11.3 ESDS Handling, Storage, Packaging and Workshop Procedures

11.3 ESDS Handling, Storage, Packaging and Workshop Procedures

For topic 5.12 at B1/B2 level 2 (A/B3 level 1), procedure is the practical study focus drawn from the former detailed description. Physics (section 11.1) and damage classes (section 11.2) exist so that the wrist strap, the bag and the hold-the-case-not-the-pins rule are not superstition. This section describes personnel protection, component protection, packaging, storage and workshop layout. Where a number such as about 1 MΩ appears, it is typical industry practice from ESD-control standards used in electronics manufacture (for example IEC 61340-5-1 and ANSI/ESD S20.20). It is not a single official EASA Appendix I ohm value and must not be quoted as a Regulation (EU) 2023/989 figure.

The ESD protected area (EPA)

An ESD protected area (EPA) is a defined space in which ESDS can be unpacked, tested, repaired and packed with a controlled electrostatic environment. A typical avionics EPA includes:

• A static-dissipative work surface bonded to an approved common-point ground.
• Personnel grounding (wrist strap and, where required, footwear and flooring).
• ESDS packaging materials at the bench, not a drawer of ordinary polythene.
• Identification (signs, EPA tape) so that untrained staff do not walk in with polystyrene cups and bubble wrap.
• Periodic checks of straps, mats and bonding leads.

The aircraft ramp is not a full EPA, but the same personal rules apply when a CMOS LRU is opened or a module is swapped: bond the person, handle by the case, keep the spare in its shielding bag until the moment of fitment, and bag the removed unit before walking it across a dry hangar.

Personnel grounding: wrist strap and the typical 1 MΩ resistor

The standard teaching control is a wrist strap whose cord contains a current-limiting resistor of about 1 MΩ, connected to an approved ground (the EPA common-point earth, not a random pipe, painted rack or live chassis return).

Two jobs, one resistor:

1. ESD job. The person is bonded so that body voltage cannot sit at kilovolts. Charge bleeds off through the 1 MΩ at a controlled rate rather than as a spark from a fingertip into a pin.
2. Electrical-safety job. If the person contacts a live conductor, the 1 MΩ limits current. A hard earth (0 Ω) on the wrist would be an ESD bond and a shock hazard.

Worked example — current through a 1 MΩ limiter

Assume a 230 V AC hangar outlet fault to the technician, and a continuous 1.00 MΩ in the strap (idealised, ignoring extra skin and cord resistance).

I = V / R = 230 / 1.00×10⁶ = 0.23 mA

A typical perception threshold is about 1 mA; let-go current is much higher. 0.23 mA is the reason the resistor exists. On a 28 V DC aircraft bus:

I = 28 / 1.00×10⁶ = 28 µA

which is negligible as a shock, but the same strap still discharges the body for ESD.

Failure modes of the strap are examinable:

• Open circuit (broken coil cord, dirty snap, unplugged banana): the person is not protected. A daily strap test is industry practice because an open cord looks fitted.
• Shorted resistor: ESD bonding is better in the naive sense, but the shock path is no longer limited. Do not defeat the resistor with a wire jumper.
• Wrong ground: clipping to a painted surface, a floating trolley or a live 115 V return is not an approved EPA ground.

Industry strap-test windows are often a band such as 750 kΩ to 35 MΩ for the person-plus-strap system (values vary by the standard and the tester). Use the tester and the organisation’s procedure. Do not treat 1 MΩ as an EASA-legal single number; treat it as the usual current-limiter teaching value in the cord.

Heel straps and dissipative flooring supplement the wrist strap when the technician must move. They do not replace a wrist strap at a seated repair bench. Never wear a strap when working on exposed high-energy electrical systems where the cord could become a current path into live equipment outside the ESD task.

Handle the component by the case, not the pins

Discharge current does damage when it is forced through a pin, pad or connector contact into the die. The case, card ejector, connector shell (if designed as a handle) or board edge away from contacts is the correct grip. Rules:

• Do not touch gold contacts, backplane pins, device leads or test-point lands.
• Do not slide a board so that pins scrape an insulator or another card.
• Fit connector ESD protective caps on open plugs.
• Keep the device in its packaging until the last moment; the bag is the Faraday cage, not dry skin.

This rule is the cheapest control in the syllabus and the one most often broken when a technician merely steadies a connector.

Packaging: shielding, conductive and dissipative

Not every pink or black plastic bag is equivalent. Using the wrong bag is a classic latent-damage route (section 11.2).

[!NOTE] Pink-bag trap: Pink dissipative polyethylene reduces charge generation compared with untreated polythene. It does not replace a metallised shielding bag for carrying a CMOS module across a hangar. If a question says anti-static bag, read whether it means dissipative or shielding.

Secondary packaging (cardboard, cushioning) must not undo the shield. Do not empty a shielding bag onto a plastic desk to read the part number and then wonder why the computer later becomes NFF.

Ionisers as typical control for essential insulators

Some objects in a real workshop cannot be bonded: plastic housings, some composite covers, process-essential insulators, tapes that the AMM still requires. A room or bench ioniser floods the area with a balanced stream of positive and negative air ions so that charge on those insulators is neutralised. Industry EPAs use ionisers as a typical control, not as a substitute for the wrist strap on a person who can be bonded.

Limits:

• An ioniser does not earth you. Wear the strap.
• An unbalanced ioniser can charge surfaces. Periodic balance checks are part of EPA maintenance.
• Do not blow ordinary (non-ionised) compressed air across a board; the flow can charge insulators and spray charged dust.

Appendix I does not mandate a particular ioniser current or decay time. Describe ionisers as typical industry tools for insulators that cannot be grounded.

Storage, transport and line practice

• Store ESDS in shielding packaging on bonded shelving, not loose on a painted metal shelf next to vinyl folders.
• Do not stack bare boards.
• Transport in closed shielding bags or conductive boxes; do not carry an open card across carpet.
• On the aircraft, bag the removed LRU before the walk to the shop; fit the spare from its bag at the rack, person bonded.
• Follow the AMM/CMM for any unit-specific shorting connectors or packing inserts.
• Keep humidity from collapsing to desert-dry if the organisation controls the EPA climate; humidity helps leakage but is not a substitute for bonding.

Putting the controls together

A defensible handling sequence for a CMOS EFIS module:

1. Identify the EPA or, on the line, establish personal bonding first.
2. Put on and test the wrist strap to approved ground through the typical ~1 MΩ limiter.
3. Open the shielding bag at the last moment; do not pre-stage naked modules on plastic.
4. Handle by the case; never by the pins.
5. Fit or test; return the unserviceable unit to a shielding bag immediately.
6. Close the EPA materials so the next shift does not inherit a bench covered in bubble wrap.

Category A and B3 at level 1 must recognise the strap, the bag and the not-the-pins rule and know that mishandling risks damage. B1 and B2 at level 2 must also explain why the resistor is about 1 MΩ (current limit plus bleed path), why pink dissipative film is not a Faraday cage, and why ionisers address insulators rather than replacing personnel grounding. Never upgrade those workshop numbers into a fictitious EASA-regulated ohm, volt or humidity statute.