By The ESD Association

Basic Principles of Static Control

We suggest that static control programs become more effective and less complex if we focus on just four basic principles of static control as follows:

In most facilities, our static control efforts center around the last three principles. In this column we will concentrate on the primary materials and procedures that eliminate and reduce generation, dissipate and neutralize charges, or protect sensitive products from ESD.

Identifying the Problem Areas

There are at least nine critical elements to successfully developing and implementing an effective ESD control program. Selecting appropriate static control materials and developing and implementing effective procedures begin with two of these critical elements:

Cleanrooms

Personnel and Moving Equipment

In many facilities, people are one of the prime generators of static electricity. The simple act of walking around or repairing a board can generate several thousand volts on the human body. If not properly controlled, this static charge can easily discharge into a static-sensitive device—a human body model (HBM) discharge.

Even in highly automated assembly and test processes, people still handle static-sensitive devices ... in the warehouse, in repair, in the lab, in transport. For this reason, static control programs place considerable emphasis on controlling personnel-generated electrostatic discharge. Similarly, the movement of carts and other wheeled equipment through the facility can also generate static charges that can transfer to the products being transported on this equipment.

Wrist Straps

Typically, the primary means of controlling static charge on personnel is with a wrist strap. When properly worn and connected to ground, a wrist strap keeps the person wearing it near ground potential. Because the person and other grounded objects in the work area are at or near the same potential, there can be no hazardous discharge between them. In addition, static charges are safely dissipated from the person to ground and do not accumulate.

Wrist straps have two major components, the cuff that goes around the person's wrist and the ground cord that connects the cuff to the common point ground. Most wrist straps have a current-limiting resistor molded into the ground cord head on the end that connects to the cuff. The resistor most commonly used is a one megohm, 1/4 watt with a working voltage rating of 250 V.

Wrist straps should be tested on a regular basis. Daily testing or continuous monitoring is recommended.

Floors, Floor Mats, Floor Finishes

A second method of controlling electrostatic charge on personnel is with the use of ESD-protective floors in conjunction with ESD-control footwear or foot straps. The combination of floor materials and footwear provides a ground path for the dissipation of electrostatic charge, thus reducing the charge accumulation on personnel and other objects to safe levels. In addition to dissipating charge, some floor materials (and floor finishes) also reduce triboelectric charging. The use of floor materials is especially appropriate in those areas where increased personnel mobility is necessary. In addition, floor materials can minimize charge accumulation on chairs, carts, walking stackers, lift trucks, and other objects that move across the floor. However, those items require dissipative or conductive casters or wheels to make electrical contact with the floor.

Shoes, Grounders, Casters

Used in combination with ESD-protective floor materials, static control shoes, grounders, casters, and wheels provide the necessary electrical contact between the person or object and the floor material. Insulative footwear, casters, or wheels prevent static charges from flowing from the body to the floor to ground.

Clothing

Clothing is a consideration in some ESD-protective areas, especially in cleanrooms and very dry environments. Clothing materials can generate electrostatic charges when they contact and separate from other objects and the clothing itself. These charges may discharge into sensitive components or create electrostatic fields that may induce charges on the human body. Although a person may be grounded, that does not mean that insulative clothing fabrics can dissipate a charge to that person's skin and then to ground. Clothing usually is electrically insulated or isolated from the body. Static control garments are intended to minimize the effects of electrostatic fields or charges that may be present on a person's clothing.

Workstations and Worksurfaces

Worksurfaces and workstations are important parts of an ESD-protective program. Many ESDS devices and assemblies are handled, assembled, or repaired at workstations.

An ESD-protective workstation refers to a single individual's work area that is constructed and equipped with materials and equipment to limit damage to ESD-sensitive items. It may be a stand-alone station in a stockroom, warehouse, or assembly area, or in a field location such as a computer bay in commercial aircraft. A workstation may also be located in a controlled area such as a cleanroom.

The workstation provides a means for connecting all worksurfaces, fixtures, handling equipment, and grounding devices to a common point ground. In addition, there may be provision for connecting additional personal grounding devices, equipment, and accessories such as constant ground monitors and ionizers. The key ESD control elements comprising most workstations are a static-dissipative work surface, 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 ohms provide a surface that is at the same electrical potential as other ESD-protective items in the workstation. They also provide an electrical path to ground for the controlled dissipation of any static potentials on materials that contact the surface. The worksurface also helps define a specific work area in which ESD-sensitive devices may be safely handled. The work- surface is connected to the common point ground.

Production Equipment and Production Aids

Although personnel-generated static is typically the primary ESD culprit in many environments, automated manufacturing and test equipment can also pose an ESD problem. For example, a device may become charged from sliding down the feeder. If the device then contacts the insertion head or another conductive surface, a rapid discharge occurs from the device to the metal object—a charged device model (CDM) event.

In addition, various production aids may also pose an ESD problem. Production aids are those materials, tools, and fixtures that help to produce finished products but do not become part of the finished product. Some examples are hand tools, soldering irons, tapes, solvents, and so forth.

Grounding is the primary means of controlling static charge on many production aids and equipment. The metal chassis or conductive enclosure of equipment that uses utility power is required by the National Electrical Code to be connected to the equipment ground (the green wire) in order to carry fault currents. This ground connection will also function for ESD purposes. All electrical tools and equipment used to process ESD-sensitive hardware require the three-prong grounded type ac plug. Hand tools that are not electrically powered, i.e., pliers, wire cutters, and tweezers, are usually grounded through the ESD worksurface and the (grounded) person using the conductive tools. Holding fixtures should be made of conductive or static-dissipative materials when possible. If a conductive fixture is not sitting on an ESD worksurface or handled by a grounded person, a separate ground wire may be required. For those items that are composed of insulative materials, the use of ionization or application of topical antistats may be required to control generation and accumulation of static charges.

Packaging and Materials Handling

Direct protection of ESDS devices from electrostatic discharge is provided by packaging materials such as bags and corrugated, rigid, or semirigid packages. The primary use of these items is to protect the product when it leaves the facility, usually when shipped to a customer. In addition, materials-handling products such as tote boxes and other containers primarily provide protection during inter- or intrafacility transport.

The main ESD function of these packaging and materials-handling products is to limit the possible impact of ESD from triboelectric charge generation, direct discharge, and electrostatic fields. The initial consideration is to have low-charging materials (antistatic) in contact with ESD-sensitive items. For example, the antistatic property would control triboelectric charge resulting from sliding a board or component into the package or container. A second requirement is that the material provide protection from direct electrostatic discharge as well as shield from electrostatic fields.

Many materials are available that provide all three benefits: antistatic, discharge protection, and electric field suppression. The inside of these packaging materials has an antistatic layer, but also has an outer layer with a surface resistance generally in the dissipative range.

A material's antistatic properties are not necessarily predicted by its resistance or resistivity. However, resistance or resistivity measurements help define the material's ability to provide electrostatic shielding or charge dissipation. Electrostatic shielding attenuates electrostatic fields on the surface of a package in order to prevent a difference in electrical potential from existing inside the package. Electrostatic shielding is provided by materials that have a surface resistance equal to or less than 1.0 x10³ ohms when tested according to EOS/ESD-S11.11 or a volume resistivity of equal to or less than 1.0 x 10³ ohm/cm when tested according to the methods of EIA 541.

Dissipative materials provide charge dissipation characteristics. These materials have a surface resistance greater than 1.0 x 10⁴ but less than or equal to 1.0 x 10¹¹ when tested according to EOS/ESD-S11.11 or a volume resistivity greater than 1.0 x 105 ohm/cm but less than or equal to 1.0 x 10¹² ohm/cm when tested according to the methods of EIA 541. Be aware that the very wide range of resistance and resistivity results in a wide range of performance.

Grounding

In our discussion to this point, we have seen how important grounding is to effective ESD control. Consequently, effective ESD grounds are of critical importance in any operation, and ESD grounding should be clearly defined and regularly evaluated.

ESD Association standard ANSI EOS/ESD 6.1—Grounding recommends a two-step procedure for grounding ESD-protective equipment.

The first step is to ground all components of the work area (worksurfaces, people, equipment, etc.) to the same electrical ground point called the common point ground. This common point ground is defined as a "system or method for connecting two or more grounding conductors to the same electrical potential."

This ESD common point ground should be properly identified. ESD Association standard EOS/ESD S8.1-1993 recommends the use of a symbol to identify the common point ground.

The second step is to connect the common point ground to the equipment ground or the third (green) wire electrical ground connection. This is the preferred ground connection because all electrical equipment at the workstation is already connected to this ground. Connecting the ESD control materials or equipment to the equipment ground brings all components of the workstation to the same electrical potential. If a soldering iron used to repair an ESDS item were connected to the electrical ground and the surface containing the ESDS item were connected to an auxiliary ground, a difference in electrical potential could exist between the iron and the ESDS item. This difference in potential could cause damage to the item.

Any auxiliary grounds (water pipe, building frame, ground stake) present and used at the workstation must be bonded to the equipment ground to minimize differences in potential between the two grounds.

Ionization

As we have seen, the primary method of static charge control is direct connection to ground for conductors, static-dissipative materials, and personnel. However, a complete static control program must also deal with isolated conductors that cannot be grounded and insulating materials (e.g., most common plastics). Topical antistats are often used to dissipate static charges from these items under some circumstances.

More frequently, however, air ionization can neutralize the static charge on insulated and isolated objects by charging the molecules of the gases of the surrounding air. Whatever static charge is present on objects in the work environment will be neutralized by attracting opposite polarity charges from the air. Because it uses only the air that is already present in the work environment, air ionization may be employed even in cleanrooms where chemical sprays and some static-dissipative materials are not usable.

Air ionization is not a replacement for grounding methods. It is one component of a complete static control program. Ionizers are used when it is not possible to properly ground everything and as backup to other static control methods. In cleanrooms, air ionization may be one of the few methods of static control available.

Cleanroom Requirements

While the basic methods of static control discussed here are applicable in most environments, there are characteristics of the semiconductor manufacturing process that require special considerations.

Many objects integral to the semiconductor manufacturing process (quartz, glass, plastic, and ceramic) are inherently charge generating. Because these materials are insulators, this charge cannot be removed easily by grounding. Many static control materials contain carbon particles or surfactant additives that sometimes restrict their use in cleanrooms. The need for personnel mobility and the use of cleanroom garments often make the use of wrist straps difficult. In these circumstances, ionization and flooring and footwear systems become key weapons against static charge.

Identification

A final element in our static control program is the use of appropriate symbols to identify static-sensitive devices and assemblies, as well as products intended to control ESD. The traditional symbols used to identify ESDS parts or ESD control materials have been replaced with newer, more appropriate symbols. ESD Association standard ANSI ESD S8.1-1993—ESD Awareness Symbols provides two symbols for ESD identification.

The ESD Susceptibility Symbol consists of a triangle, a reaching hand, and a slash through the reaching hand. The triangle means "caution" and the slash through the reaching hand means "Don't touch." Because of its broad usage, the hand in the triangle has become associated with ESD, and the symbol literally translates to "ESD-sensitive stuff, don't touch."

The ESD Susceptibility Symbol is applied directly to integrated circuits, boards, and assemblies that are static-sensitive. It indicates that handling or use of this item may result in damage from ESD if proper precautions are not taken. If desired, the sensitivity level of the item may be added to the label.

The ESD Protective Symbol consists of the reaching hand in the triangle. An arc around the triangle replaces the slash. This "umbrella" means protection. The symbol indicates ESD-protective material. It is applied to mats, chairs, wrist straps, garments, packaging, and other items that provide ESD protection. It may also be used on equipment such as hand tools, conveyor belts, or automated handlers that are especially designed or modified to provide ESD control.

Neither symbol is applied on ESD test equipment, footwear checkers, wrist strap testers, resistance or resistivity meters, or similar items that are used for ESD purposes, but which do not provide actual protection.

Summary

For Additional Information

ESD-S1.1—Evaluation, Acceptance, and Functional Testing of Wrist Straps

ANSI EOS/ESD S3.1—Ionization

ESD STM 4.1 (Revised)—ESD Protective Work Surfaces—Resistive Characterization

ANSI EOS/ESD S6.1—Grounding—Recommended Practice

ANSI ESD S7.1—Floor Materials—Resistive Characterization of Materials

ANSI ESD S8.1—ESD Awareness Symbols

ESD S9.1—Resistive Characterization of Footwear

ANSI ESD S11.1—Surface Resistance Measurement of Static Dissipative Planar Materials

ANSI ESD S11.31—Evaluating the Performance of Electrostatic Discharge Shielding Bags

ESD STM2.1—Garments

ESD SP 3.3—Periodic Verification of Air Ionizers

ESD ADV53.1—ESD Protective Workstations

ESD ADV2.0—ESD Handbook

EIA-541—Packaging of Electronic Products for Shipment

Prepared by

Quest Technology (HK) Ltd.

C/o SIMCO (HK) Ltd.