What is ESD?
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ESD - Electrostatic Discharge The effects of static electricity may macroscopically be familiar experiences, such as an electric storm, but microscopically, static events do occur everyday. Typical experiences may include the clinging of clothes, the dust build-up on our TVs or computer monitors, the unexpected "static" shock as we touch an object such as a door knob, pet or other object, or the plastic wrap that does not want to be thrown away. When contact and separation occurs between two materials, a transfer of electrons from the atoms on the surface will take place. This process is referred to as triboelectric generation. The resulting imbalance of electrons is what is called an electrostatic charge. This electrostatic surface charge is either positive or negative depending on whether there is a deficiency or abundance of free electrons respectively. We refer to this charge state as static electricity because it tends to remain at rest or static unless acted upon by an outside force. The amount of charge generated through the process of friction and separation will be influenced by the extent of the contact, the materials involved, relative humidity, and the texture of the materials. Static charges of up to 30,000 Volts are not uncommon and can be generated by the simple act of walking across a floor; yet a discharge of only 10 Volts can destroy a class 1 ESD Sensitive device. Static electricity is in essence invisible, although we often see its effects and can feel and measure its presence or electrostatic field. Since it is created by putting the surface's electrons into a state of imbalance of it is not in a natural or stable state. Material with an imbalance of electrons will, when possible, return to a balanced state. When this is done rapidly a zap or spark associated with rapid electrostatic discharge (ESD) occurs. We may feel these zaps if the discharge that occurs is over 3,000 Volts. Electrostatic discharges below that level are below the threshold of human sensation but are still lethal to electronics and associated semiconductor devices. Rapid electrostatic discharges above 6,000 Volts can be seen. One common misconception is that conductive materials do not generate charges. This is because the dissipation of static charges from grounded conductive material tends to be complete and rapid. Ungrounded conductors can generate and hold static charges. A material that inhibits the generation of static charges from triboelectric generation is classified as low charging (antistatic). An low charging (antistatic) material can be conductive (< 104 ohms), dissipative (104 to < 1011 ohms) or even insulative (< = 1011 ohms). Only conductive or dissipative low charging (antistatic) materials should be used in ESD protected areas. Insulative materials are more commonly understood to generate and hold a static charge. Since they are insulators they do not allow the charge to move or distribute throughout the object. Grounding is not an effective method of neutralizing insulators. Static fields on insulators are not necessarily permanent either; they will eventually be neutralized by gradual recombination with free ions. Free ions are charged particles that occur naturally in air. They may be in the form of atoms, molecules, or groups of molecules such as water droplets. As free ions pass near a charged object of the opposite polarity they are attracted by the field and will gradually return the material to a state of balance. A charged object is surrounded by an electrostatic field. This field can also affect nearby objects by charge induction. Charge induction lets an electrostatically charged object charge other nearby objects without actually touching them; typically as far away as several feet. |
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