Appropriate Applications for Static Control Table Covering vs. Static Control Flooring

The unwritten rule is that table covering (mats, laminates) should always measure in the static dissipative range and ESD flooring (carpet, tile and epoxy) should always measure in the static conductive range.

This rule stems from the fact that static control table coverings and ESD floors serve two entirely different functions:

• Table covering is intended to discharge objects like ESD sensitive part as well as tools and fixtures.
• Flooring is used to ground and remove static on people while they move around the work area. (Incidentally, wrist straps serve the same purpose for grounding people when they are stationary.)

Table Coverings

A static control work surface is specifically designed to eliminate charges on any item that either contacts or rests on the surface including: tools, test equipment, electronic parts, static shielding bags, circuit boards, assemblies, material handling fixtures and ESD safe packaging etc.

ESD sensitive parts can be safely charged and discharged--as long as the work surface has a sheet resistance greater than 100,000 ohms--a charged sensitive part can be discharged without negative consequences. A resistance above 100,000 ohms allows a slow and controlled voltage ramp-down within the charged part. Conversely, any table top surface with a sheet resistance below 100,000 ohms could enable a discharge that is too fast. In layman’s terms, rapid discharges will “fry the internal circuit” because the discharge will overwhelm the part.

A good analogy of damage from “too rapid static discharge” might be the physical effect on our bodies from sudden changes in atmospheric pressure. A controlled depressurization system would be the equivalent of a static dissipative table top – slow and easy.

Floor Coverings vs. Wrist Straps

The main purpose of a static control floor is to duplicate the performance of a wrist trap. Wrist straps, unlike table tops, are worn to prevent and eliminate static charges on people. Wrist straps connect people to a ground connection. But, wrist straps are cumbersome and impractical if the person needs mobility to do his job. The wrist strap cord ties down the person and greatly limits their range of motion.

For safety reasons, wrist straps are equipped with a current limiting resistor. The purpose of the resistor is the prevention of dangerous electrical shocks in the event that the person contacts live voltages while they are wearing the wrist strap. Resistors do not improve the performance of the wrist strap; they are there for safety reasons only. Properly designed ESD flooring also has built-in resistance. ESD flooring is made with static conductive materials that disperse static charges safely but rapidly. This is completely different than the function of a table top.

Why do floors need to be more conductive than a table top work surface?

Properly designed and installed ESD flooring has built-in resistance. ESD flooring is made with static conductive materials that disperse static charges safely but rapidly.(See “How Does ESD Flooring Work?) This is completely different than the function of a table top. The table top should remove static slowly; the floor needs to remove static rapidly or a charged person could discharge to a static sensitive assembly like a circuit board or a computer.

Wrist Straps

Wrist straps are worn to prevent and eliminate static charges on people, by connecting to a ground connection. For safety reasons, wrist straps are equipped with a current limiting resistor. The purpose of the resistor is the prevention of dangerous electrical shocks in the event that the person contacts live voltages. Resistors do not improve the performance of the wrist strap; they are there for personnel safety reasons only.