Chapter 12 ESD Earlier we said that advancements in technology were bringing new challenges for those involved with the operation and maintenance of modern passenger aircraft. One of those challenges is associated with the handling of semiconductor devices that are susceptible to damage from stray electric charges. This is a problem that can potentially affect a wide range of electronic equipment fitted in an aircraft (see Fig. 12.1) and can have wide ranging effects, including total failure of the LRU but without any visible signs of damage! Electrostatic Sensitive Devices (ESD) are electronic components and other parts that are prone to damage from stray electric charge. This problem is particularly prevalent with modern LSI and VLSI devices but it also affects other components such as metal oxide semiconductor (MOS) transistors, microwave diodes, displays, and many other modern electronic devices. Extensive (and permanent) damage to static sensitive devices can result from mishandling and inappropriate methods of storage and transportation. This chapter provides background information and specific guidance on the correct handling of ESD. 12.1 Static electricity Figure 12.1 Part of the avionics bay of a modern passenger aircraft containing LRUs which use large numbers of electrostatic sensitive devices (ESD) Figure 12.2 Lightning (a natural example of static electricity) results from the build up of huge amounts of static charge Static electricity is something that we should all be familiar with in its most awesome manifestation, lightning (see Fig. 12.2). Another example of static electricity that you might have encountered is the electric shock received when stepping out of a car. The synthetic materials used for clothing as well as the vehicle’s interior are capable of producing large amounts of static charge which is only released when the hapless driver or passenger sets foot on the ground! When two dissimilar, initially uncharged non- conducting materials are rubbed together, the friction is instrumental in transferring charge from one material to the other and consequently adecs12 page 1 Composite Monday, September 18, 2006 08:54
Transcript
Chapter
12 ESD
Earlier we said that advancements in technology
were bringing new challenges for those involved
with the operation and maintenance of modern
passenger aircraft. One of those challenges is
associated with the handling of semiconductor
devices that are susceptible to damage from stray
electric charges. This is a problem that can
potentially affect a wide range of electronic
equipment fitted in an aircraft (see Fig. 12.1) and
can have wide ranging effects, including total
failure of the LRU but without any visible signs
of damage!
Electrostatic Sensitive Devices (ESD) are
electronic components and other parts that are
prone to damage from stray electric charge. This
problem is particularly prevalent with modern
LSI and VLSI devices but it also affects other
components such as metal oxide semiconductor
(MOS) transistors, microwave diodes, displays,
and many other modern electronic devices.
Extensive (and permanent) damage to static
sensitive devices can result from mishandling and
inappropriate methods of storage and
transportation. This chapter provides background
information and specific guidance on the correct
handling of ESD.
12.1 Static electricity
Figure 12.1 Part of the avionics bay of a modern passenger aircraft containing LRUs which use large numbers of electrostatic sensitive devices (ESD)
Figure 12.2 Lightning (a natural example of static electricity) results from the build up of huge amounts of static charge
Static electricity is something that we should all
be familiar with in its most awesome
manifestation, lightning (see Fig. 12.2). Another
example of static electricity that you might have
encountered is the electric shock received when
stepping out of a car. The synthetic materials used
for clothing as well as the vehicle’s interior are
capable of producing large amounts of static
charge which is only released when the hapless
driver or passenger sets foot on the ground!
When two dissimilar, initially uncharged non-
conducting materials are rubbed together, the
friction is instrumental in transferring charge
from one material to the other and consequently
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132 Aircraft digital electronic and computer systems
raising the electric potential that exists between
them.
12.1.1 The triboelectric series
The triboelectric series classifies different
materials according to how well they create static
electricity when rubbed with another material.
The series is arranged on a scale of increasingly
positive and increasingly negative materials.
The following materials give up electrons and
become positive when charged (and so appear as
positive on the triboelectric scale) when rubbed
against other materials:
• Air (most positive)
• Dry human skin
• Leather
• Rabbit fur
• Glass
• Human hair
• Nylon
• Wool
• Lead
• Cat fur
• Silk
• Aluminium
• Paper (least positive).
The following are examples of materials that do
not tend to readily attract or give up electrons
when brought in contact or rubbed with other
materials (they are thus said to be neutral on the
triboelectric scale):
• Cotton
• Steel
The following materials tend to attract electrons
when rubbed against other materials and become
negative when charged (and so appear as negative
on the triboelectric scale):
• Wood (least negative)
• Amber
• Hard rubber
• Nickel, copper, brass and silver
• Gold and platinum
• Polyester
• Polystyrene
• Saran
• Polyurethane
• Polyethylene
• Polypropylene
• Polyvinylchloride (PVC)
• Silicon
• Teflon (most negative).
The largest amounts of induced charge will result
from materials being rubbed together that are at
the extreme ends of the triboelectric scale. For
example, PVC rubbed against glass or polyester
rubbed against dry human skin. Note that a
common complaint from people working in a dry
atmosphere is that they produce sparks when
touching metal objects. This is because they have
dry skin, which can become highly positive in
charge, especially when the clothes they wear are
made of man-made material (such as polyester)
which can easily acquire a negative charge. The
effect is much less pronounced in a humid
atmosphere where the stray charge can leak away
harmlessly into the atmosphere. People that build
up static charges due to dry skin are advised to
wear all-cotton clothes (recall that cotton is
neutral on the triboelectric scale). Also, moist
skin tends to dissipate charge more readily.
Human hair becomes positive in charge when
combed. A plastic comb will collect negative
charges on its surface. Since similar charges
repel, the hair strands will push away from each
other, especially if the hair is very dry. The comb
(which is negatively charged) will attract objects
with a positive charge (like hair). It will also
attract material with no charge, such as small
pieces of paper. You will probably recall
demonstrations of this effect when you were
studying science at school.
Electric charge can also be produced when
materials with the same triboelectric polarity are
rubbed together. For example, rubbing a glass rod
with a silk cloth will charge the glass with
positive charges. The silk does not retain any
charges for long. When both of the materials are
from the positive side of the triboelectric scale (as
in this example) the material with the greatest
ability to generate charge will become positive in
charge. Similarly, when two materials that are
both from the negative end of the triboelectric
scale are rubbed together, the one with the
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ESD 133
All modern microelectronic components are
prone to damage from stray electric charges but
some devices are more prone to damage than
others. Devices that are most prone to damage
tend to be those that are based on the use of field
effect technology rather than bipolar junction
technology. They include CMOS logic devices
(such as logic gates and MSI logic), MOSFET
devices (such as transistors), NMOS and PMOS
VLSI circuits (used for dynamic memory devices,
microprocessors, etc). Microwave transistors and
diodes (by virtue of their very small size and
junction area) are also particularly static sensitive
as are some optoelectronic and display devices. If
in doubt, the moral here is to treat any
semiconductor device with great care and to
always avoid situations in which stray static
charges may come into contact with a device.
Printed circuit board assemblies can also be
prone to damage from electrostatic discharge. In
general, printed circuit board mounted
components are at less risk than individual
semiconductors. The reason for this is that the
conductive paths that exist in a printed circuit can
often help to dissipate excessive static charges
that might otherwise damage un-mounted
semiconductor devices (there are no static
dissipative paths when a transistor, diode or
integrated circuit is handled on its own).
Table 12.2 provides a guide as to the relative
susceptibility of various types of semiconductor
device to damage from static voltages.
Table 12.1 Representative values of electrostatic voltages generated in typical work situations
greatest tendency to attract charge will become
negative in charge.
Representative values of electrostatic voltages
generated in some typical working situations are
shown in Table 12.1. Note the significant
difference in voltage generated at different values
of relative humidity.
Typical electrostatic voltage generated
20% relative humidity 80% relative humidity
Walking over a wool/nylon carpet 35 kV 1.5 kV
Sliding a plastic box across a carpet 18 kV 1.2 kV
Removing parts from a polystyrene bag 15 kV 1 kV
Walking over vinyl flooring 11 kV 350 V
Removing shrink wrap packaging 10 kV 250 V
Working at a bench wearing overalls 8 kV 150 V
Situation
Very large electrostatic potentials can be easily generated when different materials are rubbed together. The effect is much more pronounced when the air is dry.
Key Point
Test your understanding 12.2 Explain the importance of the triboelectric series. Give ONE example of a material from the positive end of the triboelectric scale and ONE example of a material from the negative end of the triboelectric scale.
Test your understanding 12.1 Explain, in relation to electric charge, what happens when a glass rod is rubbed with a polyester cloth.
12.2 Static sensitive devices
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134 Aircraft digital electronic and computer systems
Type of device Typical static voltage
susceptibility
CMOS logic 250 V to 1 kV
TTL logic 550 V to 2.5 kV
Bipolar junction transistors 150 V to 5 kV
Dynamic memories 20 V to 100 V
VLSI microprocessor 20 V to 100 V
MOSFET transistors 50 V to 350 V
Thin film resistors 300 V to 3 kV
Silicon controlled rectifiers 4 kV to 15 kV
Table 12.2 Representative values of static voltage susceptibility for different types of semiconductor
12.3 ESD warnings
Static sensitive components (including printed
circuit board cards, circuit modules, and plug-in
devices) are invariably marked with warning no-
tices. These are usually printed with black text on
yellow backgrounds, as shown in Figures 12.3 to
12.7.
Figure 12.3 A typical ESD warning label
Figure 12.4 ESD warning notice (third from bottom) in the avionics bay of a Boeing 737
12.4 Handling and transporting ESD
Special precautions must be taken when handling,
transporting, fitting and removing ESD. These
include the following:
1. Use of wrist straps which must be worn when
handling ESD. These are conductive bands
that are connected to an effective ground point
by means of a short wire lead. The lead is
usually fitted with an integral 1 MΩ resistor
which helps to minimise any potential shock
hazard to the wearer (the series resistor serves
to limit the current passing through the wearer
in the event that he/she may come into contact
with a live conductor). Wrist straps are usually
stored at strategic points on the aircraft (see
Figure 12.5) or may be carried by maintenance
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ESD 135
technicians. Figure 12.6 shows a typical wrist
strap being used for a bench operation whilst
Figures 12.7 and 12.8 show ESD warning
notices associated with the wearing of wrist
straps
2. Use of heel straps which work in a similar
manner to wrist straps
3. Use of static dissipative floor and bench mats
4. Avoidance of very dry environments (or at
least the need to take additional precautions
when the relative humidity is low)
Figure 12.5 Typical on-board stowage for a wrist strap
Figure 12.6 Using a wrist strap for a bench operation (note the grounding jack connector)
Figure 12.7 ESD wrist strap stowage notice
Figure 12.8 ESD wrist strap warning notice
5. Availability of ground jacks (see Fig. 12.6)
6. Use of grounded test equipment
7. Use of low-voltage soldering equipment and
anti-static soldering stations (low-voltage
soldering irons with grounded bits)
8. Use of anti-static insertion and removal tools
for integrated circuits
9. Avoidance of nearby high-voltage sources
(e.g. fluorescent light units)
10. Use of anti-static packaging (static sensitive
components and printed circuit boards should
be stored in their anti-static packaging until
such time as they are required for use).
Note that there are three main classes of
materials used for protecting static sensitive
devices. These are conductive materials (such as
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Test your understanding 12.3 Which one of the following semiconductor devices is likely to be most susceptible to damage from stray static charges?
1. A dynamic memory
2. A silicon controlled rectifier
3. A bipolar junction transistor.
1. A particular problem with the build-up of
static charge is that:
(a) it is worse when wet
(b) it is invariably lethal
(c) it cannot easily be detected.
2. The typical resistance of a wrist strap lead is:
(a) 1 Ω
(b) 1 kΩ
(c) 1 MΩ.
3. Which one of the following devices is most
susceptible to damage from stray static
charges:
(a) a power rectifier
(b) a TTL logic gate
(c) a MOSFET transistor.
4. The static voltage generated when a person
walks across a carpet can be:
(a) no more than about 10 kV
(b) between 10 kV and 20 kV
(c) more than 20 kV.
5. Which of the materials listed is negative on
the triboelectric scale?
(a) glass
(b) silk
(c) polyester.
6. When transporting ESD it is important to:
(a) keep them in a conductive package
(b) remove them and place them in metal foil
(c) place them in an insulated plastic package.
7. To reduce the risk of damaging an ESD
during soldering it is important to:
(a) use only a low-voltage soldering iron
(b) use only a mains operated soldering iron
(c) use only a low-temperature soldering iron.
8. Which one of the following items of clothing
is most likely to cause static problems?
(a) nylon overalls
(b) a cotton T-shirt
(c) polyester-cotton trousers.
12.5 Multiple choice questions metal foils, and carbon impregnated synthetic
materials), static dissipative materials (a
cheaper form of conductive material), and so-
called anti-static materials (these are materials
that are neutral on the triboelectric scale, such as
cardboard, cotton, and wood). Of these,
conductive materials offer the greatest protection
whilst anti-static materials offer the least
protection.
Test your understanding 12.5 Explain the difference between conductive and static dissipative materials for ESD protection.
Stray static charges can very easily damage static-sensitive devices. Damage can be prevented by adopting the correct ESD handling procedures.
Key Point
Test your understanding 12.4 Which one of the following situations is likely to produce the greatest amount of stray static charge?
