Until now, finding out how to set up and maintain an ESD-safe work environment has been difficult. Many documents could provide some of the information, but that was the problem: many documents with some information. Fortunately, the Electronic Industries Association (EIA) has changed all that.
The hottest new standard in static control is EIA 625, Requirements for Handling Electrostatic Discharge Sensitive Devices (November 1994). It replaces the JEDEC 108-B from the Joint Electron Device Engineering Council with concise minimum requirements, performance limits for those requirements, test references and an audit system that ensures proper implementation.
Granted, most of the information in EIA 625 is available in pieces from other documents, such as MIL-HDBK-773, MIL-HDBK-263 and the obsolete JEDEC 108-B. However, EIA 625 succeeds where the others fail in the presentation—all the necessary knowledge appears on just 24 pages, with only some mild disorder.
EIA 625 is divided into 12 sections, with Sections 1, 2 and 3 delivering the usual boilerplate about what the document covers, who should use it and where to get referenced publications. It establishes the minimum requirements for the methods and materials used to protect electronic devices from static damage to a ±200-V level.
The standard has been developed to help a broad audience, from semiconductor manufacturers to distributors, processors and device users, including board-level production and handling. All the supporting documents—ANSI, ASTM, EIA, ESD and MIL documents-are referenced.
Section 4
Section 4 provides 24 definitions of terms used in the standard, including:
Antistatic—A material which will not tribocharge more than ±200 V when used in its intended manner. Unfortunately, no guidance is given about how testing for this value should be accomplished.
ESD-Protective Packaging—Required to provide electrostatic shielding and limit tribocharging. An unprotected ESD-sensitive (ESDS) device is one that is not in ESD-protective packaging.
Section 5
Section 5 discusses inspection frequency, testing equipment and measurement methods.
Inspection—Items that must be checked and the frequency of those inspections are detailed in a table. The performance of wrist straps and footwear must be checked daily. To ensure straps and footwear are being used, personnel should be checked on a regular basis.
Weekly checks of work surfaces for cleanliness and ground-wire connections are required. A monthly check for voltages greater than ±200 V is mandated. Work surfaces, flooring and wiring must be tested quarterly. An annual audit of the entire system is also required.
Test Equipment-EIA 625 recommends a wrist-strap checker or constant monitors, a checker for footwear or grounders, an electrostatic voltmeter, meters to test resistivity, meters for resistance to ground (RTG) and resistance top to top (RTT) of the work surfaces, meters to verify ground connection, and finally, a charge plate monitor to check air ionizers.
While an electrostatic voltmeter is far more accurate and expensive than a field meter, the field meter may provide enough sensitivity in many situations.
Test Methods—EIA 625 makes good use of ESD Association test methods. ESD Standards 3.1, 4.1, 6.1 and 7.1 are referenced.
Section 6
Section 6 details the minimum requirements for an ESD-protected work area. The information is presented in Figure 1.
A static-dissipative surface is preferred. The requirements include:
Personnel must wear wrist straps or ESD footwear/heel grounders. A floor mat also is required. When a worker is seated, a wrist strap is necessary.
No item exhibiting a potential greater than ±200 V can be within 12 inches of an ESDS item.
Static caution signs must be posted.
The list of optional equipment includes smocks, which must cover the worker’s clothes from the waist up and reduce generated voltage to ±200 V; and air ionizers, which can be used to reduce static charges when the charges are not controlled by other methods.
Records of checks must be kept for at least one year. When a value is measured, it must be recorded if it is outside the acceptable limits. Passing values may be recorded or a simple “pass” can be substituted.
The EIA-471 symbol and text must appear on packaging. Work areas are required to display the symbol and the text “ESD HANDLING IS REQUIRED.” Military-oriented product can use the MIL-STD-129M symbol.
Section 7
Section 7 describes how grounding for ESD protection should be accomplished using work-surface mats, wiring, flooring and floor mats. Mobile equipment is briefly addressed. Other noteworthy topics include:
Grounding—Electrical green wire ground is preferred instead of earth ground. If both are used, they should be connected.
Work Surfaces—New and updated dissipative work surfaces are recommended to have surface resistivity and RTG values of ³ 105 to £ 109. This is lower than the £ 1010 recommendation of ESD S4.1.
Storage Areas—Devices placed in storage areas that do not meet the requirements for an ESD-protected area must be stored in ESD-protective packaging.
Carts—Mobile carts will not generate a voltage of ±200 V within 12 inches of a device. ESD floors/mats must be used if drag chains or conductive rollers are used.
Section 8
All ESDS devices must be placed in ESD-protective packaging when the device is not at an ESD-protective work area. ESD-protective packaging is required to provide electrostatic shielding and limit tribocharging.
EIA 541 Appendix E Shielding Test Results suggests that packaging without a very conductive layer, 102-3 W , will not provide adequate shielding. Pink poly bags, some clamshells and dissipative tote boxes will likely have trouble with the shielding requirement.
Trays, tubes and magazines must provide reduced charging and mechanical protection. No charging limit is set. Labels and tape may be applied or removed only if the resulting charge is less than ±200 V or if there are no devices in the container.
Section 9
Section 9 describes compliance to EIA 625 and verification of that compliance, including:
Establishing a site ESD coordinator or team.
Documenting annual audits and saving the documentation for at least two years.
Establishing a procedure at each site to deal with ESDS devices that are mishandled.
Treating rejected devices as ESDS.
Rejecting ESDS devices received without proper packaging and returning these devices in the improper packaging.
Section 10
Each worker must be trained initially and every 12 months in ESD basics and work-area applications. Training must be documented and the records retained for two years.
Section 11
Section 11 provides additional methods to reduce static charge in the work area. These methods must be applied when unprotected devices are within 12 inches of static voltages measured at ±200 V or greater. The methods include the application of antistatic solutions, control of relative humidity to 40%, the use of air ionizers and the donning of protective smocks and gloves or finger cots.
Section 12
The last seven pages of EIA 625 provide a thorough ESD audit checklist. This 120-question audit includes paragraph references back to the document.
The ±200-V Limit
One question regarding the application of this standard is the global limit of ±200 V. While based on MIL-STD-883 Human Body Model (HBM) sensitivity of devices, the noncontact measurement of ±200 V may not relate to a ±200-V discharge from an HBM. Field-induced damage from a ±200-V field does not equate to a ±200-V direct discharge from an HBM.
For argument, let’s assume that a ±200-V discharge can occur from the charged object. The capacitance and resistance of that system’s discharge path is very unlikely to match that of the HBM tester. That amount of charge and energy will be different; therefore, the damage potential will be different.
Setting this issue aside, some limit of charge must be established to define an effective protection program. A value of ±200 V could be as appropriate as any low value.
A ±200-V limit allows a 400-V potential difference to exist. If one object is at -200 V and another is at +200 V, the potential difference is 400 V. Some users may state that objects within 12 inches of an ESDS device cannot have a potential difference greater than 200 V.
Conclusions
If you are developing an internal static program, consider adopting EIA 625 or using it as a guide. This standard also is valuable if you are updating an existing ESD-control program.
This brief review of EIA 625 is no substitute for reading the document. Get your own copy of EIA 625 by contacting Global Engineering, the EIA distributor, at (800) 854-7179.
About the Author
Brent A. Beamer is the Director of Technology at Static Control Components. He is also active in ESD standards development, both with the EIA and the ESD Association, where he chairs the packaging committee and the tribocharge work group. Mr. Beamer received a degree in electronics from Indiana State University. Static Control Components, Inc., 3115 Hal Siler Dr., P.O. Box 152, Sanford, NC 27331, (800) 356-2728.
Copyright 1995 Nelson Publishing Inc.
June 1995