B1-05.12 ELECTROSTATIC SENSITIVE DEVICES

Electrostatic Sensitive Devices

• Special handling of components that are sensitive to electrostatic discharges is required.
• It is important to be aware of the risks and possible damage due to electrostatic discharges.
• Component and personnel anti-static protection devices are used.

Static Electricity

• Static electricity is an electrical charge at rest, unlike current electricity, which is in motion.
• Static charges can be generated through friction or induction.
• Triboelectric charge is a type of static electricity build-up caused by rubbing two non-conductive objects together.
• Activities like walking across a carpet can produce an electrostatic charge of up to 35,000 V on the human body.
• A person can feel a shock from an electrostatic potential difference discharge at around 3000 V.
• Electrostatic-sensitive circuits can be damaged by discharges of only a few hundred volts.
• Damage to electrostatic-sensitive devices can occur without one's knowledge because discharges less than 3000 V cannot be felt or seen.
• Induction is another way to generate static charge.
• Induction is when an isolated conductive object is brought near a charged object without physical contact.
• An Electrostatic Discharge (ESD) event occurs when the field from the charged object induces charge to flow if the isolated object touches ground.

Induced Static Charges

• Simple induction is the separation of positive and negative charges when a conductor is in the presence of an electrostatic field.
• Free or valence electrons are attracted towards a positively charged source; a strong magnetic field has the same effect.
• An electric current is produced because electrons rush back to neutralize the imbalanced charge when the field influence is removed.
• This current can cause ESD damage.
• IC (Integrated Circuits) can be damaged if placed near a strong magnetic or electromagnetic field, even without direct contact with an ESD source.
• ESD damage is caused by the imbalance of charge within the IC, induced by the external field.
• Non-conductive substances (nylon, wool, plastics, carpet, etc.) can hold an electrostatic field.
• Merely placing a nylon garment near an IC can cause damage.
• Materials prone to holding static charges must be kept away from sensitive ICs or components.
• Simple induction is the phenomenon resulting in damage.

Compound Induction

• If the conductor (circuit card) side farthest from the field source is grounded, more electrons are drawn from Earth into the conductor.
• After the ground is disconnected, a net negative charge remains on the conductor.
• Charges equalize when the field is removed, creating a negative potential across the conductor's surface.
• Compound induction can occur in electronics workshops or storage areas, posing risks to static-susceptible items.
• A conductor with a potential gradient can cause ESD damage when potentials neutralize.
• After initial induction and field source removal, the conductor may retain damaging voltage, awaiting contact with another neutral source.
• Induced potentials can cause failures on sensitive items, like a charge on a Styrofoam cushion inducing a charge on a circuit board.
• A person grounding the board can cause ESD failure because of the charge imbalance in the card as soon as the device is touched.
• Dangerous situations involving static charge generation (especially in low humidity of 10-20%) include walking on carpet (35,000 V), vinyl floors (12,000 V), working at a bench (6,000 V), using vinyl envelopes (7,000 V), handling polythene bags (20,000 V), and using work chairs padded with urethane foam (18,000 V).
• Removing static-generative materials is an ESD control measure.
• Notorious static generators include, Teflon, acetate, common plastics, polyethylene, Styrofoam, wool, silk, and nylon.
• These materials generate high potentials and retain them for extended periods.

Metal-Oxide-Semiconductor Devices

• MOSFETs (Metal-oxide-semiconductor field-effect transistors) and equipment containing them are prone to damage from electrostatic discharge.
• They must be handled with caution.
• High static charge exposure causes an arc, which destroys the silicon dioxide insulating layer, degrading the MOSFET's operation.
• Parts susceptible to ESD damage are those needing small energies for switching or voltage changes in high-impedance lines.
• Examples include NMOS, PMOS, CMOS, and low power transistor-transistor logic (TTL) items.
• All semiconductor devices have some level of susceptibility to ESD damage.

ESD Sensitivity

• HBM (Human Body Model) electrostatic discharge is a common measurement of ESD sensitivity, due to the human body originally being the most common damage source.
• A charged 100-pF capacitor is discharged into the device via a 1500-Ω resistor during testing.
• The 100-pF capacitor imitates charges stored in the human body, and the resistor imitates the human body and skin resistance.
• A device's ESD sensitivity is measured by the "ESD withstand voltage" or the maximum test voltage where the device did not suffer damage.
• Internal device sizes are reducing as component technology progresses, increasing ESD sensitivity.
• On-chip protection circuits protect modern components, which would otherwise be extremely sensitive.
• The design goal is to increase the device's ESD withstand voltage to 2 kV (2000 V), but trade-offs between ESD protection and performance can interfere with this goal.

Device Sensitivity Classification

• The HBM testing standard includes a classification system to define component sensitivity.
• It allows grouping, comparing, and also indicates the required level of ESD protection of components.
• Class ratings of components:
  • Class 0: < 250 volts
  • Class 1A: 250 volts to < 500 volts
  • Class 1B: 500 volts to < 1000 volts
  • Class 1C: 1000 volts to < 2000 volts
  • Class 2: 2000 volts to < 4000 volts
  • Class 3A: 4000 volts to 8000 volts
  • Class 3B: >= 8000 volts

Circuit Cards

• Components installed on circuit cards that are ESD-sensitive remain vulnerable.
• Circuit card assemblies are handled as ESD sensitive items.
• Equipment containing circuit cards with ESD-sensitive components (computers, receiver/transmitters, digital display units, encoder/decoders) require treatment to prevent damage.
• Special treatment prevents ESD through connector receptacles.

Types of ESD Damage

• Early electronic devices and integrated circuits were more robust but now include millions of components, increasing sensitivity due to miniaturization
• Devices have become so sensitive that signals once inconsequential can now bias PN junctions.
• Circuit switching now occurs hundreds of thousands of times a second.
• If a static charge of several thousand volts is applied to transistors designed to amplify millivolts, it will be destroyed.
• Static damage types include:
  • Direct and latent catastrophic failures
  • Upset failures which result in gate leakage

Direct Catastrophic Failures

• Direct catastrophic failures occur when a component is dead and will not function again.
• The ESD event may have caused a junction breakdown, metal melt, or oxide failure.
• The device's circuitry is damaged, causing it to fail and making it easily detectable during testing.

Latent Failures

• Latent failures occur when ESD weakens a component, and the failure weakens a component to cause poor system performance/complete failure over time.
• The component may be marginally damaged but continue to operate for some time by shortening its operational life.
• Degradation continues due to the damaged condition and ordinary operational stress.
• Repairs are costly (aircraft downtime and engineering man-hours) because latent failures occur only after final inspection, when fitted to an aircraft.
• Latent failure damage may test as serviceable at the avionics test bench and then be returned to service no-fault-found meaning it is primed to create more aircraft downtime and lost man-hours.

Upset Failure

• Upset failure occurs when static discharge causes a current flow that isn’t significant enough to cause total failure but may cause an intermittent result.
• If there is gate leakage it will cause loss of software or incorrect storage of information which presents software glitches or intermittent faults that cannot easily be replicated, and can plague aircraft.
• Upset or latent failures may pass testing in an avionics workshop, as static damage may not be felt, seen, or detected through normal testing.

ESD Damage Repair

• The images show ESD damage to C2, a MOS capacitor, that could have been prevented with the correct safeguards.
• Costs caused by ESD damage are higher than capital investments for ESD-protected workstations, making it an economic necessity to use ESD precaution measures.
• Burnt circuit track is a failed IC, and sectioning pinpointed a partial short in top silicon.

ESD Handling Precautions

• Precautions to prevent damage to devices from ESD include:
  • Never touch pins on components or sockets/pins within the plugs on any electronic equipment.
  • You should remove any static on yourself by grasping an earthed conductive surface.
  • Aircraft must be grounded and have its power removed when removing ESD-sensitive equipment.
  • The cable of the equipment should be touched to equalize all potentials before disconnecting cables.
  • Always fit conductive caps immediately to disconnected plugs.
  • When installing electronic equipment, be in contact with the outer shell plug and the outer shell of the equipment mating connector to equalize the potentials.
  • You should never expose circuit cards anywhere other than an ESD workstation or place equipment on an anti-static mat when maintenance is being performed.
  • Electronic devices should be put into a circuit immediately after removing it from the protective carrier bag.
  • Always ship electronics material in anti-static containers or bags.
  • Don't touch the edge connectors of circuit cards, and place shorting straps (shunts) across the edge connectors when transporting them.
  • Only open packaging with an ESD symbol at the ESD workstation.
  • Never store or place circuit cards near electrostatic or strong magnetic fields.

Work Areas

• Areas should minimize generation of electrostatic charges:
  • Contain non-carpeted floors, earthed storage racks/anti-static mats on shelves.
  • Have no magnetic or electrostatic fields present.

ESD protected work areas

• A grounded ESD-protective work surface, ESD-safe flooring, personnel grounding and identified ESD-safe workstations is needed.
• Best practices in sensitive area:
  • Removal of static-generating sources, wear ESD smocks, and install air ionizers.
  • Connect the chassis of instruments, soldering iron tips, and workbench to earth.
  • Wear anti-static wrist straps.
  • Keep ESD-sensitive components in aluminum foil so that no voltages can occur between any two pins.
• Not following this can result in immediate/latent failures and performance degradation.
• The only way to be 100% certain a component is safe is to do all procedures

ESD Packaging

• Circuit cards and components must be in packaging and bags that are static shielding to prevent any effects that can cause an electrostatic charge to occur.
• This bag can be moisture-locking for opened bags and can be reused or used for connectors.

Anti-Static Bags

• Pink Bags
  • Made up of Poly material that are for components with class ratings over 3, have great shelf life if heat sealed.
• Metallic bags
  • Made up of transparent laminate that are for items low for static discharge and are very resistant to the elements

Grid tape

• Anti-static tape is used in ESD-sensitive areas which is made up of a multi layer of the conductive with ESD warning symbols marked.
• This tape is used for shielding EMI, and voltage levels

Conductive Transit Trays

• These trays are for transporting ESD-sensitive components. They reduce building a static charge for transporting it in a workshop.

Shipping

• Methods for shipping include devices and PCBs to static-safe workstations.
• Tote and shipping boxes must avoid triboelectricity which can use friction.
• Shipping boxes can be both reused and effectively shield equipment.

Static Protection

• DIP (Dual in-line packages) can have static protection by being stored in a tube for shipment and storage.
• These materials are made of plastic and aluminum and use a tight, confined space to prevent static discharge.

Shipping of ESD Material:

• Shipping boxes must have specific information and shielding to prevent any damage occurring to the materials such as putting them near harmful areas or misuse

Equipment:

• Grounding
  • Wrist straps are used to prevent electrical shock and are adjustable to easily disconnect one's self.
• Gloves
  • Anti-static gloves for instrumentation and circuit boards for assembly.
• Finger Cots
  • Used for ESD-safe areas and are latex-free.

Smocks

• ESD safe smocks will minimize static and provide a conducive area when dissipating to earth.

Grounders

• Used like smocks that give a conducive are to earth so all static will build up when dissipated to earth.

Envelopes

• Safe work envelopes replace plastic ones, to prevent high voltage.

Workstation:

• Safe areas protect the components and must be readily available to control floors and tiles that may be overlooked. And must drain all damaging static safely to the parts.