2.1 ESD Basics
ESD events are a result of charge building up on a device, then discharging through some sort of conductive path. The goal of ESD mitigation procedures is to try to minimize the build-up of charge and to control and direct the discharge path.
Charge can build up and become trapped on an insulator or isolated conductor through normal handling and movement of devices through any environment. The build-up of charge can happen in one of two ways: triboelectrification or electrostatic induction.
Triboelectrification
Triboelectric charging occurs when two materials of differing electron affinities come into close contact, resulting in a transfer of electrons from one material to the other. When these two materials come into contact, a weak bond is formed and when the materials are, then, separated, electrons from one material will transfer to the other material, leaving one object positively charged and the other negatively charged. The material that retains electrons and becomes negatively charged is said to have a higher electron affinity. A ranking of different materials with respect to their electron affinities can be found in a triboelectric series table (AlphaLab Inc. TriboElectric Series Table)
The total amount of charge stored on an object depends on the size of the object and its electron affinity in relation to the other object.
Electrostatic Induction
Transfer of charge through electrostatic induction occurs when a charged object comes near a conductor. The electric field induced from the proximity of the charged object causes a charge imbalance in the conductor.
If a metal object touches the conductor while in this state, there is a static discharge. If the charged object is, then, taken away, it will, again, leave a charge imbalance in the conductor, leaving it charged in the opposite polarity state and susceptible to another static discharge.
Electrostatic Discharge
An electrostatic discharge occurs when two objects, charged at different potentials, come into contact. ESD events are very short events, on the order of less than 500 ns. Transients longer than this are typically classified as electrical overstress (EOS). A charged object may not require direct contact but only needs to be in close enough proximity to cause an arc to occur. An example of this is when walking across a room and touching a doorknob and feeling or seeing an electric shock or spark.