effects of current density on the human body are shown in the table below.
Contrary to popular belief, it is the current - not the voltage - level
which is responsible for effects. According to Ohm's Law, of course, a
certain voltage is required to cause the necessary currents to flow.
Values show vary depending on the body.
||Cannot let go
Basics of Electrostatic Discharge Part 1---An Introduction to
History & Background
The decade of the 90's may be remembered as the Decade of Quality in
the electronics industry. Increased competition, six-sigma quality, and ISO
9000 have forced a recommitment to quality even in those companies that
might not have done so willingly. As we examine our environments for quality
improvement areas, electrostatic discharge (ESD) remains a key target.
Static electricity has been an industrial problem for centuries. As early as
the 1400’s, European and Caribbean forts were using static control procedures
and devices to prevent electrostatic discharge ignition of black powder stores.
By the 1860's, paper mills throughout the U.S. employed basic grounding, flame
ionization techniques, and steam drums to dissipate static electricity from the
paper web as it traveled through the drying process. The age of electronics
brought with it new problems associated with static electricity and
electrostatic discharge. And, as electronic devices became faster and smaller,
their sensitivity to ESD increased.
Today, ESD impacts productivity and product reliability in virtually every
aspect of today’s electronics environment. Many aspects of electrostatic
control in the electronics industry also apply in other industries such as clean
room applications and graphic arts.
Despite a great deal of effort during the past decade, ESD still affects
production yields, manufacturing costs, product quality, product reliability,
and profitability. Industry experts have estimated average product losses due to
static to range from 8-33% (Table 1). Others estimate the actual cost of ESD
damage to the electronics industry as running into the billions of dollars
annually. The cost of damaged devices themselves range from only a few cents for
a simple diode to several hundred dollars for complex hybrids. When associated
costs of repair and rework, shipping, labor, and overhead are included, clearly
the opportunities exist for significant improvements.
Informal Summary of
Static Losses by Level
Static Losses Reported
Source: Stephen Halperin,
"Guidelines for Static Control Management," Eurostat, 1990.
This first in a series of six articles on ESD focuses on how electrostatic
charge and discharge occur, how various materials affect the level of charge,
types of ESD damage, and how ESD events can damage electronic components. Future
articles will cover various ways to control the problem.
Static Electricity: Creating Charge
Static electricity is defined as an electrical charge caused by an
imbalance of electrons on the surface of a material. This imbalance of electrons
produces an electric field that can be measured and that can influence other
objects at a distance. Electrostatic discharge is defined as the transfer
of charge between bodies at different electrical potentials.
Electrostatic discharge can change the electrical characteristics of a
semiconductor device, degrading or destroying it. Electrostatic discharge also
may upset the normal operation of an electronic system, causing equipment
malfunction or failure. Another problem caused by static electricity occurs in
clean rooms. Charged surfaces can attract and hold contaminants, making removal
from the environment difficult. When attracted to the surface of a silicon wafer
or a device's electrical circuitry, these particulates can cause random wafer
defects and reduce product yields.
Controlling electrostatic discharge begins with understanding how
electrostatic charge occurs in the first place. Electrostatic charge is
most commonly created by the contact and separation of two similar or dissimilar
materials. For example, a person walking across the floor generates static
electricity as shoe soles contact and then separate from the floor surface. An
electronic device sliding into or out of a bag, magazine or tube generates an
electrostatic charge as the device's case and/or metal leads make multiple
contacts and separations with the surface of the container. While the magnitude
of electrostatic charge may be different in these examples, static electricity
is indeed generated.
Figure 1: The Triboelectric Charge. Materials Make Intimate Contact
Figure 2: The Triboelectric Charge - Separation
Creating electrostatic charge by contact and separation of materials is known
as "triboelectric charging." It involves the transfer of electrons
between materials. The atoms of a material with no static charge have an equal
number of positive (+) protons in their nucleus and negative (-) electrons
orbiting the nucleus. In Figure 1, Material "A" consists of atoms with
equal numbers of protons and electrons. Material B also consists of atoms with
equal (though perhaps different) numbers of protons and electrons. Both
materials are electrically neutral.
When the two materials are placed in contact and then separated, negatively
charged electrons are transferred from the surface of one material to the
surface of the other material. Which material loses electrons and which gains
electrons will depend on the nature of the two materials. The material that
loses electrons becomes positively charged, while the material that gains
electrons is negatively charged. This is shown in Figure 2.
The actual level of charge is measured in coulombs. Commonly, however, we
speak of the electrostatic potential on an object, which is expressed as
This process of material contact, electron transfer and separation is really
a more complex mechanism than described here. The amount of charge created by
triboelectric charging is affected by the area of contact, the speed of
separation, relative humidity, and other factors. Once the charge is created on
a material, it becomes an "electrostatic" charge (if it remains on the
material). This charge may be transferred from the material, creating an
electrostatic discharge, or ESD, event.
Examples of Static Generation
Typical Voltage Levels
Means of Generation
Walking across carpet
Walking across vinyl tile
Worker at bench
Poly bag picked up from bench
Chair with urethane foam
An electrostatic charge also may be created on a material in other ways such
as by induction, ion bombardment, or contact with another charged object.
However, triboelectric charging is the most common.
Material Characteristics—How They Affect Static Charge
Virtually all materials, including water and dirt particles in the air, can be
triboelectrically charged. How much charge is generated, where that charge goes,
and how quickly, are functions of the materials' electrical characteristics.
A material that prevents or limits the flow of electrons across its
surface or through its volume is called an insulator. Insulators have an
extremely high electrical resistance. A considerable amount of charge can be
generated on the surface of an insulator. Because an insulative material does
not readily allow the flow of electrons, both positive and negative charges can
reside on insulative surface at the same time, although at different locations.
The excess electrons at the negatively charged spot might be sufficient to
satisfy the absence of electrons at the positively charged spot. However,
electrons cannot easily flow across the insulative material's surface, and both
charges may remain in place for a very long time.
A conductive material, because it has low electrical resistance,
allows electrons to flow easily across its surface or through its volume. When a
conductive material becomes charged, the charge (i.e., the deficiency or excess
of electrons) will be uniformly distributed across the surface of the material.
If the charged conductive material makes contact with another conductive
material, the electrons will transfer between the materials quite easily. If the
second conductor is attached to an earth grounding point, the electrons will
flow to ground and the excess charge on the conductor will be
Electrostatic charge can be created triboelectrically on conductors the same
way it is created on insulators. As long as the conductor is isolated from other
conductors or ground, the static charge will remain on the conductor. If the
conductor is grounded the charge will easily go to ground. Or, if the charged
conductor contacts or nears another conductor, the charge will flow between the
Static Dissipative Materials
Static dissipative materials have electrical resistance between
insulative and conductive materials. There can be electron flow across or
through the dissipative material, but it is controlled by the surface resistance
or volume resistance of the material.
As with the other two types of materials, charge can be generated
triboelectrically on a static dissipative material. However, like the conductive
material, the static dissipative material will allow the transfer of charge to
ground or other conductive objects. The transfer of charge from a static
dissipative material will generally take longer than from a conductive material
of equivalent size. Charge transfers from static dissipative materials are
significantly faster than from insulators, and slower than from conductors.
When two materials contact and separate, the polarity and magnitude
of the charge are indicated by the materials’ positions in the triboelectric
series. The triboelectric simply lists materials according to their relative
triboelectric charging characteristics. When two materials contact and separate,
the one nearer the top of the series takes on a positive charge, the other a
negative charge. Materials further apart on the table typically generate a
higher charge than ones closer together. See Table 3.
Typical Triboelectric Series
ESD Damage—How Devices Fail
Electrostatic damage to electronic devices can occur at any point from
manufacture to field service. Damage results from handling the devices in
uncontrolled surroundings or when poor ESD control practices are used. Generally
damage is classified as either a catastrophic failure or a latent defect.
When an electronic device is exposed to an ESD event it may no longer
function. The ESD event may have caused a metal melt, junction breakdown, or
oxide failure. The device's circuitry is permanently damaged causing the device
fail. Such failures usually can be detected when the device is tested before
shipment. If the ESD event occurs after test, the damage will go undetected
until the device fails in operation.
A latent defect, on the other hand, is more difficult to identify. A
device that is exposed to an ESD event may be partially degraded, yet continue
to perform its intended function. However, the operating life of the device may
be reduced dramatically. A product or system incorporating devices with latent
defects may experience premature failure after the user places them in service.
Such failures are usually costly to repair and in some applications may create
It is relatively easy with the proper equipment to confirm that a device has
experienced catastrophic failure. Basic performance tests will substantiate
device damage. However, latent defects are extremely difficult to prove or
detect using current technology, especially after the device is assembled into a
Basic ESD Events--What Causes Electronic Devices to Fail?
ESD damage is usually caused by one of three events: direct electrostatic
discharge to the device; electrostatic discharge from the device
or field induced discharges.
Discharge to the Device
An ESD event can occur when any charged conductor (including the
human body) discharges to an ESDS (electrostatic discharge sensitive) device.
The most common cause of electrostatic damage is the direct transfer of
electrostatic charge from the human body or a charged material to the
electrostatic discharge sensitive (ESDS) device. When one walks across a floor,
an electrostatic charge accumulates on the body. Simple contact of a finger to
the leads of an ESDS device or assembly allows the body to discharge, possibly
causing device damage. The model used to simulate this event is the Human Body
A similar discharge can occur from a charged conductive object, such as a
metallic tool or fixture. The model used to characterize this event is known as
the Machine Model.
Discharge from the Device
The transfer of charge from an ESDS device is also an ESD
event. The trend towards automated assembly would seem to solve the problems of
HBM ESD events. However, it has been shown that components may be more sensitive
to damage when assembled by automated equipment. A device may become charged,
for example, from sliding down the feeder. If it then contacts the insertion
head or another conductive surface, a rapid discharge occurs from the device to
the metal object. This event is known as the Charged Device Model (CDM) event,
and can be more destructive than the HBM for some devices. Although the duration
of the discharge is very short--often less than one nanosecond--the peak current
can reach several tens of amperes.
Field Induced Discharges
Another event that can directly or indirectly damage devices is
termed Field Induction. As noted earlier, whenever any object becomes
electrostatically charged, there is an electrostatic field associated with that
charge. If an ESDS device is placed in that electrostatic field, a charge may be
induced on the device. If the device is then momentarily grounded while within
the electrostatic field, a transfer of charge from the device occurs.
Device Sensitivity: How Much Static Protection is Needed?
Damage to an ESDS device by the ESD event is determined by the device's
ability to dissipate the energy of the discharge or withstand the voltage levels
involved. This is known as device "ESD sensitivity". Test procedures
based on the models of ESD events help define the sensitivity of components to
ESD. Some devices may be more readily damaged by discharges occurring within
automated equipment, while others may be more prone to damage from handling by
personnel. Defining the ESD sensitivity of electronic components is the first
step in determining the degree of ESD protection required.
Many electronic components are susceptible to ESD damage at relatively low
voltage levels. Many are susceptible at less than 100 volts, and many disk drive
components have sensitivities below 10 volts. Current trends in product design
and development pack more circuitry onto these miniature devices, further
increasing their sensitivity to ESD and making the potential problem even more
acute. Table 4 indicates the ESD sensitivity of various types of components.
Susceptibility of Electronic Components to ESD
30 - 1,200
Mosfet, GaAsfet, EPROM
100 - 300
150 - 7,000
190 - 2,500
300 - 2,500
300 - 3,000
1,000 - 2,500
In this introductory article on electrostatic discharge, we have discussed
the basics of electrostatic charge, discharge, types of failures, ESD events,
and device sensitivity. We can summarize this discussion as follows:Virtually
all materials, even conductors, can be triboelectrically charged.
- The level of charge is affected by material type, speed of contact and
separation, humidity, and several other factors.
- Electrostatic discharge can create catastrophic or latent failures in
- Electrostatic discharge can occur throughout the manufacturing, test,
shipping, handling, or operational processes.
- Component damage can occur as the result of a discharge from the component
as well as a direct discharge to the component.
- Components vary significantly in their sensitivity to ESD.
Protecting your products from the effects of static damage begins by
understanding these key concepts of ESD. Armed with this information, you can
then begin to develop an effective ESD control program.