What is ESD?
ESD stands for ElectroStatic Discharge. Static electricity is an everyday phenomenon - there can be few of us who have not experienced a static shock after walking across a room and touching the door knob, or on getting out of a car. Other static nuisance effects include the cling of some fabrics to the body, the sticking of a plastic document cover, or the attraction of dust to a TV or computer screen.
While we can feel some of these effects, static electricity is normally present at lower levels that we cannot feel, hear or see, but may nevertheless damage sensitive electronic components. It can build up rapidly on objects, in unexpected ways, to produce surprisingly high voltages. If two objects that have different voltages approach each other closely enough, charge may pass from one object to the other in a fast electrostatic discharge. While this only lasts a microsecond or less, the peak discharge current can be several Amps and the peak power can be in the kiloWatt range!
Why worry about ESD?
ESD can cause unseen damage to electronic components during manufacture of electronic assemblies and equipment. If the damaged component fails immediately, the result can be a board that fail tests and requires rework. This represents lost production and additional manufacturing costs. Worse than this, a component may be partially damaged and weakened. It may suffer a change or drift in characteristics. It may remain within specification, but fail later when in use by a customer. It has been estimated that 90% of damaged devices may be discovered in this way. This is the most expensive type of failure, as it represents:
Customer dissatisfaction, and the possibility of loss of product reputation and future sales
Customer service personnel and facility cost
Engineers time, possibly for on-site repair with travel, and parts replacement
In manual assembly most ESD arises from charged personnel, if they are not grounded. Most people do not feel an ESD shock unless they are charged to over 4000 V (the sensitivity threshold varies between people, and even over parts of the body!). This voltage is quite common in the uncontrolled environment - how many of us have not felt the occasional electrostatic shock in everyday life?