Chapter 9: Electrostatic Sensitive Devices

Static electricity

Static electricity is an imbalance of electric charge on the surface of a material. Electrostatic discharge (ESD) is the sudden flow of current when a charged object comes close to, or touches, an object at a different potential — the spark you sometimes feel when touching a door handle. To a person it is harmless. To modern electronics it can be fatal.

• Triboelectric charging: when two materials touch and then separate (rubbing, peeling, walking, sliding), electrons move from one to the other. The material that loses electrons becomes positive, the other negative. The triboelectric series ranks materials by this tendency; the further apart two materials are in the series, the larger the charge.
• Induction: a charged object brought near a conductor (a circuit card, an LRU connector) pushes charge around inside it without touching. If the conductor is then grounded, even briefly, it is left charged — and can discharge through a component later.
• Humidity: moist air lets charge leak away, so static is far worse in dry conditions — winter, air-conditioned buildings, and aircraft cabins at altitude. The same walk across a carpet can generate about 35 000 V in dry air but only about 1 500 V in humid air.
• People generally do not feel a discharge below about 3 000 V, but many components can be damaged by less than 100 V — some by a few tens of volts. You can destroy a part without knowing.
• Test standards model the person as the human body model: 100 pF charged to the test voltage and discharged through 1 500 Ω. Parts are rated by the voltage they survive.

Damage and sensitive components

• Catastrophic failure: the device stops working immediately — a punctured gate oxide, a melted junction, a vaporised metal track. It is found by testing and replaced.
• Latent damage: the device is weakened but still works. It passes every test, then fails in service — perhaps months later, perhaps intermittently, perhaps under heat or vibration. Most ESD damage is believed to be latent, and it is the reason why ESD control matters so much in aviation.
• Sensitive devices (ESDS) include MOS and CMOS integrated circuits, MOSFETs, microprocessors and memory, operational amplifiers, thin-film resistors, Schottky diodes, laser diodes and optical devices — and therefore almost every circuit card and avionics LRU containing them. Damage can enter through an LRU's connector pins, so a complete unit is also sensitive.

Protection — the ESD-protected area

The principle of ESD control is simple: keep everything that touches a sensitive part at the same potential, drain charge away slowly, and keep charge-generating materials away. Work on exposed ESDS parts is done in an ESD-protected area (EPA).

• Wrist strap: a conductive band worn in contact with the skin, connected by a coiled cord to the common ground point. The cord contains a 1 MΩ resistor, which drains static slowly and — for personal safety — limits the current if the wearer touches a live circuit. Check the strap with a wrist-strap tester before each use; never bypass the resistor.
• Dissipative bench mats and floor mats (or conductive floors with heel or foot straps) drain charge from parts and people, connected to the same common point ground. 'Dissipative' means charge flows away slowly (surface resistance roughly 10⁶–10⁹ Ω) rather than in a sudden spark.
• Ionisers blow balanced positive and negative ions over the work to neutralise charge on insulators, which cannot be grounded.
• Control the environment: keep relative humidity moderate (around 40–60%); wear ESD smocks over synthetic clothing; use ESD-safe tools and grounded soldering irons.
• Remove ordinary insulators — plastic cups, foam, sticky tape, ordinary plastic bags and folders, paper — from the work area. They generate charge and cannot be grounded.

Packaging

• Static-shielding bags have a metallised layer that acts as a Faraday cage: external fields and discharges flow around the outside, not through the part. Use them for storage and for transport outside the EPA.
• Conductive packaging (black, carbon-loaded bags, boxes and foam) also shields and drains charge. Conductive foam shorts an IC's pins together so no voltage can appear between them.
• Antistatic (dissipative) material — usually pink polyethylene — does not generate charge itself but gives no shielding. Use it only inside the EPA.
• Removed LRUs have conductive protective caps fitted over their connectors.

Handling procedures

1. Recognise the ESD caution in the AMM task or on the unit's label.
2. Remove electrical power before removing or installing the unit (unless the AMM says otherwise).
3. Put on a wrist strap and connect it to the aircraft's ESD grounding jack (fitted in the electronics bay) or to clean, unpainted structure. Touch the LRU case to equalise potential before handling it.
4. Do not touch connector pins, circuit-card tracks or component leads. Hold cards by their edges.
5. Fit protective connector caps straight away, and put the unit or card in approved ESD packaging with a label. Never use ordinary plastic bags or bubble wrap.
6. Do not slide units across seats or carpets; keep them away from clothing, and carry them in their protective packaging.
7. Open the packaging only in an EPA or with a wrist strap connected, and install the new unit the same way.