In the world of PCB design and manufacturing, we often focus on traces, vias, and signal integrity. However, the very foundation of any printed circuit board—its base material or substrate—holds secrets critical to long-term reliability. One of the most important, yet sometimes overlooked, properties of this substrate is its CTI
If you’re designing boards for high-voltage applications, harsh environments, or safety-critical systems, understanding CTI isn’t optional—it’s essential. This guide covers what CTI is, how it differs from PTI, how it’s measured, how it impacts creepage distance and compliance, and how to select the right material for your next design.
1. What Is CTI (Comparative Tracking Index)?
CTI stands for Comparative Tracking Index. It is the maximum voltage, measured in volts, at which a PCB base material can withstand 50 drops of a standardized contaminant solution (0.1% ammonium chloride) without forming a conductive tracking path on its surface
In simpler terms: CTI tells you how well your PCB substrate resists the formation of a leakage current path when exposed to moisture, contamination, and electrical stress—the exact conditions found in automotive bays, factory floors, and outdoor enclosures.
The “Tracking” Phenomenon Explained
Electrical tracking is a progressive, irreversible failure mechanism. Here’s how it happens:
Contamination accumulates
Dust, flux residue, or environmental pollutants settle on the PCB surface.
Moisture bridges the gap
Condensation or humidity creates a thin conductive film between adjacent conductors.
Leakage current flows
The voltage difference between conductors drives current through the contaminated surface layer.
Joule heating carbonizes the substrate
Localized heating breaks down the epoxy resin, depositing carbon (a conductor) on the surface.
The CTI value is the voltage threshold below which this tracking process doesn’t initiate within the 50-drop test. A higher CTI number means better tracking resistance.
Key Insight
Tracking is not the same as dielectric breakdown. Dielectric failure is an instantaneous event through the bulk of the material. Tracking is a slow, surface-level degradation caused by the combination of contamination, moisture, and voltage over time. Both must be considered, but CTI specifically addresses the tracking mode.
2. CTI vs PTI: What's the Difference?
Both CTI and PTI are defined in IEC 60112, and both measure a material’s resistance to electrical tracking—but they use fundamentally different test approaches:
| Property | CTI (Comparative Tracking Index) | PTI (Proof Tracking Index) |
|---|---|---|
| Test Method | IEC 60112 Method A: 50 drops at varying voltages to find the failure threshold | IEC 60112 Method B: 100 drops at a fixed voltage to verify pass/fail |
| Result | A specific voltage value (e.g., 600V) | Pass/fail at the specified test voltage |
| Purpose | Characterize the material's tracking resistance across a voltage range | Verify that a material meets a required threshold for a specific application |
| Contaminant | Solution A (0.1% NH₄Cl) or Solution B (0.1% NH₄Cl + 0.5% dioctyl phthalate) | Same as CTI |
| Common Usage | Material datasheets, material comparison, design selection | Compliance verification for specific standards |
| Relationship | If a material has CTI = 600V, it will pass PTI testing at 600V (100 drops). But a material that passes PTI at 300V may have a CTI slightly above 300V — the exact CTI value is not revealed by a PTI pass/fail test alone. | |
Practical Takeaway
PCB material datasheets almost always report CTI rather than PTI because CTI provides a continuous numerical value that enables material comparison. When a standard specifies “PTI ≥ 175V,” you can satisfy it by using a material with CTI ≥ 175V, since CTI ≥ PTI always holds for the same test solution.
3. How Is CTI Measured? The IEC 60112 Test Method
The IEC 60112 standard defines a precise, reproducible laboratory procedure for determining CTI. Understanding the test helps you interpret CTI values on material datasheets and assess their relevance to your application.
Test Setup
- Electrodes: Two platinum electrodes with chisel-shaped tips, spaced 4 mm apart, placed on the flat surface of the test specimen with a contact force of 1.0 ± 0.05 N each.
- Contaminant solution: Solution A = 0.1% ammonium chloride (NH₄Cl) in deionized water (conductivity ≈ 2.5 µS/cm at 23°C). Solution B adds 0.5% dioctyl phthalate for materials that may be exposed to oily environments.
- Drip mechanism: Drops of 20 ± 5 µL volume, delivered at 30 ± 5 second intervals, falling from a height of 15 ± 5 mm onto the surface between the electrodes.
- Voltage source: AC 50/60 Hz, variable from 100V to 600V (or higher), with short-circuit current ≥ 1.0 A.
Test Procedure (Method A — CTI)
- Apply a starting voltage (e.g., 300V) across the electrodes.
- Begin dripping the contaminant solution between the electrodes at the specified interval.
- Monitor for tracking failure — Failure is defined as a current flow ≥ 0.5 A persisting for ≥ 2 seconds, or sustained arcing that carbonizes the surface.
- If the specimen withstands 50 drops without failure, increase the voltage and test a fresh specimen.
- If tracking occurs before 50 drops, decrease the voltage and test again.
- Repeat until the highest voltage at which 5 consecutive specimens each withstand 50 drops is determined. This voltage is the CTI.
For Engineers
The 50-drop criterion is critical. It means CTI is not about a single catastrophic event—it’s about the material’s ability to survive repeated, cumulative exposure to contamination under voltage stress. This directly mirrors real-world conditions where condensation and contamination cycles occur repeatedly over a product’s lifetime.
Test Flow Diagram
Prepare specimen → Place electrodes (4mm gap) → Apply voltage →
Drip contaminant (every 30s) → Monitor current →
Withstands 50 drops? → YES → Increase voltage, retest →
Tracking failure? → Decrease voltage, retest →
Find highest voltage with 5/5 pass → CTI = that voltage
4. CTI PLC Grades and IEC Material Groups
Based on the CTI test result, materials are classified into Performance Level Categories (PLC) and corresponding IEC Material Groups. These classifications are referenced throughout safety standards and design rules:
| IEC Material Group |
PLC Grade |
CTI Range |
Tracking Resistance |
Typical Materials | Typical Applications |
|---|---|---|---|---|---|
| I | PLC 0 | CTI ≥ 600V | Highest | Shengyi S1600, KB-6160C, Isola 370HR (high-CTI grade), specialty polyimide | Automotive under-hood, industrial drives, outdoor telecom, medical |
| II | PLC 1 | 400V ≤ CTI < 600V | Good | Medium-CTI FR4 grades (e.g., Shengyi S1155, ITEQ IT-180A) | Industrial power supplies, indoor automotive, high-reliability consumer |
| IIIA | PLC 2 | 250V ≤ CTI < 400V | Moderate | Some higher-grade FR4 (CTI ~275–375V) | Office equipment, appliances in dry environments |
| IIIA | PLC 3 | 175V ≤ CTI < 250V | Moderate | Standard FR4 (KB-6160, KB-6160A, most common FR-4) | Consumer electronics, low-voltage indoor devices |
| IIIB | PLC 4 | 100V ≤ CTI < 175V | Low | FR-1, CEM-1, some paper-based laminates | Single-sided low-cost boards, toys |
| IIIB | PLC 5 | CTI < 100V | Poor | Low-cost paper phenolic laminates | Generally unsuitable for most PCB applications |
Critical Design Rule
Never assume all “FR-4” is the same. The FR-4 designation refers to a flame retardancy rating (UL 94V-0), not to electrical properties. Two materials both labeled “FR-4” can have CTI values ranging from 175V to 600V+. Always check the manufacturer’s datasheet for the specific CTI value of the grade you’re specifying.
5. How CTI Affects Creepage Distance (with Calculation Examples)
This is where CTI becomes a direct design parameter. Creepage distance is the shortest path between two conductive parts along the surface of the insulating material. IEC 60664-1 defines minimum creepage distances based on:
- Working voltage (RMS or DC)
- Pollution degree (1 = clean/dry, 2 = occasional condensation, 3 = conductive pollution, 4 = persistent conductive pollution)
- Material CTI group (I, II, IIIA, IIIB)
The Core Relationship
Creepagemin = f(Working Voltage, Pollution Degree, Material Group)
Higher CTI (lower material group number) → shorter required creepage distance at the same voltage and pollution degree. This enables more compact board layouts—a critical advantage in miniaturized high-voltage designs.
Creepage Distance Table (IEC 60664-1, Pollution Degree 2)
The table below shows the minimum creepage distances in millimeters for the most common design scenario—Pollution Degree 2 (occasional condensation):
| Working Voltage (V) | Group I(CTI ≥ 600V) | Group II(CTI 400–599V) | Group IIIA(CTI 175–399V) | Group IIIB(CTI 100–174V) |
|---|---|---|---|---|
| 50 | 0.40 mm | 0.56 mm | 0.78 mm | 1.10 mm |
| 100 | 0.56 mm | 0.80 mm | 1.10 mm | 1.60 mm |
| 150 | 0.73 mm | 1.05 mm | 1.50 mm | 2.10 mm |
| 300 | 1.30 mm | 1.85 mm | 2.60 mm | 3.70 mm |
| 500 | 2.00 mm | 2.80 mm | 4.00 mm | 5.60 mm |
| 800 | 3.20 mm | 4.50 mm | 6.40 mm | 9.00 mm |
Minimum creepage distances in mm per IEC 60664-1, Pollution Degree 2.
Calculation Example
Example: You're designing an industrial motor controller PCB with 500V between adjacent primary-side traces. The operating environment is Pollution Degree 2.
- Using standard FR4 (Group IIIA, CTI ~175V): Min. creepage = 4.00 mm
- Using high-CTI FR4 (Group I, CTI ≥ 600V, e.g., Shengyi S1600): Min. creepage = 2.00 mm
- Space savings: 50%
By upgrading to a high-CTI material, you can halve the creepage distance—freeing board space for additional circuitry, reducing layer count, or enabling a smaller form factor. The material cost premium is often offset by board size reduction and simplified layout.
For detailed creepage tables at other pollution degrees, refer to IEC 60664-1, Table F.2 or consult your PCB fabricator’s engineering team.
6. Why CTI Matters in PCB Manufacturing
The CTI of your base material directly impacts the board’s reliability, safety, and lifespan—especially in specific use cases:
Prevents Catastrophic Failure
Electrical tracking is a progressive failure. Once a carbon track forms, it grows more conductive over time, leading to short circuits, leakage currents, and—in the worst case—fire hazard. A high-CTI material acts as a robust barrier against this failure mode.
Enables Miniaturization
As shown above, a higher CTI rating allows reduced creepage distances, meaning you can safely place high-voltage traces closer together. This is critical for modern high-density designs where board real estate is at a premium.
Ensures Safety Certification Compliance
Many international safety standards (UL, IEC, EN) have mandatory CTI/PLC requirements. Using a material with insufficient CTI can block your product from receiving necessary certifications, preventing market entry entirely.
Critical for Harsh Environments
Any application where the PCB might be exposed to humidity, condensation, dust, or chemical contaminants demands high CTI materials. Key sectors include:
- Automotive electronics — under-hood controls, EV battery management systems
- Industrial motor drives and power supplies — factory-floor environments with airborne contaminants
- Outdoor telecommunications — base stations, remote sensors
- Medical devices — patient-connected equipment with strict safety margins
- Home appliances — washing machines, dishwashers (high humidity, temperature cyclin
7. CTI in Safety Standards: UL 796, IEC 62368-1 & More
CTI isn’t just a material property—it’s a regulatory gatekeeper. The following standards explicitly reference CTI or PLC requirements for PCBs:
| Standard | Scope | CTI / PLC Requirements |
|---|---|---|
| UL 796 | Standard for Safety: Rigid Printed Wiring Boards | Requires CTI evaluation as part of material qualification. Materials must meet minimum CTI for their intended voltage rating and end-use environment. |
| IEC 62368-1 | Audio/video, IT, and communication technology equipment — Safety | Replaces IEC 60950-1 and IEC 60065. Specifies minimum creepage distances based on CTI group (I/II/IIIA/IIIB), working voltage, and pollution degree. |
| IEC 60950-1 | IT Equipment Safety (legacy, still referenced) | Table 2N: Minimum creepage distances based on material group and pollution degree. Group IIIA materials (standard FR4) require significantly more creepage than Group I. |
| IEC 60664-1 | Insulation coordination for equipment within low-voltage systems | Defines the creepage/clearance framework and CTI material groups used by all other standards. This is the “mother standard” for CTI-based design rules. |
| IPC-4101 | Base Material Specification for Rigid PCBs | Specifies CTI as a material property in laminate datasheets. Some IPC-4101 slash sheets require minimum CTI values. |
| UL 60950-1 | US adoption of IEC 60950-1 | Mirrors IEC 60950-1 creepage requirements. CTI group classification is mandatory for product certification. |
If your product requires UL or IEC certification and you’ve used a standard FR4 material (CTI ~175V, Group IIIA) where the creepage distance was designed assuming Group I (CTI ≥ 600V), your product will fail certification.
The only fixes are: (1) redesign with wider creepage distances, (2) switch to a higher-CTI material, or (3) add physical barriers (slots/grooves). Option 2 is usually the least expensive if caught early.
8. CTI of Common PCB Materials (Comparison Table)
Not all FR-4 is created equal. Below is a comparison of commonly available laminates with their CTI values and key properties:
| Material | Manufacturer | CTI (V) | PLC / Group | Tg (°C) | Dk @ 1GHz | Best For |
|---|---|---|---|---|---|---|
| S1600 | Shengyi | ≥ 600 | PLC 0 / Group I | 170 | 4.7 | Automotive, industrial, outdoor |
| KB-6160C | Kingboard | ≥ 600 | PLC 0 / Group I | 170 | 4.6 | High-voltage power, EV |
| 370HR | Isola | ≥ 600 | PLC 0 / Group I | 170 | 4.1 | High-reliability telecom, defense |
| IT-180A | ITEQ | ≥ 400 | PLC 1 / Group II | 180 | 4.4 | Lead-free assembly, industrial |
| S1155 | Shengyi | ≥ 300 | PLC 2 / Group IIIA | 150 | 4.7 | Consumer electronics, appliances |
| KB-6160F | Kingboard | 175 | PLC 3 / Group IIIA | 140 | 4.6 | General-purpose FR4, low CTE |
| KB-6160 / 6160A | Kingboard | 175 | PLC 3 / Group IIIA | 130–140 | 4.5–4.6 | Standard FR4, UV-blocking |
| FR-1 | Various | 100–150 | PLC 4 / Group IIIB | N/A | N/A | Single-sided, low-cost products |
When evaluating materials, don’t look at CTI in isolation. A high-CTI material with a low Tg may not survive lead-free reflow. Conversely, a high-Tg material with poor CTI may fail in humid environments. Understanding prepreg and core interactions in your stackup is also critical—CTI should be consistent across all layers.
9. How to Choose the Right CTI Material — Step by Step
Step 1: Assess the Operating Environment
| Environment | Conditions | Minimum Recommended CTI |
|---|---|---|
| Benign | Air-conditioned office, controlled humidity | 175V (PLC 3, standard FR4) |
| Moderate | Consumer automotive (cabin), indoor industrial | ≥ 300V (PLC 2) |
| Harsh | Outdoor, high humidity, automotive under-hood, chemical exposure | ≥ 400V (PLC 1, Group II) |
| Very Demanding | EV power modules, medical patient-connected, defense | ≥ 600V (PLC 0, Group I) |
Step 2: Determine the Operating Voltage
The higher the voltage difference between adjacent conductors, the greater the tracking risk. For power electronics, motor controllers, or AC/DC converters, always opt for a higher CTI material—regardless of environment.
Step 3: Calculate Creepage Distance
Using the IEC 60664-1 tables (see Section 5), determine the minimum creepage for your working voltage and pollution degree at each candidate material’s CTI group. Verify that your PCB layout can accommodate the required spacing.
Step 4: Check Compliance Requirements
Verify that your material choice satisfies all applicable safety standards (UL 796, IEC 62368-1, IEC 60950-1). If certification requires a specific PLC grade, your material must meet or exceed it.
Step 5: Review Manufacturer Datasheets
Never assume CTI based on the “FR-4” label alone. Always check the datasheet for the specific grade. Laminate manufacturers like Isola, Shengyi, and ITEQ offer multiple FR-4 grades with CTI values from 175V to 600V+.
Step 6: Consult Your PCB Fabricator
An experienced manufacturer like Weller PCB can help you navigate material options, balancing CTI requirements with other critical properties—surface finish compatibility, thermal performance (Tg, thermal conductivity), signal integrity (Dk/Df), and cost.
10. Case Study: Tracking Failure in a High-Voltage Industrial Controller
The following case is based on a composite of real-world failure analyses. Details have been anonymized.
Product: 3-phase motor controller, 480VAC input, designed for factory-floor installation.
Failure mode: After 14 months in service, multiple units exhibited intermittent ground-fault trips. One unit developed visible scorch marks between primary-side traces on the PCB surface.
Root Cause Analysis
- Material selection error: The PCB was fabricated with standard FR4 (CTI ~175V, Group IIIA) because the designer assumed “FR4” was sufficient for all applications.
- Insufficient creepage: At 480VAC with Pollution Degree 2, IEC 60664-1 requires 3.82 mm creepage for Group IIIA material. The layout had only 2.5 mm between high-voltage traces and the ground plane pour.
- Environmental aggravation: The factory floor had high humidity and metallic dust from nearby CNC operations—classic Pollution Degree 2+ conditions.
- Progressive tracking: Contamination + condensation + voltage stress → carbonized tracking paths → leakage current → ground-fault trips. Eventually, sustained arcing charred the board surface.
Corrective Actions
- Material upgrade: Switched to Shengyi S1600 (CTI ≥ 600V, Group I), reducing the required creepage at 480V to 1.90 mm—well within the existing 2.5 mm spacing.
- Conformal coating: Added conformal coating as an additional barrier against contamination and moisture.
- Design rule update: All future high-voltage designs must specify CTI ≥ 400V for industrial applications.
Lesson Learned: The material cost increase from standard FR4 to S1600 was approximately
$0.50 per square foot.
The cost of the field failure—including warranty replacements, engineering investigation, and production line stoppage—was estimated at
$120,000+.
The ROI on specifying the correct CTI material from the start was clear.
11. Frequently Asked Questions
What is CTI in PCB?
CTI (Comparative Tracking Index) is the maximum voltage, measured in volts, at which a PCB base material can withstand 50 drops of a standardized contaminant solution without forming a conductive tracking path. It quantifies the material’s resistance to surface leakage current caused by contamination, moisture, and electrical stress. A higher CTI value means better resistance to electrical tracking failure.
What is the difference between CTI and PTI?
CTI (Comparative Tracking Index) is tested with 50 drops of contaminant solution (Method A per IEC 60112) and measures the voltage at which tracking first occurs. PTI (Proof Tracking Index) tests whether a material can withstand a fixed voltage for 100 drops without tracking. CTI finds the failure threshold; PTI verifies pass/fail at a specified voltage. Both are defined in IEC 60112, but CTI is more commonly used in PCB material specifications.
What CTI value does standard FR4 have?
Standard FR4 laminates typically have a CTI of 175–250V, placing them in PLC Grade 2 or 3 (IEC Material Group IIIA). This is adequate for many consumer electronics operating at low voltage in benign environments, but insufficient for high-voltage, automotive, or outdoor applications where a minimum CTI of 400V (PLC 1) or 600V (PLC 0) is required.
How does CTI affect creepage distance?
Higher CTI allows shorter creepage distances. Per IEC 60664-1, creepage distance is calculated from the working voltage, pollution degree, and the material’s CTI-based insulation group. A material with CTI ≥ 600V (Group I) requires significantly less creepage than one with CTI 175V (Group IIIA) at the same voltage, enabling more compact board layouts in high-voltage designs.
What is PLC 0 in PCB material?
PLC 0 (Performance Level Category 0) is the highest CTI rating, indicating a Comparative Tracking Index of 600V or above. Materials in this category (IEC Material Group I) provide the best resistance to electrical tracking and are required for demanding applications such as automotive under-hood electronics, industrial motor drives, outdoor telecom equipment, and medical devices. Examples include Shengyi S1600 and KB-6160C.
Why is high CTI important for automotive PCBs?
Automotive PCBs, especially under-hood controllers and EV power modules, operate in harsh environments with high humidity, temperature cycling, dust, and chemical contaminants. These conditions accelerate surface tracking on the PCB. A high CTI material (≥ 400V, PLC 1 or better) is essential to prevent conductive tracking paths that could cause leakage currents, arcing, or fire—failure modes that are unacceptable in safety-critical automotive systems.
What is the IEC 60112 standard for CTI testing?
IEC 60112 is the international standard that defines the test method for determining the Comparative Tracking Index (CTI) and Proof Tracking Index (PTI) of solid insulating materials. The test involves placing platinum electrodes on the material surface, applying a voltage, and dripping a standardized ammonium chloride solution (0.1% concentration) between the electrodes. The CTI is the highest voltage at which the material withstands 50 drops without tracking failure.
Need Help Selecting the Right CTI Material?
At Weller PCB, our engineering team has 20+ years of experience helping designers navigate base material selection for high-voltage, harsh-environment, and safety-critical applications. We’ll help you balance CTI requirements with thermal performance, signal integrity, and cost.
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