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The Basics of Electrostatic Discharge (ESD)

Introduction

We’ve all experienced the effects of static electricity— clothes that stick, hair that won’t stay down, shocks that surprise you, plastic wrap that won’t leave your hand and of course, lightening. While most people consider these things minor inconveniences, static electricity is a serious industrial problem estimated to cause more than $5 billion in damages annually. 

Electrostatic discharge (ESD) may seem to be a new problem because of the electronics and technology age we live in, but has actually been causing concerns for centuries. Old military forts had to use grounding devices to control static build-up in black powder storage rooms, and paper mills of the 1860’s used grounding devices, flame ionization and steam drums to prevent explosions.

When you feel an electrical shock, you are feeling a minimum of 3000 volts. But surprisingly, charges of even less than 1000 volts can damage electronic equipment. That means many damaging shocks cannot be felt. The following shows how easy it is to build a potentially damaging charge:

Controlling what may seem to be “harmless” activities has become a high priority in the electronics industry.

ESD Terms

Antistatic—A property (of a material) that resists charge generation. 

Dissipative—A property (of a material) that controls the rate of electrostatic discharge. 

Electrostatic Discharge—Transfer of charge between bodies at different electrical potentials. 

ESD Event—Any time a charged conductor discharges to an electrostatic, discharge-sensitive device. 

Static Electricity—Electrical charge caused by an imbalance of electrons on the surface of a material. (Separating two materials can also cause a build up of static electricity.)

ESD Effects

A new problem regarding static electricity is in clean-room applications. Charged surfaces attract oppositely-charged particles—similar to the way a magnet attracts a piece of metal. (This helps explain why clothing sometimes sticks to your body.) Removing charged particles in these rooms can be difficult. Normal ventilation and air filtration taking place in cleanrooms definitely helps keep charged particles out of the room, but charged particles attached to a person’s clothing entering the cleanroom for example, presents another problem in itself. That is why additional static-control devices should also be used when working with electrically-sensitive products.

Electrostatic discharge can change the properties of a semiconductor device, degrade the device or destroy it entirely. If the product is destroyed it is considered a catastrophic failure—causing a metal melt or a junction breakdown. If the product is degraded it is considered a latent defect—allowing a product to work for a while, but then eventually degrade over time. This ESD event is extremely difficult to detect before the product reaches the consumer.

Hazardous static electricity build-up can also occur when working with flammable liquids and gases. If enough static electricity is created, it could cause a spark and create an explosion. Standard static-control procedures should NOT go ignored.

Principles of Static Electricity Control

When working with electrical or electronic devices, procedures should be designed to remove as many variables that might increase the build-up of static electricity. For example, using a metal device in a procedure rather than a plastic device. (Common plastic generally creates the greatest static charge.) Another precautionary measure would be to remove carpeting from electrostatically- sensitive areas.

Wherever possible, processes should be changed in order to reduce static-generating procedures. Processes and materials should also be kept at the same electrostatic potential. In a situation where charge is likely to build up, proper dissipation and neutralization products should be used. This includes grounds, proper ionization and dissipative static-control measures.

Static Control Products

The primary way to control human static charge is to use a wrist strap. The strap is attached to a ground and makes the person and the piece of equipment operate at ground potential, thereby eliminating any transfer of charge from person to equipment. Wrist straps should be tested on a regular basis using a ground/circuit tester—a device that measures the continuity of grounds.

Another method to control build-up of human charge is to protect the area or type of floor that is used. This can be done by installing floor mats, floor finishes or electrostatically dissipative floors—a “must have” when mobility is important.

Anything that touches the floor has ESD properties including shoes and casters. Shoes can be grounded by using grounding straps. Insulative equipment—such as padded shoes or natural rubber casters—also prevent static charges from flowing from the body to the floor and to the ground.

ESD Workstations are made of materials and outfitted with equipment that is essential to limiting static charge. These stations generally include a way to connect all surfaces with a common ground. Table mats can be placed on a work area to help dissipate static charge. However, these mats must be properly grounded to be effective.

Various clothing material can also generate a large degree of static electricity. ESD-protective clothing and smocks keep common, static-generating clothes from coming into contact with sensitive equipment. Grounds alone cannot protect from this danger as clothing has a different electrostatic charge than the wearer. This is obvious in the winter when it is almost impossible to reduce the static charge from pants, skirts and sweaters.

Air ionizers are available for use when it is impossible to remove every static-generating object in an area. An ionizer produces an ionized air stream that neutralizes static charges over a short period of time.

All electrostatic protected areas and grounds must have proper labels—ANSI ESD S801-1993. These ESD Awareness Symbols provide standards on symbols used for identification.

Serious electrostatic damage can also occur during product shipment. The product—which has already passed inspection at the factory—may possibly become defective during transit and therefore, will not be caught until it gets to the customer. There are, however, antistatic and dissipative bags available to counteract this problem. Electrostatic shielded totes and containers are also made to protect products in transit.

An electrostatic field meter measures the presence of an electrostatic charge. This device helps identify problem areas in an ESD-control program. The meter measures the electrostatic field associated with a charged object. Sensitivity of these meters can vary. Some meters give just a general indication of a charge while others display a precise measurement of a charge.

Resistance also helps in evaluating ESD-control materials. A resistance meter will provide the data necessary in evaluating the resistance of flooring materials, work surfaces, equipment, furniture, garments and packaging materials.

ESD Standards

ESD standards are not mandatory in the United States. However, they are often written into agreements between buyers and sellers. ESD standards help assure consistency and quality of ESD-sensitive products. Standards reduce confusion in the market-place and allow for objective evaluations and comparisons between ESD products. There are three groups of standards that affect ESD products: The first group provides ESD program guidance or requirements. The second group covers requirements for specific product procedure—such as packaging or grounding. The third group covers the standardized test methods used to evaluate products and materials.

Some groups that develop ESD standards are The U.S. Military, The Electronic Industries Alliance (EIA) and EIA’s Joint Electronics Development Engineering Council (JEDEC) and The ESD Association, which is an ANSI-accredited standards development organization. 

The ESD Association is responsible for developing ESD standards and test methods. They have written eight complete standards, six standard test methods, six draft standards and five advisories.

International standards are also being incorporated into the ESD regulations. The European International Electrotechnical Commission has issued a European standard—EN100015-Protection of Electrostatic Sensitive Devices.

Commonly Asked Questions

Sources for More Information

www.borg.com 

www.desco.com 

www.esdsystems.com 

ESD-S1-1-1998: Evaluation, Acceptance, and Functional Testing of Wrist Straps. 

ANSI EOS/ESD S3.1-1991: Ionization 

ANSI EOS/ESD S6.1-1991: Grounding— Recommended Practice 

ANSI ESD S8.1-1993: ESD Awareness Symbols. 

ESD S9.1-1995: Resistive Characterization of Footwear. 

ESD ADV2.0-1994: EDS Handbook ESD Association, 7900 Turin Rd., Bldg. 3 Suite 2, Rome, NY 13440. Phone: (315) 339-6937.

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