First Published in EOS/ESD TechnologyJune/July
1991
Induction and ESD
Failures
Owen J. McAteer
Advisory Engineer
Westinghouse Electronic Systems Gp.
Baltimore, Maryland
The preceding columns on
the basics of electrostatics have discussed charge and dielectric constant,
triboelectric generation, and conductivity and resistivity. These topics
provide a good foundation from which to proceed into the subject of
induction.
Induction Misconception
Electrostatic induction is commonly misunderstood even
though the principles involved are straightforward. Sometimes otherwise-knowledgeable
individuals seem to disregard inductive effects when analyzing circumstances
involving conductors in the presence of an electric field.
A primary obstacle appears to be a misconception about equi-potential
surfaces and the behavior of charges on conductors. An equi-potential
surface has the same voltage everywhere on the surface. Do you think
that a conductor is always at the same potential at all surface locations?
What must the potential be at one end (A) of a conductor if its opposite
end (B) is hard-wired to ground? Consider these questions with the conductor
in the presence of an electric field.
Potential Gradients
From early observations of the attractive or repulsive
forces between items after rubbing, it became evident that forces were
associated with charged items. These forces are commonly represented
as lines constituting an electric field.
All electric charges in a field are under the influence of field lines
which emanate from positively charged items, or are directed toward
negatively charged items. Mobile charges will be influenced and moved
accordingly as a conductor is brought into the field.
A volt is the unit of work required to move a unit charge one centimeter
in a field of 1 dyne/coulomb. Thus the quantities of charges moved,
as well as the distance traversed, will result in a voltage or potential
gradient across the conductor.
Simple Induction
 |
| Figure 1: Simple Induction with an initially uncharged
conductor under the influence of a charged item. |
Figure 1 shoes a conductor in the presence of an electric
filed emanating from a positively charged item. The resultant separation
of positive and negative charges is defined as simple induction. Upon
removal from the field influence there is a tendency toward charge recombination
resulting from the attractive forces of separated mobile charges of
opposite polarities culminating in neutralization. Whether upon entry
or removal from a field, the rapid charge movement can lead to ESD failure.
Compound Induction
 |
| Figure 2a-d: Compound Induction. |
Simple induction is shown in Figure 2.a. In this case
the source of the field is a negatively charged item. In Figure 2.b
the conductor side farthest away from the field source is grounded while
still under the field influence. This provides the repelled electrons
with a path to ground, whereby they can get even further from the repulsive
positive field source.
Note that because of the field forces of attraction the positive charge
has its greatest concentration nearest the negative filed source. Thus
a potential gradient exists even when the conductor is grounded.
After disconnecting the ground a net positive charge remains on the
conductor, still attracted to the negative source, as shown in Figure
2.c. When the field is removed, or the conductor is taken from the field,
the charges equalize resulting in a positive equi-potential across the
conductor surface as shown in Figure 2.d. This entire sequence is called
compound induction.
Compound induction in the factory can present double jeopardy to static
susceptible items. A conductor with an induced potential gradient between
its two extreme edges can deliver a damaging potential to a susceptible
item. This electrostatic discharge (ESD) event can be analogous to grounding
the conductor as shown in Figure 2.b.
Thus after the first ESD event and removal of the field source, the
conductor may be left with a possibly damaging voltage level of the
opposite polarity awaiting a second ESD failure upon contact with an
ESD susceptible item.
Sources of Induction
One of the primary ESD control measures is to purge
static generative materials from areas where susceptible items are processed.
Notorious static generators include common insulative materials such
as teflon, acetate, common plastics, polyethylene, styrofoam, wool,
silk, and nylon. These materials tend not only to generate high potentials
but are likely to retain them for considerable time, i.e. minutes to
hours.
Since the charges on insulators are relatively immobile, a charged insulator
presents virtually no threat as a result of contact discharge to a susceptible
item. However, charged insulators can cause failures through means of
the induction phenomenon.
Induction-caused failures can occur from induced potentials on a sensitive
item itself. One example could result from a printed circuit board being
placed on a styrofoam cushion. A charge on the cushion would induce
a charge on the board. A person touching the board could bring about
an ESD failure, even if the person were grounded.
A similar example could occur with a insulative transparent template
placed over the board to check for proper part placement. A charge on
the template would induce a charge on the board resulting in static
discharge when the board is touched or grounded. If the template were
then removed from the board, a charge of opposite polarity would remain
by the process of compound induction. The charged board now presents
a second opportunity for failure upon contact.
A failure can also result form contact of a static-sensitive item to
an ungrounded conductor that has been charged by induction. Do you think
a grounded conductor would increase, decrease, or not alter the induction
threat?
Owen J. McAteer is an advisory engineer for the
Westinghouse Electronic Systems GP, Baltimore, Maryland. A past president
of the EOS/ESD Association, he chairs the Association's Professional
Development Committee and is author of he recently published "Electrostatic
Discharge Control." Extended treatments of many of the themes to
be treated here may be found in this book.