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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