CTI Material Selection Reference
| Application | Recommended CTI | Typical Material Group |
|---|---|---|
| Consumer electronics in controlled environments | 175–250V | Group IIIA |
| Industrial control equipment | ≥400V | Group II |
| Automotive electronics and under-hood systems | ≥600V | Group I |
| EV power electronics and harsh environments | ≥600V | Group I |
Do not select PCB laminate only by the term “FR-4”. FR-4 identifies a flame-retardant material classification, not a specific CTI performance level. Different FR-4 laminate grades can have significantly different tracking resistance, thermal properties, and electrical characteristics.
Weller PCB engineers can help evaluate laminate selection based on CTI requirements, working voltage, creepage distance, thermal performance, signal integrity, and manufacturing requirements.
Contact Weller PCB →1. What Is CTI in PCB Materials?
How Electrical Tracking Happens
- Contamination accumulates: Dust, moisture, or residues collect on the PCB surface.
- Surface conductivity increases: Moisture creates a temporary leakage path between conductors.
- Current generates heat: Local heating damages the resin system.
- Carbonized tracking forms: A permanent conductive path develops, increasing failure risk.
CTI evaluates resistance against surface tracking. It is different from dielectric breakdown, which occurs through the insulation material. High-voltage PCB designs must evaluate both insulation strength and tracking resistance.
2. CTI vs PTI: What Is the Difference?
| Property | CTI | PTI |
|---|---|---|
| Purpose | Determines the tracking resistance capability of a material | Verifies performance at a defined voltage level |
| Test Method | Voltage is varied to determine tracking resistance limit | Fixed voltage test with pass/fail evaluation |
| Result | Numerical value such as CTI 600V | Pass or fail result |
| Engineering Usage | Material selection and PCB design comparison | Compliance verification |
PCB laminate datasheets usually provide CTI values because they allow engineers to compare different materials. Final product compliance should always be verified according to the applicable safety standard.
3. How Is CTI Measured? The IEC 60112 Test Method
IEC 60112 defines the standardized test method used to determine Comparative Tracking Index values for solid insulating materials. The test evaluates material resistance against tracking under controlled contamination and electrical stress.
CTI Test Principle
- Electrodes: Conductive electrodes are placed on the material surface with defined spacing.
- Contamination: A standardized electrolyte solution is repeatedly applied between the electrodes.
- Voltage Stress: The applied voltage is increased to evaluate tracking resistance.
- Result: The highest voltage without tracking failure becomes the CTI value.
CTI should not be evaluated alone. PCB designers must also consider working voltage, pollution degree, creepage distance, clearance distance, and the applicable safety standard.
Test Flow Diagram
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 Grades and IEC Material Groups
| Material Group | CTI Range | Typical Applications |
|---|---|---|
| Group I | CTI ≥600V | Automotive electronics, EV systems, industrial power equipment |
| Group II | 400V ≤ CTI <600V | Industrial controls and high-reliability electronics |
| Group IIIA | 175V ≤ CTI <400V | General FR-4 PCB applications |
| Group IIIB | 100V ≤ CTI <175V | Low-cost applications with limited electrical stress |
A higher CTI rating does not automatically replace other PCB requirements. Engineers should also evaluate Tg, Dk/Df, thermal performance, assembly requirements, and manufacturing capability.
5. How CTI Affects Creepage Distance
- 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)
| Parameter | Engineering Meaning |
|---|---|
| CTI | Resistance of the material surface against electrical tracking |
| Creepage Distance | Required surface distance between conductive parts |
| Clearance Distance | Air distance required to prevent electrical breakdown |
6. CTI Requirements by PCB Application
Different PCB applications have different CTI requirements because the risk of tracking depends on voltage level, environmental exposure, and product safety requirements.
| Application | Main Risk Factors | Recommended CTI Range |
|---|---|---|
| Automotive Electronics | Humidity, temperature cycling, contamination, vibration | ≥400V–600V |
| Industrial Control Systems | High voltage, dust, factory environments | ≥400V |
| Outdoor Telecommunications | Moisture and environmental exposure | ≥600V |
| Medical Equipment | Safety-critical insulation requirements | Based on applicable standards |
| Consumer Electronics | Controlled indoor environments | 175V–300V |
For automotive controllers, EV systems, and industrial power electronics, higher CTI materials are often preferred because tracking failures can create safety risks, unexpected downtime, and expensive field repairs.
Reliability requirements should also be evaluated according to manufacturing quality standards such as IPC Class 2 vs Class 3, especially for products requiring long operating life and higher failure prevention requirements.
7. Common PCB Material CTI Comparison
FR-4 is not a single material specification. Different laminate systems can provide very different CTI performance depending on resin chemistry, construction, and manufacturer formulation.
| Material | Manufacturer | Typical CTI | Typical Applications |
|---|---|---|---|
| S1600 | Shengyi | ≥600V | Automotive, industrial, high-voltage PCB |
| KB-6160C | Kingboard | ≥600V | Power electronics and reliability-focused designs |
| 370HR | Isola | High CTI grade | High-reliability industrial and telecom applications |
| IT-180A | ITEQ | ≥400V | Industrial electronics and lead-free assembly |
| Standard FR-4 | Various | Typically 175V–250V | General electronics applications |
CTI should be evaluated together with Tg, Dk/Df, thermal performance, copper reliability, and stackup structure. Understanding prepreg and core interactions is important when selecting a reliable PCB material system.
8. How to Select the Right CTI Material
Selecting a PCB material with the correct CTI rating requires balancing electrical safety, environmental conditions, manufacturing requirements, and product cost.
Step 1: Identify Operating Environment
Determine whether the PCB will operate in a clean indoor environment or under conditions involving humidity, dust, chemicals, or temperature cycling.
Step 2: Define Voltage and Creepage Requirements
Evaluate working voltage, pollution degree, and required creepage distance according to the applicable safety standard. The selected CTI rating should support the required insulation design.
Step 3: Verify Laminate Datasheet
Never assume CTI performance based on material family names. Confirm the exact laminate grade, manufacturer specifications, and certification information before production.
Step 4: Consider Complete PCB Performance
CTI is only one part of material selection. Engineers should also consider thermal performance, signal integrity, assembly temperature, and mechanical reliability.
For complex PCB stackups, factors such as prepreg and core selection, surface finish compatibility, and manufacturing capability should be reviewed together.
PCB layout decisions such as spacing, via structures, and copper geometry should also be verified during design review. Related considerations include PCB annular ring design requirements.
9. Typical Tracking Failure Analysis
A common tracking-related failure occurs when a PCB designed for a controlled environment is later exposed to higher humidity, contamination, or electrical stress than originally expected.
A high-voltage industrial PCB using standard FR-4 may experience leakage current growth when moisture and contaminants create a conductive surface path. Over time, carbonized tracking can develop and increase the risk of intermittent faults or permanent failure.
Corrective Actions
- Select a higher CTI laminate suitable for the application environment.
- Increase creepage distance where possible.
- Use additional protection methods such as conformal coating when required.
- Review the complete insulation system during PCB design.
Surface protection methods, including conformal coating and appropriate solder mask selection, should be considered as part of the complete reliability strategy rather than replacements for correct material selection.
10. Frequently Asked Questions
What CTI rating should I use for automotive PCB applications?
Automotive PCBs exposed to humidity, temperature cycling, and contamination commonly require higher CTI materials, often 400V or above. Under-hood and EV applications may require CTI ratings of 600V depending on voltage and safety requirements.
Is standard FR-4 a high CTI material?
Not necessarily. FR-4 describes flame resistance performance, not a fixed CTI value. Standard FR-4 materials may have CTI values around 175V, while specialized high-CTI FR-4 materials can reach 600V or higher.
How does CTI affect PCB creepage distance?
Higher CTI materials provide better resistance against surface tracking and may reduce required creepage distance under the same voltage and pollution conditions. However, clearance and other insulation requirements must still be considered.
Can conformal coating replace a high CTI PCB material?
Conformal coating can provide additional protection against moisture and contamination, but it should not automatically replace selecting the correct base material. The complete insulation system must meet application requirements.
What standard defines CTI testing?
IEC 60112 defines the test method used to determine CTI and PTI values for solid insulating materials under controlled contamination and electrical stress conditions.
