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Prevent Electrostatic Damage To
Electronic Components
Unit 5934
Certificate In Electrical Engineering
Level 2
Student Name ……………………………..
Prevent Electrostatic Damage
Prevent electrostatic damage to electronic components
Element 1
Causes of static electricity in the electronics workplace, and the damage to
semiconductors
Introduction
Static electricity is defined as an electrical charge caused by an imbalance of
electrons on the surface of a material. This imbalance of electrons produces an
electric field that can be measured and that can influence other objects at a distance.
Electrostatic discharge is defined as the transfer of charge between bodies at
different electrical potentials.
Electrostatic discharge can change the electrical characteristics of a semiconductor
device, degrading or destroying it. Electrostatic discharge also may upset the normal
operation of an electronic system, causing equipment malfunction or failure. Another
problem caused by static electricity occurs in clean rooms. Charged surfaces can
attract and hold contaminants, making removal from the environment difficult.
ESD is the abbreviation of electrostatic discharge. It is more commonly referred to
as static electricity or just plain static.
Static electricity is an electrical charge at rest.
Every object has an electrical charge. This charge is either:
Positive (+) - where there are more protons around the nucleus of the atom than
electrons;
Negative (-) - where there are more electrons around the nucleus than protons;
or
Neutral - where there are the same number of protons and electrons forming the
atom.
The static charges, caused by the charge imbalances, being built up on equipment
or persons are the main causes of damage to electronic components in the
workplace.
There are three methods of generating a static voltage on a piece of equipment or
person.
1
2
3
Tribo-electric charging;
Inductive charging;
Capacitive charging.
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1.1.1 Tribo-electric Charging (Friction)
All materials have the ability to hold a charge. Materials likely to cause a problem in
the electrical/electronics industry are: nylon, human and animal hair, wool, and
asbestos with positive potentials and silicon, polythene, rayon, and polyester
with negative potentials. The intensity of the charge formed on each of these
materials is inversely proportional to the relative humidity. Walking across a carpet
is a common source of charge potential. Discharge occurs when the person touches
a conductive surface.
When a charged object comes in contact with another object, the electrostatic
charge will attempt to find a path to earth, thereby discharging the object. The
discharge current can be very low (nA) but the voltage can be very high (20-5OkV).
The Triboelectric Charge. Materials The Triboelectric Charge – Separation
Make Intimate Contact - Figure 1
Figure 2
Creating electrostatic charge by contact and separation of materials is known as
"tribo-electric charging." It involves the transfer of electrons between materials. The
atoms of a material with no static charge have an equal number of positive (+)
protons in their nucleus and negative (-) electrons orbiting the nucleus. In Figure 1,
Material "A" consists of atoms with equal numbers of protons and electrons. Material
B also consists of atoms with equal (though perhaps different) numbers of protons
and electrons. Both materials are electrically neutral.
When the two materials are placed in contact and then separated, negatively
charged electrons are transferred from the surface of one material to the surface of
the other material. Which material loses electrons and which gains electrons will
depend on the nature of the two materials. The material that loses electrons
becomes positively charged, while the material that gains electrons is negatively
charged. This is shown in Figure 2.
The actual level of charge is measured in coulombs. Commonly, however, we speak
of the electrostatic potential on an object, which is expressed as voltage.
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This process of material contact, electron transfer and separation is really a more
complex mechanism than described here. The amount of charge created by
triboelectric charging is affected by the area of contact, the speed of separation,
relative humidity, and other factors. Once the charge is created on a material, it
becomes an "electrostatic" charge (if it remains on the material). This charge may be
transferred from the material, creating an electrostatic discharge, or ESD, event.
The degree of damage caused by electrostatic discharge is dependent upon:
1
The size of the charge, which is determined by the capacitance of the object.
2
The rate at which the charge is dissipated, which is determined by the
resistance into which it is discharged.
The following table provides examples of typical static voltage levels.
Table 2
Examples of Static Generation
Typical Voltage Levels
Means of Generation
10-25% RH
65-90% RH
(Dry conditions)
(Damp)
Walking across carpet
35,000V
1,500V
Walking across vinyl tile
12,000V
250V
Worker at bench
6,000V
100V
Poly bag picked up from bench 20,000V
1,200V
Chair with urethane foam
1,500V
18,000V
1.1.2 Inductive Charging
When a charged object is brought close enough to another object it may induce or
force a charge on the other object without actually touching. Your hair standing on
end when you approach a large electrical charge is one example.
It is important to realise that you can induce a charge on anything you carry or get
close to even though you may be electrically insulated from it.
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1.1.3 Capacitive Charging
When two insulated objects become charged and the distance between them is
increased the voltage difference between the objects will also increase.
Q=CxV
Q = The charge in coulombs
C = The capacitance in farads
V = The voltage in volts.
(I x t)
Increasing the distance between the two objects decreases the capacitance. Since
the charge is constant, the voltage must increase by an amount equal to the
capacitance decrease. Lifting or moving a charged object will increase the value of
voltage and increase its potential to cause irreparable damage.
The effect that each potential static source has to produce the damaging voltages
can be reduced by the careful use of safeguards and practices that will be covered
later in this unit. However, tribo-electric charging is the most common method of
generation
Tribo-electric Series
When two materials contact and
separate, the polarity and magnitude of
the charge are indicated by the materials’
positions in the tribo-electric series. The
tribo-electric simply lists materials
according to their relative tribo-electric
charging characteristics. When two
materials contact and separate, the one
nearer the top of the series takes on a
positive charge, the other a negative
charge. Materials further apart on the
table typically generate a higher charge
than ones closer together. See the Table
Typical Tribo-electric Series
+
Positive
Negative
-
Acetate
Glass
Nylon
Wool
Lead
Aluminum
Paper
COTTON
Wood
Steel
Nickel-Copper
Rubber
Polyester
PVC
Silicon
Teflon
Virtually all materials, including water and dirt particles in the air, can be triboelectrically charged. How much charge is generated, where that charge goes - and
how quickly, are functions of the materials' electrical characteristics.
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Causes of Static Electricity in the Workplace
There are a number of key causes of static electricity in the workplace
Some of these are:







Walking across a carpet
Walking across a vinyl tiles
Using sellotape at work bench
Using a plastic – stand alone solder sucker
Personnel clothing – especially made of man-made fibres
Plastics - Styrofoam cups, lolly wrappers, common plastic items, (bags, boxes
etc) are all notorious static generators
Aerosol sprays for painting and cleaning
The build up of static potentials in the workplace can come from a vast number of
sources. Among the most common are non-conductive plastic packaging
materials, such as polyethylene, polystyrene, or mylar. Standard bubble pack and
plastic packaging can also create high static voltages - from 1200v to 20,000 volts
Even a plastic foam coffee cup can generate a damaging static voltage.
The type of clothing worn in the workplace should be carefully chosen.
Floor coverings also need to be carefully chosen. As previously stated even walking
across a carpeted floor will cause a large static potential to be induced into a person.
The use of aerosol sprays for cleaning, painting and cooling, in the area of printed
circuit boards (PCBs) and edge connectors can create significant Static voltage
levels.
At the work bench the use of plastic solder suckers and some aerosol sprays should
be kept to a minimum. These can be considered as the silent killers of the
workplace.
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Low humidity levels result in much higher static voltage levels.
Faulty air conditioning resulting in very low humidity levels can significantly
contribute to high static voltage levels.
How Damage in Semiconductors Occurs and is Prevented
An ESD event can occur when any charged conductor (including the human body)
discharges to an ESDS (electrostatic discharge sensitive) device. The most
common cause of electrostatic damage is the direct transfer of electrostatic charge
from the human body or a charged material to the electrostatic discharge sensitive
(ESDS) device. When one walks across a floor, an electrostatic charge accumulates
on the body. Simple contact of a finger to the leads of an ESDS device or assembly
allows the body to discharge, possibly causing device damage.
When a charged object comes in contact with another object, the electrostatic
charge will attempt to find a path to earth, thereby discharging the object. The
discharge current can be very low (nA) but the voltage can be very high (20-5OkV).
The degree of damage caused by electrostatic discharge is dependent upon:
1
The size of the charge, which is determined by the capacitance of the object.
2
The rate at which the charge is dissipated, which is determined by the
resistance into which it is discharged.
1.3.1. Static Generators
The elimination of static generators from the work area is a very important
consideration and is the responsibility of all personnel in the area. Among
common static generators are:
Personnel clothing - Man-made fibres make up the large percentage of the
composition of clothing, clothes have the potential to be excellent static
generators. It may be necessary to have issued ESD protective smocks,
gloves, footwear, masks and aprons.
Spraying cleaning and painting - If possible conductive solvents and paints
should be used in the servicing process. If this is impractical then it is
essential that ionized air blowers are used to prevent the build up of static
charges.
Plastics - Styrofoam cups, lolly wrappers, common plastic items, (bags,
boxes etc) are all notorious static generators. Articles of common plastic,
rubber, nylon, or untreated cotton should never be used in static sensitive
areas.
Prevention involves the reduction of materials and situations which will generate a
static charge. The use of a discharge path to earth is a key element – using
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workstations with grounded mats and wrist straps whenever electrostatically
sensitive equipment is being used or worked on is essential.
Storage of components and Printed circuit boards in ESD shielded containers/bags
is another key element in the prevention of damage.
Damage to Semiconductors
Destructive voltages and/or currents from an ESD event may cause a device to fail
because of thermal fatigue or dielectric breakdown. Metal-oxide-semiconductor
field effect transistors (MOSFETs) are usually constructed with oxide layers between
the device elements. The thickness (or thinness) of this oxide layer is the main
determining factor on the survivability of these devices to electrostatic punchthrough.
Electrostatic damage to electronic devices can occur at any point from manufacture
to field service. Damage results from handling the devices in uncontrolled
surroundings or when poor ESD control practices are used. Generally damage is
classified as either a catastrophic failure or a latent defect.
1.4.1 Catastrophic Failure
When an electronic device is exposed to an ESD event it may no longer function.
The ESD event may have caused a metal melt, junction breakdown, or oxide failure.
The device's circuitry is permanently damaged causing the device to fail – this is a
catastrophic failure. Such failures usually can be detected when the device is tested
before shipment. If the ESD event occurs after test, the damage will go undetected
until the device fails in operation.
1.4.2 Latent (delayed) Defect
A latent defect, on the other hand, is more difficult to identify. A device that is
exposed to an ESD event may be partially degraded, yet continue to perform its
intended function. However, the operating life of the device may be reduced
dramatically. A product or system incorporating devices with latent defects may
experience premature failure after the user places them in service. Such failures are
usually costly to repair and in some applications may create personnel hazards. The
device may also have intermittent faults as a result of the damage sustained.
Damage may include punch through of conductors and semiconductor material as
illustrated in the followed pictures.
1.4.3 Intermittent Failure
Perhaps the most sinister defect is partial damage which causes intermittent failure
of devices, for example, failure is only noticed when the device reaches a certain
operating temperature. Again, this type of failure is difficult to detect and usually
very costly to repair.
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1.4.4.
Device Sensitivity - Threshold Levels
Different devices are more or less susceptible to electrostatic damage – caused
mainly by their methods of manufacture.
Semiconductor Device
Type
MOSFET
VMOS
NMOS
GaAsFET
EPROM
CMOS
JFET
Schottky Diodes
Film Resistors
Bipolar Resistors
Threshold Susceptibility
(volts)
10 -100
30 - 1800
60 - 100
60 - 2000
100+
200 - 300
140 - 7000
300 - 2500
300 - 4000
300 – 7000
Note from the table above that the threshold voltage, especially for devices common
in everyday electronic appliances (MOSFETs, CMOS, NMOS and EPROMs) is
generally very low and well within the levels of prevalent static charges built up in
everyday situations.
1.4.4 Review of element 1
In element 1 it was seen that;
A static charge can be generated by one of three methods.



Tribo-electric
Inductive
Capacitive charging
Tribo-electric charge generation is the most common method.
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Electrostatic discharge is the result of the action of positively or negatively charged
bodies acting upon other charged or neutral bodies.
Large static voltages can be generated by simply walking across carpet, sitting on a
chair or pulling tape off a circuit assembly.
Increasing the distance between two charged objects decreases the capacitance but
increases the voltage between them.
1.4.5 Review questions
1.
Explain a method of generating a static voltage.
2.
State three methods of creating a static voltage.
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Element 2
Precautions used to minimize electrostatic discharge
2.1
Rules for handling of components and printed circuit boards
(PCBs)
The basis for ESD damage prevention can be covered with three golden rules:
1.
Treat all electronic parts and assemblies as static sensitive.
2.
Handle all sensitive parts and assemblies at static safe workstations.
3.
Package parts properly for storage and transportation.
All ESD sensitive components, devices and assemblies should be handled as
detailed below to prevent ESD damage to them:

Personnel handling ESD sensitive items should be suitably trained or at
least be aware of recommended ESD procedures.

Treat any part received in ESD packaging as eectrostatically sensitive.
ESD packaging and ESD sensitive parts should be identified with ESD
caution symbols.
Identification symbol for static sensitive parts

ESD items must be protected from static charges at all times. They should
have shorting links attached or be placed in conductive foam and be stored
and transported in enclosed, conductive containers at all times until they
are being installed.
They should only be installed at static safe
workstations.
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
Personnel removing ESD sensitive items from storage containers should be
adequately grounded prior to doing so.

Conductive storage containers should be bonded together before transferring
ESD sensitive items between boxes.

Charges on all ESD packages should be neutralised before opening by first
touching them to a grounded work mat prior to opening and removing parts
from them.

ESD sensitive parts should be identified as such in the appropriated
servicing instructions, parts breakdown lists, circuit diagram and on the circuit
board or assembly itself.

The leads of ESD sensitive items should not be probed without first
grounding the probe to bleed off any static charge.

Power should not be applied during removal or installation of any ESD
sensitive item.

The person handling the PCB should wear a grounded wrist strap ( having a
lead resistance of 1-10M). (see later notes)

Only the edges of the PCB should be handled.

The PCB should be placed on a grounded mat.

The PCB should be stored in an ESD protective bag.
2.1.1 Shielded bags and containers
There are essentially three types or categories of electrostatically safe container:
 antistatic bags
 dissipative bags
 metallized bags
The latter two categories are typically the high-end of ESD bag product offerings and
tend to have three combined properties of protection, (1) antistatic, (2) dissipative
and (3) shielding.
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Protective bags
The bags can be padded or non-padded.
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Static Shielded Tote boxes are also available for storage of assembled printed circuit
boards and components.
ESD sensitive electronic equipment packaging is often marked with the ESD
susceptibility symbol
The ESD Susceptibility Symbol (above) is applied directly to integrated circuits,
boards, and assemblies that are static sensitive.
ESD Protective Symbol
The symbol indicates ESD protective material. It is applied to mats, chairs, wrist
straps, garments, packaging, and other items that provide ESD protection.
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Selection of Clothing, Furnishings and Floor Coverings
To reduce electro-static charges, clothing and materials used in the workplace
should be selected to reduce static charges.
Materials likely to cause a problem in the electrical/electronics industry are: nylon,
human and animal hair, wool, and asbestos with positive potentials and silicon,
polythene, rayon, and polyester with negative potentials. The intensity of the
charge formed on each of these materials is inversely proportional to the relative
humidity. Walking across a carpet is a common source of charge potential.
Discharge occurs when the person touches a conductive surface.
2.2.1 ESD Floor Finish
An ESD Floor finish will Prevent a static charge generating on mobile personnel and
equipment which is preferable than having a static charge build and then removing it.
An effective ESD floor will provide 3 protective properties
a)
Provide a dissipative or conductive surface
To drain charges from operators or equipment that comes in contact
with its surface
b)
Provide a ground for personnel
To ground mobile operators and equipment in contact with its surface
c)
Provide a non charge generating surface
To prevent charge generation from the conductive or dissipative floor
surface to operators and equipment traveling on its surface.
It is recommended that a grounded floor mat be used as part of a workstation.
2.2.2 Treatment of Floor Coverings
Carpet should be non-flammable and antistatic. There are two types:

Carpets manufactured of antistatic material

Carpets manufactured of conventional material, then treated with an antistatic
agent after manufacture
Antistatic qualities of treated carpets are short-lived. They must be re-treated
regularly to maintain antistatic integrity.
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If using existing carpeting that is not antistatic:

Clean the carpet thoroughly.

Treat the carpet with antistatic agent.

Repeat the antistatic treatment at three-month intervals to maintain a staticfree environment.
Ionized Air Blowers
A work area can still harbor dangerous charges even after applying the proper
grounding techniques. Grounding is a very effective ESD technique for conductive
or dissipative materials, but is ineffective for non-conductors, i.e., insulators which
are materials with resistance greater than 1 x 10 12 .
Examples of insulative materials include human hair, man-made fibres in clothing,
some hand tool handles, latex and vinyl gloves, various tapes, test equipment,
computer video monitors, key boards, and many packaging and storage items. Even
the main constituent of electronic circuit boards - the board itself – is an insulator.
Ionized air is the most effective method to neutralize charged insulators. High
voltage (corona) ionization is the most common ionization technology in the
semiconductor and electronics industries. The most widely used commercial method
for ionization is the high voltage corona technology, of which there are three types:

AC

steady state DC

pulsed DC
Which type of ionization technology should be used to neutralize charges in a
particular work area depends on the application and working conditions and can be
a recommended addition for an ESD control program.
The ionized air blower emits equal levels of positive and negative ions. This
provides a neutral air environment for electronics repair, servicing and assembly.
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Bench-top air ionizers neutralize static charge on non-conductive materials and help
prevent static from damaging sensitive electronic components.
The Overhead air ionizer "bathes" a typical work area in ionized air, neutralizing
static charges on the surface of any object in the area.
Classical applications for these ionizers are:

automated machinery
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
test equipment

wave solderers

clean room areas

conveyor belts

component kitting

in shipping and receiving areas
ESD Work benches can greatly benefit from the application of Overhead Ionizers.
Static Safe Workstation
The minimum requirements for the static workstation are an ESD grounded mat with
wrist strap on the bench and an anti-static floor covering.
The workstation provides a means for connecting all work surfaces, fixtures,
handling equipment, and grounding devices to a common point ground. In addition,
there may be provision for connecting additional personal grounding devices,
equipment, and accessories such as constant ground monitors and ionizers.
The key ESD control elements comprising most workstations are a static dissipative
work surface, a means of grounding personnel (usually a wrist strap), a common
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grounding connection, and appropriate signage and labeling. A typical workstation is
shown above.
To assist with the prevention of ESD damage to components it is advisable to carry
out any servicing in an electro safe workstation.
An electro safe workstation consists of an area where all conductive objects are
electrically connected together (bonded) and earthed (ground).
SAFETY
All grounding connections should be made with cables having a resistance of
1M ± 0.2M to limit any discharge current to a low value.
SAFETY
The following are specific requirements for a static safe workstation:

Grounded workbench. This can be achieved with a conductive mat on the
bench surface or with an all-metal bench.

Personnel grounding straps. (connected through a resistance lead of 1 to
10 MΩ to an earthed mat) Wrist straps are most common but ankle and leg
straps are also available. The strap should be flexible and long enough so as
not to restrict movement. The strap should not be used when working on
live equipment.

Grounded floor mats (They are connected to earth with a lead of resistance
less than 1 Ω). These are placed around the workstation to ensure that any
static build up on personnel approaching the bench will be grounded before
it can cause any harm to equipment on the bench.

Conductive containers. The workstation should only have conductive type
storage trays and bags or conductive foam for loose ESD sensitive
components. Loose ESD sensitive components should further be protected
with shorting links or shunts.

Relative Humidity. Humid air helps to dissipate static charges. The
workshop air should be kept at approximately 50 - 60 % relative humidity
provided it does not cause any other problems within the workshop. Humidity
is simply controlled by adjustments to the workshop air conditioning unit.

Ionisers. Where the formation of static charges, at already protected
workstations, becomes critical, ionized air blowers may be used to dissipate
the charge as it forms. Ionized air blowers put out a stream of ionized
particles which attach themselves to charges of the opposite polarity and
neutralise the offending charge.
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
Grounded Tools. Mains operated power tools (soldering Irons etc) should be
supplied through an isolating transformer and then bonded to the work
bench. Hand tools (diags, pliers etc) should not be able to produce a static
charge. The handgrips on tools should have any non-conductive material
removed.

Signs. Each static safe workstation should be well signposted as such. This
should help to prevent static generators and unsafe practices being bought
into the area.
2.4.1 Wrist strap
The adjustable wrist strap and associated lead is designed to provide a discharge
path for earthing the body
Conductive and Dissipative Mats in Temporary Work Situations
When working in a remote environment the use of a portable field service kit is
recommended. The following pictures show examples of a typical portable field
service kit.
This portable field service kit provides a static-free environment and reduces risk of
static discharge during field repair of printed circuit boards and other electronic
equipment in a compact, lightweight package. It weighs less than 0.5 kg and folds
up to a compact package. The kit includes a red static dissipative mat with pocket,
an adjustable wrist strap band, a wrist strap ground cord, an extension ground cord,
a work surface ground cord and an un-insulated alligator clip
.
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The portable field service kit also provides a means of storing a PCB.
2.6
Review of element 2
In element 2 it was seen that:
1.
ESD damage prevention can be achieved by following the three golden rules:
i
ii
iii
Treat all electronic parts and assemblies as static sensitive;
Handle all sensitive parts and assemblies at static safe workstations;
Package parts properly for storage and transportation.
2.
The correct selection of clothing, furnishings and floor coverings will minimize
electrostatic generation.
3.
Ionized air blowers are used to neutralize charged insulators.
4.
A static safe work station comprises are ESD grounded mat, with wrist strap
on the bench and an anti-static floor covering.
A correctly set up static safe workstation would have the following minimum
requirements:
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i
ii
iii
iv
v
vi
vii
Grounded Work bench;
Personnel grounding straps;
Grounded floor mats;
Conductive containers;
Grounded tools;
Signs indicating the static safe area;
Personnel should wear static protective clothing in the vicinity of the
work area.
All grounding connections should be made with cables having a resistance of 1M
to limit any discharge current to a low value.
5.
In temporary workplaces Static dissipative portable field service kits should be
used, which comprises a flexible dissipative mat/storage bag and wristrap.
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2.6.1 Review Questions
1.
List the three golden rules associated with the handling of static sensitive
devices.
2.
List five precautions that should be observed when handling static sensitive
devices.
3.
List five requirements for a static safe workstation.
4.
Describe three common forms of static generator.
Element 3
Demonstrate the use of anti-static precautions
Note
Element 3 of this Unit Standard will be completed as part of the Soldering Unit
15849 element 4 - Perform soldering and de-soldering on printed circuit
boards, (PCBs)
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