Inquire: Call 0086-755-23203480, or reach out via the form below/your sales contact to discuss our design, manufacturing, and assembly capabilities.
Quote: Email your PCB files to Sales@pcbsync.com (Preferred for large files) or submit online. We will contact you promptly. Please ensure your email is correct.
Notes: For PCB fabrication, we require PCB design file in Gerber RS-274X format (most preferred), *.PCB/DDB (Protel, inform your program version) format or *.BRD (Eagle) format. For PCB assembly, we require PCB design file in above mentioned format, drilling file and BOM. Click to download BOM template To avoid file missing, please include all files into one folder and compress it into .zip or .rar format.
IPC-9691 Explained: Complete Guide to CAF Testing for PCB Reliability
Field failures are the nightmare scenario for any electronics engineer. You ship thousands of units, everything passes production testing, and then six months later the returns start coming in. Intermittent shorts. Random resets. Products that worked perfectly in the lab but fail in humid environments. More often than not, the culprit is something you cannot see on any inspection report: conductive anodic filament growth happening inside your PCB laminate.
IPC-9691 exists because CAF failures are preventable, but only if you know how to test for them. The standard provides the guidance needed to implement IPC-TM-650 Method 2.6.25 testing effectively, interpret results correctly, and make informed decisions about laminate materials and design rules. Without this guidance, CAF testing becomes an expensive checkbox exercise that tells you little about actual field reliability.
This guide explains what IPC-9691 covers, how CAF testing works, and how to use the results to prevent electrochemical migration failures before they reach your customers.
What Is IPC-9691? Understanding the CAF Test User Guide
IPC-9691 is titled “User Guide for the IPC-TM-650, Method 2.6.25, Conductive Anodic Filament (CAF) Resistance and Other Internal Electrochemical Migration Testing.” The current version is IPC-9691C, developed by the IPC Electrochemical Migration Task Group (5-32e) under the Cleaning and Coating Committee (5-30).
The standard serves as a companion document to IPC-TM-650 Method 2.6.25, which defines the actual test procedure. While the test method tells you how to run the test, IPC-9691 tells you how to design test vehicles, interpret results, determine pass/fail criteria, and relate accelerated test data to expected field life.
IPC-9691 addresses critical questions that the test method alone cannot answer. These include what test coupon design to use for your application, how to set appropriate pass/fail thresholds, how conductor spacing affects CAF resistance requirements, how to translate 596-hour test results into field life predictions, and what factors beyond laminate material affect CAF susceptibility.
IPC-9691 Table of Contents Overview
Section
Topic
Purpose
1-4
Scope, Background, Purpose, Introduction
Context and applicability
5
What Is Conductive Anodic Filament Growth?
CAF mechanism explanation
6
Other Internal ECM Modes
Related failure mechanisms
7
Increasing Internal ECM Risk Trends
Industry drivers for CAF concern
8
Adjusted CALCE CAF Failure Model
Field life prediction methodology
9
Internal ECM Test Vehicle Design
Coupon selection guidance
10
Sample Preparation and Testing
Pre-conditioning and test execution
11-12
Data Format and Analysis
Results interpretation
13
Sample Sizes
Statistical requirements
14
Relating Results to Field Life
Acceleration factor application
15-18
Specifying, Locating, and Factors Affecting ECM
Practical implementation
How Conductive Anodic Filament Formation Works
Understanding the CAF mechanism is essential for interpreting test results and implementing effective prevention strategies. CAF is not random corrosion; it is a specific electrochemical process that requires four conditions to occur simultaneously.
CAF formation proceeds in two distinct stages. The first stage involves degradation of the glass fiber and epoxy resin interface within the laminate. This degradation can result from thermal cycling stresses, drilling damage, moisture absorption, or inherent weaknesses in the glass finish and resin system. Once the interface is compromised, it provides a pathway for ionic transport.
The second stage is the electrochemical migration itself. With moisture present and a voltage applied between two conductors, copper at the anode oxidizes to form copper ions. These ions migrate along the degraded glass/resin interface toward the cathode, where they reduce back to metallic copper. Over time, this process creates a conductive filament that bridges the insulation between conductors.
The resulting failure can manifest as reduced insulation resistance, intermittent shorts that appear and disappear with humidity changes, or complete short circuits. Because the filament forms inside the laminate along glass fibers, it is invisible to surface inspection and extremely difficult to detect without destructive analysis.
IPC-9691 Test Coupon Selection
IPC provides four standard CAF test coupon designs, each optimized for different testing scenarios. IPC-9691 provides guidance on selecting the appropriate coupon for your application.
Standard CAF Test Coupon Comparison
Coupon
Primary Use
Key Features
IPC-9253
Current standard for hole-to-hole CAF
Replaces IPC-9254; smaller size; includes press-fit connector evaluation zone
IPC-9254
Legacy hole-to-hole CAF testing
Original design; larger format; includes Z-axis testing zone
IPC-9255
Line-to-hole and line-to-line CAF
Evaluates trace-to-via and trace-to-trace migration
IPC-9256
Specialized configurations
Additional test structures for specific applications
The IPC-9253 coupon has largely superseded the IPC-9254 design for new qualification testing. The newer design offers a smaller footprint while maintaining the critical test structures needed for comprehensive CAF evaluation. Both coupons include multiple test zones with different via spacings and orientations relative to the glass weave direction.
Critical Test Structure Parameters
The test coupons include various via patterns designed to evaluate CAF resistance under different geometric conditions.
Parameter
Typical Range
Why It Matters
Hole-to-hole spacing
0.3mm to 1.0mm
Smaller spacing increases electric field and CAF risk
Higher aspect ratio vias have greater drilling stress
Manhattan distance, which is the shortest orthogonal path between vias along the X and Y axes, corresponds to the glass fiber weave directions and represents the most likely CAF propagation path.
IPC-9691 provides guidance for implementing the test method defined in IPC-TM-650 Method 2.6.25. Understanding the test conditions is essential for interpreting results correctly.
Standard CAF Test Parameters
Parameter
Standard Condition
Notes
Temperature
85°C
Accelerates moisture absorption and chemical reactions
Relative humidity
85% RH
Provides moisture for electrolyte formation
Test duration
596 hours (approx. 25 days)
Sufficient for CAF initiation in susceptible materials
Bias voltage
Typically 10-100 VDC
Higher voltage increases electric field and acceleration
Measurement interval
Periodic (varies)
Monitor for resistance drops during test
Failure threshold
≤10^7 Ω (10 MΩ)
Large drop indicates CAF formation
The 85°C/85%RH condition, often called “85/85 testing,” is an industry standard for accelerated humidity testing. This environment dramatically accelerates moisture absorption compared to typical field conditions, allowing detection of CAF susceptibility within weeks rather than years.
Some OEM specifications use alternative conditions such as 65°C/85%RH or 85°C/95%RH to evaluate performance under different acceleration factors or to better match specific application environments.
What the Test Actually Measures
During the 596-hour test exposure, high-resistance measurements are performed periodically on each test circuit. The test monitors insulation resistance between biased conductors, looking for the characteristic resistance drop that indicates CAF formation.
A typical failure progression shows stable high resistance (greater than 10^9 Ω) for some period, followed by a relatively rapid drop as the conductive filament forms and bridges the gap between conductors. The resistance may stabilize at the 10^6 to 10^8 Ω range or continue dropping toward a hard short.
Relating IPC-9691 Test Results to Field Life
One of the most valuable sections of IPC-9691 addresses translating accelerated test results to expected field performance. The standard incorporates the CALCE (Center for Advanced Life Cycle Engineering) CAF failure model developed at the University of Maryland.
CALCE CAF Failure Model Parameters
The model relates time-to-failure under test conditions to expected field life using acceleration factors based on temperature, humidity, voltage, and geometry.
Factor
Effect on CAF Risk
Model Consideration
Temperature
Higher temp accelerates reaction kinetics
Arrhenius relationship
Humidity
Higher RH provides more electrolyte
Exponential relationship
Voltage/Spacing
Higher field accelerates ion migration
Linear or power-law relationship
Geometry
Conductor orientation relative to glass weave
Path length factor
The model allows engineers to estimate field life from accelerated test data, though significant uncertainty remains due to material variability and the stochastic nature of CAF initiation.
Practical Application of Acceleration Factors
For a board tested at 85°C/85%RH with 50V bias that shows no failures at 596 hours, the expected field life depends heavily on actual use conditions. A product operating in a controlled office environment (25°C/50%RH) might have acceleration factors of 100x or more, suggesting field life measured in years. The same board in a tropical outdoor enclosure (35°C/90%RH) might see acceleration factors of only 10x, with potential field failures within months.
This is why IPC-9691 emphasizes understanding your actual application environment when specifying CAF resistance requirements.
Sample Preparation and Testing Best Practices
IPC-9691 provides detailed guidance on sample preparation that significantly affects test validity and repeatability.
Pre-Test Sample Conditioning
Step
Purpose
Typical Requirement
Visual inspection
Identify obvious defects
Per IPC-A-600
Bake-out
Remove absorbed moisture
24-48 hours at 105-125°C
Initial resistance
Baseline measurement
All circuits >10^9 Ω
Assembly simulation
Replicate production thermal exposure
Reflow per IPC-9631
The bake-out step is particularly important because absorbed moisture in the laminate affects both the test results and the consistency of measurements. Without proper drying, samples may show artificially high CAF susceptibility due to pre-existing moisture.
Assembly simulation ensures that the test evaluates the material after it has experienced the thermal stresses of actual production. CAF susceptibility can increase after reflow due to thermal damage to the glass/resin interface.
IPC-9691 and IPC-4101 Laminate Qualification
CAF testing per IPC-9691 and IPC-TM-650 Method 2.6.25 is a standard component of laminate qualification to IPC-4101, the specification for base materials for rigid and multilayer printed boards.
CAF Testing in Material Qualification
Application
CAF Test Role
Typical Requirement
IPC-4101 qualification
Material capability verification
Pass 596-hour test at specified spacing
New laminate evaluation
Compare CAF resistance vs. baseline
Equivalent or better than qualified material
Supplier change
Verify alternate source performance
Match original qualification data
Process change
Confirm no degradation
No increase in CAF failures
Many OEMs specify CAF-resistant laminate materials for products destined for high-humidity environments or high-reliability applications. These materials typically use enhanced glass finishes and resin systems designed to maintain glass/resin bond integrity.
CAF Prevention Strategies Based on IPC-9691
Beyond testing, IPC-9691 provides guidance on factors that affect CAF susceptibility and strategies for prevention.
Design Rules for CAF Mitigation
Strategy
Implementation
Effectiveness
Increase via spacing
Minimum spacing based on voltage and reliability requirement
High; directly reduces electric field
Stagger vias at 45°
Orient via patterns diagonal to glass weave
Moderate; increases effective path length
Specify CAF-resistant material
High-Tg, enhanced glass finish laminates
High; addresses root cause
Control drilling parameters
Optimize feed/speed to minimize damage
Moderate; reduces initiation sites
Remove non-functional pads
Delete unused internal pads on via connections
Moderate; increases copper-to-copper distance
Process Controls for CAF Prevention
Process
Control Point
CAF Impact
Drilling
Bit wear, feed rate, spindle speed
Mechanical damage creates initiation sites
Desmear
Chemistry concentration, time
Over-aggressive desmear causes wicking
Lamination
Void-free lamination, proper cure
Voids provide moisture paths
Assembly
Pre-bake before reflow
Prevents moisture-induced delamination
Cleaning
Flux residue removal
Removes ionic contamination
Useful Resources for IPC-9691 Implementation
Official IPC Standards:
IPC-9691C User Guide for CAF Testing (shop.ipc.org) – approximately $108
IPC-TM-650 Method 2.6.25C CAF Resistance Test (free download from IPC)
IPC-9253, IPC-9254, IPC-9255, IPC-9256 Test Coupon Artwork (free from IPC)
IPC-4101 Specification for Base Materials
Related Standards:
IPC-TR-476A Electrochemical Migration in Printed Wiring Assemblies
IEC 61189-5-503 CAF Testing of Circuit Boards
Equipment and Testing Services:
Gen3 Systems AutoCAF2+ (gen3systems.com) – CAF test equipment
Element Materials Technology (element.com) – CAF testing services
CALCE, University of Maryland (calce.umd.edu) – Research and consulting
Industry Publications:
CALCE CAF research publications
IPC APEX EXPO technical papers on electrochemical migration
Circuits Assembly magazine reliability articles
Frequently Asked Questions About IPC-9691
What is the difference between IPC-9691 and IPC-TM-650 Method 2.6.25?
IPC-TM-650 Method 2.6.25 is the test method that defines how to perform CAF testing, including equipment requirements, test conditions, and measurement procedures. IPC-9691 is the user guide that explains how to implement the test method effectively. IPC-9691 covers test coupon selection, sample preparation, data analysis, pass/fail criteria determination, and relating test results to field life expectations. Think of 2.6.25 as the “how” and IPC-9691 as the “why” and “what it means.”
How long does CAF testing take?
The standard test duration per IPC-TM-650 Method 2.6.25 is 596 hours, approximately 25 days. This includes continuous exposure to 85°C/85%RH conditions with periodic resistance measurements throughout. Some OEM specifications require extended testing to 1000 hours or longer for high-reliability applications. Including sample preparation, conditioning, and post-test analysis, a complete CAF evaluation program typically requires 4-6 weeks.
What does a CAF test failure mean for my product?
A CAF test failure indicates that the combination of laminate material, board design, and fabrication process is susceptible to forming conductive filaments under accelerated humidity and bias conditions. This does not necessarily mean your product will fail in the field. The significance depends on your actual operating environment, expected product life, and the specific test conditions that caused failure. IPC-9691 provides guidance on interpreting results in context. Options include specifying CAF-resistant material, increasing conductor spacing, or accepting the risk for products used in controlled environments.
Which test coupon should I use for CAF qualification?
For general hole-to-hole CAF evaluation, IPC-9253 is the current recommended coupon, having replaced the older IPC-9254 design. Use IPC-9255 if your design includes critical trace-to-via or trace-to-trace spacings that need evaluation. Some OEMs require testing with custom coupon designs that replicate specific features of their product designs. IPC-9691 recommends that coupon designs include multiple hole sizes, spacings, and orientations relative to the glass weave to capture all potential failure modes.
Can CAF failures be repaired?
No. Once a conductive filament has formed inside the PCB laminate, it cannot be removed or repaired. The filament exists within the composite material structure along glass fiber interfaces. Unlike surface contamination that can be cleaned, CAF represents permanent internal damage. This is why prevention through proper material selection, design rules, and process control is essential. Products that have experienced CAF failures in the field must be scrapped and replaced.
Building CAF Resistance into Your Products
IPC-9691 provides the framework for understanding and preventing one of the most insidious failure modes in PCB electronics. Unlike solder joint failures that can be inspected or thermal problems that manifest quickly, CAF can take months or years to develop and remains invisible until the product fails.
The standard’s value lies not just in defining how to test, but in explaining what the test results mean for real-world reliability. By understanding the CAF mechanism, selecting appropriate test vehicles, and correctly interpreting accelerated test data, engineers can make informed decisions about material selection, design rules, and process controls.
For products destined for high-humidity environments, extended field life requirements, or safety-critical applications, CAF testing per IPC-9691 and IPC-TM-650 Method 2.6.25 should be a standard part of the qualification process. The cost of testing is trivial compared to the cost of field failures, warranty claims, and damaged reputation from products that fail due to preventable electrochemical migration.
Inquire: Call 0086-755-23203480, or reach out via the form below/your sales contact to discuss our design, manufacturing, and assembly capabilities.
Quote: Email your PCB files to Sales@pcbsync.com (Preferred for large files) or submit online. We will contact you promptly. Please ensure your email is correct.
Notes: For PCB fabrication, we require PCB design file in Gerber RS-274X format (most preferred), *.PCB/DDB (Protel, inform your program version) format or *.BRD (Eagle) format. For PCB assembly, we require PCB design file in above mentioned format, drilling file and BOM. Click to download BOM template To avoid file missing, please include all files into one folder and compress it into .zip or .rar format.
