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ESD - Electrostatic Discharge

Protection against electrostatic discharge is rapidly increasing in importance. This article provides an overview about what ESD is, where does it come from, and how can you protect your circuits from it.

In recent years, the semiconductor industry has made a huge progress in the development of new devices. The chips are continuously becoming faster, smaller, more complex, with lower consumption. On the other side, they are often more sensitive to damage caused by electrostatic discharge. The importance of preventing such a damage is increasing. Although the chip manufacturers include protective circuits in their devices, the protection works only up to a certain voltage. If this voltage is exceeded, the chip is damaged or destroyed.

What is ESD, where does it come from?

Electrostatic charge builds up, for example, when two insulative surfaces are rubbed against each other. Electrons are coming from one surface to the other one. So, the surfaces gain a different potential. When they are placed near each other, in a certain distance a discharge occurs. Electrostatic discharge is a tiny "spark" of electrostatic charge jumping from one surface to the other. It occurs if the voltage (difference between the potentials of the two surfaces) is high enough to overcome the electric insulation strength of the material which separates the surfaces (in our example it is air). Whenever the charge passes from one surface to the other, electric current exists - and damages or destroys the semiconductor device.

Electrostatic discharge can occur (and damage semiconductor circuits) in the following four ways:

  • An electrostatically charged person touches the semiconductor device (its pins).
  • A charged semiconductor device touches e.g. a grounded surface.
  • A semiconductor device is touched by e.g. a charged tool.
  • A semiconductor device finds itself inside an electrostatic field.

Hidden damages

It is possible to happen that a device is degraded by ESD (for example, its characteristics change), but is not destroyed. Repeated weak electrostatic discharges don't destroy the chip but rather cause its degradation. The lifetime of a circuit decreases with each such a discharge; eventually it is destroyed. Due to this fact, circuits need to be equipped with ESD protection.

How to prevent damage

The best way to prevent circuit damage is to ensure that the circuit has the exact same potential as its surroundings. The best level is zero voltage - meaning ground potential. So, it is necessary to carefully ground everything that the device can touch or get near of. It is recommended to follow the following guidelines:

  • People manipulating with devices should be grounded, for example by wearing a wrist-strap (a metal band attached to their wrists and connected to a grounded object). Or, they can wear ESD protective shoes and walk on an antistatic floor.
  • The workdesk should be made of antistatic material and carefully grounded.
  • All insulating materials have to be away enough from the workplace, and should be neutralized by a ionizer. Clothing that could cause charge build-up should be covered with antistatic suit.
  • Stocked parts, or parts that are being manipulated with (e.g. transported from one workplace to another or being purchased in a store) should be wrapped in an antistatic wrap (e.g. in alluminum foil) to prevent damage.

Example of a workplace layout

Example of a workplace layout.

  • Grounds G1 and G2 are allowed for ESD protection. Both of them are interconnected.
  • R1 is a resistor for protective wrist-straps, as recommended by appropriate standards.
  • R2 is a resistor for the workbench and R3 for the antistatic floormat.

Practical info

Of course, the ESD protection cannot be dogmatized. All CMOS devices have protection diodes on all pins, unless they would interfere with the device functionality. They should neutralize a possible ESD, at least the ones in common voltage range.

However, such protection is impossible to implement e.g. in HF circuits or their parts - especially in receivers and transmitters or in similar devices. In these cases, the above guidelines are in no way exaggerated. Also, they should be used in manufacturing halls etc.

A frequent topic is ESD related to long cables and transmission lines, or to industrial networks, such as RS 485. ESD issues are very important for circuits connected to cables several hundreds of meters long. For this reason you can find information about ESD protection in many datasheets of manufacturers like MAXIM, DALLAS, TI, etc.

The standard IEC 100-4-2 deals with ESD testing and final equipment parameters. It does not contain specific recommendations for IC's. For detailed description of differences between various ways of testing circuits, using a human body model, and about the IEC 100-4-2 standard, datasheets of individual devices are recommended.

Written by: Milan Drinek
English translation: Joe Hlavac

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