The phenomenon of electrostatic discharge (ESD) is not new, but its effect on today’s products certainly is. Electronic products are becoming more sensitive to ESD damage as they grow in sophistication. Considering that uncontrolled ESD sometimes causes only partial damage—the effects of which may not be discovered for months or years—ESD not only increases manufacturing costs and product liability, but also reduces product reliability, resulting in lost customers and lost profits.
ESD can strike anywhere along the line, including receiving and inspection, production, assembly, testing, packaging, shipping and storage, and service and repair. The further into the manufacturing process that the damaged components are introduced, the more costly the consequences. From production to final use, the best defense against ESD damage is a coordinated and ongoing control program.
First Steps Toward Control
The first step toward controlling ESD is an information program. Company-wide programs use a wide range of support materials such as signs, symbols, stickers, and training literature to heighten awareness among employees.
Underlying all successful control programs is the understanding that virtually every electronic component and product can be damaged by ESD. From this understanding, static-safe practices, like those provided in Table 1, should be developed.
Safe Storage
Inadequate materials handling and storage systems can be a major source of uncontrolled ESD in manufacturing and maintenance. In manufacturing, yields can be increased by eliminating uncontrolled ESD events from the inventory stores. In maintenance, repair efficiencies can be increased by eliminating uncontrolled ESD from the maintenance work area.
Regardless of whether the components are produced internally or externally, they are stored until ready for assembly. Unless adequate measures are taken to safely dissipate static electricity, components will be susceptible to damage in storage areas. Fortunately, systems are available that can alleviate ESD in materials handling.
Paint It Black
Compartmentalized drawers of modular cabinets are one of the most space-efficient and ergonomic ways to store small-to-medium sized electronic components and circuit boards. However, typical modular drawer cabinets do not discharge static in a controlled manner. As a result, they are unsuitable for storing ESD-sensitive components.
The missing element is a dissipative coating. A dissipative coating and production process developed by Stanley Storage Systems for a new cabinet series offer the benefits of conventional cabinets and solves several storage problems. The dissipative coating discharges static electricity at a controlled rate. These ESD-safe storage systems are recognizable by their distinctive black color, a characteristic of the dissipative coating.
Two distinctly different choices in performance levels are provided. The high-end ESD cabinets perform within the static-dissipative range of 106 to 109W at an open-circuit voltage of 100 V. The general-purpose ESD cabinets perform within the static-dissipative range of 106 to 109W at 10 V. The test criteria were established by the ESD Association and follow ANSI Accredited Test Standards and Advisories such as ESD Standards 4.1, 6.1, and 8.0 Advisory 53.1.
Some New Solutions
Bagless Storage
When individual compartments of an ESD-safe cabinet can be bar-coded, components no longer must be placed in labeled bags. With bagless storage, components take up less space in the cabinet and can be more readily counted. Bagless storage also uses less space in the transfer bins where components are placed in preparation for assembly.
Compartments or bins are coded according to the circuit board to be assembled. One bin can be loaded with enough components for the partial assembly of eight to 20 boards. Typically, one bin is dedicated to components that are assembled at one station, whether by hand or automatically.
Automatic Storage and Retrieval
Automated material handling may be aided by an ESD-safe powerdrawer™ system, a vertical lift system equipped with dissipative coating and an ionizing blower. Vertical lifts save space and add a measure of security since access can be restricted by procedures such as passwords.
A vertical lift functions as an automatic storage and retrieval system (AS/RS). AS/RSs store electronics components and present them to one or two points of use. They can transport materials through to a second floor, such as from an inventory storage area to an assembly area.
At some manufacturing facilities, vertical lifts are dedicated to store bins coded for various assembly jobs. They can be held in the storage area, ready to go, allowing centralized control over the manufacturing operations.
In other words, nothing goes to assembly until assembly is ready for it. This policy reduces the risk of ESD damage, because components are not left unmonitored while awaiting assembly. Unless the flow of materials is tightly controlled, the manufacturing process can get out of control.
Meanwhile, the powerdrawer system provides control over the manufacturing process since the status and location of each bin are tracked via computer. There is no searching for bins, since the job number and the bin location are synonymous within the powerdrawer.
Facility-Wide Solutions
The sheer number of different components, their high value, and their sensitivity to damage make the design of a suitable ESD-safe materials storage area for electronics components especially challenging. A satisfactory solution requires an integrated, facility-wide approach that includes ESD-safe workstations, local ESD-safe storage cabinets, and a central ESD-safe materials storage area. Accessories include ESD-safe chairs, floor mats, table mats, and wristbands.
About the Author
Greg Benton is manager of corporate account development at Stanley Storage Systems. He joined the company 13 years ago as a sales engineer and later as manager of specified products. Stanley Storage Systems, 1000 Stanley Dr., New Britain, CT 06053, (800) 523-2449, [email protected].
Table 1
1. Properly label ESD-sensitive items.
2. Prevent discrete parts from rubbing against one another to create ESD.
3. Store and transport ESD-sensitive items only in static-shielded containers.
4. Rid static-shielded containers of any items that could cause ESD, such as untreated plastics and packing materials.
5. Allow only properly-grounded personnel to handle ESD-sensitive items in static-dissipative work areas.
6. Wear proper grounding equipment, such as wrist straps and heel straps.
7. Rid work areas of dust, dirt, and all unnecessary materials.
8. Allow only minimal personnel traffic in a static-safe work area.
9. Report any potential problems regarding equipment or procedures promptly.
10. Properly ground all ESD-protective equipment.
11. Use test equipment to verify performance.
Copyright 1999 Nelson Publishing Inc.
September 1999
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