How does electrostatic discharge occur

However, latent defects are challenging to prove or detect using current technology, especially after the device is assembled into a finished product. Whether or not damage occurs to an ESD susceptible item ESDS by an ESD event is determined by the device's ability to dissipate the energy of the discharge or withstand the voltage levels involved. An ESD event can occur when any charged conductor including the human body discharges to an item. A cause of electrostatic damage could be the direct transfer of electrostatic charge from the human body or a charged material to the ESDS.

When a person walks across a floor, an electrostatic charge accumulates on their body. Simple contact or proximity of a finger to the leads of an ESDS or assembly, which is typically at a different electrical potential, can allow the body to discharge and possibly cause ESD damage to the ESDS.

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.

From the nature of the discharge, the model used to describe this event is known as the machine model MM. Static charge may accumulate on the ESDS itself through handling or contact and separation with packaging materials, worksurfaces, or machine surfaces.

This frequently occurs when a device moves across a surface or vibrates in a package. The capacitances, energies, and current waveforms involved are different from those of a. However, it has been shown that components may be more sensitive to damage when assembled by automated equipment.

For example, a device may become charged. When it contacts the insertion head or any other conductive surface, a rapid discharge occurs from the device to the metal object. Another electrostatic charging process 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 is placed in the electrostatic field and grounded while located within the electrostatic field, a transfer of charge from the device occurs as a CDM event. If the item is removed from the region of the electrostatic field and grounded again, a second CDM event will occur as the charge of opposite polarity from the first event is transferred from the device.

Damage to an ESDS by an ESD event is determined by the device's ability to dissipate the energy of the discharge or withstand the voltage levels involved in the discharge. As explained previously, these factors determine the ESD sensitivity of the device. Although it is known that there is very rarely a direct correlation between the discharges in the test procedures and real-world ESD events, defining the ESD sensitivity of electronic components gives some guidance in determining the degree of ESD control protection required.

These procedures and more are covered in Part Five of this series. Many electronic components are susceptible to ESD damage at relatively low voltage levels. Many are susceptible at less than volts, and many disk drive components withstand voltages even below 10 volts.

Current trends in product design and development pack more circuitry onto these miniature devices, further increasing the sensitivity to ESD and making the potential problem even more acute. Table 3 indicates the ESD sensitivity of various types of components. We can summarize this discussion as follows:. Protecting products from the effects of ESD damage begins by understanding these fundamental concepts of electrostatic charges and discharges.

An effective ESD control program requires an effective training program where all personnel involved understand the key concepts. By the s, paper mills throughout the U. Figure 1: The Triboelectric Charge. Materials Make Intimate Contact 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.

Table 1. Table 2. Insulative Materials A material that prevents or limits the flow of electrons across its surface or through its volume, due to having an extremely high electrical resistance, is called an insulative material. The excess electrons at the negatively charged spot might be sufficient to satisfy the absence of electrons at the positively charged spot.

Conductive Materials A material that allows electrons to flow easily across its surface or through its volume is called a conductive material. Dissipative Materials Dissipative materials have an electrical resistance between insulative and conductive materials. Catastrophic Failure When an electronic device is exposed to an ESD event, it may no longer function. The charge is released when one material comes near or in contact with another dissimilarly charged material.

We accumulate between V to V of charge in our bodies during a normal day but we can only feel static discharges of V or more. ESD can also occur without actual human contact. While most ESD occurs through direct human contact, there are also a number of cases where ESD is caused by human interaction. Some examples are when safety measures are not followed; such as when synthetic materials which tend to gather more electrostatic energy are placed near or on electronic equipment; or when rapid, non-ionic air movement occurs near electronics, typically in the form of a fan or compressed air; or when improperly grounded electronics, such as a radio or cell phone, is placed nearby.

There are three possible ways that ESD occurs: discharge to a device, discharge from a device, or a field-induced discharge. The first two alternatives are self-explanatory and are caused by the proximity of materials that are not of the same electrostatic potential. A field-induced discharge is rare but happens when a device is placed into an electrostatic fi eld, the charge is transferred to the device, and then grounded, causing a discharge. The ability of a device to dissipate the discharged energy also determines whether damage occurs to the sensitive electronics.

All of these discharges are potentially devastating to electronics, so it is important to eliminate electrostatic energy before it can build up. The most noticeable failure of devices exposed to ESD is the catastrophic failure.

These failures tend to have very visible results, such as melting or explosions. A catastrophic failure results in the immediate destruction of the device. Latent failure is the less noticeable type. Because the damage is not readily apparent, subsequent damage can continue to occur before failure results.

Since the device is partially degraded, failure can occur at a later time—even years later. Latent damage, shown in Figure 2, can be extremely difficult to detect. A device may still pass all tests after the damage has been done and may continue to work after assembly and subsequent use.

Because latent damage is difficult to determine, the location of the failure in an instrument or component is difficult to pinpoint. Furthermore, most devices you purchase e. This is document aeoh in the Knowledge Base.

Last modified on Skip to: content search login. Knowledge Base Toggle local menu Menus About the team. Voltages of this magnitude from electronic or electrical equipment where the more current can be source and for much longer will have a much greater effect and can be very dangerous.

There are several ways in which static charges can be transferred to semiconductor devices resulting in damage from ESD. The most obvious is when they are touched by an item that is charged and conductive.

The most obvious example of this possibly occurs when a semiconductor is on a work bench and someone walks across the floor building up a charge and then picks it up. The charged finger then imparts the static charge very quickly to the semiconductor with the possibility of damage. Tools can possibly be even more harmful. Metal screwdrivers are even more conductive and will impart the charge even faster and this results in higher levels of peak current.

However it is not necessary to touch components to cause damage to them. Items such as plastic cups carry a very high charge, and placing one of these near an IC can "induce" an opposite charge into the IC. This too can damage the semiconductor device. Ties made of man-made fibre are also an ESD hazard because they can charge up and easily hang near sensitive electronic equipment. There are a number of ways in which ESD can damage semiconductor components.

The most obvious results from the very high static voltage, giving rise to high levels of peak current that can cause local burn out. Even though the current flows for a very short period of time the minute feature sizes in the integrated circuit means that damage is caused very easily. The interconnecting wire links or areas in the chip itself can be fused by the high peak current. Another way in which damage can occur as a result of ESD is when the high level of voltage causes breakdown to occur in a component in the device itself.

It may breakdown an oxide layer in the device rendering the device inoperable. With dimensions in some ICs of much less that a micron, it is hardly surprising that even relatively low voltages can cause breakdown.