ElectroStatic Discharge (ESD) tutorial [3]
a tutorial or summary about the basics of Electrostatic Discharge, ESD and
the ways in which electronic components and circuits can be protected from it
affects.
There are many ways in which the effects of ESD can be
overcome. A variety of methods are employed including products including
anti-static, or static dissipative workbenches, anti-static or static
dissipative containers, static dissipative protection for operators and the
like.
To provide the best protection the problem must be addressed
from several angles:
- An area which is static free (anti-static) must be created. These areas
are often known as electro-static discharge protected areas ( ESDPA ) and
they must be used whenever SSDs or boards containing SSDs are to be handled.
- Any static sensitive devices, or boards containing them must be stored
in conditions where they are not subjected to the effects of static.
- Any boards using SSDs should be designed so that the effects of a
discharge into the board are reduced to acceptable levels.
- Finally any people who come into contact with electronic components or
assemblies should be made aware of the effects of static discharges.
The decision about the number of measures to employ can be
difficult because it is not always easy to determine the cause of any failures.
Additionally it may take many years for some of the failures to occur. However
if sufficient measures are taken then the risks of damages from ESD can be
reduced to sufficiently low levels
Work Areas
To avoid static build up in the area where electronic
components and boards are being handled the bench surfaces should be able to
remove any static build up which occurs. If there is an existing workbench then
it is possible to buy a carbon impregnated rubber ESD mat to place on the bench.
These ESD mats are relatively cheap and are very cost effective. The ESD mats
can simply be placed on top of the existing workbench and used without the need
for expensive new furniture. However, if a new bench is being installed then
special static dissipative surfaces can be used. These ESD workbenches are ideal
as they provide not only the ESD surface, but also the wiring and connection
points for the ESD wrist straps.
The level of conductivity of the work surfaces is important.
If it is too low then it may not only affect the operation of any board or
assembly placed upon it, but when a board is placed onto it, and charge that is
dissipated should not be removed too quickly otherwise damage may occur.
Accordingly the volume conductivity of the material used for the ESD mats and
ESD workbenches should fall into the static dissipative category.
Another essential element in combating static build up on
people is to use ESD wrist straps. These ensure that any charge built up on a
person working on the equipment is safely dissipated. The ESD wrist strap
consists of two sections. The band itself which is worn around the wrist. This
is connected to earth via the lead which incorporates a large value resistor,
normally in excess of 1 M Ohm. This is included for two reasons. The first is
safety, and the second is again to ensure that any static is removed in a
controlled fashion.
The ESD wrist straps should be regularly tested to ensure
they have not become open circuit. Without a test of this nature a faulty strap
could go undetected for many months. Many companies insist that every strap that
is in use is tested every day. In this way any defects can be discovered before
they cause too much damage.
ESD wrist straps, connections to workbench tops and any other
points are normally connected together using a special junction box. These
junction boxes usually have resistors of 1 M Ohm for each of the contacts. These
are joined and then taken to earth. Often a special mains plug with a connection
to only the earth pin can be used. These special plugs are usually yellow and
have two plastic pins for the live and neutral, and a metal pin for the earth.
In this way it is only possible to connect to earth.
Flooring in an electrostatic protected area, or anti-static
area also needs to be considered as specialised ESD flooring is available.
Flooring made out of acrylic materials is likely to generate very high levels of
static. For example, acrylic carpets in the home are particularly bad whereas
natural fibres like wool are much better. Even nylon is not as bad as an acrylic
floor. For an electronics production area there is a wide variety of static
dissipative coverings which can be installed if required to overcome any
problems that might be caused.
If ESD flooring is to be used then conductive footwear must
be worn. There is no point in having static dissipative flooring if peoples'
shoes act as excellent insulators. Most people will want to wear their normal
shoes and not have to wear 'regulation' footwear as this is not likely to be as
comfortable. The solution is to use a heel strap which fits over part of the
shoe. This provides an acceptable path to earth past the shoe. However ESD shoes
are also available.
Clothing is another element that must be considered. Clothes
of wool, cotton or even polyester cotton are normally not a problem. However
some synthetic clothes can develop very high levels of static of their own even
if the person wearing them is grounded by the use of a wrist strap. Acrylic ties
are particularly notable. They can collect high levels of static charge, and
this can be passed to nearby components and electronic boards causing damage. To
overcome this type of problem special static dissipative overalls can be worn.
These normally have a relatively high conductivity to contain any static fields
which might be generated.
Finally chair coverings should also be investigated. They
should not be of the type that generate high levels of static. In some instances
they may need to be dissipative and connected to ground. It is possible to
obtain special seat coverings for existing chairs, or completely new chairs.
Choices can be made dependent upon the state of the chairs and the budget
available.
Another approach that can be taken to help control static and
ESD is to control the humidity. In dry periods of the year, especially winder
when the level of water vapour held in the air drops, the possibility if ESD
rises. By introducing some humidity the levels of static can be reduced.
Although not one of the most commonly used methods of ESD control, there are
several types of humidifiers which can be installed. Some fit into heating
systems whereas others are separate units. Ideally a minimum humidity figure of
50% can be used as an aiming point. Above this the high humidity levels can lead
to other problems.
Storage
Not only do work environments need to have ESD control
measures introduced, but so too do the storage media. Whenever an electronic
component or assembly is transported or stored it should be placed in suitable
packing to ensure that it is not damaged. The dissipative ESD bags for boards
and tubes and special dissipative containers for components are now common place
in the electronics industry. Often the storage ESD bags have a pink or grey tint
to them. The older black conductive bags are used less as they may dissipate the
charge too quickly. Another problem was that they tended to discharge any
on-board batteries more quickly than intended!
In addition to this, within the stores ESD bins and other ESD
packing is required. This will ensure that no static build up occurs.
Soldering Irons
There is a wide variety of soldering irons available on the
market today. Many are quite suitable for work with static sensitive devices.
The main requirement is that the bit used for soldering should be earthed. In
general it is recommended that the resistance to earth should be less than five
[ohm]. Any irons which are thermostatically controlled should ideally use a zero
voltage switching system. This prevents large spikes caused by the switching of
the thermostat from appearing at the tip of the iron and causing damage to the
equipment.
Summary
Although some of the ESD precautions may seem to be rather
extreme, electronic development and manufacturing companies take ESD control
very seriously in view of the damage that can be caused. Investment into
equipment such as ESD wrist straps, ESD workbenches, ESD flooring,, ESD bags and
the like pays dividends in terms of lower fault finding costs during the
manufacturing cycle, and also lower warranty repair costs. This does not take
into consideration any benefits to be gained from maintaining equipment that
gives long and reliable service to customers. Accordingly investment into ESD
control is deemed to be a good investment.
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