Electrostatic Sensitive Devices (5.12) PDF

Electrostatic Sensitive Devices (5.12) Learning Objectives 5.12.1 Describe the special handling of components sensitive to electrostatic discharges (Level 2). 5.12.2.1 Describe awareness of the risks and possible damage due to electrostatic discharges (Level 2). 5.12.2.2 Describ...

Electrostatic Sensitive Devices (5.12) Learning Objectives 5.12.1 Describe the special handling of components sensitive to electrostatic discharges (Level 2). 5.12.2.1 Describe awareness of the risks and possible damage due to electrostatic discharges (Level 2). 5.12.2.2 Describe component and personnel anti-static protection devices (Level 2). 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 255 of 285 CASA Part 66 - Training Materials Only  Static Electricity Generation of Static Charges Static electricity is an electrical charge that is at rest, as opposed to electricity in motion or current electricity. Static charges can be generated by either friction or induction. The most common method of generating static charge is friction developed by rubbing two non- conductive objects together. This type of static electricity build-up is called a triboelectric charge. Triboelectric generation of static charge on the human body occurs due to activities we do all the time, e.g. walking across carpet produces an electrostatic charge of up to 35 000 V. The human body can feel a shock from the discharge of an electrostatic potential difference at approximately 3000 V however the human body frequently carries lower levels of electrostatic charge. Electrostatic- sensitive circuits can be damaged by discharge of an electrostatic potential difference of only a few hundred volts. Since we do not see or feel discharges of less than 3000 V, damage can easily occur to electrostatic-sensitive devices without our knowledge. Arcing indicates the discharge or equalisation of a potential difference In addition to triboelectric generation, another way to generate static charge is through induction, which occurs when an isolated conductive object is brought near another charged object without actually touching it. The field from the charged object can induce charge to flow if the isolated object itself touches ground. This flow of charge is an Electrostatic Discharge (ESD) event. If the isolated object is located very close to ground, the electric field from the induced charge may break down the air and cause an ESD event. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 256 of 285 CASA Part 66 - Training Materials Only  Induced Static Charges Simple Induction The illustration below shows a conductor in the presence of an electrostatic field emanating from a positively charged source. The resultant separation of positive and negative charges is defined as simple induction. The free or valence electrons are attracted towards the positively charged source. The same effect will result if there is a strong magnetic field in the vicinity. Induction-induced static charges – simple induction Upon removal from the field influence, the electrons rush back towards the positive ions to neutralise the imbalanced charge, producing an electric current. This current may be sufficient to cause ESD damage. The initial separation of the electrons when the conductor is first introduced to the electrostatic field may also result in ESD damage. If a sensitive Integrated Circuit (IC) is placed in the vicinity of a strong magnetic or electromagnetic field, the IC can be damaged even though no ESD source comes into contact with it. The ESD damage results from the imbalance of charge within the IC, induced by an external magnetic or electrostatic field. Object or person with negative charge Electronic device mounted on a circuit board. Aviation Australia Simple induction of a static charge on an electronic component 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 257 of 285 CASA Part 66 - Training Materials Only  An electrostatic field can be resident in many non-conductive substances: nylon, wool, plastics, carpet, etc. So, simply placing a nylon garment near an IC can result in some level of damage to the IC. Therefore materials known to be susceptible to holding significant static charges must be isolated from areas where sensitive ICs or components containing ICs are stored. This phenomenon is known as simple induction. Compound Induction If the conductor (circuit card) side farthest away from the magnetic or electrostatic field source is grounded, this provides a path for even more electrons to be attracted from Earth into the conductor, further increasing the electrical potential resident in the conductor. After disconnecting the ground (moving the circuit card, but not touching any conductive areas), a net negative charge remains on the conductor (overabundance of free electrons), still attracted to the positive source. When the field is removed or the conductor is taken from the field, the charges equalise throughout the conductor (circuit card), resulting in a negative potential across the conductor surface. This sequence is called compound induction. Induced static charges – compound induction 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 258 of 285 CASA Part 66 - Training Materials Only  Compound induction in an electronics workshop, or in any area where sensitive electronic equipment is stored, can present double jeopardy to static-susceptible items. A conductor with an induced potential gradient between its two extreme edges can cause ESD damage to sensitive components when the potentials neutralise. Therefore, after the first ESD event (simple induction) and removal of the field source, the conductor may be left with a possibly damaging voltage level of the opposite polarity awaiting a second ESD failure upon contact with another neutral source. Induction-caused failures can occur from induced potentials on a sensitive item itself. One example could result from a printed circuit board being placed on a Styrofoam cushion. A charge on the cushion would induce a charge on the board. A person touching the conductive area of the board (a repair technician) could bring about an ESD failure even if the person is grounded because the imbalance of charges in the circuit card (all the electrons near the inducting source) causes a current to flow from Earth into the circuit card from the uncharged or Earth potential (the repair technician) as soon as the device is touched. Day to day activities and materials that generate static voltage One of the primary ESD control measures is to purge static-generative materials from areas where susceptible items are processed. Notorious static generators include common insulating materials such as Teflon, acetate, common plastics, polyethylene, Styrofoam, wool, silk and nylon. These materials not only tend to generate high potentials but are likely to retain them for considerable time, that is, minutes to hours. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 259 of 285 CASA Part 66 - Training Materials Only  Metal-Oxide-Semiconductor Devices Aviation Australia MOSFETs are very suscepatble to ESD damage Metal-oxide-semiconductor field-effect transistors (MOSFETs) and equipment containing MOSFETs are susceptible to damage from electrostatic discharge and must be handled with care and with strict adherence to electrostatic handling precautions. If such equipment is exposed to a high static charge, an arc will jump through the silicon dioxide insulating layer and either destroy or severely degrade the operation of the MOSFET. Parts susceptible to ESD damage are those in any logic family that require small energies to switch states or small changes of voltage in high-impedance lines. For example: N-channel metal oxide semiconductor (NMOS) P-channel metal oxide semiconductor (PMOS) Complementary oxide semiconductor (CMOS) Low power transistor-transistor logic (TTL) items. MOS devices are not the only electroninc devices susceptible to ESD damage. All semiconductor devices have some level of susceptibility to ESD damage. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 260 of 285 CASA Part 66 - Training Materials Only  ESD Sensitivity As the human body was originally the most common and damaging source of electrostatic discharge, the most common measurement of ESD sensitivity is by Human Body Model (HBM) electrostatic discharge. In this test, a charged 100-pF capacitor is discharged into the device via a 1500-Ω resistor. The 100-pF capacitor simulates charges stored in the average human body, and the resistor simulates the resistance of the human body and skin. The ESD sensitivity of devices is given as an ‘ESD withstand voltage’, which is the maximum test voltage at which the device did not suffer damage. Typical HBM withstand voltages of various device technologies are given in the following table. Human Body Model (HBM) electrostatic discharge As component technology progresses, internal device sizes reduce and become more ESD sensitive. Many modern components are protected by on-chip protection circuits, without which they would be extremely sensitive. In most cases the design goal is to increase the device’s ESD withstand voltage to 2 kV (2000 V). Sometimes this goal cannot be met for various reasons – there is often a trade-off between ESD protection and device performance. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 261 of 285 CASA Part 66 - Training Materials Only  Device Sensitivity Classification The HBM testing standard includes a classification system for defining component sensitivity. This classification system has a number of advantages. It allows easy grouping and comparing of components according to their ESD sensitivity and indicates the level of ESD protection required for the component. In addition to the HBM testing standard, several others also produce classes of sensitivity. We will only present the HBM version. Aviation Australia CPU damage after the device which had undetected ESD damage was operated. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 262 of 285 CASA Part 66 - Training Materials Only  Identification of Equipment Susceptible to ESD Classes of Devices Any component which can be damaged by 16,000 V or less is considered sensitive to ESD. These components include microelectronics devices, discrete semiconductors, film resistors, resistor chips, other thick and thin film devices, and piezoelectric crystals. The three ESD-sensitive classifications are as follows: Class 1: Extremely sensitive – Ranges from 0 to 2 kV Class 2: Sensitive – Ranges from 2 to 4 kV Class 3: Less sensitive – Ranges from 4 to 16 kV. ESD Component Sensitivity Classification Voltage Range Class 0 < 250 volts Class 1A 250 volts to < 500 volts Class 1B 500 volts to < 1 000 volts Class 1C 1 000 volts to < 2 000 volts Class 2 2 000 volts to < 4 000 volts Class 3A 4 000 volts to 8 000 volts Class 3B > = 8 000 volts 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 263 of 285 CASA Part 66 - Training Materials Only  Circuit Cards ESD-sensitive components installed on circuit cards are still susceptible to ESD. For that reason, circuit card assemblies are treated as ESD sensitive. Equipment containing circuit cards with ESD- sensitive components, such as computers, receiver/transmitters, digital display units, encoder/decoders, etc., require special treatment to prevent ESD from entering through connector receptacles and damaging sensitive components. Aviation Australia ESD safe packaging of circuit cards with ESD sensitive devices mounted on the circuit board 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 264 of 285 CASA Part 66 - Training Materials Only  Types of ESD Damage Early electronic devices and integrated circuits were quite robust. Once an IC may have contained several transistors, resistors, capacitors and so on, now they contain millions. Miniaturisation of circuits has meant reduced component size and extreme sensitivity. Years ago, only significant signals could be amplified. Recently devices have become so sensitive that signals that were once too inconsequential to bias a PN junction can now be applied to extremely sensitive transistors and amplified. Where circuit switching was in the region of hundreds of times a second, circuits now switch hundreds of thousands of times a second because the responsiveness of semiconductors has increased significantly. All the improvements in semiconductor circuitry have meant that the circuits themselves have become extremely sensitive and very easy to damage. Where the base of a transistor may be designed to amplify millivolts, if a static charge of several thousand volts is applied, that transistor will be destroyed. Static damage to components can take the form of: Catastrophic failures – occur in two forms, direct and latent Upset failures – result in gate leakage. Direct catastrophic failures occur when a component is damaged to the point that it is dead and will never again function. The ESD event may have caused a junction breakdown, metal melt or oxide failure. The device's circuitry is permanently damaged, causing the device to fail. This is the easiest type of ESD damage to find since it is usually detected during testing. Latent failures occur when ESD weakens or degrades the component to the point that it will still function properly during testing, but over time the degraded component will cause poor system performance and eventually complete system failure. Latent failures due to ESD occur when a component is sufficiently damaged to shorten its operational life. The component may be only marginally damaged by the ESD event and continue to operate for some time. Degradation continues due to the damaged condition of the component and ordinary operational stress. Because latent failures occur after final inspection when the component is fitted to an aircraft, the cost of repair is very high in terms of aircraft downtime and engineering man-hours needlessly consumed. Latent failure damage may then test as serviceable at the avionics test bench, where the component will be returned to service no-fault-found, primed to create more aircraft downtime and lost man- hours. ESD damage can cause direct catastrophic failures, latent failures, or upset failures 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 265 of 285 CASA Part 66 - Training Materials Only  Upset failure occurs when an electrostatic discharge has caused a current flow that is not significant enough to cause total failure, but in use may intermittently result in gate leakage causing loss of software or incorrect storage of information. To the user, this represents the software glitches and intermittent faults which are so difficult to replicate and isolate, producing the repetitive snags that plague some aircraft. Upset or latent failures may pass testing in an avionics workshop or during on- board Built-In Testing. In other words, static damage may occur that cannot be felt, seen or detected through normal testing procedures. ESD Damage The two images show ESD damage to C2, a MOS capacitor. What appears to be a slight pinhole in the left image with 175 times magnification shows to be substantial damage to the trace when examined at 4300X magnification on the right. This damage could have been prevented had the appropriate ESD safeguards been in place. C2 metal oxide semiconductor (MOS) capacitor 175 X magnification and 4300 X magnification The economic necessity for ESD-protective measures is clear, particularly as the costs caused by ESD damage are far higher than the capital investment needed to provide an ESD-protected workstation. The burnt circuit track is a failed IC that was rejected as low input resistance (leaky) at a particular input pin. Sectioning identified the partial short through the silicon from the top creating a well on the track. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 266 of 285 CASA Part 66 - Training Materials Only  Aviation Australia Examples of direct catastrophic failures - the easiest type of ESD damage to detect. You will have no way of knowing when you cause ESD damage to a component. And if latent or upset damage has been caused, even testing the component on a workstation may not detect any failure. The only way of being certain a component has not been subjected to ESD is to take all precautions to avoid it. CAUTION SENSITIVE ELECTRONIC DEVICES DO NOT SHIP OR STORE NEAR STRONG ELECTROSTATIC, ELECTROMAGNETIC, MAGNETIC ORRADIOACTIVE FIELDS Aviation Australia ESD handling precautions ESD Handling Precautions 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 267 of 285 CASA Part 66 - Training Materials Only  For ESD a number of handling precautions should be adhered to to prevent damage to the devices: Do NOT touch the pins of any electronic component. Do NOT touch the sockets or pins within the plugs of any electronic equipment. Discharge any static electricity on yourself by grasping an earthed conductive surface. When removing ESD-sensitive equipment from the aircraft, the aircraft must be grounded and power removed. Prior to disconnecting the cables from the equipment, touch the metal case of the equipment to equalise any electrostatic potentials. Immediately fit conductive covers/caps to disconnected plugs. When installing electronic equipment on the aircraft, touch the outer shell of the plug to the outer shell of the equipment mating connector to equalise electrostatic potentials. ESD-sensitive equipment must not be opened to expose circuit cards anywhere other than at an ESD workstation. Place electronic equipment and subassemblies on a grounded anti-static mat whenever maintenance is performed. When working with electronic devices (components and cards), insert the device into the circuit or connector immediately after removing it from the protective carrier or packaging. When shipping or moving any electronic components or circuit cards, use only approved anti- static containers and bags. Do NOT touch the edge connectors of circuit cards. Place shorting straps (shunts) across the edge connectors of circuit cards when the cards are being carried or transported. At no time during shipment or storage should packaging identified by an ESD symbol be opened except at an ESD workstation. Never store or place circuit cards or electronic components (aircraft avionics components) in the vicinity of electrostatic or strong magnetic fields. Work areas where electronic components are often stored and processed must be designed to minimise generation of electrostatic charges. They should contain: Un-carpeted floors Earthed storage racks Earthed anti-static mats on storage shelves No magnetic or electrostatic fields in the vicinity. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 268 of 285 CASA Part 66 - Training Materials Only  An ESD-protected work area should include: A grounded ESD-protective work surface ESD-safe flooring Personnel grounding Identification of ESD-safe workstations. In work areas where particularly sensitive components are handled (Class 0 and 1), the following are best practices: Removal or control of static generating sources, e.g. Styrofoam cups, plastics, etc. Usage of ESD smocks Installation of air ionisers. When actually removing and installing electronic components on a circuit card or when fault- isolating a circuit card: Connect the chassis of all test instruments, soldering iron tips and your workbench (if metal) to Earth. Connect yourself to Earth with an anti-static wrist strap. Keep ESD-sensitive components in conductive foam or aluminium foil. This will keep all the pins shorted together so that no dangerous voltages can be developed between any two pins. Failure to observe these precautions could result in serious damage to electronic components with either immediate or latent operation failure or performance degradation. ESD damage is undetectable by the human eye. The only way of being certain that a component has not been subjected to ESD is to take all precautions to avoid it. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 269 of 285 CASA Part 66 - Training Materials Only  ESD Packaging Approved ESD-Protective Packaging Circuit cards and components must be packaged in ESD-protective packaging prior to leaving the ESD workstation. Static shielding bags, which have a static-dissipative inner layer and a conductive outer layer, are used for this purpose. Since the days of Michael Faraday, it has been well known that field-induced effects can be prevented by enclosure in a highly conductive material, thereby creating an electrostatic shield or Faraday cage. The inside layer of the bag is designed to prevent static build-up due to the movement of the component inside the bag. Another layer is made from metal and is referred to as a Faraday cage. This metal layer prevents damage to the bag's contents due to induction charging. The degree of shielding is a function of the conductivity of the material from which the shield is constructed – the higher the conductivity, the better the shielding. ESD protective packaging The other layers of the bag provide strength and a moisture barrier. They must be non-corrosive and must zip-lock or heat-seal closed. All static-shielding bags are identified with an ESD caution sticker over the closed seals so that broken stickers serve as indicators of opened bags. Cushion wrap (bubble wrap) used around circuit cards must also be made of static-dissipative material. Circuit cards may be packaged in reusable ESD fast-pack containers. At the equipment level, conductive connector receptacle dust caps are used to prevent ESD from entering the equipment through the connector receptacle and damaging sensitive components. When conductive dust caps are unavailable, a conductive grid tape may be used to cover connector receptacles. Shorting of connecting leads or pins of devices can be done by means of wire, spring clips or metal foil or by inserting the leads or pins into a conductive foam or material. For PCBs and card modules with edge connectors, specially formed strips called shunts are placed over connectors to keep them at the same potential. For complete electronic assemblies, covers or caps made from a conductive material are placed over the connectors. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 270 of 285 CASA Part 66 - Training Materials Only  Anti-Static Bags – Pink Poly Pink poly anti-static bags are made from amine-free polyethylene and are approximately 2 to 6 mm thick. The polyethylene resin is impregnated with an anti-static substance which renders them quasi- conductive. This additive allows the material to remain transparent and because of its colour, the bags are usually referred to as ‘pink poly’. Pink poly anti-static bags are recommended for packaging components with Class III sensitivity levels. They have a shelf life of three years, are not damaged by wrinkling or creasing, and are heat- sealable. They are available with an anti-static zipper, allowing frequent opening and closing without reducing their anti-static properties. Pink poly anti-static bags Cushion-pack bags are made from pink poly foam material. They are manufactured with two layers of laminated 4-mm anti-static pink polyethylene and one layer of anti-static cushioning. The single most important characteristic of a static-protective packaging material is its electrical resistance. The less the resistance around the container, the more static protection is provided. Anti- static plastics do not provide the electrostatic shielding required by the extremely fast response of static-sensitive devices; this is achieved by using metallised bags. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 271 of 285 CASA Part 66 - Training Materials Only  Anti-Static Bags – Metallic Metallic anti-static bags are manufactured with a transparent layered laminate that provides optimum static shielding with maximum visibility. Transparent metallised static shield bags (similar to reflective black window tinting) are designed for packaging products that are very sensitive to low- level static discharges (Class 1 sensitivity). The bag's inner layer prevents static from generating inside the bag. It is bonded to a metallised polyester layer which provides static shielding of the contents from external sources. The abrasion-resistant exterior helps the bag maintain its static- shielding capabilities even under rugged handling and reuse. These top-of-the-line static shielding bags are transparent for maximum visibility. Their layered lamination prevents static generation inside and outside of the bag while the metallised layer offers optimum static shielding. The multi-track, anti-static zipper is easily opened and closed. This allows the bag to be reused many times without loss of the anti-static properties. Aviation Australia Anti-static bag – metallic 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 272 of 285 CASA Part 66 - Training Materials Only  Grid Tape Cello tape is a high-risk source of static electricity. Grid tape is anti-static tape for use in ESD- sensitive areas. Grid tape ESD-conductive grid tape is a beige polypropylene, three-layer tape with a conductive grid middle layer. The grid pattern and ESD awareness symbols help alert recipients that packages contain ESD- sensitive components and should be handled in an ESD-safe work area. Applications for conductive shielding grid tape include the following: Applications requiring EMI shielding Use in areas where static electricity generation is a concern; using a grounded dispenser, voltage generated unrolling the tape will effectively be reduced to zero For securing (bundling) anti-static tubes containing ICs Sealing ESD bags and other ESD packaging/containers Use with ESD symbols for ESD awareness Attaching ESD paperwork to bags or product Holding notes, etc., in anti-static workstations Covering external plugs, holes or connector pins on electronic chassis (black boxes and so on) during transportation or storage. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 273 of 285 CASA Part 66 - Training Materials Only  Conductive Transit Trays Conductive Transit Trays are used for transporting ESD-sensitive components in an ESD workshop. They reduce the likelihood of building up static charge during transport around the workshop when all other ESD measures are in place (heel grounders, ESD smocks, ESD-safe flooring, etc.). These trays can be designed for return and reuse with or without an anti-static lid. Conductive transit trays 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 274 of 285 CASA Part 66 - Training Materials Only  Tote Boxes The primary use for tote boxes is for the in-plant transport of devices and PCBs to and from the controlled static-safe workstations. A tote box is an anti-static conductive box with a lid. As in the case of bags, to be effective, the box must not only prevent triboelectrically (produced by friction between two objects) generated charge, but also shield the components from external static fields. The theory of shielding effectiveness of bags (more conductive material means a more efficient shield) can be extended to cover any closed container, including a tote box. These boxes may also be used for storage of small, discrete ESD-sensitive items at an ESD workbench. Tote boxes with lids provide greater protection than a conductive tray alone. ESD safe tote boxes 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 275 of 285 CASA Part 66 - Training Materials Only  Static Protection in DIP Tubes A common carrier used for shipment, storage and automatic handling of Dual In-Line Packages (DIPs) is the storage rail or tube. These tubes are fabricated from a variety of materials, including aluminium, plain plastic, carbon-loaded plastic and anti-static plastic. Although these tubes are rarely capped on the ends, they may, for the purposes of static shielding, be considered closed containers because of the relatively small size of the end openings. Therefore, the theory developed for protective bags can be applied to DIP tubes as well. Aviation Australia Static protection in DIP tubes 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 276 of 285 CASA Part 66 - Training Materials Only  Anti-Static Shipping Materials Transportation of ESD Sensitive Devices An enormous number of products are available with anti-static or ESD reduction/elimination incorporated. Just a few examples are listed below. Shipping Boxes The black static-protected shipping box is an effective means for shipping expensive circuit boards and devices. These shippers are designed with a conductive layer buried in the corrugated fibreboard to help dissipate static surfaces. The interior has special static-dissipative foam cushioning on the top and bottom to help protect against shock and vibration during transportation. Aviation Australia ESD safe shipping boxes 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 277 of 285 CASA Part 66 - Training Materials Only  Anti-Static Clamshells Anti-static clamshell containers are used to package circuit boards as a replacement for anti-static bags and foam. Anti-static clamshells 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 278 of 285 CASA Part 66 - Training Materials Only  Safe ESD Equipment and Work Practices People are Prime Sources of ESD Electrostatic charges generated by rubbing or separating materials are readily transmitted to a person's conductive sweat layer, causing that person to be charged. When a charged person handles or comes in close proximity to an ESD-sensitive component, that component can be damaged by a direct discharge when it is touched or subjected to an electrostatic field. Humans are prime sources of ESD 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 279 of 285 CASA Part 66 - Training Materials Only  ESD Grounding Strap The wrist strap must have a resistance of 1 MΩ to prevent the worker from conducting an electric shock. These straps are comfortable and adjustable, and you can even snap the cord off of the wristband when you have to leave the workbench for a minute. The worker must wear the wrist strap directly on their skin (not on top of clothing) and check the conductivity of the wrist strap at least once a day to make sure the cable is not broken. Check the conductivity of the wrist strap at least once a day to make sure the cable is not broken Anti-Static Gloves Anti-static gloves are ideal for handling delicate or sensitive parts, films, electronic instruments, circuit boards and components. They are suited for assembly and repair work in electronics, telecommunications, precision instrumentation and optics. Anti-static gloves 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 280 of 285 CASA Part 66 - Training Materials Only  Finger Cots Ideal for ESD-safe areas, powder-free pink latex finger cots offer a 3-mm thickness. ESD-safe finger cots ESD-Safe Smocks and Lab Coats ESD-safe smocks and lab coats minimise static build-up on normal clothing and provide a conductive path for charges in normal clothing to dissipate to Earth (when connected to Earth). Aviation Australia ESD-Safe smocks and lab coats 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 281 of 285 CASA Part 66 - Training Materials Only  Heel Strap Grounders Like ESD-safe smocks, heel strap grounders provide a conductive path to Earth so any charge built up when walking will be dissipated to Earth with each step. ESD-safe shoes are also available, as are ESD-safe over-shoes that resemble large gumboots. Heel strap grounders ESD-Safe Work Envelopes ESD-safe work envelopes replace plastic envelopes which can develop very high static charges. They are used to protect maintenance instructions and certification paperwork utilised in ESD-safe work areas. Aviation Australia ESD-safe work envelopes 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 282 of 285 CASA Part 66 - Training Materials Only  Workshop Anti-Static Devices The best ESD controls are not only ones that protect sensitive components and equipment but are at hand, readily available and easily maintained. For these reasons, carpets and tile floors should not be overlooked as points of electrostatic control. Existing carpet or tile floors can be easily included in an ESD control program. An ESD-safe workstation protects your sensitive products by providing pathways to safely drain damaging static charge to ground. A wrist strap and coil cord remove static from the person as it is generated. Parts placed on a static-dissipative mat decay their static charge to ground at a safe rate. Both the mat and coil cord are connected to ground by the Common Ground Cord System. An ESD safe workstation 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 283 of 285 CASA Part 66 - Training Materials Only  ESD Protection Workplace Requirements ESD device repair facilities use standard ESD-protection procedures based on the most susceptible device they expect to repair. The ESD workstation is an essential part of ESD protection and is the only safe location to repair, package or handle ESD-sensitive components or circuit cards. The purpose of the workstation is to keep potential differences below the level that can damage ESD- sensitive components. This is accomplished in several ways. The bench top, floor mat and personnel wrist strap are electrically connected together through resistors to ground. In addition, the floor mat, bench top, chair, component containers and all other materials in the area are made from static-dissipative material. No static generators, such as plain plastic wrap, Styrofoam, plastic coffee cups, etc., are allowed in the area. Humid air helps to dissipate electrostatic charges by keeping surfaces moist and increasing surface conductivity. Example of ESD safe work bench The workstation and surrounding area should be kept between 40% and 60% relative humidity for this purpose. Ionised air generators which produce both positive and negative ions may be used at the ESD workstation to dissipate any static charge. Personnel are often required to wear static- dissipative smocks and should avoid wearing synthetic clothing under the smock. ESD workstations must be periodically monitored to ensure all components are functional. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 284 of 285 CASA Part 66 - Training Materials Only  Ionisers The action of air ionisers is to add very nearly equal numbers of positive and negative ions to the air in order to provide charge carriers that increase the electrical conductivity of the air. If only a small percentage of air molecules are ionised (0.0001% to 0.000001%), the time for static discharge is reduced from hours to seconds. The theory of operation for one simple type of ioniser is as follows: A very high voltage (5–20 kV) is applied to a set of sharp points within the ioniser and an intense electric field is established in the very near vicinity (~100 mm) of the points. This field accelerates free electrons to a sufficiently high energy to allow them to ionise molecules that they collide with. When the voltage on a point is positive, positive ions are repelled into the environment, and when the point is negative, negative ions are delivered. Corona ionisers are simpler and therefore cheaper to manufacture. They utilise a step-up transformer to create the high voltage for ion generation. Because the AC type ioniser produces the positive and negative ions in sequence from the same emitter, these ions are separated in time by half the period of the AC power line (half the frequency, i.e. 1/100 or 1/120 s). This means the waves of positive and negative ions are rather close to each other, making loss through recombination a large factor. AC ionisers typically utilise fast airflow velocity to minimise recombination. This is not always desirable in a clean room environment. Aviation Australia Air ionisers increase the electrical conductivity of the air 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 285 of 285 CASA Part 66 - Training Materials Only