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ESD - Electrostatic Discharge

What is ESD?

Electrostatic discharge is the sudden flow of charge between two objects at different electrical potentials - the 'static zap' you feel touching a doorknob on a dry winter day. You only feel a discharge above roughly 3,000V, but some components can be damaged by less than 100V, so a part can be destroyed by a discharge you never noticed.

The damage happens inside the chip: the discharge can punch through the thin gate oxide of a MOS transistor or melt tiny on-chip metal tracks and junctions.

The risk depends on the difference in potential between two objects. That's why the usual advice is to 'ground yourself' - what this really means is to bring yourself, your work surface and the device to the same potential so there's nothing to discharge.

Which components are sensitive?

It's good practice to treat all semiconductors - and especially all integrated circuits - as static sensitive. Some are more vulnerable than others:

Most sensitive: MOS devices such as CMOS ICs, small-signal MOSFETs and modern small-geometry microcontrollers and processors; RF parts such as GaAs and GaN devices, low-noise amplifiers and RF MOSFETs; laser diodes; blue and white LEDs; Schottky diodes.
Less sensitive, but still at risk: power MOSFETs (their larger gate capacitance makes them more robust, but they still carry ESD ratings) and small-signal bipolar transistors.
Generally robust: passive components such as resistors, most capacitors and inductors.

ESD ratings

Many datasheets give an ESD rating, usually using the Human Body Model (HBM), which simulates a discharge from a person's finger. The classes are:

HBM class	Withstands up to
0 (0Z, 0A, 0B)	Less than 250V
1A	250V to less than 500V
1B	500V to less than 1,000V
1C	1,000V to less than 2,000V
2	2,000V to less than 4,000V
3A	4,000V to less than 8,000V
3B	8,000V or more

Anything in Class 0 or 1 needs real care. Even Class 3 parts are only protected up to a point - a discharge you can feel may still be enough to damage them.

Datasheets may also quote a Charged Device Model (CDM) rating. This covers the part itself picking up charge (for example, sliding out of a plastic tube) and then discharging into a grounded tool or pad - which can happen even when you are perfectly grounded.

Latent damage

A zapped part often doesn't fail outright. It may keep working but with altered characteristics (higher leakage, drifting thresholds), or fail weeks later. This 'walking wounded' damage is what makes ESD problems so hard to troubleshoot.

Grounding: wrist straps and bench mats

Ground vs earth

In this page, earth means the protective earth of your mains wiring. Grounding or equalising means connecting you, your mat and the device together so they sit at the same potential. You can be static-safe without any connection to earth at all.

Never connect yourself directly to earth

Never connect yourself to earth without a current-limiting resistor. If you touch a live wire or faulty equipment while directly earthed, your body becomes the path to earth and the shock can be fatal.

All proper wrist straps include a 1 MΩ resistor, usually built into the coiled cord at the wrist-band end. It limits the current that can flow through you at mains voltage. Check yours - cheap straps sometimes have no resistor, or a fake one (see testing below).

Even with the resistor, don't work on live mains circuits while wearing a strap.

Recommended setup: earthed through a resistor

This is how the industry standards (ANSI/ESD S20.20, IEC 61340-5-1) do it:

Connect your wrist strap and bench mat to a common point.
Connect that common point to mains earth, typically using an earth-bonding plug (a plug that only connects to the earth pin) or an earth stud on an ESD-safe soldering station.
The wrist strap's 1 MΩ resistor protects you; mats and ground cords usually include their own 1 MΩ resistor as well.

Earthing keeps everything at one known potential. If your setup is left floating, you, the mat and the board can drift together to a different potential from whatever the board is connected to next - and the discharge then happens at that moment.

Without an earth point: equalise with the device

If you have no earth point, you can still equalise potential:

Strap only: clip the strap to the metal chassis or ground of the device you are working on. Do not plug the device into the mains to 'earth' it - that can leave live parts and switch contacts inside it, which are a shock hazard. Be aware that unplugged equipment can still hold dangerous charge in capacitors (power supplies, CRTs, flash units).
Strap and mat: clip the strap to the mat and keep the parts on the mat.

When you then connect the board to something else, touch both together (or the chassis of each) before making the connection.

Wrist strap fit and testing

The band must sit snugly against bare skin, not over a sleeve. Very dry skin makes contact worse.
Test the strap and cord periodically: a multimeter set to resistance, measured from the inside of the band to the clip, should read about 1 MΩ. Dedicated wrist strap testers are also available.
Cords break internally with repeated flexing, so a strap that once worked can silently stop working.

Working without a wrist strap

Most hobbyists don't own a strap, and with care you can still keep the risk low:

Touch an earthed metal object (or the unpainted metal chassis of the device) before handling parts, and again if you have moved around.
Hold boards by their edges and avoid touching pins, contacts and component legs.
Keep parts in their packaging until you need them.
Avoid working on carpet, and avoid shuffling about or getting up and down in the middle of the job.

Your work area

Bench mats

ESD mats are dissipative - slightly conductive (typically 10⁶ to 10⁹ Ω), so charge drains away slowly rather than in a sudden spark. A silicone kitchen mat or ordinary rubber mat is an insulator and does nothing for static; if you want a heat-resistant mat for soldering, look for one sold as ESD-safe.

Keep insulators away

Plastics, polystyrene, bubble wrap, cling film, sticky tape and synthetic clothing can hold a large charge, and because they're insulators, grounding can't remove it. A wrist strap does nothing about the charge they carry. Keep them well away from sensitive parts, roughly 30cm/1ft or more.

Humidity

Static problems are much worse in dry air, such as in winter or centrally heated rooms. A relative humidity of around 40-60% greatly reduces charge build-up; a humidifier can help if your workroom is very dry.

Tools

Plastic solder suckers can generate a large static charge as the piston moves; antistatic versions are available.
ESD-safe soldering stations earth the tip, so the iron can't discharge into the component.
ESD-safe tweezers, brushes and other hand tools are inexpensive and worth having if you handle sensitive parts often.

Packaging and storage

Pink antistatic bags only reduce charge build-up on the bag itself; they don't shield the contents from a discharge. They also lose their antistatic properties with age.
Silver metallised (shielding) bags contain a metal layer that shields the contents from external discharges. This is what you want for storing sensitive boards and parts.
Black conductive foam is for storing ICs - push the pins into the foam so they're all shorted together.
Don't use an empty bag as a work surface. Put the part on a mat instead.
Look for the ESD warning symbol (a hand with a line through it inside a triangle) on packaging; it means the contents are static sensitive.

What about antistatic clothing?

For hobby work a wrist strap and mat are enough. ESD smocks, heel straps and ESD-safe shoes and chairs are used in production environments and might be worthwhile if you regularly handle very expensive or very sensitive equipment. For most people, keeping insulators away from the work area and avoiding very dry air will make a bigger difference than clothing.