Electrostatic Sensitive Devices
Special handling of components sensitive to electrostatic discharge; awareness of risks and possible damage; component and personnel anti-static protection measures.
Electrostatic sensitive devices, summary notes
- Electrostatic sensitive devices (ESDS), MOSFETs, CMOS ICs, microprocessors, memory and most modern avionics boards, are damaged by static discharges far below the ~3 kV a person can feel. Simply walking across a carpet can generate 10-15 kV.
- Damage may be catastrophic (immediate failure) or LATENT, the device passes test but fails later in service. Latent damage is why the precautions are mandatory even when the unit "still works".
- The wrist strap is the primary control. It contains a series resistor, typically 250 kΩ to 1.5 MΩ, so that charge on the operator DRAINS AWAY SLOWLY. The resistor also limits the current through the wearer if they contact a live conductor, it is deliberately not a short to ground.
- The full ESD-protected workstation adds a dissipative (not metallic) bench mat, a common bonding point, conductive or static-shielding packaging, ionisers where charge cannot be bonded away, and controlled humidity.
- ESDS items are identified by the black-and-yellow hand-in-a-triangle symbol, are shipped in shielding bags, and must only be removed from that packaging at a protected workstation.
- Never handle a board by its connector pins or edge-connector fingers; handle by the edges, and refit removed items into their original protective packaging.
- ⚠ Exam trap: the strap resistor is there to bleed charge gradually and protect the operator, a zero-ohm "hard short to ground" is wrong and dangerous.
- Wrist-strap series resistance
- ≈ 250 kΩ - 1.5 MΩ
- Human perception threshold
- ≈ 3 kV, damage occurs well below this
Why is a wrist strap fitted with a series resistor rather than bonded directly to earth?
Two reasons. The resistance drains accumulated static gradually instead of in a damaging high-current pulse, and it limits the current through the wearer to a safe value should they contact a live conductor while strapped.
An avionics card is removed and left on a normal workbench for an hour before refit, then works normally. Is any action required?
Yes. The ESD precautions have been breached, so the card may have suffered latent damage that testing will not reveal. It must be handled per the organisation's ESD procedure, quarantined or rejected as the maintenance data requires, rather than being accepted because it currently functions.
ESDS handling concept map
Electrostatic Sensitive Devices
Electrostatic sensitive devices quiz
Electrostatic Sensitive Devices, quiz
1. Electrostatic-sensitive devices are most at risk from:
A static discharge through the deviceHigh humidityLow current2. The correct packaging for an electrostatic-sensitive device is:
Conductive or antistatic bagsOrdinary plastic bagsLoose in a cardboard box3. The resistor in an ESD wrist strap is there to:
Drain charge slowly and limit current through the wearerShort the operator hard to groundMeasure body voltage4. The typical series resistance of an ESD wrist strap is:
250 kΩ to 1.5 MΩ1 Ω to 10 Ω10 MΩ to 100 MΩ5. Which family of devices is most static-sensitive?
CMOS and MOSFET devicesWire-wound resistorsBipolar power transistors6. Latent ESD damage means the device:
Passes test now but fails later in serviceFails immediately on installationRecovers after a power cycle7. An ESDS item should be removed from its protective packaging:
Only at an ESD-protected workstationAnywhere, provided it is handled quicklyOnly after it is fitted to the aircraft8. The correct way to hold an avionics circuit card is:
By its edgesBy the edge-connector fingersBy the largest component