25 – CTI Comparative Tracking Index and PTI Proof Tracking Index of base material

The increased packing density of components on the printed circuit boards leads to increasingly smaller conductor track spacings. Therefore, the CTI / PTI value in the base material data sheets gets more important. This is the index for tracking resistance.

What is tracking resistance?

An example: 2 tracks with different potentials run at a small distance parallel to each other. The base material acts as insulation. The insulation capability of the base material is generally highly resistive, but may decrease accordingly by environmental influences such as dirt and moisture, so that current can flow between the two potentials. The so-called leak current. This ultimately leads to a short circuit, which can destroy components, thereby causing an error in the circuit.

The amount of CTI / PTI indicates how resistant the material against environmental influences is. The higher the value, the more resistant the material is. The occurrence of errors due to leakage is minimized. The default CTI value for FR4 is 175 and goes up to 600 on special materials. The CTI / PTI value is determined using standard test methods.

Test procedure (IEC 60112)

On an insulating material, in this case FR4, 2 electrodes are applied with an adjustable test voltage between 100 and 600 V. Now 50 drops of a test solution are applied between the electrodes and the voltage is gradually increased. The failure criterion is a cutoff current 𕠆,5 A during a time >2s.

Results of the testing method:
CTI value: maximum value of the voltage that caused in five testing samples – each with 50 drops – no failure.
PTI value: the amount of the applied voltage, which has resulted in five samples – each with 50 drops – at no failure.

We supply PCBs in FR4 with a standard CTI value of 175 as well as printed circuit boards with a maximum CTI value of 600.