19.7 Bench Repair Practices, ESD & Rework Tools
Key Takeaways
When working with CMOS, the pool's answer is that a wrist strap must have less than 100,000 ohms of resistance to prevent static electricity
The preferred method of cleaning solder from plated-through circuit board holes is a vacuum device
Plastic wire ties are cut with flush-cut diagonal pliers, cut flush, so no sharp stub remains
The ideal method of removing insulation from wire is the thermal stripper
A hot gas bonder is used for non-contact melting of solder, and safety glasses are the most important precaution when repairing circuit board assemblies
19.7 Bench Repair Practices, ESD & Rework Tools
Quick Answer: Wrist strap — the pool's keyed answer is less than 100,000 ohms. Clear plated-through holes with a vacuum device. Cut wire ties flush, with flush-cut diagonal pliers. Strip insulation with a thermal stripper. A hot gas bonder does non-contact melting of solder. When repairing boards, wear safety glasses.
Sub-topic 3-L-079 (Repair Procedures) is the most hands-on sub-topic in Element 3. These are shop-floor practices, and the pool tests them because a licensed maintainer who damages equipment or injures themselves is a genuine risk.
ESD and the wrist strap
When soldering or working with CMOS electronics products or equipment, a wrist strap must have less than 100,000 ohms of resistance to prevent static electricity.
That is the exam answer, and it is the one to select. Understand the two competing requirements it sits between:
| Requirement | Pushes resistance which way |
|---|---|
| Drain static charge from the operator to ground before it can damage a device | Lower |
| Protect the operator by limiting current if they contact a live circuit | Higher |
A direct wire to ground would drain charge instantly and electrocute the technician; an open circuit protects them and drains nothing. The compromise is a resistive path. Note for the bench that current industry practice (ANSI/ESD S20.20 and IEC 61340) specifies a 1 megohm series resistor in the strap cord, tested to a path resistance typically under 35 megohms — a higher figure than the pool's. On the exam, answer 100,000 ohms; in the shop, use a 1 MΩ commercial strap.
Why CMOS specifically? A CMOS gate's insulated gate oxide is only nanometres thick and punctures at a few tens of volts across it. Simply walking across a carpeted floor can put several kilovolts on a human body. The damage is frequently latent — the device works on the bench and fails weeks later in service, which is the worst possible outcome on a vessel at sea.
Desoldering plated-through holes
Which of the following is the preferred method of cleaning solder from plated-through circuit-board holes? Use a vacuum device.
A plated-through hole carries a thin copper barrel connecting the layers. That barrel is the fragile part. A vacuum desoldering tool — a solder sucker or, better, a vacuum desoldering station with a heated hollow tip — melts the solder and draws it out axially, leaving the barrel intact and the hole open for the new lead.
| Method | Problem |
|---|---|
| Vacuum device | None — the pool's answer |
| Solder wick / braid alone | Works on pads, but drags across the barrel and can lift it |
| Pushing the lead out with a pick | Tears the barrel and breaks the interlayer connection |
| Drilling | Destroys the plating entirely |
Once a barrel is torn out the layer-to-layer connection is gone and the repair needs a through-board jumper — far more work than doing it right the first time.
Wire ties
What is the proper way to cut plastic wire ties? With flush-cut diagonal pliers and cut flush.
Both halves matter: the right tool and the right result. An ordinary side-cutter leaves an angled stub that is genuinely sharp — sharp enough to lacerate a hand reaching into a loom, and sharp enough to chafe insulation on an adjacent conductor over time with vibration. On an aircraft or a vessel, both consequences are serious.
Cut flush with the head, and cut ties hand-tight rather than crushing the bundle; over-tensioned ties deform insulation and create the very chafe point the tie was meant to prevent.
Stripping insulation
The ideal method of removing insulation from wire is the thermal stripper.
A thermal stripper melts or vaporises the insulation with a heated element and lifts it away without touching the conductor at all. That matters because a mechanical stripper set even slightly undersized nicks the strands, and a nicked strand is a stress riser that eventually fails from vibration — in a mast loom or an airframe, exactly where you cannot reach it.
| Method | Risk to conductor |
|---|---|
| Thermal stripper | None — the pool's ideal |
| Precision mechanical stripper, correct gauge | Low if set correctly |
| Adjustable stripper set by eye | Nicks strands |
| Knife | High — the worst option |
Thermal stripping also handles the tough insulations found in aviation and marine wiring (PTFE, Kapton, Tefzel) that resist mechanical strippers.
The hot gas bonder
A "hot gas bonder" is used for non-contact melting of solder.
Hot gas — usually filtered air or nitrogen — is directed at a joint through a nozzle, melting the solder without any tool touching the board. Two reasons this matters:
- Surface-mount rework. A fine-pitch QFP or a BGA cannot be removed with an iron; hot gas reflows every joint of the package simultaneously.
- Fragile assemblies. No mechanical force is applied to pads, so nothing lifts.
Key the answer on the phrase non-contact. The distractors describe contact methods.
The overriding safety rule
When repairing circuit board assemblies it is most important to: wear safety glasses.
The pool asks what is most important, and the answer is eye protection. The hazards are specific and common:
- Solder spits when flux boils, especially on a damp or contaminated joint
- Clipped component leads launch at speed when cut
- Wire tie ends and stripped insulation flick unpredictably
- Solvents and flux removers splash
- Electrolytic capacitors vent, and a reverse-biased or overvoltaged one can rupture
None of these is exotic; all of them happen at ordinary benches, and the eye has no protection of its own. Every other good practice in this section protects the equipment — this one protects you, which is why it ranks first.
When soldering or working with CMOS equipment, what resistance does the pool specify for a wrist strap, and why is a resistive path used at all?
Zero ohms, because a direct ground drains charge fastest
Less than 100,000 ohms, balancing the need to drain static against the need to limit current if the operator contacts a live circuit
Infinite resistance, because any conductive path damages CMOS
Exactly 50 ohms, to match the bench impedance
What is the preferred method of cleaning solder from plated-through circuit-board holes, and what is at risk if it is done badly?
Push the lead through with a pick; nothing is at risk
Use a vacuum device; the thin copper barrel that connects the board layers can be torn out, destroying the interlayer connection
Drill the hole to the next size up; only cosmetic damage results
Apply solder wick with heavy pressure; the pad may discolour
What is the ideal method of removing insulation from wire, and why does the alternative matter on a vessel or aircraft?
A knife, because it handles any insulation type
A thermal stripper, because it never touches the conductor - a mechanical stripper can nick strands, creating a stress riser that fails from vibration where you cannot reach it
An adjustable stripper set by eye, because it is fastest
Pulling the insulation off by hand, because no tool can damage the strands
A hot gas bonder is used for what purpose, and what is the single most important precaution when repairing circuit board assemblies?
Contact soldering of heavy connections; wearing gloves
Non-contact melting of solder; wearing safety glasses
Drying flux residue after cleaning; grounding the bench
Curing conformal coating; using a fume extractor
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