others accumulate excessive electrons. When two
Table 2-7.-Triboelectric Series
substances are rubbed together, separated or flow
relative to one another (such as a gas or liquid
over a solid), one substance becomes negatively
charged and the other positively charged. An
electrostatic field or lines of force emanate
between a charged object to an object at a
different electrostatic potential (such as more or
less electrons) or ground. Objects entering this
field will receive a charge by induction.
The capacitance of the charged object relative
to another object or ground also has an effect on
the field. If the capacitance is reduced, there is
an inverse linear increase in voltage, since the
charge must be conserved. As the capacitance
decreases, the voltage increases until a discharge
occurs via an arc.
CAUSES OF STATIC ELECTRICITY
Generation of static electricity on an object
by rubbing is known as the triboelectric effect.
Table 2-7 lists substances in the triboelectric series.
The size of an electrostatic charge on two
different materials is proportional to the separa-
tion of the two materials. Typical prime charge
generators commonly encountered in a manu-
facturing facility are shown in table 2-8.
Electrostatic voltage levels generated by
nonconductors can be extremely high. However,
air will slowly dissipate the charge to a nearby
conductor or ground. The more moisture in the
air the faster a charge will dissipate. Table 2-9
shows typical measured charges generated by
personnel in a manufacturing facility. Note the
decrease in generated voltage with the increase in
humidity levels of the surrounding air.
NOTE: The triboelectric series is arranged in
an order that when any two substances in the list
contact one another and are separated, the
substance higher on the list assumes a positive
charge.
EFFECTS OF STATIC ELECTRICITY
The effects of ESD are not recognized.
COMPONENT SUSCEPTIBILITY
Failures due to ESD are often misanalyzed as
being caused by electrical overstress due to
All solid-state devices (all microcircuits and
transients other than static. Many failures, often
most semiconductors), except for various power
classified as other, random, unknown, infant
transistors and diodes, are susceptible to damage
mortality, manufacturing defect, etc., are actually
by discharging electrostatic voltages. The
caused by ESD. Misclassification of the defect is
discharge may occur across their terminals or
often caused by not performing failure analysis
through subjecting these devices to electrostatic
fields.
to the proper depth.