ESD testing (IEC 61000-4-2): practical tips and countermeasures

Electrostatic discharge (ESD) testing checks immunity to discharges from people or objects. The discharge has a very fast edge (nanoseconds), so board layout and grounding strongly influence the result.

Test methods

• Contact discharge: the tip touches a conductive part (metal enclosure, screw, connector shell) before the discharge. It is more repeatable, so the standard prefers it.
• Air discharge: the tip approaches an insulating surface (plastic seams, buttons) and discharges through air. Used where contact discharge is not possible.
• Indirect discharge: contact-mode discharges to the horizontal coupling plane (HCP) and vertical coupling plane (VCP) evaluate the effect of discharges from nearby objects, and are performed together with direct discharges.

Sequence and pass criteria

• Raise the level step by step from low to the specified value; the basic standard requires all lower levels to be satisfied too, unless the product standard says otherwise.
• Test with the number of discharges and polarities set in the product standard and test plan (the basic standard commonly uses 10 per polarity per point), and respect the interval between discharges.
• Classify the result with the criteria in the product standard (normal operation kept, temporary malfunction with automatic recovery, operator intervention needed). Define them in the test plan beforehand.

Board-level countermeasures

• Place the TVS diode right at the connector and keep the trace to the protected pin as short as possible. Trace inductance raises the clamping voltage, so keep the ground return path short as well.
• Route discharge current to chassis ground first so it does not pass sensitive circuits, and design the path between chassis and board ground.
• Add an RC filter and software debounce on sensitive signals such as reset and interrupt (calculator: RC filter).
• Enclosure seams are paths for discharge to enter. Consider conductive gaskets or overlapping joints.

Field notes

• During an air-discharge test we saw the discharge go in through a gap in the enclosure. Fastening with bolts alone was not enough; putting a conductive gasket or an insulating material in the gap is what mattered.