature
- Continuous Bias
- Hundreds Of Hours
- A Damaged Internal Path
A PCB can pass continuity and isolation testing at shipment and later fail in the field.
Electrical testing verifies the present conductor network. CAF qualification evaluates how insulation behaves under accelerated environmental and electrical stress.
How Can PCB Designers Reduce CAF Risk?
Increase Critical Conductor Spacing
Increase hole-to-hole, hole-to-plane, and hole-to-trace distance where voltage and environment justify it.
Separate Opposite-Polarity Nets
Do not place high-potential and low-potential conductors adjacent simply because layout software permits the spacing.
Stagger Via Arrays
Staggering may reduce direct alignment along glass bundles.
Review Nonfunctional Pads
Removing selected nonfunctional pads may increase internal copper clearance. However, their removal must be reviewed for manufacturability, registration, signal integrity, and mechanical reliability.
Define The Production Stackup
Glass style, resin system, dielectric thickness, and copper construction affect CAF performance.
Our PCB stackup design article explains why nominal board thickness alone does not define internal reliability.
Perform Voltage-Aware DFM
The PCB supplier should receive net-voltage or high-voltage-region information when CAF spacing is critical.
Ordinary Gerber data shows copper geometry but does not explain which neighboring conductors carry the largest voltage difference.
How Can PCB Manufacturers Control CAF Risk?
Use Qualified CAF-Resistant Materials
The selected laminate should have evidence relevant to the construction and application.
Control Prepreg Storage
Expired, wet, or improperly stored prepreg can affect cure, resin flow, adhesion, and moisture behavior.
Control Inner-Layer Cleanliness
Ionic residue and chemical contamination should be prevented before lamination seals the layers together.
Maintain Lamination Quality
Temperature, pressure, vacuum, resin flow, cure time, and cooling must match the material.
Control Drilling
Tool wear, feed, speed, stack height, entry material, backup material, and hit count should be monitored.
Control Desmear
The process should remove smear without excessively attacking resin-glass interfaces.
Maintain Traceability
The manufacturer should connect test results to:
- Laminate Lot
- Prepreg Lot
- Press Cycle
- Drilling Program
- Drill Tool
- Desmear Line
- Plating Line
- Production Panel
- Test Coupon
Traceability allows a CAF failure to be investigated at the material, process, and panel level.
How Can PCBA Manufacturers Reduce CAF Risk?
Assembly suppliers cannot redesign the internal laminate, but they can avoid increasing susceptibility.
Recommended controls include:
- Use A Verified Reflow Profile
- Avoid Excessive Thermal Exposure
- Limit Rework Cycles
- Clean Ionic Residue When Required
- Dry Boards Only Under Approved Procedures
- Prevent Mechanical Bending
- Apply Conformal Coating Correctly
- Protect Routed Edges
- Maintain Environmental-Test Records
Repeated BGA or connector rework should be reviewed because localized heating can damage internal interfaces.
A controlled PCB reflow soldering process should measure actual board temperature.
How Should A CAF Failure Be Investigated?
Step 1: Confirm The Electrical Symptom
Record resistance, leakage current, operating voltage, temperature, humidity, and failure time.
Step 2: Identify The Failed Nets
Determine the polarity and voltage difference between the affected conductors.
Step 3: Map The Geometry
Measure actual hole, pad, trace, and plane separation using production data and cross-section measurements.
Step 4: Review Environmental History
Check humidity, condensation, fluid exposure, enclosure sealing, temperature cycling, and powered operating time.
Step 5: Review Manufacturing Records
Examine material lots, lamination, drilling, desmear, plating, and thermal history.
Step 6: Exclude Surface Leakage
Clean and inspect the surface, connectors, coating, and test fixture.
Step 7: Locate The Internal Path
Use electrical localization, imaging, or targeted sectioning.
Step 8: Confirm The Chemistry
Use SEM or elemental analysis when the failure mechanism remains uncertain.
Step 9: Compare Good And Failed Samples
Compare products from different lots, panel positions, materials, and manufacturing processes.
Step 10: Verify Corrective Action
Repeat the representative biased humidity test using a controlled trial lot.
A structured PCBA failure analysis helps separate CAF from surface contamination, corrosion, dendritic growth, and dielectric breakdown.
Common CAF Corrective-Action Mistakes
Increasing Spacing Without Reviewing Material
Spacing improvement helps, but poor drilling or weak resin-glass adhesion can remain.
Changing To High-Tg FR-4 Only
High Tg is not the same as CAF resistance.
Applying Conformal Coating
Coating may reduce surface moisture but cannot repair an internal pathway.
Cleaning The Assembled PCB
Cleaning removes surface contamination, not internal contamination sealed during lamination.
Passing A Short Electrical Test
Short-duration testing does not reproduce humidity, bias, and time.
Testing Only Raw Laminate
Finished-PCB drilling, desmear, plating, lamination, and reflow can change CAF performance.
Testing A Nonrepresentative Coupon
A coupon with different spacing, glass style, drill diameter, or process history may provide misleading results.
PCB CAF DFM Checklist
Electrical Design
- Are Maximum Voltage Differences Identified?
- Are Continuous And Transient Voltages Included?
- Is Internal Spacing Reviewed Separately From Creepage?
- Are Opposite-Polarity Vias Closely Spaced?
- Are High-Voltage Nets Separated From Planes?
- Is Voltage Gradient Considered?
PCB Layout
- Can Critical Via Pitch Be Increased?
- Can Via Rows Be Staggered?
- Are High-Risk Vias Aligned In Long Rows?
- Are Nonfunctional Pads Necessary?
- Are Conductors Too Close To Routed Edges?
- Are Internal Plane Clearances Adequate?
Material And Stackup
- Is CAF-Resistant Material Specified?
- Is The Exact Material Or Approved Equivalent Defined?
- Are Glass Styles Known?
- Is Resin-To-Glass Adhesion Qualified?
- Are Moisture And Thermal Properties Suitable?
- Is The Production Stackup Approved?
Manufacturing
- Are Drill Parameters Controlled?
- Is Drill Hit Count Defined?
- Is Hole-Wall Damage Inspected?
- Is Desmear Controlled?
- Are Lamination Materials Traceable?
- Are CAF Coupons Included?
Qualification
- Is IPC-TM-650 2.6.25 Referenced?
- Are Temperature And Humidity Defined?
- Is Bias Voltage Defined?
- Is Test Duration Defined?
- Is The Resistance Threshold Defined?
- Is Thermal Preconditioning Defined?
- Is Physical Failure Confirmation Required?
These questions should be included in the PCB DFM review before the design is released.
What Should Buyers Include In A PCB CAF RFQ?
A complete RFQ should provide:
- PCB Part Number And Revision
- Fabrication Data
- Fabrication Drawing
- Approved Stackup
- Laminate Manufacturer And Grade
- Allowed Material Substitutions
- Glass Style
- Layer Count
- Finished Thickness
- Copper Weight By Layer
- Finished Hole Diameter
- Minimum Hole Pitch
- Minimum Hole-To-Copper Distance
- Maximum Operating Voltage
- Transient Voltage
- Operating Temperature
- Operating Humidity
- Condensation Risk
- Expected Service Life
- Reflow And Rework Cycles
- CAF Test Method
- Coupon Geometry
- Test Temperature And Humidity
- Bias Voltage
- Test Duration
- Resistance Threshold
- Sample Quantity
- Failure-Analysis Requirements
- Applicable IPC Class
- End-Product Application
If voltage and environmental requirements are confidential, buyers can identify critical regions or net groups without disclosing the complete product design.
The PCB assembly file requirements should keep fabrication data, stackup, voltage notes, and revision status consistent.
Frequently Asked Questions About PCB CAF
What Does CAF Mean In PCB Manufacturing?
CAF means conductive anodic filament. It is an internal electrochemical migration mechanism that can create leakage or a short between conductors.
What Causes PCB CAF?
CAF requires moisture, electrical bias, a conductive source, mobile ionic material, and a susceptible internal pathway.
Does CAF Grow From Anode To Cathode?
CAF is generally described as filament growth from the copper anode toward the cathode under an applied electric field.
Can Low-Voltage PCBs Develop CAF?
Yes. Lower voltage does not eliminate the risk when conductor spacing is extremely small, humidity is high, and exposure time is long.
Is CAF Visible On The PCB Surface?
Usually not. The filament may be microscopic and located inside the laminate.
Can X-Ray Find CAF?
Conventional X-ray may help locate structural features but may not directly identify a small conductive filament. Targeted microsectioning and SEM analysis are more suitable for confirmation.
Is High-Tg FR-4 CAF Resistant?
Not automatically. Tg does not directly measure resin-glass adhesion, ionic purity, moisture behavior, or CAF performance.
Does Increasing Via Spacing Prevent CAF?
It reduces risk by increasing the migration distance and lowering the voltage gradient, but material and process controls remain necessary.
Can Conformal Coating Stop CAF?
It can reduce surface moisture exposure but cannot guarantee protection against internal CAF.
Can PCB Cleaning Remove CAF?
No. Cleaning can remove surface residue but cannot remove a filament or internal contamination inside the laminate.
Does CAF Happen Only Between Vias?
No. CAF can develop between holes, traces, pads, and internal planes.
Can A PCB Pass Electrical Test And Still Fail From CAF?
Yes. CAF may develop only after extended humidity and voltage exposure.
What IPC Method Is Used For CAF Testing?
IPC-TM-650 Method 2.6.25C covers X-Y-axis conductive anodic filament resistance testing.
How Long Does CAF Testing Take?
The duration depends on the qualification requirement. Accelerated tests may run for hundreds or more hours while resistance is monitored.
Is A Material Datasheet Enough To Prove CAF Reliability?
No. Finished-board construction, drilling, lamination, spacing, reflow, and environmental conditions also affect performance.
Final Recommendations For Preventing PCB CAF
CAF prevention should connect electrical design, PCB layout, material engineering, fabrication, assembly, and environmental protection.
For reliable results:
- Identify The Actual Voltage Difference Between Adjacent Conductors.
- Evaluate Internal Spacing Separately From Surface Creepage.
- Increase Critical Via-To-Via And Via-To-Copper Distance.
- Stagger Opposite-Polarity Via Arrays Where Practical.
- Select Materials With Documented CAF Performance.
- Do Not Use Tg As A Substitute For CAF Data.
- Approve The Glass Style And Production Stackup.
- Control Prepreg Storage, Lamination, And Cleanliness.
- Prevent Drilling Damage And Excessive Tool Wear.
- Control Desmear Without Attacking Glass-Resin Interfaces.
- Include Representative CAF Coupons In High-Risk Projects.
- Reproduce Actual Assembly Thermal Exposure During Qualification.
- Monitor Insulation Resistance Throughout Biased Humidity Testing.
- Confirm Electrical Failures Through Physical Analysis.
- Validate Corrective Actions On A Controlled Production Lot.
The key reliability relationship is:
Moisture + Electrical Bias + Ionic Material + Vulnerable Internal Path + Time = PCB CAF Risk
A CAF-resistant PCB is not created by one material label or one spacing rule. It results from a coordinated design and manufacturing system that protects insulation integrity from fabrication through the product’s complete service life.
