An Introduction to ESD

8/14/2019 An Introduction to ESD

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An Introduction to ESD

By The ESD AssociationStatic electricity has been an industrial problem for centuries. As early

as the 1400s, European and Caribbean forts were using static controlprocedures and devices to prevent electrostatic discharge ignition ofblack powder stores. By the 1860s, paper mills throughout the U.S.employed basic grounding, flame ionization techniques, and steamdrums to dissipate static electricity from the paper web as it traveledthrough the drying process. The age of electronics brought with it newproblems associated with static electricity and electrostatic discharge.And, as electronic devices became faster and smaller, their sensitivityto ESD increased.Today, ESD impacts productivity and product reliability in virtuallyevery aspect of the electronic environment. Many aspects of

electrostatic control in the electronics industry also apply in otherindustries such as cleanroom applications and graphic arts.Despite a great deal of effort during the past decade, ESD still affectsproduction yields, manufacturing costs, product quality, productreliability, and profitability. Industry experts have estimated averageproduct losses due to static to range from 8 to 33% (see Table I).Others estimate the actual cost of ESD damage to the electronicsindustry as running into the billions of dollars annually. The cost of thedamaged devices themselves ranges from only a few cents for asimple diode to several hundred dollars for complex hybrids. Whenassociated costs of repair and rework, shipping, labor, and overhead

are included, clearly the opportunities exist for significantimprovements.

Static Losses Reported

Description Min. loss Max. lossEst. avg.

loss

Componentmanufacturers

4% 97% 1622%

Subcontractors 3% 70% 915%

Contractors 2% 35% 814%

User 5% 70% 2733%Source: Stephen Halperin, "Guidelines for Static Control

Management," Eurostat, 1990.

Table I. Informal summary of static losses by level.

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.


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Static Electricity: Creating ChargeStatic electricity is defined as an electrical charge caused by animbalance of electrons on the surface of a material. This imbalance ofelectrons produces an electric field that can be measured and that caninfluence other objects at a distance. Electrostatic discharge is defined

as the transfer of charge between bodies at different electricalpotentials.Electrostatic discharge can change the electrical characteristics of asemiconductor device, degrading or even destroying it. Electrostaticdischarge may also upset the normal operation of an electronicsystem, causing equipment malfunction or failure. Another problemcaused by static electricity occurs in cleanrooms. Charged surfaces canattract and hold contaminants, making removal from the environmentdifficult. When attracted to the surface of a silicon wafer or a device'selectrical circuitry, these particulates can cause random wafer defectsand reduce product yields.

Controlling electrostatic discharge begins with understanding howelectrostatic charge occurs in the first place. Electrostatic charge ismost commonly created by the contact and separation of two similaror dissimilar materials. For example, a person walking across the floorgenerates static electricity as shoe soles contact and then separatefrom the floor surface. An electronic device sliding into or out of a bag,magazine, or tube generates an electrostatic charge as the device'scase and/or metal leads make multiple contacts and separations withthe surface of the container. While the magnitude of electrostaticcharge may be different in these examples, static electricity is indeedgenerated.

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


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Figure 1. The triboelectric

charge of two materials incontact.

Figure 2. The triboelectriccharge when two materialsplaced in contact are thenseparated.

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


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The actual level of charge is measured in coulombs. Commonly,however, we speak of the electrostatic potential on an object, which isexpressed as voltage.This process of material contact, electron transfer, and separation isreally a more complex mechanism than described here. The amount of

charge created by triboelectric charging is affected by the area ofcontact, 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 maybe transferred from the material, creating an electrostatic discharge,or ESD, event.

Typical Voltage Levels at Different Relative Humidities

Means of Generation 025% RH 6590% RH

Walking across carpet 35,000 V 1500 V

Walking across vinyl tile 12,000 V 250 V

Worker at bench 6000 V 100 V

Poly bag picked up from

bench20,000 V 1200 V

Chair with urethane foam 18,000 V 1500 V

Table II. Examples of static generation.

An electrostatic charge may also be created on a material in other ways such as by

induction, ion bombardment, or contact with another charged object. However,

triboelectric charging is the most common.

Material Characteristics: How They Affect Static Charge

Virtually all materials, including water and dirt particles in the air, canbe triboelectrically charged. How much charge is generated, wherethat charge goes, and how quickly, are functions of the material'selectrical characteristics.InsulatorsA material that prevents or limits the flow of electrons across itssurface or through its volume is called an insulator. Insulators have anextremely high electrical resistance. A considerable amount of chargecan be generated on the surface of an insulator. Because an insulativematerial does not readily allow the flow of electrons, both positive andnegative charges can reside on an insulative surface at the same time,although at different locations. The excess electrons at the negativelycharged spot might be sufficient to satisfy the absence of electrons atthe positively charged spot. However, electrons cannot easily flowacross the insulative material's surface, and both charges may remainin place for a very long time.Conductive Materials


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A conductive material, because it has low electrical resistance, allowselectrons to flow easily across its surface or through its volume. Whena conductive material becomes charged, the charge (i.e., thedeficiency or excess of electrons) will be uniformly distributed acrossthe surface of the material. If the charged conductive material makes

contact with another conductive material, the electrons will transferbetween the materials quite easily. If the second conductor is attachedto an earth grounding point, the electrons will flow to ground and theexcess charge on the conductor will be "neutralized."Electrostatic charge can be created triboelectrically on conductors thesame way it is created on insulators. As long as the conductor isisolated from other conductors or ground, the static charge will remainon the conductor. If the conductor is grounded, the charge will easilygo to ground. Or, if the charged conductor contacts or nears anotherconductor, the charge will flow between the two conductors.Static-Dissipative Materials

Static-dissipative materials have an electrical resistance that liesbetween insulative and conductive materials. There can be electronflow across or through the dissipative material, but it is controlled bythe surface resistance or volume resistance of the material.As with the other two types of materials, charge can be generatedtriboelectrically on a static-dissipative material. However, like theconductive material, the static-dissipative materialwill allow the transfer of charge to ground or otherconductive objects. The transfer of charge from astatic-dissipative material will generally take longerthan from a conductive material of equivalent size.

Charge transfers from static-dissipative materialsare significantly faster than from insulators, andslower than from conductors.Triboelectric SeriesWhen two materials contact and separate, thepolarity and magnitude of the charge are indicatedby the materials' positions in the triboelectricseries, which lists materials according to theirrelative triboelectric charging characteristics. Whentwo materials contact and separate, the one nearerthe top of the series takes on a positive charge, the

other a negative charge. Materials further apart onthe table typically generate a higher charge thanthose that are closer together (see Table III).ESD Damage: How Devices FailElectrostatic damage to electronic devices canoccur at any point from manufacture to fieldservice. Damage results from handling the devices in uncontrolled

Positive

Negative

AcetateGlassNylonWool

LeadAluminumPaperCottonWoodSteelNickelCopperRubberPolyester

PVCSiliconTeflon

Table III. Typical

triboelectric series.


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surroundings or when poor ESD control practices are used. Generallydamage is classified as either a catastrophic failure or a latent defect.Catastrophic FailureWhen an electronic device is exposed to an ESD event it may no longerfunction. The ESD event may have caused a metal melt, junction

breakdown, or oxide failure, permanently damaging its circuitry andresulting in failure. Such failures usually can be detected when thedevice is tested before shipment. If the ESD event occurs after test,the damage will go undetected until the device fails in operation.Latent DefectA latent defect, on the other hand, is more difficult to identify. A devicethat is exposed to an ESD event may be partially degraded, yetcontinue to perform its intended function. However, the operating lifeof the device may be reduced dramatically. A product or systemincorporating devices with latent defects may experience prematurefailure after the user places them in service. Such failures are usually

costly to repair and in some applications may create personnelhazards.With the proper equipment, it is relatively easy to confirm that adevice has experienced catastrophic failure. Basic performance testswill substantiate device damage. However, latent defects areextremely difficult to prove or detect using current technology,especially after the device is assembled into a finished product.Basic ESD Events That Cause Electronic Device FailureESD damage is usually caused by one of three events: directelectrostatic discharge to the device, electrostatic discharge from thedevice, or field-induced discharges.

Discharge to the DeviceAn ESD event can occur when any charged conductor (including thehuman body) discharges to an ESDS (electrostatic discharge sensitive)device. The most common cause of electrostatic damage is the directtransfer of electrostatic charge from the human body or a chargedmaterial to the ESDS device. When one walks across a floor, anelectrostatic charge accumulates on the body. Simple contact of afinger to the leads of an ESDS device or assembly allows the body todischarge, possibly causing device damage. The model used tosimulate 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 eventis known as the machine model.Discharge from the DeviceThe transfer of charge from an ESDS device is also an ESD event. Thetrend towards automated assembly would seem to solve the problemsof HBM ESD events. However, it has been shown that components maybe more sensitive to damage when assembled by automatedequipment. A device may become charged, for example, from sliding


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down a feeder. If it then contacts an insertion head or anotherconductive surface, a rapid discharge occurs from the device to themetal 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 shortoften less than

one nanosecondthe peak current can reach several tens of amperes.Field-Induced DischargesA very rare event that can directly or indirectly damage devices istermed field induction. As noted earlier, whenever any object becomeselectrostatically charged, there is an electrostatic field associated withthat charge. If an ESDS device is placed in that electrostatic field, acharge may be induced on the device. If the device is thenmomentarily grounded while within the electrostatic field, a transfer ofcharge from the device occurs.Device Sensitivity: How Much Static Protection Is Needed?Damage to an ESDS device by an ESD event is determined by the

device's ability to dissipate the energy of the discharge or withstandthe voltage levels involved. This is known as device "ESD sensitivity."Test procedures based on the models of ESD events help define thesensitivity of components to ESD. Some devices may be more readilydamaged by discharges occurring within automated equipment, whileothers may be more prone to damage from handling by personnel.Defining the ESD sensitivity of electronic components is the first stepin determining the degree of ESD protection required.Many electronic components are susceptible to ESD damage atrelatively 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 circuitryonto these miniature devices, further increasing their sensitivity to ESDand making the potential problem even more acute. Table IV indicatesthe ESD sensitivity of various types of components.

Device Type

ESD

Susceptibility

(Volts)

VMOSMOSFET, GaAsFET,

EPROM

JFETOp-Amp

Schottky diodes

Film resistorsSchottky TTL

301200

1003001507000190250030025003003000

10002500

Table IV. Susceptibility of electronic

components to ESD.

Summary


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

Virtually all materials, even conductors, can be triboelectrically

charged. The level of charge is affected by material type, speed of contact

and separation, humidity, and several other factors. Electrostatic discharge can create catastrophic or latent failures

in electronic components. Electrostatic discharge can occur throughout the manufacturing,

test, shipping, handling, or operational processes. Component damage can occur as the result of a discharge from

the component as well as a direct discharge to the component. Components vary significantly in their sensitivity to ESD.

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

An Introduction to ESD

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