Detecting ESD events: how do you find discharges that resistance testing cannot show?

Why does a resistance test not reveal discharges?

A resistance test is a snapshot of a static condition: it checks whether a work surface, person or tool is connected to ground at the moment of measurement. Charge generated during the running process, isolated metal parts inside machines and the fast discharge of a charged device into grounded metal all stay invisible. A good ground can even be the target of a hard discharge.

This follows from the Charged Device Model (CDM): the device itself carries the charge, for example after sliding along a feeder rail or after a cover tape has been peeled off. When a pin then touches a low-resistance grounded surface, the charge drains in a fraction of a nanosecond. The surface passed its last resistance check, and the damage still happens.

• Charge build-up from friction, separation and motion at machine cycle rate.
• Conductive parts isolated from ground by bearings, plastic holders or coatings.
• Intermittent breaks at sliding contacts, cable carriers or quick-change tooling.

How does a detector recognise a discharge?

Every discharge is a current pulse with a very short rise time. It radiates a broadband electromagnetic field, much like a tiny radio transmitter. An event detector receives this pulse with an antenna, compares its amplitude with an adjustable threshold and counts every crossing as an event. No contact with the object under test is needed.

What determines threshold and sensitivity?

The received signal strength depends on the energy of the discharge, its speed and, above all, the distance between source and antenna. A weak discharge close to the antenna can therefore look as strong as a powerful one further away. The threshold defines the amplitude from which a signal is counted.

• Threshold too low: switching of relays, motors, frequency inverters or radios is counted as well.
• Threshold too high: small discharges that still matter for CDM-sensitive devices are missed.
• Proven approach: with the machine stopped, set the threshold just above the background noise, measure in operation, record antenna distance and threshold.

Handheld meter, fixed monitor or indicator: which one when?

The three instrument types differ in whether they only count, rate the strength of an event or log continuously. An indicator suits a quick spot check, a handheld meter a targeted search along the line, and a fixed monitor the supervision of critical process steps over many shifts.

ESD meters

Event detectors, field meters and resistance meters for audits and troubleshooting.

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Monitoring systems

Permanently installed units that keep grounding and process under continuous watch.

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Where do unnoticed discharges typically occur?

Discharges cluster where devices or assemblies are moved, separated or picked by machines and then touch metal. Typical locations are placement machines, feeders, handlers and test fixtures, but also hand tools and packing steps. In an EPA with verified personnel grounding, the risk often shifts from people to machines.

• Placement machines: peeling the cover tape from the carrier, nozzles that pick and place devices.
• Conveyors, chutes and feeder rails on which devices or boards slide with friction.
• Isolated metal parts: grippers, fixtures, rails and covers without a defined ground connection.
• Test fixtures and pogo pins where charged assemblies meet low-resistance contacts.
• Hand tools such as screwdrivers, tweezers and pliers with insulating handles or coatings.

The Charged Device Model is particularly critical. Small devices with fine structures often withstand only low CDM voltages, and the discharge when they are set down on a grounded part is extremely fast. How devices are classified under HBM and CDM is explained in the guide ESD sensitivity classes: HBM, CDM and MM explained.

Why should voltage measurement at soldering irons and tools be included?

Not every failure comes from an electrostatic discharge. Leakage currents and noise voltages from power supplies, heating elements or poorly grounded equipment cause electrical overstress (EOS). An event detector does not see such steady or mains-frequency voltages. That is why the voltage present at the soldering tip and on tools relative to ground is measured as well.

• Measure the voltage between soldering tip and ground in both AC and DC ranges, with the tip at temperature.
• Check tip-to-ground resistance, because a broken ground leaves the tip floating.
• Include electric screwdrivers, desoldering tools and mains-powered test equipment.

Procedure and limits for soldering iron checks are covered in the guide Grounding a soldering iron and testing tip-to-ground resistance.

What should you do when the detector triggers?

An event is first of all an indication, not proof of device damage. What matters is to pinpoint the source, confirm the cause with field and resistance measurements, implement a countermeasure and show with a repeat detection that the events have disappeared or become significantly weaker.

• Locate: move the antenna closer step by step; the signal rises towards the source.
• Rule out interference: stop the machine, compare events with switching operations.
• Confirm the cause: measure charge with a field meter, check grounding of isolated metal parts.
• Act: ground metal parts, replace hard metal contacts with dissipative materials, add ionizers, reduce peel speed.
• Verify: measure again with the same antenna position and threshold.

For choosing a field meter, see Choosing an electrostatic field meter.

ESD test stations

Check wrist strap and footwear before each shift with a documented result.

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Workstation monitor

Why continuous monitoring of personnel grounding closes the gaps left by daily testing.

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Frequently asked questions