A Guide to ESD Testing

What does ESD stand for? Electrostatic discharge: a sudden, brief electric current between unequally charged objects.

And what is ESD testing? Electrostatic discharge testing is a means of determining how susceptible equipment is to ESD, and thus what potential it has for suffering damage from ESD or discharging onto ESD-sensitive equipment.

So just what is electrostatic discharge (ESD)? It is an instantaneous flow of electric current between two objects of unequal charge, that is, one object carrying excess electrons and another able to take them on.

ESD can occur when the objects contact, are brought close to each other, or when an insulating material between them experiences dielectric breakdown: the insulator experiencing more voltage than it can withstand, briefly turning it into a conductor. ESD can produce a visible spark, though its more significant effect is the electrical energy it transfers, which can damage delicate electronics.

You’re likely familiar with electrostatic discharge in both its small forms (like zapping yourself on a light switch after walking across a carpet) and its most extreme form (lightning). Rubbing objects together, like socks and carpet, can transfer charge between them (tribocharging), and then this static electricity can discharge into a conducting object upon contact. In the case of lightning, an electric potential of hundreds of millions of volts between a cloud and the ground produces a massive, visible, audible, and destructive electrostatic discharge.

Visible finger zaps around the house aren’t as small as ESD gets, though. Tiny electrostatic discharge can be imperceptible to us but still damaging to equipment: unless it has ESD protection.

What does ESD mean for workplace safety? In our high-tech world’s reliance on electronic equipment for so many things, their ESD protection is serious business. ESD precautions are also essential for environments that must avoid all forms of sparks.

Electrostatic discharge (ESD) is caused by which three sources?

ESD-sensitive devices experience electrostatic discharge in the following three ways, all of which ESD testing seeks to prevent:

• Discharge from another device: When an external object, including a person, accumulates a charge and then contacts an ESD-sensitive device. This is known as the human body model (HBM).
• Discharge to another device: When the ESD-sensitive device itself builds up a charge due to its operations, handling, or contact with other objects, and then discharges to an object of lower electrostatic potential. This is also called the charged device model (CDM) or machine model (MM).
• Induction: Charged objects (which emanate an electrostatic field) brought into proximity with an ungrounded ESD-sensitive device can make the device’s electrons redistribute at the surfaces near the charged object, giving it a charge imbalance (all without contacting the charged object). An ESD can then occur when the device contacts some other object: one that can conduct this charge. This is the field-induced charged board model (FCBM).

ESD testing procedures need to consider devices’ potential to experience electrostatic discharge in all three of these modes. People working in electronics production or cleanrooms need to take careful ESD precautions against these modes to prevent ruining their work.

ESD precautions like ESD testing seek to avoid the following types of incidents, some of which can be catastrophic:

• Damage to electronic equipment: If electrostatic discharge fries your electronics, this means replacement and service costs.
• Failed automated equipment: Any equipment dependent on control signals from ESD-sensitive devices will fail to operate if electrostatic discharge cripples those devices. This can mean lights, alarms, doors, ventilation, robotic equipment, and telecommunications operating incorrectly or not operating when they’re needed. That leads to inconvenience, crippled business operations, or serious safety concerns.
• Explosions: If electrostatic discharge sparks occur in atmospheres rich in natural gas, gasoline vapour, or coal dust, the spark can ignite this fuel and produce disastrous explosions.

Electrostatic discharge prevention thus plays a serious role in workplace safety.

However, the ESD’s shock itself is not typically a safety hazard for people (unless you include lightning). Even with shocks from contacting industrial equipment, the voltage producing the shock may be high, but it can’t sustain current long enough to harm you. Electronic equipment is much more sensitive to ESD than humans are.