For electronic assembly areas, proper grounding is essential because it ensures safe operation of equipment and protects users from harmful voltages. It also prevents damage to electronic devices by keeping personnel working on static-sensitive products close to a zero voltage level and at the same electrical potential as the surrounding area.
The first step to ensure that everything is at the same electrical potential is to ground all components of the work area, including work surfaces, personnel and equipment, to the same electrical ground point. This is called the common point ground and is defined in ANSI EOS/ESD-S6.1 Grounding as a system or method for connecting two or more grounding conductors to the same electrical potential.
Figure 1 shows the symbol used to identify and label this point.1 Other documents that address grounding include EIA-625 Requirements for Handling Electrostatic-Discharge-Sensitive Device and MIL-HDBK-263B ESD Control Handbook, 31 July 1994.
“To ensure proper ESD grounding in a working environment, evaluate the static dissipation and grounding performance of the complete work system, including the work surface, personnel, packaging material and flooring,” said Jim Novak of Static Technologies. “Next, use industrial standards to select and evaluate the performance of the individual grounding product.”
Standards such as EOS/ESD-S1.0 Wrist Straps; EOS/ESD-S4.1, Worksurfaces; EOS/ESD-S6.1, Grounding; and EOS/ESD-S7.1, Floor Materials – Resistive Characterization of Materials are available from the ESD Association and provide the specific information you need.
The work area should have a defined procedure to ensure safe handling of static-sensitive items. Design a step-by-step program to test the entire grounding system, perhaps like the one suggested by Lawrence Lepore of the Walter G. Legge Co.:
1. Put on heel straps or ESD-control shoes prior to entering an ESD-safe area.
2. Ensure that proper contact is made between the heel strap and the user.
3. Wear dissipative shoes or conductive cloth inserts when direct skin-contact grounding is inappropriate. After donning the shoes, wait approximately five minutes to guarantee proper ground connection before beginning the test procedure.
4. Clean the heel straps and the shoes with a conductive cleaner to prevent increased resistance from dirt buildup.
5. Wear heel straps on both shoes to ensure constant discharge.
6. Verify proper operation of the shoe-/heel-strap tester. Contact the manufacturer for the appropriate calibration procedure.
7. Inspect wrist straps for loose connectors, dirty inner bands or stretched material and correct as needed.
8. Apply conductive lotion to wrists to help achieve proper contact between the skin and the strap.
9. Learn the proper procedure for operating the wrist- and heel-strap test equipment.
10. Ensure that workstations, floors and equipment are properly grounded.
Common Point Ground
The most important item on an ESD audit checklist is verifying the integrity of the workstation’s ground connection, said Timothy O’Connell, Electrical Engineer at K-TEC. A common point ground system is typically used for grounding workstations, equipment and personnel in an electronics test and production area.The integrity of the connections from the common point ground to the equipment ground can be measured with an ohmmeter, said Steve Koehn, Advanced Technical Service Engineer for 3M. The resistance between the two connections should be <1 W . The total AC resistance for the equipment ground also should be <1 W .
The ground connection can be achieved in a number of ways, according to John Kiedaisch of Richmond Technology, but the best and safest is a dedicated earth ground. Sinking a conductive metal rod at least 3 feet into the ground and connecting all conductive surfaces to this point makes a safe, dedicated ground. It guarantees equal electrostatic potential at all ground connections and allows rapid and safe dissipation of all accumulated charges.
Alternative grounds in facilities include connections to water pipes, metal support stanchions and electrical outlets, said Mr. Kiedaisch. Each may be functional in your circumstance, but should be certified for operator safety by a qualified electrician.
However, the preferred grounding method per EIA 625 and EOS/ESD-6.1 is to connect the common point ground to the equipment ground or the electrical green-wire ground. It is preferred because all electrical equipment at a workstation is already connected to this ground. Connecting ESD protective products to equipment ground brings all components of the workstation to the same electrical potential.
In some cases, an exposed metal water pipe provides the best hard ground connection, although most companies have better access to the third prong on the electrical outlet for the hard ground connection, said Mr. O’Connell. The electrical ground wiring, however, can become dirty if it carries voltage transients or offset voltages caused by motors or other heavy equipment. When using the electrical wiring as the ESD ground, verify the correct polarity of the electrical outlet with a commercially available tester, said Mr. O’Connell.
The ESD ground connection for many companies is a dedicated 14 awg or 16 awg bare stranded copper wire, continued Mr. O’Connell. The wire is strung throughout the facility and ultimately attached to a large metal stake driven into the ground. The bare copper wire allows ground connections at any point on the wire. It is dedicated to ESD ground connections and provides a clean ground, he added.
Grounding needs are not the same for everyone working with static-sensitive devices. There are some important differences according to job function.
Mobility is the first consideration when investigating personnel grounding needs, according to Mr. Kiedaisch.
Mobile personnel include stock-room, engineering and support workers, whose duties require them to move freely throughout many departments, including static-sensitive areas.
The primary method of grounding for these workers is ESD footwear, heel or toe grounders and conductive flooring. The floor surface can be mats, conductive tile or poured epoxy.
For stationary personnel, such as a workstation operator assigned exclusively to a bench, the primary line of defense against static discharge is the wrist strap and a properly grounded dissipative work surface. The most commonly used wrist band is an adjustable knit or crocheted type.
The critical evaluation for the wrist strap and cord is the life-test result assessed per EOS/ESD-S1.0. It provides a wear estimate for electrical and physical characteristics for the band and the cord.Bench mats used in the workstation environment also must be tested because they vary in base material, texture and electrical characteristics, continued Mr. Kiedaisch. The recommended resistance range is >1 x 106 W to <1 x 1010 W , per EOS/ESD-S4.1.
Another category of personnel works at the assembly bench but also has access to stockrooms, test areas or other ESD-sensitive departments. These users require wrist straps and mats while working at the bench, and must wear ESD footwear while in the proximity of or transporting ESD-sensitive products.
1. ESD Association Advisory 2.0-1994 Handbook, Grounding, ESD Association, 1994.
ESD Grounding Products
Ground Unit Replaces
SURE-GROUNDä replaces drag chains used on chairs, carts and other mobile equipment in ESD-sensitive areas. It is manufactured from stainless steel, does not corrode and is impervious to cleaning chemicals found in electronic assembly areas. The device may be used in conjunction with or in place of conductive casters. It will not damage conductive tiles or wax coatings. Custom Products & Services, Inc., (800) 848-4912.
Ground Cord Accommodates
Bench Mats and Wrist Straps
A common point ground cord for grounding static-control bench mats and wrist straps features a low-profile head, a 10-mm snap connection for mats, two banana plug jacks, and a 1 MW resistor. The cord is 15’ long and equipped with an alligator clip or a ring terminal for customizing the cord length. Plastic Systems, (508) 485-7390.
Wrist-Strap Assembly Has
Heavy-Duty 14-Foot Cord
The PWS-610M Wrist Strap Assembly provides a knit, two-way fabric cuff that is adjustable and a 14’ heavy-duty cord. The strap withstands more than 200,000 bending cycles in accordance with ESD Association Standard 1.0-1987. Prostat Corp., (708) 238-8883.
Monitor Checks Dual- and
Single-Cord Wrist Straps
The Dual Single Continuous Monitor (DS-CM) checks single- or dual-cord wrist straps. It illuminates a green light if the wrist strap makes contact with the operator’s wrist and flashes a red LED and beeps if there is a failure, disconnect from ground or improper contact of the wrist strap. The unit adjusts to monitor resistance from 750 kW to 10 MW . A standby switch allows you to turn off the alarm while leaving the red LED lit. The wrist-strap cord is available in 6’ and 12’ versions and provides a minimum current-carrying capacity of 25 mA. The DS-CM uses a 9 VDC adapter power supply. Richmond Technology, Inc., (800) 829-2942.
Resistive Wrist Strap
Accommodates Dual-Lead Units
The OhmScan™ Resistive Wrist Strap Monitor accommodates dual-lead, dual-section wrist straps. It detects intermittent, broken or degraded wrist straps, dual-lead cords and system grounds. The monitor provides an audible and visual alarm if a fault condition is detected. A three-prong AC adapter provides a continuous ground source. The Simco Co., Inc., ESD Consumable Products Division, (215) 822-2171.
Disposable Heel Grounder
Fits Under User’s Foot
The STATIC TECH™ disposable heel grounder fits under the wearer’s foot, providing a path to ground. It is a bright yellow felt strip that provides a conductive path. The grounder connects from inside the shoe under the wearer’s heel to outside the shoe under the heel. A low-tack adhesive holds the strip in place. Static Technologies Corp., (617) 357-6444.
Copyright 1995 Nelson Publishing Inc.