ElectroStatic Discharge, ESD tutorial and basics [1]
a tutorial or summary about the basics of what is ElectroStatic Discharge,
ESD and why ESD products including anti-static bags, ESD mats, ESD wrist straps,
etc are needed.
ElectroStatic Discharge or ESD awareness is particularly
important for anyone associated with electronics. As integrated circuits become
more compact, and feature sizes shrink, active devices as well as some passive
devices are becoming more prone to damage by the levels of static that exist. To
combat its effects, industry is spending many millions of pounds to prevent
damage to electronic components from the effects of static. Anti-static areas
using protective antistatic workbenches, as well as measures for ensuring people
are not carrying static are all used. Using what are termed ESD PA or
Electrostatic Discharge Protected Areas, the destructive effects of static on
electronics equipment during manufacture can be virtually removed.
In view of the fact that ESD us so important electronics
manufacturing and development companies go to significant lengths to overcome
the effects of ESD. Specially protected areas using a variety of ESD products
including anti-static mats, ESD benches, ESD bags and packaging, ESD wrist
straps, soldering irons adapted to absorb static, and much more are all
implemented. These ESD precautions enable the effects of static to be overcome,
and ensure the long term reliability of the products being developed and
manufactured.
ESD overview
Although awareness has grown considerably in recent years,
the problem has existed for a long time. It came to light in a major way with
the introduction of the first MOSFET devices. In view of the very high gate
impedances that existed it was found that they were easily damaged. Originally
it was thought that only devices such as MOSFETs were at risk, but studies soon
revealed that far more damage was being done that had been originally imagined.
The problem of ESD became more acute as feature sizes on ICs dropped and they
became more prone to damage.
What is static?
Static electricity is a natural phenomenon which occurs as
part of everyday life. Its effects can often be felt when touching a metal door
handle having walked across a nylon carpet. Another effect can be seen when hair
stands up after it has been combed. The most dramatic effect is lightning. Here
the scale is many orders of magnitude greater than those seen in and around the
home. Colossal powers are dissipated in every strike, and its effects can be
heard for many miles around. This is a particularly impressive form of ESD.
Static is created when there is movement. When objects rub
together there is friction and this causes the surfaces to interact. An excess
of electrons appears on one surface while there will be a deficiency on the
other. The surface with the excess of electrons becomes negatively charged,
whereas the surface with the deficit becomes positively charged.
These charges will try to flow and neutralise the charge
difference. They may leak away slowly, or the discharge may take place more
quickly. However as many substances exhibit a very high resistance these charges
can remain in place for a very long time and wait until a suitable path is
created for the discharge to take place. When charges find a path through an
electronic circuit, the high instantaneous currents can give rise to damage. As
a result ESD is of great importance.
ESD and the tribo-electric series:
The size of the charge which is generated is determined by a
variety of different factors. One is obviously the conductivity of the two
materials and also whether the charge between them can leak away. However one of
the major influences is the materials themselves and their position of the two
materials in what is called the tribo-electric series. The position of the two
materials which are in rubbing against one another in this series governs the
size of the charge and the relative polarities. The further apart they are in
the series, then the greater the charge. The material that is higher up the
series will receive the positive charge, whereas the one lower in the series
will receive the negative charge. Materials such as human hair, skin, and other
natural fibres are higher up the series and tend to receive positive charges,
whereas man made fibres together with materials like polythene, PVC and even
silicon are towards the negative end. This means that when combing hair with a
man made plastic comb, the hair will receive a positive charge and the comb will
become negative.
positive charge
skin
hair
wool
silk
paper
cotton
wood
rubber
rayon
polyester
polythene
pvc
teflon
negative charge
Practical examples of ESD
One of the most commonly visible examples of generating
charge is when walking across a room. Even this everyday occurrence can generate
some surprisingly high voltages. Walking on an ordinary vinyl floor might
generate a voltage in the region of 10kV. Walking on a nylon carpet is much
worse with voltages in the region of 30kV to be expected. Other actions can also
generate very high voltages. For example moving a polythene bag can generate
voltages of around 10kV.
These voltages seem to be very high, but they usually pass
unnoticed. The smallest discharge that can be felt is around 5kV, and even then
this magnitude of discharge may only be felt on occasions. The reason is that
even though the resulting peak currents may be very high, they only last for a
very short time and the body does not detect them.
ESD effects on electronics
With most electronics ICs and components being designed to
operate at voltage so 5 V or less, it is hardly surprising that these discharges
can cause some damage. As a result ESD is of major importance to all who are
involved in the electronics industry. The next page in this tutorial looks more
closely at the discharges and they way in which they cause damage to
electronics. The final page looks at ways of protecting against ESD. This
includes looking at ESD products including anti-static bags, ESD wrist straps,
ESD workbenches, ESD packaging and the like.
|
