Electrostatic Discharge
A sudden discharge of static electricity from a finger or
other conductor can destroy static-sensitive devices or microcircuitry. Often the spark is
neither felt nor heard, but damage occurs. An electronic device exposed to electrostatic
discharge (ESD) may not be affected at all and will work perfectly throughout a normal
cycle. Or it may function normally for a while, then degrade in the internal layers,
reducing its life expectancy.
Networks built into many integrated circuits provide some
protection, but in many cases, the discharge contains enough power to alter device
parameters or melt silicon junctions.
Click on the topics below for
additional ESD information.
Generating Static
The following table shows how different activities generate
static electricity and and at different electrostatic voltage levels.
Typical Electrostatic Voltages |
Relative
Humidity |
Event |
10% |
40% |
55% |
Walking across carpet |
35,000 V |
15,000 V |
7,500 V |
Walking across vinyl
floor |
12,000 V |
5,000 V |
3,000 V |
Motions of bench
worker |
6,000 V |
800 V |
400 V |
Removing DIPS from
plastic tubes |
2,000 V |
700 V |
400 V |
Removing DIPS from
vinyl trays |
11,500 V |
4,000 V |
2,000 V |
Removing DIPS from
Styrofoam |
14,500 V |
5,000 V |
3,500 V |
Removing bubble pack
from PCBs |
26,000 V |
20,000 V |
7,000 V |
Packing PCBs in
foam-lined box |
21,000 V |
11,000 V |
5,000 V |
NOTE:
700 volts can degrade a product. |
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Preventing
Electrostatic Damage to Equipment
Many electronic components are sensitive to ESD. Circuitry
design and structure determine the degree of sensitivity. The following proper packaging
and grounding precautions are necessary to prevent damage:
- Protect all electrostatic parts and assemblies with
conductive or approved
containers or packaging.
- Keep electrostatic sensitive parts in their containers until
they arrive at
static-free stations.
- Place items on a grounded surface before removing them from
their
container.
- Always be properly grounded when touching a sensitive
component or
assembly.
- Place reusable electronic-sensitive parts from assemblies in
protective
packaging or conductive foam.
Use transporters and conveyors made of antistatic belts and
metal roller bushings. Mechanized equipment used for moving materials must be wired to be
grounded, and proper materials must be selected to avoid static charging. When grounding
is not possible, use an ionizer to dissipate electric charges.
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Preventing
Damage to Drives
To prevent damage to hard drives, use the following
precautions:
- Handle drives gently, using static-guarding techniques.
- Store drives in the original shipping containers.
- Avoid dropping drives from any height onto any surface.
- Handle drives on surfaces that have at least one inch of
shockproof foam.
- Always place drives PCB assembly side down on the foam.
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Grounding
Methods
The method for grounding must include a wrist strap or a
foot strap at a grounded workstation. When seated, wear a wrist strap connected to a
grounded system. When standing, use footstraps and a grounded floor mat.
Static-Shielding
Protection Levels |
Method |
Voltages |
Antistatic Plastic |
1,500 |
Carbon-Loaded Plastic |
7,500 |
Metallized Laminate |
15,000 |
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Grounding
Workstations
To prevent static damage at the workstation, use the
following precautions:
- Cover the workstation with approved static-dissipative
material. Provide a
wrist strap connected to the work surface and properly grounded tools and
equipment.
- Use static-dissipative mats, heel straps, or air ionizers to
give added protection.
- Handle electrostatic-sensitive components, parts, and
assemblies by the case or
PCB laminate. Handle them only at static-free workstations.
- Avoid contact with pins, leads, or circuitry.
- Turn off power and input signals before inserting and
removing connectors or test equipment.
- Use fixtures made of static-safe materials when fixtures
must directly contact dissipative surfaces.
- Keep work area free of nonconductive materials such as
ordinary plastic assembly aids and Styrofoam.
- Use field service tools, such as cutters, screwdrivers, and
vacuums, that are conductive.
- Use a portable field service kit with a static dissipative
vinyl pouch that folds out of a work mat. Also use a wrist strap and a ground cord for the
work surface. Ground the cord to the chassis of the equipment undergoing test or repair.
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Grounding
Equipment
Use the following equipment to prevent static electricity
damage to the unit:
Wrist Straps are flexible straps with a minimum of 1
mega ohm +/- 10% resistance
to the ground cords. To provide proper ground, a strap must be worn snug against the skin.
On grounded mats without banana-plug connectors, connect a wrist strap with alligator
clips.
Heelstraps/Toestraps/Bootstraps can be used at
standing workstations and are compatible with most types of boots and shoes. On conductive
floors or dissipative floor mats, use them on both feet with a minimum of 1 mega ohm
resistance between operator and ground. To be effective, the conductive strips must be
worn in contact with the skin.
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Recommended
Materials and Equipment
Other materials and equipment that are recommended for use
in preventing static electricity include:
- Antistatic tape
- Antistatic smocks, aprons, or sleeve protectors
- Conductive bins, and other assembly or soldering aids
- Conductive foam
- Conductive tabletop workstations with ground cord of 1 mega
ohm
of resistance
- Static dissipative table or floor mats with hard tie to
ground
- Field service kits
- Static awareness labels
- Wrist straps and footwear straps providing 1 mega ohm +/-
10% resistance
- Material handling packages
- Conductive plastic bags
- Conductive plastic tubes
- Conductive tote boxes
- Metal tote boxes
- Opaque shielding bags
- Transparent metallized shielding bags
- Transparent shielding tubes
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