ElectroStatic Discharge (ESD) tutorial [2]

- a tutorial or summary about the basics of Electro Static Discharge, ESD and the affects it has on electronic components and electronic circuits

This ESD tutorial is in three pages dealing which address the different topics:
ESD Tutorial detailing the basics of ESD and how it arises [1]
The sensitivity of electronics to ESD [2]
Overcoming ESD [3]

ESD can have disastrous effects on electronic components. With ICs operating of supply voltages of 5 V and less these days, and with the feature sizes measured in fractions of a micron the static charges that go unnoticed in everyday life can easily destroy a chip. Worse still these effects may not destroy the chip instantly, but leave a defect waiting to cause a problem later in the life of the equipment.

In view of their sensitivity to static, most semiconductor devices today are treated as static sensitive devices ( SSD ). To prevent damage they must be handled in anti-static areas, often called ESDPAs (Electrostatic Protected Areas). Within these areas a variety of precautions are taken to ensure that static is dissipated and that the SSD, static sensitive devices do not experience any static discharges. Benches with dissipative surfaces, anti-static flooring, wrist straps for oerpators and many more items all form part of these anti-static areas.

Sensitivity

Some electronic devices are more sensitive to ESD than others. However to put the problem in perspective it is worth relating the levels of static to those to supply voltages. One would not consider applying a voltage of even fifty volts to a logic device. Yet static voltages of several kilovolts are often applied to them by careless handling.

The devices that are most sensitive to ESD are generally those which include FETs. These devices have very high impedances which do not allow the charge to dissipate in a more controlled fashion. However this does not mean that bipolar devices are immune from damage.

Standard CMOS chips can be damaged by static voltages of as little as 250V. These include the 74HC and 74HCT logic families are widely used in many designs using "glue logic" because of their lower current consumption. However many of the new microprocessors and LSI chips use very much smaller feature sizes, and cannot withstand anything like these voltages, making them very sensitive to ESD. Many new devices would be destroyed by operating them with a supply voltage of 5 V, and they are corresponding more susceptible to damage from ESD.

Logic devices are not the only devices requiring anti static precautions to be taken. GaAsFETs which are used for RF applications are very susceptible to damage, and can be destroyed by static voltages as low as 100V. Other forms of discrete FETs are also affected by ESD. MOSFETs which are again often used for many RF applications are very sensitive.

Even ordinary bipolar transistors can be damaged by potentials of around 500V. This is particularly true of the newer transistors which are likely to have much smaller internal geometries to give higher operating frequencies. This is only a broad indication of a very few of the ESD susceptibility levels. However it indicates that all semiconductor devices should be treated as static sensitive devices ( SSD ).

It is not only semiconductor devices that are being treated as SSDs these days. In some areas even passive components are starting to be treated as static sensitive. With the ever quickening trend to miniaturisation individual components are becoming much smaller. This makes them more sensitive to the effects of damage from ESD. A large discharge through a very small component may cause overheating, or breakdown in the component.

Discharge mechanisms

The way in which the electrostatic discharge, ESD, takes place is dependent on a large number of variables. Most of these are difficult to quantify. The level of static which is built up varies according to the materials involved, the humidity of the day, and even the size of the person has an effect. Each person represents a capacitor on which charge is held. The average person represents a capacitor of about 300 pF but this will vary greatly from one person to the next.

The way in which the discharge takes place also varies. Often the charge will be dissipated very quickly: typically in less than a hundred nanoseconds. During this time the peak current can rise to as much as twenty or thirty amps. The peak current and the time for the discharge are dependent upon a wide variety of factors. However if a metal object is used, like a pair of tweezers or thin nosed pliers the current peak is higher and reached in a shorter time than if the discharge takes place through a finger. This is because the metal provides a much lower resistance path for the discharge. However whatever the means of the discharge, the same amount of charge will be dissipated.

Failure mechanisms

The way in which ICs fail as a result of ESD also varies, and it is also dependent upon a number of factors including the way in which the charge is dissipated to the topology within the IC.

One of the most obvious way in which an IC can fail as a result of ESD occurs when the static charge represented as a very high voltage gives rise to a high peak current causing burn out. Even though the current passes for a very short time, the minute sizes within ICs can mean that the small interconnecting links wires or the devices in the chip itself can be fused by the amount of heat dissipated. In some instances the connection or component may not be completely destroyed. Instead it may only be partly destroyed. When this happens the device will continue to operate and may have no detectable reduction in its performance. At other times there may be a slight degradation in operation. This is particularly true of analogue devices where small fragments of material from the area of damage can spread over the surface of the chip. These may bridge or particularly bridge other components in the chip causing the performance to be altered or degraded.

When damage has been caused to the device, but it still remains operational, the defect leaves it with what is termed a 'latent defect' which may lead to a failure later in its life. Subsequent current surges resulting from turning the equipment on, or even as a result of normal operation may stress the defect and cause it to fail. This may also be brought about by vibration in some cases.

Latent damage cause inside an IC by ESD

These latent defects are particularly damaging because they are likely to lead to failures later in the life of the equipment, thereby reducing its reliability. In fact manufacturing plants with poor anti-static protection are likely to produce low reliability equipment as a result of this. In fact it is estimated that for every device which suffers instant damage at least ten are affected by latent damage and will fail at a later date.

Another way in which ESD can cause failure is when the voltage itself causes breakdown within the IC. It is quite possible for the voltage to breakdown an insulating oxide layer leaving the IC permanently damaged. Again this can destroy the chip immediately, or leave a partly damaged area with a latent failure.

Charge can also be transferred to electronic components in other ways and cause damage. It may result in damage either from voltage breakdown or by generating current to flow in the device. This may occur because a highly charged item will tend to induce an opposite charge in any article near it. Plastic drinks cups are very susceptible to carrying high static voltages and if they are placed on a work surface next to a sensitive piece of electronics they can induce a charge which may lead to damage.

Investigations

Although it is not easy to determine the cause of destruction of a device, some specialist laboratories have the means of making these investigations. They accomplish this by removing the top of the IC to reveal the silicon chip beneath. This is inspected using a microscope to reveal the area of damage.

These investigations are relatively costly. They are not normally undertaken for routine failures. Instead they are only undertaken when it is necessary to determine the exact cause of the failure.

Protection

With ICs being prone to damage so easily, it is necessary to consider all semiconductor devices, and often many passive devices as static sensitive devices SSD. They should only be handled in the special anti static ESDPAs. The next page in this tutorial ( Page [3] )summarises some of the methods and techniques that can be used. These include measures such as the use of ESD workbenches or ESD mats, ESD wrist straps, ESD bags and ESD packaging, etc..

Further pages from this tutorial
Page:   (1) >> (2) >> (3)
Next