ESD: Back to Basics
When Coulomb and Priestly observed and measured
the attractive and repulsive forces between two charged balls,
and Coulomb subsequently formulated his famous Coulomb's Law,
little did he know that he laid the foundation on which much of
modern Physics is built. Electrostatic charging (ESC) and discharging
(ESD) phenomena, already observed and described by the Greeks
in 600 BC, can be directly related to Coulomb's observations.
Charged elementary particles of like polarity,
electrons in our case, repel each other. This is actually an understatement.
Electrons forced to live together on a small conductive object
repel each other with, proportionally speaking that is, unimaginably
strong forces. As a matter of fact, if enough electrons are present
on this object, the expulsion force may be so great that they
create an escape path in the form of an arc.
The Electrostatic Discharge or Event.
An electrostatic discharge is basically a more
or less sudden and violent redistribution of electrons between
bodies such that in their new spatial equilibrium position, the
electrons end up as far away from each other as they could possibly
get. Because of the repulsion between like charges, the charges
will always position themselves at the outside skin of a conductive
body, which is ultimately as far away from each other as they
can possibly get.
If the path for this sudden electron rush happened
to include sensitive electronic devices, destruction or latent
damage may well be the result. If, in their quest to redistribute,
they forced themselves through an arc, and if this happened in
an explosive atmosphere, the damage could be far more than just
a dead electronic component or system.
The suddenness, or better, the time duration during
which such a charge redistribution or ESD event takes place depends
on numerous parameters. If for example, the interacting bodies
are excellent metallic conductors suddenly touching each other,
the electrons are probably redistributed in a matter of picoSeconds,
and the peak current during the sudden massive flow is substantial.
It is not readily understood how fast ESD events really are because
of bandwidth limitations of today's existing equipment.
The electromagnetic fields associated with an
impulsive current of this magnitude are simply huge, and have
the ability to upset electronic circuits quite easily. Measured
frequency spectra of fast discharges extend all the way up to
40 to 50 Ghz. If on the other hand, there is resistance or impedance
in the path between the interacting bodies, the electrons will
require more time to work themselves through these obstacles.
The peak current will hence be lower, and the duration of the
discharge longer. Most ESD protective gear such as wrist straps,
conductive footwear, and the like, attempt to do just that. By
inserting known resistive paths, the currents are kept to the
microAmpere levels, and the duration are extended to the millisecond
or second range.
Note that redistribution of charge, whether sudden
or not, always involve dissipation of energy. Slowing the flow
of the electrons with a resistor is but converting some of their
kinetic energy into heat. Other forms of released energy are of
electromagnetic and mechanical nature, and if an arc is involved,
acoustic, and light as well.
Charge Is The Enemy
The culprits in ESD events, are electrons, either
too much or not enough of them, and the bodies on which they reside.
The term body is obviously not restricted to the human body. It
is a standard expression in Electrostatic Physics, used to describe
an isolated entity of matter, conductive or not, surrounded by
either vacuum or an infinite mass of a different matter. For a
Physicist, an integrated circuit is a body, a shoe is another
one, an elephant is a third one.
As stated earlier, on conductive bodies the charges
will end up at the outer skin of that body, because that is as
far as they can possibly go. If the body is irregular in shape,
the electrons will not be equidistantly positioned. They will
spread themselves around the surface such that all repulsive forces
on each individual electron are in equilibrium. In other words,
a giant balancing act frequently involving billions of players.
Only on a perfect conductive sphere will the electrons be spaced
equidistantly. For example, a charge of I microCoulomb is typically
found as a result of an interaction between a pair of shoe soles
and a floor, distributed over a perfect sphere with 1 meter diameter
involves 6.2 trillion electrons, each spaced 0.7 micrometer apart.
One of the implications of this electron behavior
is that any randomly shaped body can be fairly accurately modeled
as a sphere with a diameter such that its skin surface equals
the total skin surface of the body being modeled. A typical human
body, for example, becomes a 1 meter diameter conductive sphere.
A 64 pin Dual In Line IC becomes an 18 mm sphere. And, taking
it one step further, the electrostatic properties of a sphere
can be easily described and predicted with well understood mathematical
expressions. Also, the interaction between multiple spheres, when
a charged human picks up the IC for example, and between a sphere
and a plane, a human and the earth for example, can be readily
described and predicted.
Probably the most important property of the sphere
is its capacitance. Note that a "capacitor", seen through
the eyes of a physicist, is a totally different kind of animal
then the "capacitor" seen by most electrical engineers.
There is a very important distinction, and some electrical engineers
have a hard time understanding and visualizing what a capacitor
in the electrostatic physics sense of the word really is. As a
result, a fair amount of ESD related products and test equipment
designs are based on debatable and erroneous (concepts and models
For a physicist, a capacitor is basically a physical
body with certain dimensions, composed of real molecules, and
immersed in a dielectric of a certain capacity. If the total number
of electrons contained in the molecular structure of that body
exceeds the total number of protons, then the body is negatively
charged, or, expressed differently, its capacitance holds a negative
From the electrostatic physics point of view,
there is thus no need for a second plate parallel in close proximity
of a first one to make a capacitor. This is the device that the
electrical engineer usually thinks of when he hears the word capacitor.
In fact, the parallel plate capacitor is for the physicist but
a special case in the general field of electrostatic physics.
The following examples will illustrate the notion
of spherical capacitor as the physicist sees it. The capacitance
of the earth in the solar planetary context equals 710 microFarad.
A 1 meter diameter sphere, which is the model for the human body,
has a free space capacitance of approximately 110 pF. By free
space is meant far away from any other body or plane. If the same
1 meter sphere comes down to earth and floats approximately 5
centimeter above it, it's capacitance will have increased by 2.5
Charge, Capacitance and Potential Difference
If charge is stored in a body, it will develop
a potential difference with respect to the body from which the
charge was extracted. For example, when a person with conductive
footwear walks over a non conducting floor surface, the shoe sole
to floor interaction will result in electrons being extracted
by one from the other. Assuming it is the shoe soles that get
the electrons, then the extracted electrons will end up distributed
over that person's body. In other words, stored in his/her body
capacitance. Since potential difference, charge and capacitance,
are related by the equation U = Q/C, we can calculate that a 1
microCoulomb charge stored into the 110 picoFarad of the human
body would result in a 9010 Volt potential difference. And, since
the capacitance at 5 cm from the ground plane is 2.5 times higher,
the potential would drop to 3600 Volts at that distance.
The potential difference on an unprotected human
body in motion will vary continuously. It varies because the body
bobs up and down above the ground plane, it varies because the
dimensions and the geometry of the body are continuously changing,
it varies because of different amounts of charge being extracted
at each shoe sole to surface interaction, and it varies because
of dozens of other interfering parameters. Predicting the potential
difference of a human body in motion and interacting with its
environment is a nearly impossible task. It is important to realize
that ESD damage is in the first place caused by the amount of
electrons that suddenly flow, rather than by the potential differences
being so high as to cause an arc. An arc is nothing but an unusual
form of a low resistance conductive path through which the excess
electrons can rush and redistribute themselves. In the electronics
industry, it is not so much the arc that is the culprit, it is
in the first place the electron rush. In an ammunition plant,
one might be tempted to say that the arc is more important.
In a way, potential difference plays somewhat
in our favor. Electrostatic physics shows that field strength
and curvature of a body are related, the field is always more
intense at sharp protrusions. If the field becomes sufficiently
high as to cause ionization and corona, charge will be able to
escape into the ambient air, thus limiting the potential difference
excursion on the human body. It would however not be prudent to
rely on this phenomena as a protection against ESD. Another interesting
aspect of potential difference excursion on the human body is
that it can be easily measured with a suitable Voltmeter. This
can be non-contact Electrostatic Voltmeters or Electrometers.
Potential differences due to charge on the human
body can be made with a Novx Electrometer for example.
In the Electronics Industry, charged human bodies
are the component or system killer, and the charges come almost
invariably from the interaction between shoe soles and the floor.
Seldom does charge come from friction in clothing.
Matter of fact, the clothing worn in most electronic factories
are relatively thin, and from the electrostatics point of view,
it is "drenched" in highly conductive compounds containing
bromine and chlorine. Clothing actually becomes the effective
conductive outer skin of the human, and that is where the excess
charges will ultimately end up. It also has serious implications
on the magnitude of the equivalent body capacitance.
Probably one of the biggest misconceptions about
electrostatics was planted in the minds of factory workers and
engineers alike by high school science teachers, demonstrating
electrostatics by rubbing a comb, or something on their shirt
sleeves. Rubbing an object on ones shirt sleeves has become the
stereotype for electrostatics. Although there is nothing wrong
with that demonstration from the pure electrostatic physics point
of view, it has been totally misunderstood, and it must have cost,
and is still costing the industrialized world an untold fortune
to unteach and reteach the true basics of electrostatics as they
apply to the industrial establishment. Rubbing the handle of a
screwdriver on a shirt sleeve seldom is detrimental to your electronic
circuits. Shuffling your feet on the floor while sitting in a
chair at the workbench very likely is. Good self explanatory,
back to basics teaching tools are sorely needed in the industry
as well as in the science classes of our high schools and institutions
of higher learning.
The best protection against ESD obviously is to
eliminate ESC. If there are no electrons lost or gained in an
interaction between bodies, there can be no electron rush in between
them. How can we reliably and acceptably cost protect ourselves
from ESC? The easiest solution is to rely on the electron backflow
phenomena. When two materials physically interact, whether they
are different or the same, or whether they are in their gaseous,
liquid or solid states, one of them is likely to loose electrons
to the other. The amount of electrons lost is, amongst others,
a function of a materials property called the workfunction. There
is also a table classifying the materials according to a Tribolectric
Series, attempting to predict which one of the materials will
gain the electrons and thus become negatively charged. If the
interacting materials are both conductors, then electron backflow
occurs at the moment of separation, and no net charge exists on
the interacting bodies.
Foot Straps and Conductive Shoes.
The interaction between conductive footwear and
conductive flooring is an example of electron backflow in action.
To be noted, a conductive pair of shoes on a conductive floor
works. A conductive pair of shoes on a non-conductive floor does
NOT work. A non-conductive pair of shoes on a conductive floor
does NOT work either. And a non-conductive pair of shoes on a
non-conductive floor is totally out of the question. Conductive
shoes work well as long as the person wearing them does not use
insulating inserts. Some workers frequently object against wearing
them because they tend to be clumsy and unfashionable. ESD shoe
suppliers typically cater to the steel toe crowd and fashion is
obviously the least of their concern. Foot straps on BOTH feet
on a conductive floor work as long as the foot straps are touching
the conductive floor. Heel straps do NOT work when a person stands
on her/his toes, and toe straps do not work when a person stands
on his/her heels.
These conditions happen more often them one thinks.
The best foot straps cover both toes and heels. They are widely
used in ammunitions plants for example. Velcro on the foot straps
often interferes with the shoe laces. Foot straps frequently loosen
on the heels of personnel sitting at workbenches.
Nothing rivals a wrist strap when it comes to
protection against ESC. It clearly is the uncontested winner.
The biggest problem is that wearers frequently forget to connect
them when they arrive at their workstations, and then it does
not really matter whether the wrist strap is a dual conductor
type connected to an appropriate continuous workstation monitor
or just a standard ground strap. The wrist strap grounding cords
frequently are a source for aggravation. They tend to be in the
way, snag, and sweep tools and components off the workbench.
A smock is basically a conductive outer skin or
Faraday Cage. One has to be careful with smocks as they can become
the equivalent of a concentric spherical capacitor around the
human body, able to store enough charge and with sufficient capacitance
to become a threat of comparable magnitude to that of an unprotected
human body. This occasionally happens when they are worn over
thick winter clothing in dry weather and when the smock does not
make reliable contact with the skin of the person wearing it.
Friction between the smock and the clothing causes the smock to
become charged with respect to the human body, which, in turn,
can have a charge of its own. Contact with the skin of the human
body is very important and is usually done at the cuffs of the
smock. The electrical connection between the sleeves and the body
of the smock is another source for concern.
We have attempted to put ESD and ESC related phenomena
into a slightly different perspective. ESC is in the first place
electrostatic in nature, whereas ESD belongs more in the domain
of electromagnetism. For the longest time, ESC and ESD were a
neglected, poorly understood, under funded and stigmatized niches
of physics and electrical engineering. This is changing rapidly
and dramatically, as dense IC structures and MR Heads become more
and more prone to ESD induced damage. Remember, if there is no
ESC, there is no ESD.