Preliminary Considerations

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.

Generating Static

The table below shows the different amounts of static electricity generated by different activities .

Event	Relative Humidity
	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!

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 electrostatic-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 ground and proper materials 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 static 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 shock-proof foam.
• Always place drives PCB assembly side down on the foam.

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	Antistatic Plastic	Carbon-Loaded Plastic	Metallized Laminate
Voltages	1,500	7,500	15,000

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.

Grounding Equipment

Use the following equipment to prevent static electricity damage to the equipment:

Wrist Straps are flexible straps with a minimum of 1 megohm ±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 megohm resistance between operator and ground. To be effective, the conductive strips must be worn in contact with the skin.

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 megohm 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 megohm ±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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