What Causes ESD?

Static charges accumulate on the surface of conductors - the skin of a person, the surface of a conductive tote box, and an insulative surface - practically all plastics and textiles. Both conductors and insulators may become charged with static electricity.

These static charges "stay put". They cannot move because there is not path to ground. The charge is called "static" - not moving. A static charge will stay in place on the surface of an object until it has a chance to move. When a non-grounded conductor of static charges, a person or an object, comes close to a ground plane, the charge will "jump" from, a point on the non-grounded conductor to the grounded object, causing an ESD event. As you walk across the carpet and touch a metal door knob, the electrostatic charge that is built up on your body is discharged through the doorknob.

A charged object is surrounded by an electrostatic field. A charge from an electrostatically charged object can charge a component without actually touching the component. It may "jump" from the conductor to the electronic component from several feet away. This process is known as induction. When a charged insulator comes close to a conductive object, the charge can be induced onto the conductor, which can then rapidly discharge to other conductors - an ESD event.

For Example: When plastic, an insulating material, is picked up, a large charge is created on its surface. The charge doesn't drain, even if the person is grounded. Insulative materials are more commonly understood to generate and hold static charge. Since they are insulators they do not allow the charge to move or distribute throughout the object. Insulators do not usually discharge to conductors except in rare high voltage situations. Grounding is not an effective method of neutralizing insulators. (We will discuss personal grounding and other methods of controlling ESD later in this course.).

With conductive materials such as metal, the static will be discharged from any grounded person picking up the metal. One common misconception is that conductive materials do not generate charges. Ungrounded conductors can generate and hold static charges. When a conductor is charged it can discharge rapidly when it comes close to another conductor with a different electrical potential.