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How to Check ESD Resistance Requirements

Do you use a Surface Resistivity Meter to test your ESD Smocks? How do you check your ESD grounding? In this article I explain how to check lab coats, mats, wrist straps, and heel grounders using ESD test equipment.

1. How do I test my ESD Products?

In the United States, the rules for ESD testing are introduced by the ESD Association and adopted by the American National Standards Institute (ANSI). Hence, most of the standard referred to are “ANSI / ESD” standards.

• Mats

• Workstations

• Smocks

• Wrist Straps and Heel Grounders

The current standard is ANSI / ESD S20.20 2014. In addition to this document, there are many more specific ESD Association Standard Test Methods that you should use when creating your ESD Control Program Plan.

We typically perform surface resistance tests to determine if a mat, workstation, smock, wrist strap, or heel grounder can be used safely without causing static electricity.

Theoretically, surface resistivity is 10 times greater than resistance. For example, a material that measures 108 ohms in surface resistance should measure 109 ohms / square in surface resistivity. For testing purposes, per ESD TR 20.20, the correct unit of measurement is surface resistance (just ohms).

These tests look for the resistance the product has when you run an electrical current through it.

2. ESD Mat Requirements

The “ESD Resistance” test standard for mats is ANSI / ESD S4.1 Worksurfaces – Resistance Measurements. The standard requires us to perform two types of tests: a resistance top-to-top (RTT) test, and a resistance-to-groundable point (RTG) test. You can perform both tests with an ESD meter kit.

How to use an ESD Resistance Meter to Test ESD Mats gives step-by-step instructions on how to perform S4.1 compliant ESD resistance testing on your mats.

ESD Resistance Chart for 5300 Series Anti Static Mats

Generally, you should find the top surface of static dissipative mats measure in the lower end of the static dissipative spectrum: between 1 x 106 and 1 x 109 ohms / square. As they state in section 8.0, “these guidelines represent a range of resistance that has generally been proven to provide protection in the manufacturing environment.”

3. ESD Workstation Grounding Requirements

The standard used for evaluating a grounded workstation is “ANSI / ESD 6.1 ESD Association Standard for the Protection of Electrostatic Discharge Susceptible Items – Grounding”. As they explain in the foreword:

“The single most important concept in the field of static control is grounding. Attaching all electrically conductive and dissipative items in the workplace to ground allows built-up electrostatic charges to equalize with ground potential. A grounded conductor cannot hold a static charge.”

Wrist straps can be grounded at a workstation together with a mat using a common point ground or grounded separately from the mat using a bench mount ground or by clipping an alligator clip on the end of the coil cord to connect it to ground.

ANSI ESD S6.1 Grounding Diagram with ESD Wrist Strap and Grounding

Mats can be grounded at a workstation together with a wrist strap using a common point ground or grounded separately from the mat using a using a mat ground.

Grounding for ESD Mat with an Anti Static Wrist Strap

Grounding for ESD Mat separate from Wrist Strap

You should connect all grounded equipment to an electrical outlet. Electrical outlets follow the piping to the Earth underneath the building. When you are wearing the wrist strap standing on a mat, you are connected to the outlet electrically. The electrical field underneath the building is now in balance with you.

ANSI ESD S6.1 Grounding Diagram with ESD Wrist Strap and Mat

4. ESD Smock Test Methods and Definitions

A garment is considered “static dissipative” if it measures above 1 million ohms (or 1 x 106 ohms) per square, and below 1 trillion ohms (or 1 x 1012 ohms) per square.

ESD Resistance Chart for Smocks, Lab Coats

However, we cannot actually measure surface resistivity in ohms per square, so we apply voltage through ESD smocks to measure their surface resistance in just ohms.

Theoretically, surface resistivity is 10 times greater than resistance. For example, a material that measures 108 ohms in surface resistance should measure 109 ohms / square in surface resistivity. For testing purposes, per ESD TR 20.20, the correct unit of measurement is surface resistance (just ohms).

Garments used for ESD Safety are defined in ANSI / ESD S20.20 2014 as either a “Static Control Garment” (surface resistance of < 1 x 1011 ohms / square); a “Groundable Static Control Garment” (surface resistance of < 1 x 109 ohms / square); or a “Groundable Static Control Garment System” (surface resistance of < 3.5 x 107 ohms square).

An ESD meter kit is used to perform resistance top-to-top (RTT) and point-to-groundable point (RTG) tests across the panels of an ESD garment.

S20.20 Static Control Garment

S20.20 Groundable Static Control Garment

S20.20 Groundable Static Control Garment System

The specifics for performing the tests are outlined in ESD TR53‘s “Garments” section, the test for the “Groundable Static Control Garment System” is found in the “Personal Grounding with Garments” section.

5. How to Check ESD Grounding of Wrist Straps and Foot Grounders

To test heel grounders and wrist straps you would typically use a combo tester at the entrance to a facility.

ESD Wrist Strap and Heel Ground Tester

The ESD test equipment, like the combo tester, is itself typically setup to meet the requirements of ESD TR53‘s Wrist Strap and Footwear sections.

ESD Wrist Strap in Ohms / Square

The testing therefore looks for a reading from the wrist strap system of less than 3.5 x 107 (35,000,000) ohms / square, and a reading from the foot grounder of less than 1 x 109 (1 billion or 1,000,000,000) ohms / square.

ESD Foot Grounder in Ohms / Square

 
 

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