ESD Controls     

The ESD Association suggests focusing on just six basic principles for the development and implementation of an effective ESD control program, namely, 1) ESD Immunity Design-in; 2) definition of the desired level of ESD control; 3) identification of electrostatic protected areas (EPA's); 4) reduction, if not elimination, of static generation; 5) static dissipation and neutralization; and 6) protection of products from ESD.

ESD Immunity Design-in

Prevention of ESD-related problems starts with the ability to produce robust devices that can withstand ESD events. This requires the proper determination of the ESD sensitivity levels of new semiconductor devices prior to their production release.  This is best achieved by subjecting representative samples of these devices to industry-standard ESD sensitivity tests using an ESD sensitivity testing system (see example in Fig. 1). 

There are several ESD sensitivity testing procedures available today, each one depending on the ESD model being tested for. It is good practice to test a device in terms of different ESD models. Devices that exhibit inadequate immunity to ESD must be redesigned, if possible. This is known as ESD immunity design-in.

Identification of EPA's

Aside from designing ESD immunity into its products, a company must have a sound ESD control program.  To begin with, a company must identify all electrostatic protected areas (EPA's) in its factory. An EPA is an area where ESD-sensitive devices will be handled.  Every EPA must be adequately protected by ESD controls, the major ones of which are discussed below.

Static Reduction/Dissipation/Neutralization

Over-all Grounding

The backbone of static generation reduction and static dissipation is proper grounding of everything a device touches. The primary means of grounding ESD susceptible (ESDS) items (personnel, equipment, workstations, carts, shelves, etc.) is to provide electrically conductive paths between these items and a common ground.  Thus, every factory must have a common grounding point. Detailed information on ESD grounding can be found in ESD Association standard ESD-S6.1, Grounding-Recommended Practices.

Connecting everything to a common ground point essentially puts everything at the same potential (the potential of the common grounding system). As long as everything is in equipotential balance, charging/discharging events will be prevented. It is important to note though that insulators in an Electrostatic Protected Area (EPA) cannot be grounded, so insulative materials must be avoided in EPA's as much as possible.

ESD Association Standard ANSI EOS/ESD 6.1-Grounding recommends a two-step procedure for grounding equipment. The first step is to ground all components of the work area (worksurfaces, people, equipment, etc.) to the common ground, which is also referred to as the 'ESD common point ground'. This ESD common point ground should be properly identified. ESD Association standard EOS/ESD S8.1-1993 recommends its own symbol to identify the common point ground.

The second step is to connect the common point ground to the equipment ground or the third wire (green) 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.  Thus, 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.

Grounding of Personnel

People are one of the primary generators of static electricity. The simple act of walking around or repairing a board can generate several thousands of volts on the human body. If not properly controlled or dissipated, the accumulated static charge on a person can easily discharge onto a static sensitive device. Such an event is known as human body model (HBM) discharge.

Wrist straps are generally the primary means of controlling static charge build-up on personnel. 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 mega-ohm, 1/4 watt resistor with a working voltage rating of 250 volts. This resistor would protect the person wearing it from electric shock in case the point to which the wrist strap is grounded accidentally gets 'live.'

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

A second method of controlling electrostatic charge on personnel is with the use of conductive floors in conjunction with conductive shoes or foot straps. Any charge build-up on a person wearing conductive shoes will be dissipated to the conductive floors through the sweat layer between each foot and shoe.  The conductive floors must be properly grounded to the common ground system.

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.  When used as the primary personnel grounding system, the resistance to ground including the person, footwear and floor must be the same as specified for wrist straps (< 35 x 10E6 ohms) or the voltage accumulation on a person must be less than 100 volts.

The use of ESD-protective clothing is another way to control charge build-up on a person.

Grounding of Moving Equipment

Moving equipment such as carts, chairs, and lifters can likewise easily generate static charges. Thus, these moving equipment also need to be grounded to the common ground. One way of doing this is by providing these equipment with conductive wheels, casters, or drag chains that creates an electrical path between the equipment and the conductive flooring. Thus, moving equipment must only be used over conductive floors, since this is the only way they can be grounded with a drag chain or conductive wheels or casters.

Grounding Work Stations and Work Surfaces

A work station must be equipped with materials and equipment to limit ESD events. The key ESD control elements used in most work stations are:  1) properly grounded static dissipative surfaces; 2) a means of grounding personnel (usually a wrist strap); 3) appropriate signage and labeling; and 4) air ionizers if there are insulative materials in the work station. As usual, the static dissipative surfaces and personnel grounding ports must be properly grounded to the common grounding system.

Static protective work surfaces must have a resistance to ground of 10⁶ to 10⁹ ohms to 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 build-up on materials that contact the surface.

Grounding Production Equipment

Although personnel-generated static is usually the primary source of ESD in many environments, automated manufacturing and test equipment can also pose an ESD problem. For example, a device may become charged from sliding down an input track. If the device then contacts a grounded conductive surface, a rapid discharge occurs from the device to the metal object. Such a discharge is known as a Charged Device Model (CDM) ESD event.

Proper grounding is the primary means of controlling static charge on production equipment. Electrical equipment are 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 can also serve as a means to dissipate static charges on the equipment.

All electrical tools and equipment used to process ESD sensitive hardware require the 3 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-protective work surface and the (grounded) person using the conductive tools.

Holding fixtures should be made of conductive or static dissipative materials as much as possible. A separate ground wire may be required for conductive fixtures not sitting on an ESD-protective work surface or handled by a grounded person. Again, this wire must be connected to the common ground system.

For those items that are composed of insulative materials, the use of ionization or application of topical antistats may be required to control the generation and accumulation of static charges.

Ionization

It is not possible to eliminate all insulative materials or isolated conductors that cannot be grounded from the production area. Thus, there must be a way to control their ESD generation tendencies because these items can not discharge to the common ground. Air ionizers serve this purpose.

Air ionizers neutralize the static charge on insulated and isolated objects by charging the molecules 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 clean rooms where chemical sprays and some static dissipative materials are not useable.

Air ionization is just one component of a complete static control program. It must not be a  substitute for grounding or other methods. Ionizers are used when it is not possible to properly ground everything, or simply as a backup to other static control methods. In clean rooms, air ionization may be one of the few methods of static control available.

The ionization characteristics of air ionizers must be checked regularly, since an improperly working air ionizer can emit a unbalanced stream of  positive and negative ions. If that happens, the air ionizer itself would accelerate a static charge build-up, and be the root cause of a serious ESD problem on the line.

Fig. 7.  Examples of  bench-top ionizers