ESD on two different materials

Question

I am conducting ESD testing on two types of materials with a setup that closely follows the 61000-4-2 standard. We have incorporated both isolation material and a ground plane into our setup.

When I tested the first material using air discharge at 16kV, the ESD simulator did not register any discharges. However, when I tested the second material, which is supposed to have better ESD isolation, I observed discharges. I even saw discharges as low as 2kV. Both the materials we are testing are not grounded, it is sitting on the isolation sheet.

Here are the specs of the material that was susceptible to discharges.

This is the material that did not see a discharge from the ESD gun.

What is the parameter/parameters that will help me get a better idea of what I need to look into when buying a ESD isolation material next time?

Answer 1

The first sample has a funky specification. It specifies Surface resistivity in ohms rather than ohms/square. Assuming this is a typo, then the 2nd sample actually shows a much better surface resistivity than the 1st material (10E14 vs <1E9). This is a massive difference! The 1st sample doesn't specify volume resistivity, but I would expect it to be similar to the surface resistivity.

Note that there are many variables to consider when looking at ESD discharge. Resistivity or dielectric breakdown are 2 key parameters, but the shape of a sample can also be a factor. Remember that resistance is measured in ohms per square, so the shape will affect the total resistance.

Also, while air discharge is the correct method of testing insulators it has several shortfalls. It is a lot less repeatable than contact discharge and there are several variables to consider when testing. These include size and thickness of the sample, relative humidity, approach speed, approach angle, location of the discharge on the sample and there are probably others.