Title Page Prev Page Next Page TOC Submit PCR
MAINTENANCE MANUAL
ELECTROSTATIC DISCHARGE STANDARD PRACTICES — SYSTEM DESCRIPTION
1. Electrostatic Discharge System Description
A. General
Aircraft systems include installed equipment with electrical and electronic parts or assemblies that
are susceptible to damage from Electrostatic Discharge (ESD) and these components are ESD
sensitive. Maintenance personnel who remove, inspect, test or install equipment containing ESD
sensitive components should follow special handling procedures to protect them from damage due
to ESD events. An ESD event is a release of a stored electrostatic charge caused by direct contact
with a stored electrostatic charge or exposure to charges induced from electrostatic fields.
All personnel handling ESD sensitive components should be aware of the risk of damage from ESD
events and follow proper handling procedures to diminish the exposure to static generating sources
for ESD sensitive components. Lack of protection for ESD sensitive components throughout the
equipment life may potentially result in increased repair costs, equipment downtime and reduced
mission readiness.
B. Purpose
(1) Handling of Electrostatic Discharge Sensitive Components and Equipment
Only trained personnel are permitted to handle ESD sensitive devices.
(a) All ESD sensitive devices and equipment should be marked appropriately with an ESD
symbol. See Figure 1.
(b) Electrostatic Discharge caution placard / sign must be in place anytime work is being
performed on the aircraft that could expose ESD sensitive devices to ESD events or
damage. Maintenance personnel must follow handling procedures to diminish exposure
to static generating sources for ESD sensitive components or components with internal
ESD sensitive devices.
(c) Personnel are required to wear a wrist grounding strap when opening ESD protective
packaging. Avoid touching circuit components or connector pins when handling ESD
sensitive components or equipment. Protect ESD sensitive components and equipment
with protective containers, conductive caps and / or pin shorting devices. All loose ESD
sensitive components and equipment shall be placed into protective containers prior to
removing grounding wrist strap. Store and transport ESD sensitive components and
equipment in protective containers and seal all containers with an ESD warning label.
See Figure 2.
(d) All tools used for removal and installation of ESD sensitive devices should be grounded
out by being momentarily touched to a grounded point before use. Tools with plastic or
insulated handles can carry a static charge, which does not readily discharge during the
grounding process.
(e) When using air tools, ensure that tool discharge air does not blow directly onto ESD
sensitive devices. Vacuums shall not be used directly on ESD sensitive devices.
(f) When using test equipment, discharge all test leads to ground prior to connection to the
ESD sensitive circuit under test. Use only grounded, electrically isolated and
temperature controlled soldering irons that have been rated for use with ESD sensitive
devices. Test equipment, such as scopes and meters, must be rated for use around
ESD sensitive devices.
(g) Prior to removing an ESD sensitive device from its protective packaging, ground outer
surface in order to dissipate any surface charge on it.
20-23-00
Page 1
July 30/16
Title Page Prev Page Next Page TOC Submit PCR
MAINTENANCE MANUAL
(h) Avoid touching ESD sensitive circuit card assembly components or electrical connector
pins. Only handle circuit card assemblies by the jackscrews, front faceplate or edges.
(i) All conductive protective connector covers shall remain in place until the component is
installed and aircraft wiring is attached. After component installation, protective covers
shall be installed anytime aircraft wiring is detached from ESD sensitive devices. If
conductive protective covers are not immediately available, it is permissible to cover the
ESD sensitive devices connector with conductive tape. Ensure that the entire connector
is covered and that the conductive tape adheres to the connector body and does not
contact the connector pins.
Use of conductive tape is for short intervals until conductive protective connector
covers can be located and installed. Conductive tape is not a substitute for approved
conductive protective connector covers. No ESD sensitive devices shall be stored long
or short term or shipped without approved conductive protective connector covers.
(j) For ESD sensitive component other than circuit card assemblies, install conductive
protective conductive covers on component connectors prior to removal. Place
removed ESD sensitive component into a conductive protective bag and seal the bag
with conductive tape.
(k) If an ESD sensitive device is installed on a removable shelf or equipment rack, covers
must be installed anytime the shelf or rack is detached from its support structure.
(l) Use ESD workstations when removing ESD sensitive devices from protective
packaging for part number and serial number verification. The ESD workstation should
be kept free of any material not required to accomplish the assigned task.
(2) Conductors
All conductors, including personnel, shall be bonded or electrically connected and attached to
a known ground or fixed ground point (as on aircraft). This attachment creates a balance or
uniform potential between all items and personnel.
Electrostatic protection can be maintained at a potential above a 0 voltage ground potential
as long as all items in the system are at the same potential.
(3) Nonconductors
Nonconductors cannot lose their electrostatic charge by attachment to ground. Ionization
systems provide neutralization of charge on these necessary nonconductive items (circuit
board materials and some device packages are examples of necessary nonconductors).
Assessment of the ESD hazard created by electrostatic charge on the necessary
nonconductors in the work place is required to ensure that appropriate actions are
implemented, commensurate with risk to ESD sensitive items.
Necessary nonconductors (i.e., process-required insulators) in the environment cannot lose
their electrostatic charge by attachment to ground. Assessment of the ESD hazard created by
electrostatic charge on the necessary nonconductors in the work place is required to ensure
that appropriate actions are implemented, commensurate with risk to ESD sensitive items.
Electrostatic protection can be maintained at a potential above a 0 voltage ground potential
as long as all items in the system are at the same potential.
C. Background
(1) Nature of Static Electricity
20-23-00
Page 2
July 30/16
Title Page Prev Page Next Page TOC Submit PCR
MAINTENANCE MANUAL
Static electricity is an electrical charge at rest. A object is electrical charged as the result of
one of the following events:
• Electrons can move or migrate within an object causing polarization even when there is
a net overall charge of zero
• Transfer of electrons from one object to another resulting in a net positive or negative
charge (conductive charging)
The movement or transfer of electrons is due to the interaction of charged bodies or charged
and uncharged bodies. The magnitude of an electrical charge is primarily dependent on the
size, shape, composition and electrical properties of the substances which make up the
object. Some substances readily give up electrons while others tend to accumulate electrons.
An object with an excess of electrons is charged negatively and an object with an electron
deficit is charged positively. Any two materials in contact or rubbed together result in each
material becoming charged. One object gains electrons and the other loses electrons. When
the two materials are separated, the net positive or negative charge on each object can be
measured. Although these charges are equal and of opposite polarity, the charges on
nonconductors tend to remain in the localized area of contact. The charges on conductors are
rapidly distributed over its surface and the surfaces of other conductive objects which it
contacts.
Electrostatics (static electricity) and the associated phenomena are extremely complex
physical events. Electrical charge is the fundamental physical problem causing damage to
ESD sensitive parts, assemblies and equipment. Electrostatic charges are generated by any
relative motion, physical separation of materials, movement of solids or flow of liquids and
gases. Release of a stored electrostatic charge or ESD event may occur in numerous ways,
but primarily occurs during he following:
• Any charged object (including a person) coming into contact with an ESD sensitive item
• A charged ESD sensitive item making contact with ground or another conductive object
at a different potential
• An ESD sensitive item grounded while being exposed to an electrostatic field
(a) An electrostatic field (lines of force) are present around any charged object. Conductive
and nonconductive objects in this field will be polarized by induction.
In a conductive (or dissipative) object, electrons are repelled by the negative part of the
electrostatic field leaving that area relatively positively charged. Electrons are also
attracted the positive part of the electrostatic field leaving that area relatively negatively
charged. The net charge on the object remains zero with negatively and positively
charged areas.
In a nonconductive (insulative) object electrons are less mobile, but dipoles tend to
align with the electrostatic field creating apparent surface charges.
If a conductive polarized object is grounded, electrons will flow to or from the polarized
surface near the ground and upon removal of the ground the object retains a net charge
(becoming inductively charged) due to the excess or deficit of electrons. A
nonconductor cannot be inductively charged.
(b) Capacitance of a charged object relative to another object or ground also has an effect
on the electrostatic voltage. When capacitance is reduced for a given charge (Q), there
is an inverse linear increase in voltage based on the relationship (Q = CV), where C is
the capacitance and V is the voltage. As capacitance (C) is decreased the voltage (V)
will increase until a discharge occurs via an arc.
20-23-00
Page 3
July 30/16
Title Page Prev Page Next Page TOC Submit PCR
MAINTENANCE MANUAL
For example, when polyethylene bags are rubbed together the electrostatic charge
potential may be only a few hundred volts while in contact with a work surface.
However, when the same bag is picked up by an technician, the electrostatic charge
potential may be several thousand volts due to the decrease in capacitance.
(2) Source of Static Electricity (Triboelectric Effect)
When two different materials are pressed or rubbed together, the surface of one material will
generally steal some electrons from the other surface. The material with a stronger attraction
for electrons will become negative charged as the other material becomes positively charged.
Generation of electrostatic charges in this manner is called the triboelectric effect. The
triboelectric effect is not very predictable and only broad generalizations can be made.
A triboelectric series is a list of materials arranged by the polarity of charge generated by the
triboelectric effect. A material higher on the list is positively charged (loses electrons) when
contacted with a substance lower on the list which is negatively charged (gains electrons).
See Table 1.
The order of ranking is not always a constant or repetitive. The separation of any two
materials on the list does not necessarily indicate the magnitude of the charges created by
triboelectric effect. Order in the series and magnitude of the charges are dependent upon the
properties of the material. These properties are modified by factors such as purity, ambient
conditions, pressure of contact, speed of rubbing or separation and the contact area over
which the rubbing occurs.
Substantial electrostatic charges can also be generated triboelectrically when two pieces of
the same material are separated as occurs when separating the sides of a plastic bag.
Table 1: Sample Triboelectric Series
Charge MATERIALS
Positive Human hands
Rabbit fur
Glass
Human hair
Nylon
Wool
Fur
Silk
Aluminum
Paper
Cotton
Steel
Wood
Hard rubber
20-23-00
Page 4
July 30/16
Title Page Prev Page Next Page TOC Submit PCR
MAINTENANCE MANUAL
Charge MATERIALS
Nickel, Copper
Brass, Silver
Gold, Platinum
Acetate rayon
Polyester
Orlon®
Polyurethane
Polyethylene
Polypropylene
PVC (vinyl)
KEL-F®
Silicon
Negative Teflon®
NOTE: Sample triboelectric series was extracted from MIL-HDBK-263 and edited to remove
materials no longer used in the manufacture of aircraft and aircraft interiors.
(3) Prime Sources of Static Electricity
Sources of static electricity are essentially insulators and are typically synthetic materials.
Damaging electrostatic voltage levels are commonly generated by contact and subsequent
separation of commonly used materials by processes and personnel movement. Materials
which are prime generators of electrostatic voltages include, but are not limited to the
following:
• Glass
• Fiberglass
• Foam
• Plastics
• Polyurethane
• Rubber
• Synthetic textiles
• Vinyls
Electrostatic voltage levels generated with these insulators can be extremely high since they
are not readily distributed over the entire surface of the substance or conducted to another
contacting substance. The conductivity of some insulators increases with absorption of
moisture under high humidity conditions onto the otherwise insulating surface, creating a
slightly conductive sweat layer which tends to dissipate static charges over the material
surface. The generation of 15,000 volts from common plastics in a typical facility is not
unusual.
Table 2 identifies a few typical activities and the potential electrostatic charge levels that can
be generated. The conductivity of some insulators increases in higher humidity conditions,
20-23-00
Page 5
July 30/16
Title Page Prev Page Next Page TOC Submit PCR
MAINTENANCE MANUAL
creating a slightly conductive sweat layer which tends to dissipate static charges over the
material surface. The generation of 15,000 volts from common plastics is not unusual.
Table 2: Electrostatic Charge Levels
ACTIVITY MOST COMMON HIGHEST READING
READING (VOLTS) (VOLTS)
Walking across carpet 12,000 39,000
Walking across vinyl floor 4000 13,000
Seated in polyuerthane foam chair 1800 18,000
Picking up poly bag 1500 20,000
Inserting paperwork into vinyl 800 7000
envelopes
Data based on ambient relative humidity = 15 - 36%
(4) Susceptibility of Parts, Assembles and Equipment
Parts, assembles and equipment susceptible to damage when an ESD event occurs or when
these parts are exposed to electrostatic fields are ESD sensitive. These ESD sensitive
devices can be degraded or damaged by an ESD event regardless of their electrical and
ground connections. A ground connection is not required to damage an ESD sensitive device.
Parts with pins connected to ground and their voltage and signal sources applied can be
damaged by an ESD event even when installed in their parent assemblies and equipment.
Assemblies and equipment containing ESD sensitive devices are often as sensitive as the
most sensitive ESD sensitive devices they contain. Incorporation of protective circuitry in
assemblies and equipment provides varying degrees of protection from ESD events applied
to their terminals. Such assemblies and equipment are still vulnerable from induced ESD
caused by strong electrostatic fields or by direct contact of a part, assembly or equipment with
a charged object.
(5) Types of Electrostatic Discharge Induced Failures
Electrostatic discharge can cause intermittent or transient failures as well as hard
(catastrophic) and latent failures.
(a) Intermittent or transient failures can occur on certain types of parts. Such failures occur
during equipment operation and are usually characterized by a loss of information or
temporary distortion of its functions. No apparent hardware damage occurs. Proper
operation resumes automatically after the ESD exposure or after re-entry of the
information by resequencing the equipment. This type of failure can be a result of the
electrical noise associated with an ESD spark in the vicinity of the equipment. The
electrical noise may enter electronic equipment by either conduction of radiation.
Equipment operation becomes intermittent or transient if the ESD induced voltage and
/ or current exceed the signal levels in the electronic circuit. In high impedance circuits
the signals are voltage levels, thus capacitive coupling will dominate and ESD induced
voltage will be the major problem. In low impedance circuits the signals are current
based, thus inductive coupling will dominate and the ESD induced currents will cause
the problem.
(b) Catastrophic failures can be the result of electrical overstress of electronic parts caused
20-23-00
Page 6
July 30/16
Title Page Prev Page Next Page TOC Submit PCR
MAINTENANCE MANUAL
by an ESD event such as follows: a discharge from a person or object, an electrostatic
field or a high voltage spark discharge. Some catastrophic failures may not occur until
after exposure to multiple ESD events. Marginally damaged ESDS parts, which require
operating stress and time to cause further degradation, may ultimately experience
catastrophic failure. The result of an ESD induced hard failure in electronic components
may be metal melt, junction breakdown on oxide failure, rendering the device
permanently damaged.
(c) Latent defects are very difficult to detect. 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 a premature failure after the
user places them in service. Latent defects are virtually impossible to prove or detect
using current technology, especially after the device is assembled into a finished
product.
(6) Electrostatic Discharge Testing Models
The ESD testing of electronic devices provides essential information regarding the sensitivity
of these devices to the electrostatic discharge phenomenon. This information aids in
designing appropriate protection structures and procedures. The main purpose of component
testing standards are as follows:
(a) Develop suitable models for the more common ESD threats.
(b) Enable comparisons to be made between devices.
(c) Provide a system of ESD sensitivity classification to assist in the design and monitoring
requirements of the manufacturing and assembly environments.
(d) Document a procedure to ensure reliable and repeatable results.
Different ESD testing models have been developed, which represent three major sources of
ESD. People are the prime source of ESD damage. Another source is the electrostatic charge
on a machine, assembly or manufacturing equipment. A third source is the charge on the
component itself. The component may exhibit different levels of ESD sensitivity depending
upon the type of component, the design of the protection structures and the source and
nature of the discharge.
(7) Classifications of Electrostatic Discharge
Classifications are established for ESD sensitivity and provided by the equipment
manufacturer. The susceptibility of components to ESD is defined by the following three
separate models:
(a) The human body model is the most commonly used model for classifying device
sensitivity to ESD. This testing model considers the principle source of ESD damage is
from the human body and represents the discharge from the fingertip of a standing
individual delivered to the device.
CLASS VOLTAGE RANGE
1 0 - 1999 Volts
2 2000 - 3999 Volts
3 4000 - 15,999 Volts
20-23-00
Page 7
July 30/16
Title Page Prev Page Next Page TOC Submit PCR
MAINTENANCE MANUAL
(b) The machine model represents the discharge occurring from the charged cables of a
device or board tester. Machine model originated after it was recognized that the
human body model does not represent a worst-case condition.
(c) The charged device model is the most important model for modern automated
assembly and manufacturing environments. Although all components can be subjected
to device model discharges, integrated circuit devices are most vulnerable.
Components can become charged in various ways. The most common occurrence is
that of a component sliding down a delivery rail to a tester, automated insertion
machine or for marking and branding. The charge generated on the component
consists of both mobile and immobile charges. If the component touches a metallic
surface while it is charged, a very rapid discharge occurs during which current levels
can reach several tens of amperes.
20-23-00
Page 8
July 30/16
Title Page Prev Page Next Page TOC Submit PCR
MAINTENANCE MANUAL
Electrostatic Discharge Symbols
Figure 1
20-23-00
Page 9
July 30/16
Title Page Prev Page Next Page TOC Submit PCR
MAINTENANCE MANUAL
Electrostatic Discharge Caution Label
Figure 2
20-23-00
Page 10
July 30/16
