An Introduction to ESD(Ⅱ)

Fundamentals of Electrostatic Discharge
Part One—An Introduction to ESD
© 2013, EOS/ESD Association, Inc., Rome, NY

Figure 3 Induction

ESD Damage—How Devices Fail

Electrostatic damage is defined as “change to an item caused by an electrostatic discharge that makes it fail to meet one or more specified parameters.” and can occur at any point from manufacture to field service. Typically, 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.
Catastrophic Failure

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. The device's circuitry is permanently damaged causing the device to stop functioning totally or at least partially. Such failures usually can be detected when the device is tested before shipment. If a damaging level ESD event occurs after test, the part may go into production and the damage will go undetected until the device fails in final test.

Latent Defect

Per ESD ADV1.0 latent failure is “a malfunction that occurs following a period of normal operation. The failure may be attributable to an earlier electrostatic discharge event. The concept of latent failure is controversial and not totally accepted by all in the technical community.” A latent defect is 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. 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.

It is relatively easy with the proper equipment to confirm that a device has experienced a 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.

Basic ESD Events—What Causes Electronic Devices to Fail?

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. Whether or not damage occurs to an ESD sensitive 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. The level at which a device fails is known as the device’s ESD sensitivity or ESD susceptibility.

Discharge to the Device

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 one walks across a floor, an electrostatic charge accumulates on the body. Simple contact (or close proximity) of a finger to the leads of an ESDS or assembly which is typically on a different electrical potential can allow the body to discharge, possibly causing ESD 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. From the nature of the discharge, the model used to describe this event is known as the Machine Model (MM).

Discharge from the Device

The transfer of charge from an ESDS to a conductor is also an ESD event. 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 model used to simulate the transfer of charge from an ESDS is referred to as the Charged Device Model (CDM). The capacitances, energies, and current waveforms involved are totally different from those of a discharge to the ESD sensitive item, resulting very likely in different failure modes.

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 the feeder. If it then contacts the insertion head or any other conductive surface, a rapid discharge occurs from the device to the metal object.

Field Induced Discharges

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 that electrostatic field, a charge may be induced on the item. If the item is then grounded while 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.

How Much Static Protection is Needed?

Damage to an ESDS by the ESD event is determined by the device's ability to dissipate the energy of the discharge or withstand the voltage levels involved—as explained previously these factors determine the parts ESD sensitivity or susceptibility. Test procedures based on the models of ESD events help define the sensitivity of components to ESD. 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.

The ESD withstand voltage is "the highest voltage level that does not cause device failure; the device passes all tested lower voltages." Many electronic components are sensitive or susceptible to ESD damage at relatively low voltage levels. Many are susceptible at less than 100 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 their sensitivity to ESD and making the potential problem even more acute. Table 3 indicates the ESD sensitivity of various types of components.

Summary

In this “An Introduction to ESD”, we have discussed electrostatic charge and discharge, the mechanisms of creating charge, materials, types of ESD damage, ESD events, and ESD sensitivity. We can summarize this discussion as follows:

1. Virtually all materials, even conductors, can be triboelectrically charged.

2. The amount of charge is affected by material type, speed of contact and separation, humidity, and several other factors.

3. Charged objects have electrostatic fields.

4. Electrostatic discharge can damage devices so a parameter fails immediately, or ESD damage may be a latent defect that may escape immediate detection, but may cause the device to fail prematurely.

5. Electrostatic discharge can occur throughout the manufacturing, test, shipping, handling, or operational processes, and during field service operations.

6. ESD damage can occur as the result of a discharge to the device, from the device, or from charge transfers resulting from electrostatic fields. Devices vary significantly in their sensitivity or susceptibility to ESD.

Protecting products from the effects of ESD damage begins by understanding these key concepts of electrostatic charges and discharges. An effective ESD control program requires an effective training program where all personnel involved understand the key concepts. See Part Two for the basic concepts of ESD control.

References

ESD-ADV 1.0, Glossary, ESD Association, Rome NY.
ESD TR20.20, Handbook, ESD Association, Rome, NY.
ESD ADV 11.2, Triboelectric Charge Accumulation Testing, ESD Association, Rome, NY.
ANSI/ESD S20.20—Standard for the Development of Electrostatic Discharge Control Program, ESD Association, Rome, NY.