As electrons transfer, the absence
or surplus of electrons creates
an electrical field known as
static electricity. The simple
separation of two materials,
as when tape is pulled off a
roll, can also create this same
transfer of electrons between
materials, generating static
electrical fields.
The amount of static electricity
generated depends upon the materials
subjected to friction or separation,
the amount of friction or separation
and the relative humidity of
the environment. Common plastic
generally will create the greatest
static charge. Low humidity
conditions such as those created
when air is heated during the
winter will also promote the
generation of significant static
electrical charges.
Materials that easily transfer
electrons (or charge) between
atoms are called conductors
and are said to have "free"
electrons. Some examples of
conductors are metals, carbon
and the human body's sweat layer.
Materials that do not easily
transfer electrons are called
insulators. Some well known
insulators are common plastics,
glass and air. Both conductors
and insulators may become "charged"
with static electricity. When
a conductor is charged, the
free electrons give it the ability
to discharge rapidly when it
comes close to another conductor
with a different potential.
Typical Electrostatic
Voltages
Many of the common activities
you perform daily may generate
charges on your body that are
potentially harmful to components.
Some of these activities include:
Walking
across a carpet, 1,500 to 35,000
volts
Walking
over untreated vinyl floor,
250 to 12,000 volts
Worker
at a bench, 700 to 6,000 volts
Vinyl
envelope used for work instructions,
600 to 7,000 volts
Picking
up a common plastic bag from
a bench, 1,200 to 20,000 volts
Costly Effects of ESD
When you feel a static shock,
you are experiencing a minimum
of 3,000 volts of electricity.
This "shock" known
as Electro Static Discharge
or ESD also may be responsible
for damaging many of the rejected
electronic components in your
company.
While you can feel electrostatic
discharges of 3,000 volts, smaller
charges are below the threshold
of human sensation. Unfortunately,
smaller charges can and do damage
semiconductor devices. Many
of the CMOS technology components
used in your facility can be
damaged by charges of less than
1,000 volts. Some of the more
sophisticated components can
be damaged by charges as low
as 10 volts. You should be aware
of the relative sensitivity
to ESD damage of devices you
may be working with.
As electronic technology advances,
electronic components tend to
become smaller and smaller.
As the size of the components
is reduced, so is the microscopic
spacing of insulators and circuits
within them, increasing their
sensitivity to ESD. As you can
predict, the need of proper
ESD protection increases everyday.
Types of ESD Damage
Static damage to components
can take the form of upset failures
or catastrophic failures.
Upset failures - result in gate
leakage
Catastrophic
failures - occur in two forms,
Direct and Latent
Direct catastrophic failures
occur when a component is damaged
to the point where it is DEAD
NOW and it will never again
function. This is the easiest
type of ESD damage to find since
it usually can be detected during
testing.
Latent failures occur when ESD
weakens or wounds the component
to the point where it will still
function properly during testing,
but over time the wounded component
will cause poor system performance
and eventually complete system
failure. Because latent failures
occur after final inspection
or in the hands of your customer,
the cost for repair is very
high. Not only is this type
of damage hard to find, but
it severely affects the reputation
of your company's product.
An upset failure occurs when
an electrostatic discharge has
caused a current flow that is
not significant enough to cause
total failure, but in use may
intermittently result in gate
leakage causing loss of software
or incorrect storage of information.
