Electrical Power

Lesson 2:

Electrostatic Discharge

This lesson discusses a phenomenon that can damage or ruin sensitive electronic equipment—electrostatic discharge (ESD), sometimes referred to as static electricity.

Fortunately, it is one of the easiest things to protect against.


Objectives:

After this lesson, you will be able to

Define ESD
Avoid ESD


Causes of ESD


The human body has an electric field and, under the right (and very easy to obtain) conditions, can generate a tremendous amount of voltage, often referred to as static electricity.

ESD occurs when an imbalance in the amounts of positive and negative electrical charges on the surface of an object is released.

The most dramatic example of ESD is lightning, which splits trees as easily as it lights up the sky.

The amount of energy released when you touch a metal object can be quite large.

The buildup of energy with nylon clothes can easily reach 21,000 volts.

About 750 volts are required to produce a visible spark with ESD, and a mere 10 volts or so can ruin a computer chip.

The actual amount of energy in a given ESD event depends on the types of materials involved (wool fabrics generate less than nylon), the humidity (low humidity offers less resistance to the discharge), the amount of physical energy (friction) involved, and how quickly the energy is released.

NOTE

ESD does not have to be seen (as a spark) to do damage to electronic components. Voltages lower than 10 volts can damage some parts.


ESD Damage


Over the years, engineers have produced smaller and smaller components, which operate at lower and lower voltages.

Their goal is to reduce size, cost, and operational heat production.

Those are worthy goals but, because of reduced component size, they present a smaller target with less resistance to power surges.

This makes the parts more susceptible to damage from ESD.

The amount of damage and resulting problems caused by ESD can be divided into three categories:

Catastrophic failure.

This is sometimes referred to as "frying" or "smoking" a part because of the heat (and sometimes the noise and smoke) generated during the failure.

Mishandling and misapplication of a power source, cable, or test instrument are the most likely causes.

Care in opening, installing, cabling, and testing are the best ways to prevent this type of ESD damage.


Upset failure.

An ESD can produce an erratic fault in a component.

This kind of problem is very difficult to detect and repair because the failure is intermittent.

It is easy to blame the operating system or a program operation for the data loss or system crash.

The best way to diagnose and correct this type of problem is to remove and replace suspected parts until the failure stops appearing.


Latent failure.

This type of failure weakens the actual transistor.

The affected part will seem normal in most operations and will frequently pass quality control and conformance tests.

Like upset failures, these can be very difficult to isolate.


Preventing ESD


Prevention is the best defense against ESD, and the first step in prevention is to understand the source.

The leading cause of ESD damage is improper handling of electronic devices.

A semiconductor device can be damaged by ESD during handling before it is installed.

The key to ESD prevention is to keep all electronic components—and yourself—at a common electrical potential.

This usually means ground potential, or zero volts.

Maintain a habit of "grounding" yourself to the computer chassis whenever you attempt a repair.

There are times when it is not practical, or convenient, to wear a ground strap.

At such times, touching part of the metal chassis before removing devices will bring you and the computer chassis to a common voltage.

Don't move around while installing or handling a part; doing so can generate additional voltages, negating any effort you have made to eliminate ESD.

All repair shops and workbenches should have proper ESD suppression devices, and technicians should use them whenever working with exposed parts.

These devices include:

Antistatic mats.

Nonconducting pads placed on the work surface and on the floor in front of the work area.


Antistatic wristband.

A wristband with a grounding strap connected to the chassis of the PC.


Antistatic pouches.

A sealed, antistatic pouch used to store any sensitive electronic device, including hard disk drives, when they are not installed in a computer.


Antistatic pad.

An insulating foam pad in which individual chips with exposed pins should be embedded when they are not installed in a computer.


CAUTION


AC voltage can kill.

Although the power used by the computer components is no more than 12 volts DC, many computers have 110 VAC wired from the power supply to the on–off switch at the front of the computer case.

This wiring can present a hazard.

Never disconnect or remove boards from a computer with the power applied.

This can damage the components.


CAUTION


Safety precautions are different for computer monitors.

Never work on a monitor with the cabinet removed, power applied, and a wrist strap on; a wrist strap coming in contact with the high-voltage wire (30,000 volts) can cause electrocution.


Lesson Summary


The following points summarize the main elements of this lesson:

ESD damages computer components.
ESD can occur without detection.
ESD can be prevented.