Reliable electronics do not depend only on correct schematics, layouts, and assembly. The quality of the bare PCB itself also has a major influence on long-term field reliability. Two often overlooked but important indicators are the CTI value of the base material and the ionic cleanliness of the finished board. Together, they help reduce the risk of leakage currents, corrosion, electrochemical migration, and premature failure, especially in humid or contaminated environments.
At Newmatik, these topics are relevant because we do not just assemble boards. We are responsible for delivering electronic products that remain stable and safe in real-world operating conditions. For this reason, CTI and ionic cleanliness are not abstract lab values; they are useful indicators when selecting suppliers, defining procurement requirements, and assessing risk for demanding applications.
What is the CTI value?
CTI stands for Comparative Tracking Index. It describes how resistant an insulating material is to the formation of conductive tracks on its surface under electrical stress. The standard test method is defined in IEC 60112. In this test, electrodes are placed on the material, an AC voltage is applied, and a conductive test liquid is dropped onto the surface. The result indicates how well the material resists surface tracking before failure occurs.
In simpler terms, CTI describes how difficult it is for electricity to "find a path" across the surface of the PCB material when moisture, contamination, or residues are present. A helpful mental image is water slowly finding a path over a porous surface. The lower the material resistance, the easier such a path can form. The higher the CTI value, the more resistant the material is. This matters because unwanted surface conduction can eventually lead to insulation breakdown, arcing, or carbonized tracks that permanently reduce insulation performance.
What does a high CTI value mean?
According to UL's summary of the IEC 60112 method, insulating materials are commonly grouped as follows:
| Material Group | CTI Range |
|---|---|
| Group I | CTI >= 600 |
| Group II | 400 <= CTI < 600 |
| Group IIIa | 175 <= CTI < 400 |
| Group IIIb | 100 <= CTI < 175 |
A CTI value above 600 is considered very good and places the material in Material Group I, the most resistant group in this classification. In practice, that means better resistance to surface tracking and better robustness in unfavorable environmental conditions. For higher-voltage or more demanding applications, this can support safer insulation design and reduce reliability risks.
That said, CTI should not be misunderstood. IEC 60112 explicitly notes that CTI results are not by themselves sufficient to determine safe creepage distances in a finished product. Proper insulation design must also consider the applicable equipment standard, working voltage, pollution degree, overvoltage category, and creepage distance requirements, for example under IEC 60664-1. A high CTI value is important, but it is only one part of the insulation picture.
Why CTI matters for PCB procurement
For PCB procurement, CTI is mainly a material quality and risk-management parameter. If a board will be used in an environment with humidity, dust, condensation, or other contamination risk, a better base material provides an extra safety margin. This is especially relevant for products with higher voltages, small spacings, long service life expectations, or safety-related functions.
From a business perspective, specifying a higher CTI material can help avoid hidden cost later: fewer reliability problems, fewer returns, and less investigation effort in the field. For that reason, requesting a defined CTI class from the PCB manufacturer can be a reasonable requirement, especially when the product's operating conditions are demanding.
What is ionic cleanliness?
Even a good base material can become unreliable if the PCB surface is contaminated. During PCB manufacturing, many chemicals are used, including etchants, cleaners, salts, and acids. If these residues are not removed sufficiently, they can remain on the finished board surface. In the presence of moisture, such residues can become conductive and promote leakage current, corrosion, dendrite growth, and other electrochemical failure mechanisms.
This is why ionic cleanliness is an important quality characteristic. It is not primarily about visible dirt. A board may look perfectly clean and still carry enough ionic residue to create long-term problems under the wrong environmental conditions.
How is ionic cleanliness tested?
A widely used method is the ROSE test, which stands for Resistivity of Solvent Extract. The IPC method for this is IPC-TM-650 2.3.25, titled Detection and Measurement of Ionizable Surface Contaminants by Resistivity of Solvent Extract (ROSE). In this method, ionic residues are extracted from the board using a controlled solvent mixture based on 2-propanol (isopropyl alcohol) and deionized water, and the conductivity or resistivity change is measured. The result is typically expressed as micrograms of NaCl equivalent per square centimetre.
An important detail is that this value does not mean the board literally contains sodium chloride. IPC explains that the result is expressed as a sodium chloride equivalent because the contamination shows an electrical conductivity comparable to that amount of dissolved NaCl.
A practical limit: 1.56 ug/cm2 NaCl equivalent
For bare PCBs, the value of 1.56 ug/cm2 NaCl equivalent is widely referenced as a practical cleanliness requirement and is associated with IPC-6012 bare board cleanliness requirements in industry usage. Secondary industry sources and IPC-related training material continue to reference this value for bare boards, even though the broader IPC discussion around cleanliness has become more nuanced over time.
A reasonable incoming-quality requirement based on this limit can be formulated as follows:
The PCB manufacturer shall provide evidence of ionic cleanliness according to IPC-TM-650 2.3.25 (ROSE test). The measured contamination shall not exceed 1.56 ug/cm2 NaCl equivalent. One ionic contamination report per batch should be supplied with each delivery.
This gives a documented, measurable basis for supplier control.
Important limitation of the ROSE test
The ROSE test is useful, but it is not perfect. IPC notes that the method will not detect residues that are not dissolved by the extraction process, including some trapped or insoluble ionic contaminants, and it does not measure non-ionic residues. In other words, a passing ROSE result is valuable, but it is not an absolute guarantee that every possible contamination-related reliability risk has been eliminated.
IPC has also moved away from treating the old 1.56 ug/cm2 number as a universal reliability proof for all assembled electronics. J-STD-001 now emphasizes that using the ROSE number alone, without other supporting objective evidence, is not a sufficient basis for qualifying an assembly cleaning process. That change is mainly relevant to assembled boards and process qualification, but it is still useful context: ROSE is best understood as a screening and process-control tool, not a complete substitute for deeper reliability validation when risk is high.
Practical recommendations
The most sensible approach is not to overcomplicate the requirement, but also not to oversimplify it:
- Specify the base material CTI when insulation robustness matters, especially for products exposed to humidity, contamination, or higher electrical stress. A CTI above 600 indicates a high-grade material with strong resistance to surface tracking.
- Require ionic cleanliness evidence from the PCB supplier, preferably one ROSE report per batch. This helps ensure process consistency and reduces the risk of contamination-related failures.
- Treat ROSE as a practical control measure, not a universal guarantee. If an application is especially sensitive, additional methods such as ion chromatography or broader reliability validation may be justified. IPC and industry guidance increasingly support this more evidence-based approach.
Example specification language
The following wording is suitable for procurement documents or technical requirements:
CTI value: The PCB base material shall have a CTI value > 600 in accordance with IEC 60112, unless otherwise agreed for the specific application.
Ionic cleanliness: The PCB manufacturer shall provide evidence of ionic cleanliness in accordance with IPC-TM-650 2.3.25 (ROSE test). The contamination level shall not exceed 1.56 ug/cm2 NaCl equivalent. An ionic contamination report per batch shall be provided with each delivery.
Conclusion
CTI value and ionic cleanliness address two different but related reliability risks. The CTI value describes how well the base material resists conductive path formation under electrical stress, while ionic cleanliness indicates whether manufacturing residues could trigger leakage currents, corrosion, or electrochemical migration over time.
Specifying both requirements for bare-board procurement supports what matters most in production: stable processes, reliable products, and fewer field failures.
Sources
- IEC 60112:2025 -- Method for the determination of the proof and the comparative tracking indices of solid insulating materials
- UL Solutions -- Comparative Tracking Index (CTI) IEC 60112
- IPC-TM-650 2.3.25 -- Detection and Measurement of Ionizable Surface Contaminants by Resistivity of Solvent Extract (ROSE)
- J-STD-001 amendment -- ROSE / objective evidence requirements for assembly process qualification
- IPC-6012 -- Bare board cleanliness requirements (industry reference for the 1.56 ug/cm2 limit)