Insulators, Conductors and Static Dissipative materials …

Published: February 04, 2011, By Kelly Robinson

We often use jargon to communicate using “technical short-hand.” It is very important, now and then, to take a step back and think about what these terms mean. Does the term “static dissipative” mean the same thing to you as it does to me?

Here is my understanding. Electrical resistivity is one of the most widely varying physical properties in the universe. Well, that is a big claim. Here is some data.

The electrical resistivity of silver is about 2 E-8 ohm-m.

The electrical resistivity of PTFE exceed 1 E+22 ohm-m.

This is an astonishing large range of almost 10^(+30) … 30 decades!!!

It is natural for humans to categorize or group similar things. Conductors are materials with resistivities at the low end of this range. Insulators are materials with resistivities at the high end of this range.

Well, this grouping is not really very useful. Here is a better way to think about insulators, conductors and static dissipative materials. Static charge on the surface of materials will dissipate with a characteristic time constant; the charge relaxation time. The charge relaxation time is proportional to resistivity. Charge on conductors dissipates very quickly while charge on insulators persists for a long time.

So, what is a short time and what is a long time? Well, it depends on the application. Think about the time it takes for a physical or mechanical phenomena to occur; the clock time in an integrated circuit, the time it takes for you to walk across the carpet during the winter months, or the time that a static charged balloon will stick to the ceiling.

For “conductors” in an integrated circuit, charge must move very quickly relative to the clock speed of the circuit.

For “static dissipative” shoes, charge must dissipate before you can walk across a room and touch a door knob.

For “insulators,” charge must persist on the surface of the balloon long enough to get a good laugh.

If charge dissipates from a material much faster than the time needed for the physical or mechanical process, then the material is a conductor. If charge persists on the surface of materials for times much longer than the physical or mechanical process, then the material is an insulator. And, if charge dissipates from a material just a little bit faster than the mechanical of physical process, then it is static dissipative.

I know that this is unsettling, because we must think about the application to decide if a material is “static dissipative.” However, this is exactly the point. Materials might be static dissipative for one application, and insulating for different, faster application.

This is why the term “static dissipative” is so general and hard to define.