topic 6 esd & emi considerations electrostatic sensitive devices (m4.2, 5.12 &5.14)_1

ESD & EMI Considerations

Aerospace Electronics2

Specific Learning Outcomes

• Explain electrostatic discharge (ESD).• Understand the risks and possible damages

involved due to improper handling of ESD components.

• Describe the special handling of components sensitive to ESD discharges and anti-static protection devices for components and personnel.

• Explain the use of PCB and the materials used in the manufacturing processes.

• Discuss the use of PCB and protective measures required.

Reprinted with permission of the copyright owner, Barry College ICAT. All rights reserved. .

• Discuss the influence of EMC – Electromagnetic Compatibility, EMI – Electromagnetic Interference, HIRF – High Intensity Radiated Field, Lightning/lightning protection on maintenance practice for electronic systems.

• Discuss the units installed in the aircraft and portable units like mobile phones, electronic notebooks that contribute to electromagnetic radiation.

REFERENCESIR Part 66 Aircraft Maintenance License (Module 5) by Barry College ICAT

3Aerospace Electronics

Introduction

Advancements in technology challenge is associated with the handling of semiconductor devices that are susceptible to damage from stray electric charges.

Problem can potentially affect a wide range of electronic equipment fitted in an aircraft including total failure of the LRU but without any visible signs of damage!


Avionics bay of passenger aircraft containing LRUs and electrostatic sensitive devices (ESD)


Electrostatic Sensitive Devices (ESD) are electronic components and other parts that are prone to damage from stray electric charge.

Extensive (and permanent) damage to static sensitive devices can result from mishandling and inappropriate methods of storage and transportation.


Static electricity

When two dissimilar, initially uncharged non-conducting materials are rubbed together, the friction is responsible in transferring charge from one material to another and thus raising the electric potential between them.


Triboelectric charging

The process of electron transfer as a result of two objects coming into contact with each other and then separating is known as 'triboelectric charging'.

The amount of charge depends on factors like the rate of separation, humidity etc.


AIRHUMAN HANDSASBESTOSRABBIT FURGLASSMICAHUMAN HAIRNYLONWOOLLEADSILKALUMINIUMPAPERCOTTON -----STEEL WOOD AMBER SEALING WAX HARD RUBBER NICKEL COPPER BRASS SILVER GOLD PLATINUM SULFURACETATE RAYON POLYESTER CELLULOID ORLON SARANPOLYURETHANEPOLYPROPYLENE

PVC (VINYL) KEL-F (CTFE) SILICON TEFLON

Increasingly Positive

Increasingly Negative

Neutral

The Triboelectric series


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 & silver• Gold and platinum • Polyester• Polystyrene • Saran• Polyurethane • Polyethylene• Polypropylene • Polyvinylchloride (PVC)• Silicon • Teflon (most negative)


When both of the materials are from the positive side, the material with the greatest ability to generate charge will become positive in charge.

Similarly, when two materials that are both from the negative side are rubbed together, the one with the greatest tendency to attract charge will become negative in charge.


Electrostatic voltages due to triboelectric effect

Table 1 Representative values of electrostatic voltages generated in typical work situations

Situation Typical electrostatic voltage generated

20% relative humidity 80% relative humidity

Walking over a wool/nylon carpet 35kV 1.5kV

Sliding a plastic box across a carpet

18kV 1.2kV

Removing parts from a polystyrene bag

15kV 1kV

Walking over vinyl flooring 11kV 350V

Removing shrink wrap packaging 10kV 250V

Working at a bench wearing overalls

8kV 150V


Electrostatic Sensitive Devices (ESD)

All modern microelectronic components are prone to damage from stray electric charges.

These devices that can be damaged or destroyed by static electricity discharges due to improper handling are referred to as Electrostatic Sensitive Devices (ESD).


Static voltage susceptibility Type of Device Typical static voltage susceptibility

CMOS Logic 250V to 1kV

TTL Logic 550V to 2.5kV

Bipolar Junction Transistors 150V to 5kV

Dynamic memories 20V to 100V

VLSI microprocessor 20V to 100V

MOSFET transistors 50V to 350V

Thin Film resistors 300V to 3kV

Silicon Controlled Rectifiers 4kV to 15kV

Table 2Representative values of static voltage susceptibility for different semiconductor devices


ESD Damage

In the aircraft, damage to the internal components of an ESDS LRU• Changes in system characteristics and / or

performance degradation also known as latent defect

• Complete destruction or catastrophic failure

Latent defect refers to a device that is partially degraded yet can still perform its intended function.


1. Electrostatic discharge from a human body or charged material to the ESDS device.

2. Electrostatic discharge from the device3. Field induced discharges

Events that causes ESD


1. Discharge from human body or charged material

When one walks across a floor

High Voltage potential of the static charge in insulators and semiconductor junctions


2. Device electrostatic discharge

Electro Static field induced current or voltage on the ESD device

Movement across surface or vibration in a package

3. Field induced discharges


ESD warning label

ESD warning notice in the avionics bay of a Boeing 737

ESD warnings


Various types of ESDS devices identifiers that can be found in the aircraft LRUs.

Caution Yellow is the background for all 3 symbols


Boeing ESD decals


Placards installed on the ESD LRU shows that you must take the necessary precautions from static discharges when removing, installing and moving ESD LRU’s.

The ESD printed circuit boards are identified with a static sensitive placard.

General rules when handling ESD


ESD LRU room


Removal of ESD LRU’s and ESD circuit boards

Contaminated or unwanted material should be kept clear of the sensitive devices.

Connect the wrist strap into the Electrostatic Ground Jack of the card file or recognised aircraft ESD ground point.

The electrical connectors are to be covered using the correct conductive blanks.

LRU or circuit boards are to be placed in the correct conductive bag or container which must display an ESD label.


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 ESD2. Use a heel strap


Figure 5 Typical on-board stowage for a wrist strap

Figure 6 Using a wrist strap for a bench operation (note the grounding jack connector)


3. Use of static dissipative floor and bench mats.

4. Avoidance of very dry environments

5. Availability of ground jacks

6. Use of grounded test equipment


7. Use of low-voltage soldering equipment and anti-static soldering stations

8. Use of anti-static insertion and removal tools for integrated circuits.

9. Avoidance of nearby high-voltage sources

10.Use of anti-static packaging


Static Controlled work station


ESD wrist strap


Electromagnetic Environment (EMC)

Every electronic equipment with alternating current radiates unintentional signals and therefore all electronic circuits radiate some noise.

This process of preventing signal radiation and signal ingression is called Electromagnetic Compatibility, or EMC.


During initial aircraft certification, EMC testing looks at Intra-System EMC and Inter-system EMC.

Intra-system EMC examines the capability of subsystems inside the aircraft to operate without interfering with each other.

Inter-system EMC examines the capability of each subsystem in the aircraft, and the aircraft as a total system, to operate in its intended EM environment without degradation.

Intra-system and inter-system EMC


Intra-system and inter-system EMC

•Examples of susceptibilities:• component burnout• navigation and communication system degradation• loss of engine power or flight controls• computer memory or data losses

•To prevent EMI affecting an operational aircraft, the aircraft and system component manufacturers conduct intersystem EMC testing , also known as Electromagnetic Vulnerability (EMV), Electromagnetic Radiation (EMR), or High Intensity Radiated Fields (HIRF) testing.


Lightning and its protection

•Potential source of interference •Travel the path of least resistance and seek out tall

or metal objects.•Unpredictable •Aircraft often trigger lightning when flying through

heavily charged regions of clouds, especially at night.

•Originates at the aircraft and extends away in opposite directions.


Lightning at night (video)http://www.youtube.com/watch?v=036hpBvjoQw


Electromagnetic Interference (EMI)• defined as any electromagnetic disturbance that interrupts, obstructs

or otherwise degrades or limits the effective performance of electronic and electrical equipment or avionics.

• sources of EMI which produce Extremely Low Frequency (ELF) magnetic fields called H fields comes from video display units (VDU), photocopiers, air conditioning units, fluorescent lights and electric wiring.

• Inside the aircraft, EMI can come from computers, headsets, radios and navigation aids, as well as the electrical power distribution system of the aircraft.

• External EMI include ground transmitters such as radio, radar, TV and telephone.


Personal Electronic Device (PED)•Personal items such as notebooks, game consoles,

mobile hand phones, etc can produce signals at a range that could affect various avionic equipment like the gyro compass, inertial navigation system (INS), etc.

•Airlines have implemented a general ban on using any portable electronic device below 10,000 feet and practically inhibit their use during the aircraft’s takeoff and landing phases.


EMI protection•Can minimised by ensuring that all electronic equipment

is operated with a good electrical ground system.•Cords and cables connecting the peripherals must be

shielded to keep unwanted RF energy from entering and leaving.

•Effective shielding of avionic equipment must anticipate both radiated susceptibility (the degree to which outside interference affects the reliable functioning of equipment), and radiated emissions (the extent to which the device creates EM waves that can affect its function).


Lightning protection zones on an aircraft


•These defined lightning zones are required to ensure adequate protection of fuel systems, structure and primary control systems.

•Aircraft protection relies on low resistance and low inductance paths and joints between all aircraft surfaces and these are designed at initial manufacture.

•This is achieved by the good earthing of all the system components and good bonding between all parts of the airframe.


•Electrical bonding of the aircraft’s structure protects the aircraft from static buildup and provides a low resistance current path for any lightning strike.

•During normal flying, static builds up on the fuselage, wings and other structures in two ways

• Precipitation Static due to friction with rain particles, snow and ice crystals, dirt, volcanic ash and other atmospheric contaminants.

• Electrostatic Induction

Electrical bonding


Typical bonding methods


•Static buildup can occur when two objects rub together and the aircraft friction with the air molecules caused a negative charge on the aircraft skin.

•This creates an EMI wave in the region of 10 KHz to 350 MHz

•If properly bonded, these excess charges gather around the aircraft’s extremities or any sharp edges on the fuselage.

Static protection


•This means they are at the greatest potential at the wing and tail plane trailing edges and at the outboard ends of all the control surfaces.

•These static wick act as conduits for the electrons to travel through from the airframe to the surrounding air.


Typical static wick arrangement


Summary

1. ESD is caused by static electric charge (Tribo).

2. Improper handling can cause latent and catastrophic damage to LRUs and electronic devices.

3. Precautions to take when handling ESD.

4. Sources of EMI/EMC due to transmitters, PEDs, computers and electronic equipment, etc.

5. Ways to minimise and reduce EMI/EMC through bonding, earthing and using static wicks in aircrafts.

topic 6 esd & emi considerations electrostatic sensitive devices (m4.2, 5.12 &5.14)_1

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