Electrostatic Discharge: An Overview

Anyone designing or assembling electronic components must deal with the effects of electrostatic discharge (ESD), which can damage products and require countless hours of costly redesign if its effects aren’t factored into the initial plan. This article features a series of questions and answers to explain ESD and describe current standards and protective measures for avoiding its adverse consequences.

What is ESD?

The EOS/ESD Association defines ESD as “the rapid, spontaneous transfer of electrostatic charge induced by a high electrostatic field.” ESD can occur anytime one charged object approaches another. It’s what makes clothes stick together after coming out of the dryer, or causes a minor shock if you walk across a carpet and touch a metal surface.

How does ESD Affect Electronics?

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.

How is ESD Related to Coupling?

There are several ways ESD can affect electronic equipment. They are usually differentiated by the type of coupling mechanism that occurs, that is, the way electrical energy is transferred from one circuit to another (or within different parts of a single circuit).

In conductive coupling, a path between the ESD current and susceptible equipment is formed by direct contact—for example, through a transmission line, wire, cable, PCB trace, or antenna lead. Typical sources of conducted interference include switching power supplies, AC, motors, and microprocessors.

Inductive coupling occurs when two nearby conductors interact via a magnetic field. In this scenario, a change in current in the source conductor induces a voltage in the receiving conductor. For example, two cables running in parallel can become inductively coupled.

Radiated coupling occurs when the source of ESD and the equipment are separated by a large distance, typically greater than a wavelength.

How Can You Measure ESD and Gauge a Device’s Vulnerability?

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).

What is a Good Strategy for Protecting Against ESD Damage?

The goal of an ESD protection strategy is grounding—bringing all elements in a protected area to the same potential, so that there is no difference that might cause a current to flow. The American National Standards Institute (ANSI) and the EOS/ESD Association have developed and continue to update a standard for establishing, implementing, and maintaining an ESD control program.

What is an ESD Protected Area?

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.

Where are ESD Countermeasures Necessary?

ESD controls are necessary at any point where an electronic device comes into contact with a person or an object. That includes output terminals, USB connectors, LAN connectors, places where the button of an electrical product is touched, or places where a device touches a board during production.

Engineers and product designers have several options for protection, including isolation circuits and filtering circuits. Connectors can also be retrofitted with ESD protections.

Suppression components include multilayer varistors, silicon diodes, and polymer-based suppressors. Connected in series, suppression components protect the circuit by clamping the ESD voltage at a level it can handle. Connected in parallel with the signal lines, they shunt some of the ESD current away from the data line or the protected chip.

While these controls do not eliminate ESD events entirely, they validate the compatibility of components in sensitive electronics devices, greatly reducing the possibility of a catastrophic field failure.

What Kind of Materials Can Protect Against ESD?

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.

What Are Some of the Essential Considerations for ESD Circuit Protection?

ESD protection falls into two categories: protection during manufacturing and protection in real-world use.

As electronics evolve, signal speeds are becoming faster, accommodating higher data throughput. It is important to guard very high-speed data lines by taking into account the capacitance and placement of ESD. Designers also need to take into account stray characteristics, such as capacitance, in designing circuit boards. And they must consider the clamping voltage and the nature of typical voltage and current surge waveforms.

Do LEDs Need ESD Protection?

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.

How Can You Prevent ESD in LEDs?

Discrete protective components have traditionally been used to protect LED lighting systems, depending on the series and parallel connections. Transient-voltage-suppression (TVS) diodes offer a low-voltage clamping capability, preventing a sudden spike of voltage that could cause system failure. In conventional LED lights, the TVS diode as an ESD protective component is positioned next to the LED on the substrate.

TVS diodes can also protect a wide variety of other circuits and components from DC power line threats.

How Do You Decide Which Device Protections to Use?

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.

What’s the Best Place to Install ESD Suppressors?

Key locations for suppressors include: on the circuit board immediately behind the connector, at the point where circuit board traces interact with the pins of the connector, or near the IC and/or ASIC.

Optimal placement of ESD suppressors begins at the location of ESD penetration into the system. This reduces the ESD voltage and current initially experienced by the circuit and attenuates the ESD pulse. Placing an ESD suppressor too far away from the line it’s protecting can reduce its effectiveness, so you should install it as close to a protected line as possible.

Even a small amount of trace inductance can translate into substantial impedance when a high-frequency signal like ESD is run through it. For that reason, designers should put as much distance as possible between the ESD suppressor and a protected chip.

How Can Chip Manufacturers Protect Against ESD?

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.

What is the VRM/VRWM Parameter of an ESD Protection, and Why Should You Pay Attention to it?

VRM, also known as VRWM, is the maximal operating voltage of the protection to ensure good transparency of the application voltage signal in normal operation. The VRM of the ESD protection must be higher than the normal signal voltage amplitude.

If this voltage level is exceeded for more than a few seconds, the depletion layer of the diode will break down and allow current to flow backwards. This can cause the p-n junction of the diode to overheat and will destroy the device.

What are Varistors?

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.

Why is Protecting Military Hardware from ESD Important?

The modern battlefield is full of electromagnetic noise, some of it unintentional and some used for radio jamming and other electronic warfare procedures. As the use of electronics in military equipment increases, so does the need for protecting them from ESD interference.

Different environments require different forms of electromagnetic compatibility (EMC). There are two major military EMC requirements. MIL-STD-461, currently updated as MIL-STD-461G, and MIL-STD-464, currently updated as MIL-STD-464C.

What is MIL-STD-461?

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.

What is MIL-STD-464?

MIL-STD-464 is more broad-based. It was created to ensure system, subsystem, and platform compatibility. A system can include related facilities, equipment, subsystems, materials, services, and personnel, and is considered self-sufficient within its operational or support environment. The standard sets requirements for managing electromagnetic effects in and surrounding equipment such as launch vehicles, submarines, ships, spacecraft, and ground and air vehicles.

Getting Started with ESD Protection

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.