Controlling Static Charge on Personnel and Moving Equipment

People can be one of the prime generators of static electricity. The simple act of walking around or the motions required in repairing a circuit board can generate several thousand volts of electrostatic charge on the human body. If not properly controlled, this static charge can easily discharge into an ESD sensitive device­ – a typical Human Body Model discharge. Also, a person can transfer charge to a circuit board or other item making it vulnerable to Charged Device Model events in a subsequent process.

Even in highly automated assembly and test processes, people still handle ESDS… in the warehouse, in repair, in the lab, in transport. For this reason, ESD control programs place considerable emphasis on controlling personnel generated electrostatic discharge. Similarly, the movement of mobile equipment (such as carts or trolleys) and other wheeled equipment through the facility also can generate substantial static charges that can transfer to the products being transported on this equipment.

Wrist Straps

Typically, wrist straps are the primary means of grounding 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 removed from the person to ground and do not accumulate. When personnel are seated on a chair which is not EPA appropriate, they are to be grounded using a wrist strap.

Wrist straps have two major components, the wristband that goes around the person’s wrist and the ground cord that connects the wristband to the common point ground. Most wrist straps have a current limiting resistor molded into the ground cord on the end that connects to the wristband. This resistor is most commonly one megohm, rated at least 1/4 watt with a working voltage rating of 250 volts.

Wrist straps have several failure mechanisms and therefore should be tested on a regular basis. Either daily testing at specific test stations or using a continuous monitor at the workbench is recommended.

Flooring, Floor Mats, Floor Finishes

A second method of grounding personnel is a Flooring/Footwear System using ESD flooring in conjunction with ESD control footwear or foot grounders. This combination of conductive or dissipative floor materials and footwear provides a safe ground path for the dissipation of electrostatic charge, thus reducing the charge accumulation on personnel. In addition to dissipating charge, some floor materials (and floor finishes) also reduce triboelectric charging. The use of a Flooring/Footwear System is especially appropriate in those areas where increased personnel mobility is necessary. In addition, floor materials can minimize charge accumulation on chairs, mobile equipment (such as carts and trolleys), 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, and components to be electrically connected. When used as the personnel grounding system, the resistance to ground including the person, footwear and floor must be the same as specified for wrist straps (<35 megohms) and the accumulation body voltage in a standard walking voltage test (ANSI/ESD STM97.2) must be less than 100 volts.

Shoes, Foot Grounders, Casters

Used in combination with ESD flooring, static control shoes, foot grounders, casters and wheels provide the necessary electrical contact between the person or object and the flooring. Insulative footwear, casters, or wheels prevent static charges from flowing from the body or mobile equipment to the floor to ground and, therefore, have to be avoided.

Clothing

Clothing is a consideration in some ESD protective areas, especially in cleanrooms and very dry environments. Clothing materials, particularly those made of synthetic fabrics, can generate electrostatic charges that may discharge into ESDS or they may create electrostatic fields that may induce charges. Because clothing usually is electrically insulated or isolated from the body, charges on clothing fabrics are not necessarily dissipated to the skin and then to ground. Static control garments may suppress or otherwise affect an electric field from clothing worn underneath the garment. Per ANSI/ESD S20.20 and the Garment standard ANSI/ESD STM2.1, there are three categories of ESD garment:

• ESD Category 1 garment; a static control garment without being attached to ground. However, without grounding, a charge may accumulate on conductive or dissipative elements of a garment, if present, resulting in a charged source.
• ESD Category 2 garment; a groundable static control garment, when connected to ground, provides a higher level of suppression of the affects of an electric field from clothing worn underneath the garment.
• ESD Category 3 garment; a groundable static control garment system also bonds the skin of the person to an identified ground path. The total system resistance including the person, garment and grounding cord shall be less than 35 megohms.

Workstations and Worksurfaces

An ESD protective workstation refers to the work area of a single individual 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 also may be located in a controlled area such as a cleanroom. The key ESD control elements comprising most workstations are a static dissipative worksurface, a means of grounding personnel (usually a wrist strap), a common point ground, and appropriate signage and labeling. A typical workstation is shown in Figure 2.

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 personnel grounding devices, equipment, and accessories such as constant or continuous monitors and ionizers.

Static protective worksurfaces with a resistance to ground of 1 megohm to 1 gigohm provide a surface that is at the same electrical potential as other ESD control items at the workstation. They also provide an electrical path to ground for the controlled dissipation of any static charges on materials that contact the surface. The worksurface also helps define a specific work area in which ESDS are to be handled. The worksurface is connected to the common point ground.

Continuous or Constant Monitors

Continuous (or constant) monitors are designed to provide ongoing testing of the wrist strap system. While a number of technologies are utilized, the goal remains consistent: electrical connections are tested between the ground point, ground cord, wristband and person’s body while the wearer handles ESDS. Continuous monitors may also provide a monitoring circuit for the ESD worksurface or other equipment connection to the ground reference.

Typical test programs recommend that wrist straps that are used daily should be tested daily. However, if the products that are being produced are of such value that knowledge of a continuous, reliable ground is needed, and then continuous monitoring should be considered or even required. Daily wrist strap testing may be omitted if continuous monitoring is used.

Production Equipment and Production Aids

Although personnel can be the prime generator of electrostatic charge, automated manufacturing and test equipment also can pose an ESD problem. For example, an ESDS device may become charged from sliding down a component part 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. If charging of the ESDS cannot be avoided – which is quite often the case in modern assembly lines due to the insulative IC packages – charge storage should be reduced by the use of ionizers. In addition, various production aids such as hand tools, tapes, or solvents can also be ESD concerns.

Grounding is the primary means of controlling static charge on equipment and many production aids. Much electrical equipment 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 also will function for ESD control purposes. 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 worksurface and the grounded person using the conductive/dissipative tools. Holding fixtures should be made of conductive or static dissipative materials when possible. Static dissipative materials are often suggested when very sensitive devices are being handled. A separate ground wire may be required for conductive or dissipative fixtures not in contact with an ESD worksurface or handled by a grounded person. For those items that are composed of insulative materials, the use of ionization or application of topical antistats may be required to control electrostatic charge generation and accumulation of static charges.

Gloves and Finger Cots

Certainly, grounded personnel handling ESDS should not be wearing gloves or finger cots made from insulative material. If gloves or finger cots are used, the material should be dissipative or conductive. Compliance Verification ESD TR53 provides test procedures for measuring the electrical resistance of gloves or finger cots together with personnel in a system.