The effects of static electricity may macroscopically
be familiar experiences, such as an electric storm, but microscopically, static
events do occur everyday. Typical experiences may include the clinging of
clothes, the dust build-up on our TVs or computer monitors, the unexpected
"static" shock as we touch an object such as a door knob, pet or
other object, or the plastic wrap that does not want to be thrown away.
When contact and separation occurs between two
materials, a transfer of electrons from the atoms on the surface will take
place. This process is referred to as triboelectric generation. The resulting
imbalance of electrons is what is called an electrostatic charge. This
electrostatic surface charge is either positive or negative depending on
whether there is a deficiency or abundance of free electrons respectively. We
refer to this charge state as static electricity because it tends to remain at
rest or static unless acted upon by an outside force.
The amount of charge generated through the process of
friction and separation will be influenced by the extent of the contact, the
materials involved, relative humidity, and the texture of the materials. Static
charges of up to 30,000 Volts are not uncommon and can be generated by the
simple act of walking across a floor; yet a discharge of only 10 Volts can
destroy a class 1 ESD Sensitive device.
Static electricity is in essence invisible, although
we often see its effects and can feel and measure its presence or electrostatic
field. Since it is created by putting the surface's electrons into a state of
imbalance of it is not in a natural or stable state. Material with an imbalance
of electrons will, when possible, return to a balanced state. When this is done
rapidly a zap or spark associated with rapid electrostatic discharge
(ESD) occurs. We may feel these zaps if the discharge that occurs is over 3,000
Volts. Electrostatic discharges below that level are below the threshold of
human sensation but are still lethal to electronics and associated
semiconductor devices. Rapid electrostatic discharges above 6,000 Volts can be
One common misconception is that conductive materials
do not generate charges. This is because the dissipation of static charges from
grounded conductive material tends to be complete and rapid. Ungrounded
conductors can generate and hold static charges.
A material that inhibits the generation of static
charges from triboelectric generation is classified as antistatic. An
antistatic material can be conductive (<10E5 ohms/square), dissipative
(10E5 - 10E11 ohms/square) or even insulative (>10E12 ohms/square). Only
conductive or dissipative antistatic materials should be used in ESD safe
Insulative materials are more commonly understood to
generate and hold a static charge. Since they are insulators they do not allow
the charge to move or distribute throughout the object. Grounding is not an
effective method of neutralizing insulators. Static fields on insulators are
not necessarily permanent either; they will eventually be neutralized by
gradual recombination with free ions.
Free ions are charged particles that occur naturally
in air. They may be in the form of atoms, molecules, or groups of molecules
such as water droplets. As free ions pass near a charged object of the opposite
polarity they are attracted by the field and will gradually return the material
to a state of balance. A charged object is surrounded by an electrostatic
field. This field can also affect nearby objects by charge induction. Charge
induction lets an electrostatically charged object charge other nearby objects
without actually touching them; typically as far away as several feet.
II. STATIC ELECTRICITY & PRODUCT QUALITY
In the processing of film materials or plastics, static electricity can cause
materials to cling to each other causing product quality problems or production
slowdown. In clean rooms, charged materials can hold static-laden dust,
preventing these dust particles from being circulated and picked up by the
Microelectronics suffer a different type of quality
problem due to static electricity. Electronic components are composed of
micro-miniature traces and structures of alternating layers that may be
insulative, conductive or semi-conductive. Rapid electrostatic discharge (ESD)
can cause damage to these underlying structures via the traces of the
Unfortunately, ESD damage to electronic components is
not as readily apparent as the effects of static electricity in other
industries. This is because ESD damage is not generally visible as it occurs
and may be latent or not show up in functional testing of electronic devices.
ESD damage may lead to premature or intermittent failure. Estimates of the cost
of ESD damage to electronic based equipment run as high as $5 billion annually.
The cost of ESD damage is not simply the cost of the
components, but includes the cost of labor and may include all of the expenses
associated with field repair. Another cost is that of lost business due to
Many companies have implemented ESD control programs
that have reduced their quality defects, resulting in significant cost savings.
ISO 9000 certification is also driving the need for proper ESD control
III. ESD PRODUCTS
Products that control ESD work by:
Charge prevention is accomplished by reducing the exposure to charge generating
materials. The prevention of charge generation is an important part of any ESD
control program. We can prevent charge generation through the elimination of
unnecessary activities that create static charges, the removal of unnecessary
materials that are known charge generators and the use of antistatic materials.
Antistatic materials are those materials that are shown to create minimal
static charges - generally less than 200 Volts - when exposed to friction and
separation. Antistatic material may be naturally low in charge generation
properties or have been made or treated with an antistatic agent.
Grounding works only on conductors. It simply means that we tie all conductors
together (at a common point) so that electrostatic charges will flow from and
through conductors to a common point and will therefore all end up at the same
level. This is much like water seeking its own level. One of the conductors we
must ground is the human body. The common point we normally use is the common
Shielding is used to prevent a sensitive device from being charged by exposure
to an external electrostatic field or being touched by a charged object during
transport or storage. This is done using the Faraday Cage concept.
Metallized shielding bags are commonly used to protect static sensitive
electronic components and assemblies by creating a Faraday Cage effect.
Nonconductors must be neutralized in some other manner. As they do not conduct
electricity, grounding won't work. The most common method of neutralizing
insulators is through ionization. We flood an area with alternating positive
and negative charged particles (ions). A charged material will then attract
ions of the opposite polarity and quickly become neutralized.
Education is absolutely necessary for a successful ESD protection program. As
with all quality functions, ESD prevention depends upon the understanding and
commitment of every person working with sensitive components.
From the moment components arrive in the receiving
department until the completed product is shipped, everyone must understand the
danger of ESD and know the part each individual plays in preventing ESD