| Electrostatic
Discharge (ESD) |
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!
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
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Analysis
of non-conforming or defective devices showed that 60-75%
were damaged by EOS (electrical overstress) or ESD.
This rises to 90% for newer technologies. About 70%
of these failures were attributed to damage from incorrectly
grounded people.
Toshikazu
Namaguchi, Hideka Uchida.
EOS/ESD Symposium EOS-20 1998 pp 245-251
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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?
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