Electrostatic Discharge: An Overview

In electronics, ESD is a very high voltage (generally >500V) and moderate peak current (~1A to 10A) that occurs in a short time frame (generally <1µs). It comes into play when two conductive objects approach each other and form a strong electric field, which can cause a field-induced breakdown. An arc can occur when the voltage between the objects exceeds the breakdown voltage of the air and the insulation between them. In environments where the relative humidity is very low, an ESD event may have a peak voltage as high as 15,000 volts. The arc continues until the objects touch, shorting out the arc, or until the current drops too low to sustain the arc.

ESD can occur throughout a product’s manufacturing, testing, shipping, handling processes, or during field service operations. It can result from a discharge to the device, from the device, or from induction to the item from an electromagnetic field.

If the ESD pulse finds its way into an electronic device, the circuitry inside can be physically damaged. The current injected by the arc can break through the thin insulating layers inside components, damaging the gates of MOSFETs and CMOS devices. It can also trigger latch-up in CMOS devices and short-circuit reverse-biased PN junctions

Engineers often fail to account for ESD until product release is imminent, requiring them to make expensive last-minute fixes or completely revamp the product design. The cost of repairing an ESD-damaged electronic device can range from a few cents to hundreds of dollars, according to the EOS/ESD Association. When combined with its other costs, including lost production time, shipping, and labor, the cost impact of ESD can be quite significant.

Traditional ESD measurement procedures use an oscilloscope to determine the steady-state high and low values of the pulse and compute its parameters. For all digitizing oscilloscopes, the vertical resolution of an acquired waveform on the display screen is proportional to the percentage of the screen vertically occupied by the waveform.

You can also refer to IEC Standard 61000-4-2, which provides a testing and rating system to determine the robustness of a device against ESD. Two types of tests may be conducted: a contact test and an air gap test.

In the contact test, an ESD pulse is discharged directly from a gun touching the device being tested. The resulting rating shows the maximum voltage the device can withstand when the source of ESD is charged directly onto it.

In the air gap test, the ESD test gun is brought close to the device under test until a discharge occurs. The rating shows the maximum voltage the device can withstand when the source of ESD is discharged over a gap of air.

In either test, the higher the rating, the higher the voltage the device can withstand.

JEDEC also publishes standards and device-level tests to measure ESD, including the human body model (HBM), the charged device model (CDM), and the machine model (MM).

An ESD Protected Area (EPA) is a defined space within which all surfaces, objects, people and devices are kept at the same electrical potential. A protected area could be a portable work surface or mat, a permanent workstation in a room, or a factory floor encompassing thousands of workstations. Everything in the area must be protected, including tools, mats, surfaces, electronics assemblies, components, and people working on circuit boards. You can establish protection by using only materials that can be grounded, or those with an electrical resistance of less than 10⁹ ohms, for the covering of surfaces and the manufacture of containers and tools.

ESD materials are generally subdivided into three categories based on their surface resistance: anti-static, conductive, and dissipative.

Anti-static materials prevent the buildup of static electricity while still enabling electrical conduction. Resistivity is generally between 10¹⁰ and 10¹² ohms per square.

Conductive materials are defined as those having a surface resistivity less than 1 x 10⁵ ohms per square, or a volume resistivity less than 1 x 10⁴ ohm-cm. With a low electrical resistance, electrons flow easily across the surface or through the material.

Dissipative materials allow ESD to flow to ground slowly and in a more controlled manner than conductive materials. Dissipative materials have a surface resistivity equal to or greater than 1 x 10⁵ ohms per square, but less than 1 x 10¹² ohms per square, or a volume resistivity equal to or greater than 1 x 10⁴ ohm-cm but less than 1 x 10¹¹ ohm-cm.

LED-based solid-state lighting (SSL) systems, common in consumer products such as smartphones, camera flashes, car headlights, and home lighting, are gaining popularity because they are more long-lasting and reliable than their incandescent counterparts. But like all semiconductors, they are sensitive to ESD and to overvoltage spikes. They also require over-temperature protection.

Choosing the right form of protection depends on the device. While suppressors can provide effective ESD protection, the signal integrity of the system shouldn’t be compromised. The capacitance of an ESD suppressor must be considered before adding it to the circuit design.

For example, headset terminals on a cell phone operate at relatively low frequencies (in the audio range), while ESD in the cell-phone operating frequencies is much higher. In this case, high-capacitance multilayer varistors and diodes are well-suited for ESD protection.

But these benefits may disappear when signal speed is increased. The goal is to provide ESD protection to the circuit while maintaining the integrity of the data and not interfering with circuit operation. The need for higher informational throughput for video and audio data requires an increase in transmitted data rates, which often exceed 100 Mbits/s. At these speeds, the capacitance that helped to eliminate unwanted noise will also begin filtering the data signals themselves, with distortion taking the form of rounded leading and trailing edges of high/low state transitions due to slower rise and fall times.

Polymeric ESD suppressors, with their low capacitance (around 0.05pF) and fast voltage clamping ability, are therefore a good choice for high-speed digital I/O and RF lines. Low capacitance helps prevent signal loading and distortion.

To help reduce losses due to ESD, chip manufacturers can incorporate TVS diodes, which provide a safe path to ground for the energy in ESD strikes. TVS diodes are designed based on constant-voltage diodes—a type of p-n diode—specifically to protect devices from ESD. While a system is in normal operation, with no ESD pulse being introduced, ESD protection diodes should be disconnected from devices so as not to affect their operation.

When higher levels of protection are required, external components such as Y-capacitors and gas discharge tubes (GDT) can be used to attenuate and dissipate energy from a discharge. While they provide extra protection, these external solutions may also increase board size and introduce operational constraints, increasing costs.

Varistors (variable resistors) divert current created by excessive transient voltage away from sensitive components.

Multi-layer varistors provide protection from low to medium energy transients (0.05 to 2.5 Joules) in sensitive equipment operating at 0 to 120VDC. They are surface-mounted and can withstand temperatures up to 125°C without derating. You can use a single soldering process for mounting, instead of wire bonding on printed circuit boards.

MIL-STD-461 describes how to test and rate equipment for EMC. It was first issued in 1967 to facilitate EMC in defense communications and has since become the basis for many industrial and commercial applications. There have been many updates to the standard, but the goal remains limiting unintentional emissions and ensuring that devices are not vulnerable to interference from natural and machine-made electronic signals and noise.

Controlling ESD in electronics is a complex and challenging endeavor. By gaining a good overview of the dangers it poses and the array of defenses available, you can begin to focus on developing the protections that make the most sense for your own applications.