a single-event, rapid transfer of electrostatic charge between two
objects, usually resulting when two objects at
different potentials come
into direct contact with each other.
ESD can also occur when a high electrostatic field develops
between two objects in close proximity.
ESD is one of the major causes of device failures in the semiconductor
Electrostatic charge build-up occurs
as a result of an
of electrons on the surface of a material. Such a charge build-up develops an electric field that has
measurable effects on other objects at a distance. The process of electron transfer as a result of two objects
coming into contact with each other and then separating is known as
This charging process
results in one object gaining electrons on its surface, and therefore
becoming negatively charged, and another object losing electrons from
its surface, and therefore becoming positively charged. A person can get triboelectrically charged in a number of ways, even by just walking
across a room. The tendencies of
various materials to charge up either positively or negatively are shown
in a Triboelectric
There are three (3) predominant
models for IC's: 1) the
Human Body Model (HBM); 2) the
Model (CDM); and 3) the
The HBM simulates the ESD event when
a person charged either to a positive or negative potential touches an
IC that is at another potential. The
CDM simulates the ESD event wherein a device charges to a certain
potential, and then gets into contact with a conductive surface at a
different potential. The MM
simulates the ESD event that occurs when a part of an equipment or tool
comes into contact with a device at a different potential. HBM and CDM
are considered to be more 'real world' models than the MM.
manifest in a
number of ways, exhibiting one or more of these attributes:
junction leakage, short, or burn-out; dielectric rupture;
resistor-metal interface rupture; resistor/metal fusing; and die surface
controls come in a vast variety of forms.
However, they may be classified into three major categories:
1) prevention of static charge build-up; 2) safe dissipation of any
charge build-up; and 3) improvements in the ESD robustness of the product.
Example of a bench-top ionizer; see
ESD Controls for more examples
first category works on the basic premise of
'No Charge/No discharge.'
Elimination of charge build-up would include the use of materials
that have less tendency to generate static charges in the work area, i.e.,
antistatic and static dissipative materials. All equipment must be free of
moving parts that may generate charges, e.g., rubber rollers, plastic
stoppers, etc. Things that
the devices may come in contact with or get transported on must also be
antistatic or conductive. The use of
ionizers to neutralize newly
generated charges will also prevent charge build-up.
The minimization of movements in the work area, as well as the use of
ESD-safe apparel, will help in minimizing static charges generated by
in the production line, from equipment to work tables to cabinets and
racks, must be connected to this common ground. If the factory uses conductive flooring, then this should also be
connected at regular intervals to this common ground. Having a single or common ground will ensure that everything in the
production floor will remain at the same potential. Any charge build-up will immediately be dissipated by a good
grounding system. The use of
properly grounded wrist and foot straps or conductive shoes will also fall
under this category, since these will bring any charge build-up on
personnel to the common ground.
Examples of personnel grounding accessories:
strap, sole grounder, and conductive shoes
is also important, since the moisture in the air acts as a
conductive path that can bring static charges to the common ground.
Thus, a very dry environment is inviting ESD. Care must be exercised though because excessive RH might
third category does not actually control the ESD phenomenon per se, but
pertains to making devices more
resistant to ESD damage.
This involves incorporating ESD protection cells in the design of
the IC, and the use of physically robust features that can withstand the
high current brought about by an ESD event.
training of personnel on ESD precautions is also a must. A good ESD
control program therefore incorporates a training scheme that will
ensure that everyone is aware of the company's ESD controls and
SOP's. A regular audit of the manufacturing line for ESD control
compliance is important. Check out our