ESD:

ESD: The Basics

e-gizmo.com website is javascript driven. Web page may not show correctly if scripting is blocked.

Learning the Basics

Static electricity is electric charge at rest. As long as it stays resting, it will not cause harm to anything or anyone. The static charge builds up in nature rather slowly. The trouble starts when the charge becomes too much, it will seek some way to unload the charge. When a discharge happens, it will be quick, violent, and destructive. This event is what we call as the Electrostatic Discharge, or ESD. A good example of ESD event occurring in majestic scale is lightning.

Static Electricity is generated and transferred to our body in various ways. Even mundane task like walking across a polished floor may generate kilovolts of static charge in our body, enough to zap an IC to its untimely death when a sudden discharge occur as you touch it. This event may be too weak for us to even feel and notice it, but is too much and devastating for an IC. To better understand the threat, it may help if we take a look on some basics of static electricity generation.

There are several ways static electricity is generated, but triboelectric charging provides the most efficient means, and expectedly is the most common and troublesome.

Triboelectric charging occurs when two materials are rubbed or separated from each other. Note that this condition essentially happens every time something move! We and the object we come in contact with collect static charge whenever we walk (your shoe separating from the floor), get off our seat, comb our hair, grab a bottled drink, to name a few. Table 1 illustrates the amount of static we can produce while doing some common tasks. The amount of charge generated depends on the kind materials, environmental conditions, and how fast the separation occurs. Generally speaking, insulating materials, such as plastics, generates static electricity several magnitudes more compared to conducting materials. A good illustrative example is a plastic (‘scotch’) tape. Notice how dirt comes rushing toward the plastic tape whenever you pull a portion from the roll. This attraction is caused by, you guessed it, the static charge generated over the separation. Even the piece of paper you are taping over will impatiently lift itself toward the tape as it hover, making the landing a little off the desired mark.

Semiconductor Devices ESD susceptibility

Today’s advances in semiconductor technology enables the chip maker to shrink electronics circuit to sub micron scale, allowing them to produce silicon chips containing a mind boggling hundred of millions of transistors on a thin silicon slab smaller than a 25 centavo coin. This scale of miniaturization also results in chips that runs faster, consumes less power, and topping that, costs less.

But as space between components shrink, it becomes easier for voltage potential to puncture the ever thinning insulation and cross the gap between them, starting the chain of events leading to the destruction of the device. In other words, as things get smaller, the risk of ESD damage proportionately increases. Table 3 shows the comparative listing of technologies available over the indicated years, and the maximum amount of voltage allowable between the embedded components.

Table 3.

Table 1 Examples of Static Generation Typical Voltage Levels
Means of Generation	10-25% RH	65-90% RH
Walking across carpet	35,000V	1,500V
Walking across vinyl tile	12,000V	250V
Worker at bench	6,000V	100V
Poly bag picked up from bench	20,000V	1,200V
Chair with urethane foam	18,000V	1,500V

Source: www.esda.org

Table 2

Triboelectric series

Rabbit fur

Glass

Mica

Human Hair

Nylon

Wool

Fur

Lead

Silk

Aluminum

Paper

COTTON

Steel

Wood

Amber

Sealing Wax

Nickel, Copper Brass, Silver

Gold, platinum

Sulfur

Acetate rayon

Polyester

Celluloid

Silicon

Teflon

Technology Trends
Year	1995	1998	2001	2004	2007
Feature size (mm)	0.35	0.25	0.18	0.12	0.10
Voltage (V)	3.5	2 - 3.5	1.50 – 1.9	1.0 – 1.5	1.0

Source: Terry Welsher, Bell Labs, Lucent Technologies, 12/2/97 [5]

IC ESD protection provides only limited protection. Most semiconductor devices are manufactured with some level of ESD protection built-in. Although this significantly reduces the risk of damage, it still does not provide total protection. Table 4 list the susceptibility level of some common semiconductor devices. At this point, we can illustrate the threat by first looking back at Table 1. Let us pick a very common situation - a worker seated on a urethane foam chair. Even in humid environment, the worker can collect static charge up to 1,500V. If this worker is working on electronic components, we can see from Table 4 that not a single component is safe!

Table 4. Device Susceptibility

Device Type	VOLTS
VMOS	20-1800
MOSFET	100-200
GaAsFET	100-300
EPROM	100
JFET	140-7000
SAW	150-500
OPAMP	190-2500
CMOS	250-3000
SCHOTTKY DIODE	300-2500
FILM RESISTORS	300-3000
BIPOLAR TRANSISTOR	380-7000
ECL	500-1500
SCR	680-1000
SCHOTTKY TTL	1000-2500

ESD DAMAGE

A semiconductor devices subjected to electrical overstress, or zapped by ESD, can be damaged in two ways - by catastrophic failure or latent damage.

Catastrophic failure immediately shows itself as an inoperative part hence is quite easy to spot. Device with latent damage, on the other hand, are difficult to detect, because even with the defect, the device seems to work properly. This is not a cause for celebration, however, as this defect is substantially worse than catastrophic failure. A device with latent defect is a diseased part waiting for a spectacular moment to say its final goodbye. This moment will come sooner than you think, and as Murphy would have it, it will happen at the worst possible time and place.

This article contains 5 pages. Previous Page <1> <2> <3> <4> <5> Next Page

03/24/2007

Last Update

Page 2 of 5 pages

Rev A

Initial Release

Written By:

Henry L. Chua

comments?

hlc@e-gizmo.com

Disclaimer

All information contained herein are believed to be correct and accurate. The author or e-Gizmo Mechatronix Central cannot be held liable for any damage arising from the use or misuse of these information. Use of any information from this site imply your agreement to these conditions..