LATENT FAILURE MECHANISMS
handling ESDS material. Means of doing this are
described in this section.
The ESD overstress can produce a dielectric
breakdown of a self-healing nature when the
Smocks
current is unlimited. When this occurs, the device
may retest good but contain a hole in the gate
Personnel handling ESDS items should wear
oxide. With use, metal will eventually migrate
long sleeve ESD protective smocks, short sleeve
through the puncture, resulting in a shorting of
shirts or blouses, and ESD protective gauntlets
this oxide layer.
banded to the bare wrist and extending toward
Another structure mechanism involves highly
the elbow. If these items are not available, use
limited current dielectric breakdown from which
other anti-static material (such as cotton) that will
no apparent damage is done. However, this
cover sections of the body that could contact an
reduces the voltage at which subsequent break-
ESDS item during handling.
down 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
Personnel Ground Straps
lower voltage ESD will cause further degradation
or a functional failure.
Personnel ground straps should have a
minimum resistance of 250,000 ohms. Based upon
ESD ELIMINATION
limiting leakage currents to personnel to 5
milliamperes, this resistance will protect personnel
The heart of an ESD control program is the
from shock from voltages up to 125 volts RMS.
ESD-protected work area and ESD grounded
The wrist, leg, or ankle bracelet end of the ground
work station. When you handle an ESD-sensitive
strap should have some metal contact with the
(ESDS) device outside of its ESD protective
skin. Bracelets made completely of carbon-
packaging, provide a means to reduce generated
impregnated plastic may burnish around the area
electrostatic voltages below the levels at which the
in contact with the skin, resulting in too high an
item is sensitive. The greater the margin between
impedance to ground.
the level at which the generated voltages are
limited and the ESDS item sensitivity level, the
ESD PROTECTIVE MATERIALS
greater the probability of protecting that item.
There are two basic types of ESD protective
PRIME GENERATORS
materials--conductive and anti-static. Conductive
materials protect ESD devices from static
Look at table 2-8. It lists ESD prime
discharges and electromagnetic fields. Anti-static
generators. All common plastics and other prime
material is a nonstatic generating material. Other
generators of static electricity should be prohibited
in the ESD protected work area. Carpeting should
than not generating static, anti-static material
also be prohibited. If you must use carpet, it
offers no other protection to an ESD device.
should be of a permanently anti-static type.
Perform weekly static voltage monitoring where
CONDUCTIVE ESD PROTECTIVE
carpeting is in use.
MATERIALS
Conductive ESD protective materials consist
CAUTION
of metal, metal-coated, and metal-impregnated
materials (such as carbon particle impregnated,
Anti-static cushioning material is
conductive mesh or wire encased in plastic). The
acceptable; however, the items cited shall
most common conductive materials used for ESD
be of conductive material to prevent
protection are steel, aluminum, and carbon-
damage or destruction of ESDS devices.
impregnated polyethylene and nylon. The latter
two are opaque, black, flexible, heat sealable,
PERSONAL APPAREL AND
electrically conductive plastics. These plastics are
GROUNDING
composed of carbon particles, impregnated in the
plastic, which provides volume conductivity
An essential part of the ESD program is
throughout the material.
grounding personnel and their apparel when
