An Introduction to ESD

Today, ESD impacts productivity and product reliability in virtually every aspect of the electronic environment. Many aspects of electrostatic control in the electronics industry also apply in other industries such as cleanroom applications and graphic arts.

Despite a great deal of effort during the past decade, ESD still affects production yields, manufacturing costs, product quality, product reliability, and profitability. Industry experts have estimated average product losses due to static to range from 8 to 33% (see Table I). Others estimate the actual cost of ESD damage to the electronics industry as running into the billions of dollars annually. The cost of the damaged devices themselves ranges from only a few cents for a simple diode to several hundred dollars for complex hybrids. When associated costs of repair and rework, shipping, labor, and overhead are included, clearly the opportunities exist for significant improvements.

This article focuses on how electrostatic charge and discharge occur, how various materials affect the level of charge, types of ESD damage, and how ESD events can damage electronic components.

Static electricity is defined as an electrical charge caused by an imbalance of electrons on the surface of a material. This imbalance of electrons produces an electric field that can be measured and that can influence other objects at a distance. Electrostatic discharge is defined as the transfer of charge between bodies at different electrical potentials.

Electrostatic discharge can change the electrical characteristics of a semiconductor device, degrading or even destroying it. Electrostatic discharge may also upset the normal operation of an electronic system, causing equipment malfunction or failure. Another problem caused by static electricity occurs in cleanrooms. Charged surfaces can attract and hold contaminants, making removal from the environment difficult. When attracted to the surface of a silicon wafer or a device's electrical circuitry, these particulates can cause random wafer defects and reduce product yields.

Controlling electrostatic discharge begins with understanding how electrostatic charge occurs in the first place. Electrostatic charge is most commonly created by the contact and separation of two similar or dissimilar materials. For example, a person walking across the floor generates static electricity as shoe soles contact and then separate from the floor surface. An electronic device sliding into or out of a bag, magazine, or tube generates an electrostatic charge as the device's case and/or metal leads make multiple contacts and separations with the surface of the container. While the magnitude of electrostatic charge may be different in these examples, static electricity is indeed generated.

Creating electrostatic charge by contact and separation of materials is known as triboelectric charging. It involves the transfer of electrons between materials. The atoms of a material with no static charge have an equal number of positive (+) protons in their nucleus and negative (–) electrons orbiting the nucleus. In Figure 1, Material A consists of atoms with equal numbers of protons and electrons. Material B also consists of atoms with equal (though perhaps different) numbers of protons and electrons. Both materials are electrically neutral.

Figure 1. The triboelectric charge of two materials in contact.

Figure 2. The triboelectric charge when two materials placed in contact are then separated.

When the two materials are placed in contact and then separated, negatively charged electrons are transferred from the surface of one material to the surface of the other material. Which material loses electrons and which gains electrons will depend on the natures of the two materials. The material that loses electrons becomes positively charged, while the material that gains electrons is negatively charged (see Figure 2).

The actual level of charge is measured in coulombs. Commonly, however, we speak of the electrostatic potential on an object, which is expressed as voltage.

This process of material contact, electron transfer, and separation is really a more complex mechanism than described here. The amount of charge created by triboelectric charging is affected by the area of contact, the speed of separation, relative humidity, and other factors (see Table II). Once the charge is created on a material, it becomes an "electrostatic"charge (if it remains on the material). This charge may be transferred from the material, creating an electrostatic discharge, or ESD, event.

Virtually all materials, including water and dirt particles in the air, can be triboelectrically charged. How much charge is generated, where that charge goes, and how quickly, are functions of the material's electrical characteristics.

A material that prevents or limits the flow of electrons across its surface or through its volume is called an insulator. Insulators have an extremely high electrical resistance. A considerable amount of charge can be generated on the surface of an insulator. Because an insulative material does not readily allow the flow of electrons, both positive and negative charges can reside on an insulative surface at the same time, although at different locations. The excess electrons at the negatively charged spot might be sufficient to satisfy the absence of electrons at the positively charged spot. However, electrons cannot easily flow across the insulative material's surface, and both charges may remain in place for a very long time.

A conductive material, because it has low electrical resistance, allows electrons to flow easily across its surface or through its volume. When a conductive material becomes charged, the charge (i.e., the deficiency or excess of electrons) will be uniformly distributed across the surface of the material. If the charged conductive material makes contact with another conductive material, the electrons will transfer between the materials quite easily. If the second conductor is attached to an earth grounding point, the electrons will flow to ground and the excess charge on the conductor will be "neutralized."

Electrostatic charge can be created triboelectrically on conductors the same way it is created on insulators. As long as the conductor is isolated from other conductors or ground, the static charge will remain on the conductor. If the conductor is grounded, the charge will easily go to ground. Or, if the charged conductor contacts or nears another conductor, the charge will flow between the two conductors.

Static-dissipative materials have an electrical resistance that lies between insulative and conductive materials. There can be electron flow across or through the dissipative material, but it is controlled by the surface resistance or volume resistance of the material.

As with the other two types of materials, charge can be generated triboelectrically on a static-dissipative material. However, like the conductive material, the static-dissipative material will allow the transfer of charge to ground or other conductive objects. The transfer of charge from a static-dissipative material will generally take longer than from a conductive material of equivalent size. Charge transfers from static-dissipative materials are significantly faster than from insulators, and slower than from conductors.

When two materials contact and separate, the polarity and magnitude of the charge are indicated by the materials' positions in the triboelectric series, which lists materials according to their relative triboelectric charging characteristics. When two materials contact and separate, the one nearer the top of the series takes on a positive charge, the other a negative charge. Materials further apart on the table typically generate a higher charge than those that are closer together (see Table III).

Electrostatic damage to electronic devices can occur at any point from manufacture to field service. Damage results from handling the devices in uncontrolled surroundings or when poor ESD control practices are used. Generally damage is classified as either a catastrophic failure or a latent defect.

When an electronic device is exposed to an ESD event it may no longer function. The ESD event may have caused a metal melt, junction breakdown, or oxide failure, permanently damaging its circuitry and resulting in failure. Such failures usually can be detected when the device is tested before shipment. If the ESD event occurs after test, the damage will go undetected until the device fails in operation.

A latent defect, on the other hand, is more difficult to identify. A device that is exposed to an ESD event may be partially degraded, yet continue to perform its intended function. However, the operating life of the device may be reduced dramatically. A product or system incorporating devices with latent defects may experience premature failure after the user places them in service. Such failures are usually costly to repair and in some applications may create personnel hazards.

With the proper equipment, it is relatively easy to confirm that a device has experienced catastrophic failure. Basic performance tests will substantiate device damage. However, latent defects are extremely difficult to prove or detect using current technology, especially after the device is assembled into a finished product.

ESD damage is usually caused by one of three events: direct electrostatic discharge to the device, electrostatic discharge from the device, or field-induced discharges.

An ESD event can occur when any charged conductor (including the human body) discharges to an ESDS (electrostatic discharge sensitive) device. The most common cause of electrostatic damage is the direct transfer of electrostatic charge from the human body or a charged material to the ESDS device. When one walks across a floor, an electrostatic charge accumulates on the body. Simple contact of a finger to the leads of an ESDS device or assembly allows the body to discharge, possibly causing device damage. The model used to simulate this event is the human body model (HBM).

A similar discharge can occur from a charged conductive object, such as a metallic tool or fixture. The model used to characterize this event is known as the machine model.

The transfer of charge from an ESDS device is also an ESD event. The trend towards automated assembly would seem to solve the problems of HBM ESD events. However, it has been shown that components may be more sensitive to damage when assembled by automated equipment. A device may become charged, for example, from sliding down a feeder. If it then contacts an insertion head or another conductive surface, a rapid discharge occurs from the device to the metal object. This event is known as the charged device model (CDM) event, and can be more destructive than the HBM for some devices. Although the duration of the discharge is very short—often less than one nanosecond—the peak current can reach several tens of amperes.

A very rare event that can directly or indirectly damage devices is termed field induction. As noted earlier, whenever any object becomes electrostatically charged, there is an electrostatic field associated with that charge. If an ESDS device is placed in that electrostatic field, a charge may be induced on the device. If the device is then momentarily grounded while within the electrostatic field, a transfer of charge from the device occurs.

Damage to an ESDS device by an ESD event is determined by the device's ability to dissipate the energy of the discharge or withstand the voltage levels involved. This is known as device "ESD sensitivity." Test procedures based on the models of ESD events help define the sensitivity of components to ESD. Some devices may be more readily damaged by discharges occurring within automated equipment, while others may be more prone to damage from handling by personnel. Defining the ESD sensitivity of electronic components is the first step in determining the degree of ESD protection required.

Many electronic components are susceptible to ESD damage at relatively low voltage levels. Many are susceptible at less than 100 V, and many disk-drive components have sensitivities below 10 V. Current trends in product design and development pack more circuitry onto these miniature devices, further increasing their sensitivity to ESD and making the potential problem even more acute. Table IV indicates the ESD sensitivity of various types of components.

Summary

In this introduction to electrostatic discharge, we have discussed the basics of electrostatic charge, discharge, types of failures, ESD events, and device sensitivity. We can summarize this discussion as follows:

Protecting your products from the effects of static damage begins by understanding these key concepts of ESD. Armed with this information, you can then begin to develop an effective ESD control program.