Electrostatic Discharge (ESD)
Certain components used in electronic assemblies are sensitive to static electricity and can be damaged by its discharge. Static charges are created when non-conductive materials are separated, such as when plastic bags are picked up or opened, when friction occurs between articles of synthetic clothing, when plastic tapes are dispensed and many other causes.
Destructive static charges are induced on nearby conductors, such as human skin, and delivered in the form of sparks passing between conductors, such as when the surface of printed board assembly is touched by a person having a static charge potential. If touched at the right solder joint or conductive pattern, the circuit board assembly can be damaged as the discharge passes through the conductive pattern to a static sensitive component. It is important to note that usually the static damage level for components cannot be felt by humans. (Less than 3,000 volts.)
Electrical Overstress (EOS)
Electrical overstress damage can be caused by generation of unwanted energy; such as spikes, occurring within soldering irons, solder extractors, testing instruments and other electrically operated equipment. This equipment must be designed as to prevent unwanted electrical discharges.
ESD/EOS Safe Work Areas
The purpose of an ESD/EOS safe work area is to prevent damage to sensitive components from spikes and static discharges. These areas must be designed and maintained to prevent ESD/EOS damage.
Handling and Storage Methods
- Circuit board assemblies must always be handled at properly designated work areas.
- Designated work areas must be checked periodically to ensure their continued protection. Areas of main concern include:
- Proper grounding methods.
- Static dissipation of work surfaces.
- Static dissipation of floor surfaces.
- Operation of ion blowers and ion air guns.
- Designated work areas must be kept free of static generating materials including Styrofoam, vinyl, plastic, fabrics and other static generating materials.
- Work areas must be kept clean and neat. To prevent contamination of circuit board assemblies, there must be no eating or smoking in the work area.
- When not being worked on, sensitive components and circuit boards must be enclosed in shielded bags or boxes. There are three types of ESD protective enclosure materials including:
Static Shielding - Prevents static electricity from passing through the package.
Antistatic - Provides antistatic cushioning for electronic assemblies.
Static Dissipative - An "over-package" that has enough conductivity to dissipate any static buildup.
- Whenever handling a circuit board assembly the operator must be properly grounded by one of the following:
- Wearing a wrist strap connected to earth ground.
- Wearing 2 heel grounders and have both feet on a static dissipative floor surface.
- Circuit board assemblies should be handled by the edges. Avoid touching the circuits or components. (See Figure 1)
- Components should be handled by the edges when possible. Avoid touching the component leads.
- Hand creams and lotions containing silicone must not be used since they can cause solderability and epoxy adhesion problems. Lotions specifically formulated to prevent contamination of circuit boards are available.
- Stacking of circuit boards and assemblies should be avoided to prevent physical damage. Special racks and trays are available for handling.