ESD Control in Handling and Sorting -- Even minor, and seemingly unimportant, actions can produce ESD. Read on to ensure that your actions aren't the culprit.

Many think electrostatic discharge (ESD) is restricted to electronics assembly operations, but ESD problems can occur throughout a facility and encompass processes such as receiving, final inspection, automated assembly and rework. While specific functions may require adaptation of, or emphasis on, particular ESD control techniques, many basic principles of ESD control remain constant-regardless of the function or process involved. Concerning one specific area, handling and sorting, certain procedures are needed to provide ESD protection.

What Options Are Available?

Begin by reducing or eliminating as many static generating processes or materials, such as common plastics, as quickly as possible from the work environment. Remove styrofoam cups, sheet protectors and other non-essential insulators and sources of static fields. Process-essential items should be kept away from unprotected devices. Ionization may be necessary to neutralize charges on some items, such as computer monitors, in the work area.

Provide a ground path to bring ESD-protective materials and personnel to the same electrical potential. All conductors in the environment, including personnel, must be bonded or electrically connected and attached to a known or contrived ground, thus creating an equipotential balance between all items and personnel.

Control electrostatic charge in personnel who handle or contact sensitive devices. In many processes, including handling and sorting, people are one of the prime generators of static electricity. The simple act of removing components from their protective packaging or carrying a tote box from one work area to another can generate several thousand volts on the human body. Wherever people are handling unprotected ESD-sensitive devices, a properly worn and grounded wrist strap keeps employees at or near ground potential. If the process requires a high degree of mobility by those handling the devices, static control flooring and footwear can supplement the use of wrist straps.

Unprotected devices should be handled only at an ESD protective workstation. Key ESD control elements, are a static dissipative worksurface, a means of grounding personnel (usually a wrist strap), a common grounding connection and appropriate signage and labeling. Static protective worksurfaces, with a resistance to ground of 106 to 109, provide a surface that can be maintained at the same electrical potential as other ESD protective items in the workstation. Such surfaces also provide an electrical path to ground for the controlled dissipation of any static potential on materials that contact the surface.

All ESD-sensitive devices should be packaged and transported in static control packaging and materials handling products. Dissipative tote boxes, bins and dip tubes help shield the product from charge and also reduce the generation of charge caused by any movement within the container. Protection should be provided for products within a facility, as well as for products shipped to an outside customer.

Automated handling and sorting operations often require special considerations. Moving parts, such as conveyors, can generate high levels of static that may damage sensitive components. Even the sliding of a component down a feeder can generate a charge. Materials handling products, such as dip tubes, should be low charge generating. Equipment frames may require ESD grounding to keep them at the same electrostatic potential as personnel and other ESD control materials in the environment. In circumstances where the components themselves become charged and then contact highly conductive surfaces in the equipment, conductive surfaces may need to be replaced with dissipative surfaces to reduce the impact of charged device model (CDM) discharges. Spot ionization may be necessary to neutralize charges in specific areas where grounding or other procedures may not be practical.

Conclusion

Although functions may vary from area to area in a facility, the process of implementing effective ESD control remains the same. Identify areas and functions where ESD control is required, then evaluate and implement the options that are available.

Michael T. Brandt is is publicity consultant to the ESD Association and president of Marketing Resources Ltd., Rockford, IL; e-mail: [e-mail]. This column is a regular contribution of the ESD Association, Rome, NY; [phone]; e-mail: [e-mail].

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