What is Comparative Tracking Index (CTI)?

While PCB design which involve routing, placement and Gerber creation is somewhat tedious and challenging job, ensuring that the other design parameters related to the PCB material, standards are also followed is important. There are many rugged applications of electronics and understanding specifications and designing PCB for such applications is very critical to quality. PCB material selection is always a starting step before PCB designer or an electrical design goes ahed with his further PCB related design activities. One of the critical parameter of the PCB material is the Comparative Tracking Index (CTI). 

Comparative tracking Index measures the electrical properties of the PCB insulating material. The requirement is that insulating should not breakdown under electrical and thermal stress in an application. Comparative Tracking Index is the voltage at which electrical breakdown of the material occurs when 50 drops of 0.1% ammonium chloride are dropped on the material. Based on the CTI value., the material groups are classified.

If we want to understand comparative tracking Index from PCB routing perspective, when two PCB traces are laid in parallel, the dielectric material should not breakdown at any time and both traces should be able to carry current without any issues. The chance of dielectric breaking down and two traces having a direct short is very less. The dielectric ensures that resistance is present between the two traces. The extent to which the material can resist the unintended current to flow between the traces is what comparative tracking Index indicates. One of the primary reasons why the two traces might have leakage current is due to the impurities forming between traces, environmental conditions, voltage applied, temperature.