Electrostatic discharge (ESD) is the release of static electricity when two objects come into contact. Familiar examples of ESD include the shock we receive when we walk across a carpet and touch a metal doorknob and the static electricity we feel after drying clothes in a clothes dryer. A more extreme example of ESD is a lightening bolt. While most ESD events are harmless, it can be an expensive problem in many industrial environments.
ESD first requires a build-up of an electrostatic charge. This occurs when two different materials rub together. One of the materials becomes positively charged; the other becomes negatively charged. The positively-charged material now has an electrostatic charge. When that charge comes into contact with the right material, it is transferred and we have an ESD event. The heat from the ESD event is extremely hot, although we do not feel it when we are shocked. However, when the charge is released onto an electronic device such as an expansion card , the intense heat from the charge can melt or vaporize the tiny parts in the card causing the device to fail. Sometimes an ESD event can damage a device, but it continues to function. This is a called a latent defect, which is hard to detect and significantly shortens the life of the device
Many electronic devices are susceptible to low voltage ESD events. For example, hard drive components are sensitive to only 10 volts. For this reason, manufacturers of electronic devices incorporate measures to prevent ESD events throughout the manufacturing, testing, shipping, and handling processes. For example, an employee may wear a wrist strap when working with devices or may wear ESD control footwear and work on an ESD floor mat that causes the electrostatic charge to go into the ground instead of into the device. Sensitive devices can be packaged with materials that shield the product from a charge.
Labels to identify electrostatic discharge sensitive (ESDS) devices:
The following labels are commonly used on containers and packaging to alert
anyone who handles the ESDS devices on the need to use static-safe
procedures before handling the devices. The one on the left is preferred.
Protection for Electrostatic Discharge Sensitive (ESDS) devices Work area:
- It is essential to handle ESDS devices at static-safe workstations. This will prevent yield loss (through catastrophic damage) or wrose,, potential reliability failures in the field (through latent damage).
- Where it is impractical or impossible to use antistatic wrist-straps or remove items that are composed of
- Insulative materials at a static- safe work station use an air ionizer designed to neutralize electro static charge or apply topical antistats to control generation and accumulation of static charges.
- When an air ionizer is utilized, it is vital that maintenance procedures and schedules are adhered to in order to ensure that ions generated by the ionizer are sufficiently balanced.
- Avoid bringing sources of static electricity within 1 meter of a static-safe work bench Where it is necessary to use air-guns, use special models that do not generate static charges in the air stream
Any accumulated charge on the body of the human operator should be discharged first before opening the protective container with ESDS devices inside The discharge can be accomplished by putting a hand on a grounded surface or, ideally, by wearing a grounded antistatic wrist-strap
- The use of an antistatic smock for each worker is highly recommended.
- Education and training on ESD preventive measures is invaluable.
- A regular audit is also helpful in supporting an ESD program.Packaging and Transportation:
- ESDS devices should be contained in a static protective bag or container at all times during storage or transportation.
Reliable storage system of goods requires Electrostatic Discharge (ESD) protection. System engineers must choose all ESD components that are correctly suited to their requirements. Picking the right ESD protection elements is not simple. Protecting devices on the PCB against ESD stress has become an increasingly complex task. It is therefore important for system engineers to understand the datasheet parameters before choosing the right ESD protection. An ESD device that is not chosen properly will not only be ineffective, but can interfere with the normal operation of the circuit. For transient voltage suppression in portable systems, the circuit protection device must provide the following characteristics:
- Quick response time
- Ability to handle high peak ESD currents
- Ability to operate at low voltage
- Capacity to handle large number of ESD instances
- Minimal reverse leakage current and minimal size
Therefore, understanding the datasheet parameters of ESD protection elements is paramount to the task of selecting the right protection element for a successful design. Selection of a suitable component will depend on the number of lines to be protected, the available board space, and the electrical characteristics of the circuit to be protected.
Today, there are many ESD devices available in the market. They are available in a variety of surface mount packages as well as configurations.
Considerations for choosing the proper ESD protection:
>PC Board Layout
This is an important part of transient immunity design. This is especially critical in systems where the threat of ESD exists. Parasitic inductance in the protection path can result in significant voltage overshoot and may exceed the damage threshold of the protected IC. An ESD induced transient reaches a peak in less than 1ns. All inductive paths must be considered including the ground return path, the path between the ESD and the protected line, and the path from the connector to the device. It is also important to avoid running critical signal lines near board edges or next to protected lines.
Absolute Maximum Ratings and Electrical Characteristics
The absolute maximum ratings table of the device must be reviewed carefully. ESD protectors are typically not designed to handle DC overstress and an excess voltage above the absolute maximum ratings will most likely trigger the ESD device. While short time pulses (0 – 300 ns) will not damage the protector; DC like stress will result in damage if no current limitation is provided (typically 500 mA).
- Breakdown Voltage
- Standoff Voltage
- Dynamic Resistance
- ESD Characteristics Plots
- Transmission Line Pulsar Plots
- Peak Pulse Waveform
- Eye Diagram