Tracking resistance

The tracking resistance of PCB base material is specified as a CTI value (Comparative Tracking Index) and refers to the highest voltage (volts) that does not cause a failure due to tracking according to fixed test criteria. A high tracking resistance is essential if high voltages are to flow on the tracks and the track spacing is specified due to the available space.

1. General information
2. Leakage current in detail
3. Testing the tracking resistance
4. Leakage current resistance: CTI / PLC / insulating material groups

General information

Moisture or impurities can cause a measurable current to flow between two points on the - in itself insulating - PCB base material if a correspondingly high voltage is applied. This current is called leakage current.

The higher the voltage and the smaller the distance, the greater the probability of leakage current formation.

Since the operating voltage will increase in many places with increasing electrification in order to meet the power requirements and the increasing packing density of the components on the PCBs leads to ever smaller conductor spacing, the tracking resistance is becoming increasingly important for base materials (in addition to heat resistance and dielectric strength).

Material with a high tracking resistance allows significantly smaller distances between current-carrying conductors and helps with miniaturisation.

Leakage current in detail

The distance between the conductors, which are insulated from each other by the base material (and the air),is critical for the leakage current. The typical spacing here is ≥ 0.1 mm.

Example:

Two conductive tracks with different potentials run in parallel at a small distance. The base material (usually FR4) has an insulating effect. The insulating capacity of the base material is generally high resistance, but can be reduced by a number of factors so that current flows between the two potentials. This is known as leakage current.

The following factors can reduce or even bridgethe insulation:

• Metallic contamination that remains between the conductor tracks, e.g. from the soldering process
• Grease, e.g. fingerprints
• other contaminants, e.g. dust or abrasion

If high humidity and condensation create a conductive coating, this can lead to leakage current paths (creepage distances). These consist of carbonised residues of the contamination and the destroyed insulating material. Creepage paths usually spread further and further in branches (treeing) and an increasingly conductive connection is created between the different electrical potentials, which can ultimately lead to failure of the insulation.

The higher the voltages and the resulting field strength, the faster this development takes place. In the worst case scenario, a short circuit occurs and the electronic component or the entire assembly fails.

Testing the tracking resistance

The Comparative Tracking Index (CTI) is measured to determine the tendency of a material to form a creepage path. In this standardised test method (IEC 60112, alternatively ASTM D3638), a standardised test solution is dripped onto the material to be tested between two electrodes. The measurement is repeated several times.

The CTI value corresponds to the maximum voltage at which no failure was caused after 50 drops. Failure criteria: no current flow ≥ 0.5 A > 2 seconds (IEC 60112).

The practical benefit of a high CTI value is primarily the reduction in the required insulation distances = conductor track distances for current-carrying conductors.

Source: UL IEC 60112

Tracking resistance: CTI / PLC / insulating material groups

The test values determined for the base material are divided into PLC groups(Performance Level Categories). Printed circuit board FR4 material has PLC 3 (CTI 175V - 249V) as standard, particularly creep-resistant KF variants have a CTI ≥400V to >600V.

The CTI value is categorised differently when designing medical products. The insulating material groups according to EN 60601-1 apply here.