Notes on Static Control by Bertech

Static Control
by Bertech

BERTECH NOTES ON ESD
All of us are familiar with static electricity in some form. Objects become “charged” as a result of being separated. Atoms near the surfaces of separation will end up with either a surplus of electrons which we call a negative charge or a deficiency in electrons which is a positive charge. Friction will also agitate surface atoms and bring greater numbers in close contact. Objects can easily be charged to potentials as high as 30,000 volts by simple acts such as walking or sliding.

Conductors and Insulators
Materials that easily transfer electrons (or charge) between atoms are called conductors and are said to have “free” electrons. Materials that do not easily transfer electrons are called insulators. Both conductors and insulators may be “charged” with static electricity. When a conductor is charged, the free electrons give it the ability to discharge rapidly when it comes close to another conductor with a different potential.

Electrostatic Discharge or ESD
This is what we feel when our charged bodies come close to a doorknob. Your body is a conductor. An ESD event occurs which you feel as a spark of about 3,000 volts.

The very tiny and closely spaced traces in modern microchips are susceptible to damage by ESD with low potential levels. Some devices can be damaged by ESD events caused by potentials as low as 30 volts.

Dead or Wounded
ESD damage to microchips takes two forms: Catastrophic failure in which the ESD event destroys the microchip and Degradation in which the ESD event only “wounds” it. Degradation is the worst situation because the finished product may pass inspection in the factory only to cause problems or fail later in the field. ESD is a kind of contamination to microchips. During the past decade control of ESD has become an accepted practice in most modern facilities dealing with microelectronic products.

Grounding
Grounding works successfully when a conductor becomes charged. The most common case of this is the human body. The body is a conductor and is normally isolated from ground by clothing, flooring, etc. It can easily be charged as high as 35,000 volts by the simple acts of sliding or walking. By grounding the human body with devices such as wrist straps or footwear, we can drain off the charge as it occurs and thereby prevent the spark or ESD event from happening.

We can also ground work stations and other items by making certain that surfaces are conductive (have free electrons) and by connecting them all to a common ground. This allows any charges on conductors to be neutralized by “draining” them to the common ground.

Substances that are normally insulators such as rubber or plastic can be turned into “semiconductors” by adding certain compounds. We call these materials “static dissipative.” Grounded static dissipative or conductive materials are used for work surfaces and material handling equipment.

Ground will not work on insulators, so we must also:
• Isolate work areas from insulators.
• Neutralize insulators which can’t be isolated

Isolation
Isolation simply means keeping microchips away from objects (usually insulators) that might become electrically charged and can’t be drained by grounding because they are insulators. The rule here is to simply avoid having plastics and other potentially chargeable materials near any microchips; a good practice whenever possible. Use of ESD bags is the best example of isolation practices.

Neutralization
Neutralization is necessary because some insulators are a necessary part of the electronic manufacturing and isolation isn’t possible. The most notable example is the circuit board itself. It can be charged by handling and movement. Grounding doesn’t work because it is an insulator.

Neutralization is normally accomplished by ionization. Ions are simply charged particles that are ever present in normal air, such as water droplets. They are charged by natural energy events including sunlight, lightning, open flames, and radiation. Ions that are close enough to a charged insulator will be attracted to it. This process can be speeded up by creating high volumes of ions and blowing them toward the charged insulator with moving air. This is accomplished by electronic ionizers which use high voltage to create billions of ions and fans to assist the air flow toward the object being neutralized. Ionization can neutralize insulators in seconds thereby reducing the window during which an ESD event can occur.

Ionization is not a substitute for grounding and isolation. Ionization cannot neutralize an insulator fast enough in many cases to prevent harm to nearby microchips. All procedures must be practiced in a work area for it to be truly ESD safe.

Tel#: 310-787-8346/Fax: 310-787-0854
Tel#: 1-800-882-8665 /Fax: 1-800-625-2399
� Copyright 1999 by INTERSTATE GROUP, Inc.