Electrostatic Discharge, or ESD, is one of the leading causes of damage to digital equipment. Electrostatic discharges are not harmful to humans. Most people have experienced an electrostatic discharge after walking across a carpet and touching a doorknob. The small spark that jumps between your finger and the knob is an electrostatic discharge.

Causes of ESD

ESD can be generated from several sources. Movement between two materials can generate a difference in potential (voltage) between the two objects. An example of this is the aforementioned carpet/doorknob discharge, where the human body can generate approximately 1000 volts. Low humidity also contributes to electrostatic potential buildup. Relative humidity above 50% will generally keep most electrostatic potentials from developing. Relative humidity below 50% will create an environment that will allow objects to accumulate electrostatic potentials.

Some objects are natural sources of electrostatic energy. Styrofoam cups actually have approximately 8,500 volts of electrostatic potential. The foam cushions in a lot of chairs can have 5,000 volts of electrostatic energy.

What Can ESD Do To My Computer?

When an electrostatic discharge is applied to the integrated chips, or ICs, in your computer, the charge damages the internal circuitry of that IC. The circuits inside ICs are microscopic. When a 10,000 volt source is separated from a ground by a layer of semiconductor material thousands of times thinner than a human hair, the semiconductor layer will break down. This breakdown is irrepairable. Devices constructed from metal oxide semiconductor (MOS) material, such as the CMOS, the CPU and memory modules in your computer, are especially vulnerable to ESD damage.

Many computer manufacturers are integrating many devices into their motherboard designs (such as ATX, which incorporates serial, parallel, USB, and sometimes video and sound modules). If one of these items is damaged by ESD, you normally will replace the motherboard.

How Can I Prevent ESD Damage To My Computer?

The most important thing you can do is ground yourself. Standing on a carpet will not suffice. Before opening your computer, touch the metal cabinet of the computer while it is still plugged into a grounded outlet. This will equalize the difference in potentials between you and your computer. Unplug your computer from the outlet, then keep one hand on the chassis as you work on the computer. You should invest in an ESD strap as well, which allows you to use both hands while working on computers. These cost about $3 to $6 (US) each. NEVER USE A PIECE OF WIRE TO GROUND YOURSELF! The ESD strap only conducts electrostatic electricity. If you accidently brush against a power supply or an exposed terminal with the cord of the ESD strap, you will not be electrocuted. I know of one individual in Thailand who had his foot cooked off because he grounded himself to his workbench with speaker wire.

Another way to avoid ESD damage is to avoid touching the pins of connectors and wires. Also, avoid hot plugging devices like keyboards and your mouse. Hot plugging is connecting devices with the power on. USB devices are designed to allow hot plugging.

ESD is a problem for IC's made using MOS/CMOS technologies, especially since the continual refinement of semiconductor manufacturing technology permits smaller and smaller transistors on an IC; smaller devices are more susceptible to ESD, even at a level which is 'invisible', or imperceptible, to humans.
Here's why ESD is so bad:

Sufficient build up of electrical charge on an MOS transistor's gate will cause the transistor's gate voltage to increase to the point where the gate oxide breaks down and provides a conductive path.

This breakdown of the insulating oxide layer in the transistor can cause catastrophic damage such that the IC stops working. Alternatively, the damage to the gate oxide may not have been catastrophic, but may have been sufficient to leave a leakage path in the material. Over time, the damaged gate oxide layer will deteriorate further until it eventually fails completely. So, even though a chip has sustained ESD damage, it may continue to function with no apparent problems for some time before failure: a reliability problem.

Measurement Standards

The electronics industry has several standards to measure an IC's resistance to ESD damage. Each recreates a common ESD damage mechanism:

  • HBM: Human Body Model - The most commonly used standard. Simulates ESD from a human by discharging a capacitor through a 1500 Ohm resistor.
  • MM: Machine Model - Simulates ESD from a metallic surface. Zero resistance discharge path.
  • CDM: Charged Device Model - Simulates discharging of a charged device through one of its leads.

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