Effects of Static Electricity
The effects of ESD are not recognized. Failures
due to ESD are often misanalyzed as being caused by
electrical overstress due to transients other than static.
Many failures, often classified as other, random,
unknown, etc., are actually caused by ESD.
Misclassification of the defect is often caused by not
performing failure analysis to the proper depth.
All solid-state devices, except for various power
transistors and diodes, are susceptible to damage by
discharging electrostatic voltages. The discharge may
occur across their terminals or through subjection of
these devices to electrostatic fields.
LATENT FAILURE MECHANISMS
The ESD overstress can produce a dielectric
breakdown of a self-healing nature when the current
is unlimited. When this occurs, the device may retest
good, but contain a hole in the gate oxide. With use,
metal will eventually migrate through the puncture,
resulting in a shorting of this oxide layer.
Another structure mechanism involves highly
limited current dielectric breakdown from which no
apparent damage is done. However, this reduces the
voltage at which subsequent breakdown occurs to as
low as one-third of the original breakdown value.
ESD damage can result in a lowered damage
threshold at which a subsequent lower voltage ESD
will cause further degradation or a functional failure.
The heart of an ESD control program is the
ESD-protected work area and ESD grounded work
station. When you handle an ESD-sensitive device
outside of its ESD protective packaging, you need to
provide a means to reduce generated electrostatic
voltages below the levels at which the item is
sensitive. The greater the margin between the level at
which the generated voltages are limited and the
ESDS item sensitivity level, the greater the
probability of protecting that item.
All common plastics and other generators should
be prohibited in the ESD protected work area.
Carpeting should also be prohibited. If you must use
carpet, it should be of a permanently antistatic type.
Perform weekly static voltage monitoring where
carpeting is in use.
PERSONAL APPAREL AND GROUNDING
An essential part of the ESD program is
grounding personnel and their apparel when handling
ESDS material. Means of doing this are described in
Personnel handling ESDS items should wear long
sleeve, ESD-protective smocks, short sleeve shirts or
blouses, and ESD-protective gauntlets banded to the
bare wrist and extending toward the elbow. If these
items are not available, use other antistatic material
(such as cotton) that will cover sections of the body
that could contact an ESDS item during handling.
Personnel Ground Straps
Personnel ground straps should have a minimum
resistance of 250,000 ohms. Based upon limiting
leakage currents to personnel to 5 milliamperes, this
resistance will protect personnel from shock from
voltages up to 125 volts RMS. The wrist, leg, or
ankle bracelet end of the ground strap should have
some metal contact with the skin. Bracelets made
completely of carbon-impregnated plastic may
burnish around the area in contact with the skin,
resulting in too high an impedance to ground.
There are two basic types of ESD-protective
materialconductive and antistatic. Conductive
materials protect ESD devices from static discharges
and electromagnetic fields. Antistatic material is a
nonstatic generating material. Other than not
generating static, antistatic material offers no other
protection to an ESD device.
Conductive ESD-Protective Materials
Conductive ESD-protective materials consist of
metal, metal-coated, and metal-impregnated
materials. The most common conductive materials
used for ESD protection are steel, aluminum, and
carbon-impregnated polyethylene and nylon. The
latter two are opaque, black, flexible, heat sealable,
electrically conductive plastics. These plastics are