Tìm kiếm:
Giới thiệu

Đăng ký nhận tin

Nhập địa chỉ Email của Bạn

Thống kê

Tổng lượt truy cập
: 301.610
Thành viên Online
: 1
Khách
: 38
Bookmark and Share

Basics of Electrostatic Discharge (update on Jun 11 2013)

11/06/2013 08:03 CH (Lượt truy cập: 166830)
Tác giả : Ralph N.


ESD Basics VN Version (Copyright 2003 DESCO)





Part 1---An Introduction to ESD

History & Background
The decade of the 90's may be remembered as the Decade of Quality in the electronics industry. Increased competition, six-sigma quality, and ISO 9000 have forced a recommitment to quality even in those companies that might not have done so willingly. As we examine our environments for quality improvement areas, electrostatic discharge (ESD) remains a key target.

Static electricity has been an industrial problem for centuries. As early as the 1400’s, European and Caribbean forts were using static control procedures and devices to prevent electrostatic discharge ignition of black powder stores. By the 1860's, paper mills throughout the U.S. employed basic grounding, flame ionization techniques, and steam drums to dissipate static electricity from the paper web as it traveled through the drying process. The age of electronics brought with it new problems associated with static electricity and electrostatic discharge. And, as electronic devices became faster and smaller, their sensitivity to ESD increased.

Today, ESD impacts productivity and product reliability in virtually every aspect of today’s electronics environment. Many aspects of electrostatic control in the electronics industry also apply in other industries such as clean room 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 8-33% (Table 1). Others estimate the actual cost of ESD damage to the electronics industry as running into the billions of dollars annually. The cost of damaged devices themselves range 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.

Static Losses Reported

Description

Min.
Loss

Max.
Loss

Est. Avg.
Loss

Component Manufacturers

4%

97%

16-22%

Subcontractors

3%

70%

9-15%

Contractors

2%

35%

8-14%

User

5%

70%

27-33%

Table 1: Informal Summary of Static Losses by Level

Source: Stephen Halperin, "Guidelines for Static Control Management," Eurostat, 1990.

 

This first in a series of six articles on ESD 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. Future articles will cover various ways to control the problem.

Static Electricity:

Creating Charge

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 destroying it. Electrostatic discharge also may upset the normal operation of an electronic system, causing equipment malfunction or failure. Another problem caused by static electricity occurs in clean rooms. 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.


Figure 1: The Triboelectric Charge. Materials Make Intimate Contact

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.

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 nature of the two materials. The material that loses electrons becomes positively charged, while the material that gains electrons is negatively charged. 

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. 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.

 

Typical Voltage Levels

Means of Generation

10-25% RH

65-90% RH

Walking across carpet

35,000V

1,500V

Walking across vinyl tile

12,000V

250V

Worker at bench

6,000V

100V

Poly bag picked up from bench

20,000V

1,200V

Chair with urethane foam

18,000V

1,500V

Table 2: Examples of Static Generation

 

An electrostatic charge also may 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, can be triboelectrically charged. How much charge is generated, where that charge goes, and how quickly, are functions of the materials' electrical characteristics.

Insulators

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 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.

Conductive Materials
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
Static dissipative materials have electrical resistance 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.

Triboelectric Series
When two materials contact and separate, the polarity and magnitude of the charge are indicated by the materials’ positions in the triboelectric series. The triboelectric simply 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 ones closer together. See Table 3.


Table 3: Typical Triboelectric Series

 

ESD Damage—How Devices Fail
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.

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 fail. 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.

Latent Defect
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.

It is relatively easy with the proper equipment 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.

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.

Discharge to the Device
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 electrostatic discharge sensitive (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.

Discharge from the Device
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 the feeder. If it then contacts the 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.

Field Induced Discharges
Another 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.

Device Sensitivity:

How Much Static Protection is Needed?
Damage to an ESDS device by the 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 volts, and many disk drive components have sensitivities 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 4 indicates the ESD sensitivity of various types of components.

 

Device Type

ESD Susceptibility
(Volts)

VMOS

30 - 1,200

Mosfet, GaAsfet, EPROM

100 - 300

JFET

150 - 7,000

OP-AMP

190 - 2,500

Schottky Diodes

300 - 2,500

Film Resistors

300 - 3,000

Schottky TTL

1,000 - 2,500

Table 4: Susceptibility of Electronic Components to ESD


Summary

In this introductory article on 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:

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.

  1. Electrostatic discharge can create catastrophic or latent failures in electronic components.
  2. Electrostatic discharge can occur throughout the manufacturing, test, shipping, handling, or operational processes.
  3. Component damage can occur as the result of a discharge from the component as well as a direct discharge to the component.
  4. Components vary significantly in their sensitivity to ESD.

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. In Part Two we will focus on some basic concepts of ESD control.

ESD Basics:

 Part 2---Principles of ESD Control

 

Basic Principles of ESD control.

 

Basic Principles of Static Control
Sometimes, controlling electrostatic discharge (ESD) in the electronics environment seems to be a formidable challenge. However, the task of designing and implementing ESD control programs becomes less complex if we focus on just 4 basic Principles of control. In doing so, we also need to keep in mind the ESD corollary to Murphy’s law, "no matter what we do, static charge will try to find a way to discharge."

 

1. Design In Immunity
The 1st  Principle is to design products and assemblies to be as immune as reasonable from the effects of ESD.

This involves such steps as using less static sensitive devices or providing appropriate input protection on devices, boards, assemblies, and equipment. For engineers and designers, the paradox is that advancing product technology requires smaller and more complex geometries that often are more susceptible to ESD.

 

2. Eliminate and Reduce Generation
Obviously, product design isn’t the whole answer. The 2nd Principle of control is to eliminate or reduce the generation and accumulation of electrostatic charge in the first place.

It’s fairly basic: no charge -- no discharge. We begin by reducing as many static generating processes or materials, such as the contact and separation of dissimilar materials and common plastics, as possible from the work environment. We keep other processes and materials at the same electrostatic potential. Electrostatic discharge does not occur between materials kept at the same potential or at zero potential. We provide ground paths, such as wrist straps, flooring and work surfaces, to reduce charge generation and accumulation.

 

3. Dissipate and Neutralize
Because we simply can’t eliminate all generation of static in the environment, our 3rd  Principle is to safely dissipate or neutralize those electrostatic charges that do occur.

Proper grounding and the use of conductive or dissipative materials play major roles. For example, workers who "carry" a charge into the work environment can rid themselves of that charge when they attach a wrist strap or when they step on an ESD floor mat while wearing ESD control footwear. The charge goes to ground rather than being discharged into a sensitive part. To prevent damaging a charged device, the rate of discharge can be controlled with static dissipative materials.

For some objects, such as common plastics and other insulators, grounding does not remove an electrostatic charge because there is no conductive pathway. Typically, ionization is used to neutralize charges on these insulating materials. The ionization process generates negative and positive ions that are attracted to the surface of a charged object, thereby effectively neutralizing the charge.

 

4. Protect Products
Our 4th Principle is to prevent discharges that do occur from reaching susceptible parts and assemblies.

  • to provide our parts and assemblies with proper grounding or shunting that will "dissipate" any discharge away from the product.
  • to package and transport susceptible devices in proper packaging and materials handling products.

These materials may effectively shield the product from charge, as well as reduce the generation of charge caused by any movement of product within the container.

 

Elements of an Effective ESD Control Program
While these 4 principles may seem rather basic, they can guide us in the selection of appropriate materials and procedures to use in effectively controlling ESD. In most circumstances, effective programs will involve all of these principles. No single procedure or product will do the whole job, rather effective static control requires a full ESD control program.

  •  How do we develop and maintain a program that puts these basic principles into practice?
  • How do we start?
  • What is the process?
  • What do we do first?

Ask a dozen experts and you may get a dozen different answers. But, if you dig a little deeper, you will find that most of the answers center around similar key elements. You will also find that starting and maintaining an ESD control program is similar to many other business activities and projects. Although each company is unique in terms of its ESD control needs, there are at least 9 critical elements to successfully developing and implementing an effective ESD control program.

 

1. Establish an ESD Coordinator and ESD Teams.
As the problem-solving style of the decade, the team approach particularly applies to ESD because the problems and the solutions cross various functions, departments, divisions and even suppliers in most companies. Team composition includes line employees as well as department heads or other management personnel. ESD teams or committees help assure a variety of viewpoints, the availability of the needed expertise, and commitment to success. An active ESD committee helps unify the effort and brings additional expertise to the project. Committee or team membership should include representation from areas such as engineering, manufacturing, field service, training, and quality.

Heading this team effort is an ESD Program Coordinator. Ideally this responsibility should be a full-time job. However, we seldom operate in an ideal environment and you may have to settle for the function to be a major responsibility of an individual. The ESD coordinator is responsible for developing, budgeting, and administering the program. The coordinator also serves as the company’s internal ESD consultant to all areas.

 

2. Identify Your Losses.
Before seeking solutions to your problems, you will need to determine the extent of your losses to ESD. These losses may be reflected in receiving reports, QA and QC records, customer returns, in-plant yields, failure analysis reports, and other data that you may already have or that you need to gather. This information not only identifies the magnitude of the problem, but helps to pinpoint and prioritize areas that need attention.

Document your actual and potential ESD losses in terms of DOA components, rework, customer returns, and failures during final test and inspection. Use data from outside sources or the results of your pilot program for additional support. Develop estimates of the savings to be realized from implementing an ESD control program.

 

3. Evaluate Your Facility, Processes and Needs
Your next step is to gain a thorough understanding of your environment and its impact on ESD. Armed with your loss and sensitivity data, you can evaluate your facility, looking for areas and procedures that may be contributing to your defined ESD problems. Be on the lookout for things such as static generating materials and personnel handling procedures for ESD-sensitive items.

Document your processes. Observe the movement of people and materials through the areas. Note those areas that would appear to have the greatest potential for ESD problems. Remember, that ESD can occur in the warehouse just as it can in the assembly areas. Then conduct a thorough facility survey or audit. Measure personnel, equipment, and materials to identify the presence of electrostatic fields in your environment.

 

4. Identify ESD Sensitive Items
You will also want to identify those items (components, assemblies, and finished products) that are sensitive to ESD and the level of their sensitivity. You can test these items yourself, use data from suppliers, or rely on published data for similar items.

 

5. Build Justification for the Program
Once you understand your environment and the impact that ESD has on your products and services, you then build justification for your ESD control program. You may even need to conduct a pilot program if the experience of other companies is not sufficient to help prove your point.

 

6. Get the Support of Top Management
To be successful, an ESD program requires the support of your top management, at the highest level possible.

What level of commitment is required?

Recently, the CEO of a major electronics manufacturer was asked to leave the production area when he failed to wear the ESD control smock required by the company’s ESD procedures. He did so willingly and returned later wearing the proper apparel.

That’s the type of commitment you would like to achieve. To obtain it, your program justification will need to emphasize quality and reliability, the costs of ESD damage, the impact of ESD on customer service and product performance. Prepare a short corporate policy statement on ESD control. Have top management co-sign it with the ESD coordinator. Periodically, reaffirm the policy statement and management’s commitment to it.

 

7. Establish and Implement Procedures and Specifications
Now you can develop and implement the appropriate procedures that will help solve the ESD problems you have identified in your company. Prepare and distribute written procedures and specifications so that everyone has a clear understanding of what is to be done. Fully documented procedures will help you with ISO 9000 certification as well.

 

8. Train Personnel
Train and retrain your personnel in ESD and your company’s ESD control program and procedures. Proper training for line personnel is especially important. They are often the ones who have to live with the procedures on a day to day basis. A thorough understanding of what is expected and why will have a significant positive impact on proper long-term implementation.

 

9. Review, Audit, Analyze, Report, Feedback, Improve
Developing and implementing the program itself are obvious. What might not be so obvious is the need to continually review, audit, analyze, feedback and improve. You will be asked to continually identify the return on investment of the program and to justify the savings realized. Technological changes will dictate improvements and modifications. Feedback to employees and top management is essential. Management commitment will need reinforcement.

Remember to provide both reporting and feedback to management, the ESD team, and other employees. Management will want to know that their investment in time and money is yielding a return in terms of quality, reliability and profits. Team members need a pat on the back for a job well done and need to know where to put their next efforts. Other employees will want to know that the procedures you have asked them to follow are indeed worthwhile. Conduct periodic evaluations of your program and audits of your facility. You will find out if your program is successful and is giving you the expected return. You will spot weaknesses in the program and shore them up. You will discover whether the procedures are being followed.

As you find areas that need work, be sure to make the necessary adjustments to keep the program on track.

4 principles of static control and 9 key elements to program development and implementation: your guideposts for effective ESD control programs.

Part 3---An Overview of ESD Control Procedures and Materials

 

In Part 3, we will cover some of the primary specific static control procedures and materials that become part of your program. First, a quick review.

Basic Principles of Static Control
We suggested that static control programs become more effective and less complex if we focus on just 4 basic principles of static control as follows:

  1. Design In Immunity by designing products and assemblies to be as immune as reasonable from the effects of ESD.
  2. Eliminate and Reduce Generation by reducing and eliminating static generating processes, keeping processes and materials at the same electrostatic potential, and by providing appropriate ground paths to reduce charge generation and accumulation.
  3. Dissipate and Neutralize by grounding, ionization, and the use of conductive and dissipative static control materials.
  4. Protect Products from ESD with proper grounding or shunting and the use of static control packaging and materials handling products.

In most facilities, our static control efforts center around the last three principles. In this column we will concentrate on the primary materials and procedures that eliminate and reduce generation, dissipate and neutralize charges, or protect sensitive products from ESD.

 

Identifying the Problem Areas
You may also recall from our previous article that we suggested that there were at least 9 critical elements to successfully developing and implementing an effective ESD control program. Selecting appropriate static control materials and developing and implementing effective procedures begins with two of these critical elements:

  • Identifying and classifying ESD sensitive items used in your facility.
  • Evaluating your facility and processes to determine the areas that need to be protected.

One of the first questions answered with this information is "Which areas of our facility need ESD protection?" Often you will find that there are more areas that require protection than you originally thought, usually wherever ESDS devices are handled. Typical areas requiring ESD protection are shown in Table 1.

 

Receiving

Inspection

Stores and warehouses

Assembly

Test and inspection

Research and development

Packaging

Field service repair

Offices and laboratories

Clean rooms

Table 1: Typical Facility Areas Requiring ESD Protection

Personnel and Moving Equipment

In many facilities, people are one of the prime generators of static electricity. The simple act of walking around or repairing a board can generate several thousand volts on the human body. If not properly controlled, this static charge can easily discharge into a static sensitive device—a human body model (HBM) discharge.

 

ESD

Even in highly automated assembly and test processes, people still handle static sensitive devices…in the warehouse, in repair, in the lab, in transport. For this reason, static control programs place considerable emphasis on controlling personnel generated electrostatic discharge. Similarly, the movement of carts and other wheeled equipment through the facility also can generate static charges that can transfer to the products being transported on this equipment.

Wrist Straps

Typically, the primary means of controlling static charge on personnel is with a wrist strap. When properly worn and connected to ground, a wrist strap keeps the person wearing it near ground potential. Because the person and other grounded objects in the work area are at or near the same potential, there can be no hazardous discharge between them. In addition, static charges are safely dissipated from the person to ground and do not accumulate.

Wrist straps have two major components, the cuff that goes around the person’s wrist and the ground cord that connects the cuff to the common point ground. Most wrist straps have a current limiting resistor molded into the ground cord head on the end that connects to the cuff. The resistor most commonly used is a one megohm, 1/4 watt with a working voltage rating of 250 volts.

Wrist straps should be tested on a regular basis. Daily testing or continuous monitoring is recommended.

 

Floors, Floor Mats, Floor Finishes
A second method of controlling electrostatic charge on personnel is with the use of ESD protective floors in conjunction with ESD control footwear or footstraps. The combination of floor materials and footwear provides a ground path for the dissipation of electrostatic charge, thus reducing the charge accumulation on personnel and other objects to safe levels. In addition to dissipating charge, some floor materials (and floor finishes) also reduce triboelectric charging. The use of floor materials is especially appropriate in those areas where increased personnel mobility is necessary. In addition, floor materials can minimize charge accumulation on chairs, carts, walking stackers, lift trucks and other objects that move across the floor. However, those items require dissipative or conductive casters or wheels to make electrical contact with the floor.

 

Shoes, Grounders, Casters
Used in combination with ESD protective floor materials, static control shoes, grounders, casters and wheels provide the necessary electrical contact between the person or object and the floor material. Insulative footwear, casters, or wheels prevent static charges from flowing from the body to the floor to ground.


Clothing
Clothing is a consideration in some ESD protective areas, especially in clean rooms and very dry environments. Clothing materials can generate electrostatic charges when they contact and separate from other objects and the clothing itself. These charges may discharge into sensitive components or create electrostatic fields that may induce charges on the human body. Although a person may be grounded, that does not mean that insulative clothing fabrics can dissipate a charge to that person's skin and then to ground. Clothing usually is electrically insulated or isolated from the body. Grounded static control garments are intended to minimize the effects of electrostatic fields or charges that may be present on a person’s clothing.


Workstations and Worksurfaces
Worksurfaces and workstations are important parts of an ESD protective program. Many ESDS devices and assemblies are handled, assembled or repaired at workstations.

An ESD protective workstation refers to the work area of a single individual that is constructed and equipped with materials and equipment to limit damage to ESD sensitive items. It may be a stand-alone station in a stockroom, warehouse, or assembly area, or in a field location such as a computer bay in commercial aircraft. A workstation also may be located in a controlled area such as a clean room.

The workstation provides a means for connecting all worksurfaces, fixtures, handling equipment, and grounding devices to a common point ground. In addition, there may be provision for connecting additional personal grounding devices, equipment, and accessories such as constant ground monitors and ionizers. The key ESD control elements comprising most workstations are a static dissipative worksurface, a means of grounding personnel (usually a wrist strap), a common grounding connection, and appropriate signage and labeling. Static protective worksurfaces with a resistance to ground of 106 to 109 provide a surface that is at the same electrical potential as other ESD protective items in the workstation. They also provide an electrical path to ground for the controlled dissipation of any static potentials on materials that contact the surface. The worksurface also helps define a specific work area in which ESD sensitive devices may be safely handled. The worksurface is connected to the common point ground.

 

Production Equipment and Production Aids
Although personnel generated static is typically the primary ESD culprit in many environments, automated manufacturing and test equipment can also pose an ESD problem. For example, a device may become charged from sliding down the feeder. If the device then contacts the insertion head or another conductive surface, a rapid discharge occurs from the device to the metal object--a Charged Device Model (CDM) event.

In addition, various production aids may also pose an ESD problem. Production aids are those materials, tools and fixtures that help to produce finished products but do not become part of the finished product. Some examples are hand tools, soldering irons, tapes, solvents, and so forth.

Grounding is the primary means of controlling static charge on many production aids and equipment. The metal chassis or conductive enclosure of equipment that uses utility power is required by the National Electrical Code to be connected to the equipment ground (the green wire) in order to carry fault currents. This ground connection also will function for ESD purposes. All electrical tools and equipment used to process ESD sensitive hardware require the 3 prong grounded type AC plug. Hand tools that are not electrically powered, i.e., pliers, wire cutters, and tweezers, are usually grounded through the ESD worksurface and the (grounded) person using the conductive tools. Holding fixtures should be made of conductive or static dissipative materials when possible. If a conductive fixture is not sitting on a ESD worksurface or handled by a grounded person, a separate ground wire may be required. For those items that are composed of insulative materials, the use of ionization or application of topical antistats may be required to control generation and accumulation of static charges.

 

Packaging and Materials Handling
Direct protection of ESDS devices from electrostatic discharge is provided by packaging materials such as bags, corrugated, and rigid or semi-rigid packages. The primary use of these items is to protect the product when it leaves the facility, usually when shipped to a customer. In addition, materials handling products such as tote boxes and other containers primarily provide protection during inter or intra facility transport.

The main ESD function of these packaging and materials handling products is to limit the possible impact of ESD from triboelectric charge generation, direct discharge, and electrostatic fields. The initial consideration is to have low charging materials (antistatic) in contact with ESD sensitive items. For example, the antistatic property would control triboelectric charge resulting from sliding a board or component into the package or container. A second requirement is that the material provide protection from direct electrostatic discharge as well as shield from electrostatic fields.

Many materials are available that provide all three benefits: antistatic, discharge protection, and electric field suppression. The inside of these packaging materials have an antistatic layer, but also have an outer layer with a surface resistance generally in the dissipative range.

A material’s antistatic properties are not necessarily predicted by its resistance or resistivity. However, resistance or resistivity measurements help define the material’s ability to provide electrostatic shielding or charge dissipation. Electrostatic shielding attenuates electrostatic fields on the surface of a package in order to prevent a difference in electrical potential from existing inside the package. Electrostatic shielding is provided by materials that have a surface resistance equal to or less than 1.0 x 10E3 when tested according to EOS/ESD-S11.11 or a volume resistivity of equal to or less than 1.0 x 10E3 ohm-cm when tested according to the methods of EIA 541. In addition, shielding may be provided by packaging materials that provide an air gap between the package and the product.

Dissipative materials provide charge dissipation characteristics. These materials have a surface resistance greater than 1.0 x 10E4 but less than or equal to 1.0 x 10E11 when tested according to EOS/ESD-S11.11 or a volume resistivity greater than 1.0 x 10E5 ohm-cm but less than or equal to 1.0 x 10E12 ohm-cm when tested according to the methods of EIA 541. Be aware that the very wide range of resistance and resistivity results in a wide range of performance.

 

Grounding
In our discussion to this point, we have seen how important grounding is to effective ESD control. Consequently, effective ESD grounds are of critical importance in any operation, and ESD grounding should be clearly defined and regularly evaluated.

ESD Association Standard ANSI EOS/ESD 6.1-Grounding recommends a two-step procedure for grounding ESD protective equipment.

The first step is to ground all components of the work area (worksurfaces, people, equipment, etc.) to the same electrical ground point called the "common point ground." This common point ground is defined as a "system or method for connecting two or more grounding conductors to the same electrical potential."

Figure 1--Common Point Ground Symbol

This ESD common point ground should be properly identified. ESD Association standard EOS/ESD S8.1-1993 recommends the use of the symbol in Figure 1 to identify the common point ground.

The second step is to connect the common point ground to the equipment ground or the third wire (green) electrical ground connection. This is the preferred ground connection because all electrical equipment at the workstation is already connected to this ground. Connecting the ESD control materials or equipment to the equipment ground brings all components of the workstation to the same electrical potential. If a soldering iron used to repair an ESDS item were connected to the electrical ground and the surface containing the ESDS item were connected to an auxiliary ground, a difference in electrical potential could exist between the iron and the ESDS item. This difference in potential could cause damage to the item.

Any auxiliary grounds (water pipe, building frame, ground stake) present and used at the workstation must be bonded to the equipment ground to minimize differences in potential between the two grounds.

 

Ionization
As we have seen, the primary method of static charge control is direct connection to ground for conductors, static dissipative materials, and personnel. However, a complete static control program must also deal with isolated conductors that cannot be grounded, insulating materials (e.g., most common plastics). Topical antistats often are used to dissipate static charges from these items under some circumstances

More frequently, however, air ionization can neutralize the static charge on insulated and isolated objects by charging the molecules of the gases of the surrounding air. Whatever static charge is present on objects in the work environment will be neutralized by attracting opposite polarity charges from the air. Because it uses only the air that is already present in the work environment, air ionization may be employed even in clean rooms where chemical sprays and some static dissipative materials are not usable.

Air ionization is not a replacement for grounding methods. It is one component of a complete static control program. Ionizers are used when it is not possible to properly ground everything and as backup to other static control methods. In clean rooms, air ionization may be one of the few methods of static control available.

 

Clean Room Requirements
While the basic methods of static control discussed here are applicable in most environments, there are characteristics of the semiconductor manufacturing process that require special considerations.

Many objects integral to the semiconductor manufacturing process (quartz, glass, plastic, and ceramic) are inherently charge generating. Because these materials are insulators, this charge cannot be removed easily by grounding. Many static control materials contain carbon particles or surfactant additives that sometimes restrict their use in clean rooms. The need for personnel mobility and the use of clean room garments often make the use of wrist straps difficult. In these circumstances, ionization and flooring/footwear systems become key weapons against static charge.

 

Identification
A final element in our static control program is the use of appropriate symbols to identify static sensitive devices and assemblies, as well as products intended to control ESD. The traditional symbols traditionally used to identify ESDS parts or ESD control materials have been replaced with newer, more appropriate symbols. ESD Association Standard ANSI ESD S8.1-1993 — ESD Awareness Symbols provides two symbols for ESD identification.

 

Figure 2--ESD Susceptibility Symbol

The ESD Susceptibility Symbol (Figure 2), consists of a triangle, a reaching hand, and a slash through the reaching hand. The triangle means "caution" and the slash through the reaching hand means "Don’t touch." Because of its broad usage, the hand in the triangle has become associated with ESD and the symbol literally translates to "ESD sensitive stuff, don’t touch."

The ESD Susceptibility Symbol is applied directly to integrated circuits, boards, and assemblies that are static sensitive. It indicates that handling or use of this item may result in damage from ESD if proper precautions are not taken. If desired, the sensitivity level of the item may be added to the label.

 

Figure 3-- ESD Protective Symbol

The ESD Protective Symbol (Figure 4), consists of the reaching hand in the triangle. An arc around the triangle replaces the slash. This "umbrella" means protection. The symbol indicates ESD protective material. It is applied to mats, chairs, wrist straps, garments, packaging, and other items that provide ESD protection. It also may be used on equipment such as hand tools, conveyor belts, or automated handlers that is especially designed or modified to provide ESD control.

Neither symbol is applied on ESD test equipment, footwear checkers, wrist strap testers, resistance or resistivity meters or similar items that are used for ESD purposes, but which do not provide actual protection.

 

Summary
Effective static control programs require a variety of procedures and materials. In this column, we have provided a brief overview of the most commonly used elements of a program. Additional in-depth discussion of individual materials and procedures can be found in publications such as the ESD Handbook published by the ESD Association.

Your program is up and running. How do you determine whether it is effective? How do you make sure your employees follow it? In Part 4, we will cover the topics of Auditing and Training.

 

Part 4 ---Auditing and Training

Your static control program is up and running.

How do you determine whether it is effective?

How do you make sure your employees follow it?

Previously, we suggested that there were at least nine critical elements to successfully developing and implementing an effective ESD control program. In Part Four, we will focus on two more of these elements: training and auditing.

 

Personnel Training

The procedures are in place. The materials are in use. But, your ESD control program just does not seem to yield the expected results. Failures declined initially, but they have begun reversing direction. Or perhaps there was little improvement at all. The solutions might not be apparent in inspection reports of incoming ESD materials. Nor in the wrist strap log-in sheets. Sometimes simply turning the pages of the calendar will provide significant clues. When was the last time your employees attended an ESD training program? In fact, have your employees been trained at all?

In large companies or small, it is hard to underestimate the role of training in an ESD control program. There is significant evidence to support the contribution of training to the success of the program (References 1, 5, 9, 10, 14, 15, 16). We would not send employees to the factory floor without the proper soldering skills or the knowledge to operate the automated insertion equipment. We should provide them with the same skill level regarding ESD control procedures.

 

Elements of Effective Training Programs
Although individual requirements cause training programs to vary from company to company, there are several common threads that run through the successful programs.

 

1 -- Successful training programs cover all affected employees.
Obviously we train the line employees who test their wrist straps or place finished products in static protective packaging. But we also include department heads, upper management, and executive personnel in the process. Typically they are responsible for the day-to-day supervision and administration of the program or they provide leadership and support. The training will vary for these different groups, but awareness and understanding at all levels is key.

2 -- Effective training is comprehensive and consistent.
Training not only covers specific procedures, but also the physics of the problem and the benefits of the program as well. Consistent content across various groups, plants, and even countries (adjusted for cultural differences, of course) reduces confusion and helps assure conformance.

3 -- Use a variety of training tools and techniques.
Combine live instruction with training videos or interactive CD-ROM programs. Effective training involves employees in the process. Reinforce the message with demonstrations of ESD events and their impact. Bulletin boards, newsletters, and posters provide additional reminders and reinforcement.

4 -- Test, certify and retrain.
Your training should assure material retention and emphasize the importance of the effort. If properly implemented, testing and certification motivates and builds employee pride. Retraining is an ongoing process that reinforces, reminds, and provides opportunities for implementing new or improved procedures.

5 -- Feedback, auditing, and measurement.
Motivate and provide the mechanism for program improvement. Sharing yield or productivity data with employees demonstrates the effectiveness of the program and of their efforts. Tracking these same numbers can indicate that it’s time for retraining or whether modifications are required in the training program.

Design and delivery of an effective ESD training program can be just as important as the procedures and materials used in your ESD control program. A training program that is built on identifiable and measurable performance goals helps assure employee understanding, implementation and success.

 

Auditing

Developing and implementing an ESD control program itself is obvious. What might not be so obvious is the need to continually review, audit, analyze, feedback and improve. You will be asked to continually identify the program’s return on investment and to justify the savings realized. Technological changes will dictate improvements and modifications. Feedback to employees and top management is essential. Management commitment will need reinforcement.

Like training, regular auditing becomes a key ingredient in the successful management of ESD control programs. The mere presence of the auditing process spurs compliance with program procedures. It helps strengthen management’s commitment. Audit reports trigger corrective action and help foster continuous improvement.

The benefits to be gained from regular auditing of our ESD control procedures are numerous. They allow us to prevent problems before they occur rather than always fighting fires. They allow us to readily identify problems and take corrective action. They identify areas in which our programs may be weak and provide us with information required for continuous improvement. They allow us to leverage limited resources effectively. They help us determine when our employees need to be retrained. They help us improve yields, productivity, and capacity. And they help us bind our ESD program together into a successful effort.

An ESD audit measures performance to the defined standards and procedures of the ESD control program. Typically, we think of an ESD audit as a periodic review and inspection of the ESD work area covering use of the correct packaging materials, wearing of wrist straps, following defined procedures, and similar items. However, a good audit program is a bit more complicated.

 

Types of Audits
There are several types of ESD audits: the work place audit, program management audits, and quality process checking. Each type is distinctively different and each is vitally important to the success of the ESD program

Program management audits measure how well a program is managed and how strong management commitment is. The program management audit emphasizes factors such as the existence of an effective implementation plan, realistic program requirements, ESD training programs, regular audits, and other critical factors of program management. The program management audit typically is conducted by a survey specifically tailored to the factors being reviewed. Because it’s a survey, the audit can be conducted without actually visiting the site. The results of this audit indirectly measure work place compliance and are particularly effective as a means of self-assessment for small companies as well as large global corporations.

Quality process checking applies classical statistical quality control procedures to the ESD process and is performed by operations personnel. This is not a periodic audit, but rather daily maintenance of the program. Visual and electrical checks of the procedures and materials, wrist strap testing for example, are used to monitor the quality of the ESD control process. Checking is done on a daily, weekly or monthly basis.

Trend charts and detailed records trigger process adjustments and corrective action. They help assure that specified procedures are followed on a regular basis. The records are essential for quality control purposes, corrective action and compliance with ISO-9000.

Work area audits verify that program procedures are followed and that ESD control materials and equipment are within specification or are functioning properly. Audits are performed on a regular basis, often monthly, and utilize sampling techniques and statistical analysis of the results. The use of detailed checklists and a single auditor assures that all items are covered and that the audits are performed consistently over time.

 

Basic Auditing Instrumentation
Special instrumentation will be required to conduct work area audits. The specific instrumentation will depend on what you are trying to measure, the precision you require and the sophistication of your static control and material evaluation program. However, as a minimum, you will need an electrostatic field meter, a wide range resistance meter, a ground/circuit tester, and appropriate electrodes and accessories. Additional instrumentation might include a charge plate monitor, footwear and wrist strap testers, chart recorders and timing devices, discharge simulators, and ESD event detectors.

Although this equipment must be accurate, it need not be as sophisticated as laboratory instruments. The audit is intended to verify basic functions and not as a full qualification of ESD control equipment or materials. Remember, you want the right tool for job. Just as you would not buy a hammer if you are were planning to saw wood, you would not purchase an electrometer to measure static voltages on a production line. If you are making measurements according to specific standards, be sure the instrumentation meets the specifications of these standards.

With a hand held electrostatic field meter, you can measure the presence of electrostatic charge in your environment allowing you to identify problem areas and monitor your ESD control program. These instruments measure the electrostatic field associated with a charged object. Many field meters simply measure the gross level of the electrostatic field and should be used as general indicators of the presence of a charge and the approximate level of this charge. Others will provide more precise measurement for material evaluation and comparison.

For greater precision in facility measurements or for laboratory evaluation, a charge plate monitor can be attached to some field meters or connected to a voltmeter in the laboratory. With these additional tools you can evaluate the performance of flooring materials or balance ionizing equipment, for example.

Because resistance is one of the key factors in evaluating ESD control materials, a wide range resistance meter becomes a crucial instrument. Most resistance measurements are made at 100 volts, and some at 10 volts. The equipment you choose should be capable of applying these voltages to the materials being tested. In addition, the meter should be capable of measuring resistance ranges of 105 to 1011 ohms. With the proper electrodes and cables, you will be able to measure the resistance of flooring materials, worksurfaces, equipment, furniture, garments, and some packaging materials.

The final instrument is a ground/circuit tester. With this device you can measure the continuity of your ESD grounds and also check the impedance as well as neutral to ground shorts.

 

Areas, Processes, and Materials to be Audited
In our last column we stated that ESD protection was required wherever "wherever ESDS devices are handled." Obviously, our audits need to include these same areas. Table 1 indicates some of the physical areas that require ESD protection and auditing of the program.

Receiving

Inspection

Stores and Warehouses

Assembly

Test and Inspection

Research and Development

Packaging

Field Service Repair

Offices and Laboratories

Clean Rooms

Table 1: Typical Facility Areas To Be Audited

Similarly, we need to conduct work area and program management audits all of the various processes, materials, and procedures that are used in our ESD control programs.
Some of these are shown in Table 2.

Personnel

Moving Equipment (Carts, lift trucks)

Wrist Straps

Floors, Floor Mats, Floor Finishes

Shoes, Grounders, Casters

Clothing

Workstations

Worksurfaces

Packaging and Materials Handling

Ionization

Grounding

Production Equipment

Production Aids (Soldering irons, fixtures, etc.)

Labeling and Identification

Purchasing Specifications and Requisitions

ESD Control Program Procedures and Specifications

ESD Measurement and Test Equipment

Personnel Training

Engineering Specifications and Drawings

Table 2: Typical Processes, Materials and Procedures To Be Audited

Check Lists
Check lists can be helpful tools for conducting work place and program audits. However, it is important that ESD control program requirements are well documented and accessible to avoid a tendency for checklists becoming de facto lists of requirements. Table 3 indicates the type of questions and information that might be included in an auditing check list. Your own check lists, of course, will be based on your specific needs and program requirements. They should conform to your actual ESD control procedures and specifications and they should be consistent with any requirements you may have for ISO 9000.

In addition to check lists, you will use various forms for recording the measurements you make: resistance, voltage generation, etc. Part of your audit will also include the daily logs used on the factory floor such as those used for wrist strap checking.

Sample Audit Check List
ESD Control Program

Function/Area Audited: Personnel

Date:

Auditor:

Audit Questions

 

Y

 

 

1. Where ESD protective flooring is used for personnel grounding, are foot grounding devices or conductive footwear worn?

 

 

 

2. Where conductive floors and footwear are used for personnel grounding, do personnel check continuity to ground upon entering the area?

 

 

 

3. Are personnel wearing grounded wrist straps at the ESD protective workstations?

 

 

 

4. Are personnel checking wrist straps for continuity or using a continuous ground monitor?

 

 

 

5. Where continuous ground monitors are not used, are wrist straps checked and logged routinely and at frequent intervals?

 

 

 

6. Are wriststrap checkers and continuous ground monitors checked and maintained periodically?

 

 

 

7. Do wrist straps and foot grounders fit correctly?

 

 

 

8. Are wrist straps and foot grounders working correctly?

 

 

 

9. Are wrist strap cords checked, on the person, at the workstation?

 

 

 

10. Are disposable foot grounders limited to one time use?

 

 

 

11. Are test records for wrist straps and foot grounders kept and maintained?

 

 

 

12. When required, are ESD protective garments correctly worn?

 

 

 

13. Are nonessential personal items kept out of ESD controlled areas?

 

 

 

14. Are finger cots and gloves being used on ESD sensitive hardware made of ESD protective material, or treated to be ESD protective?

 

 

 

15. Are personnel working in the ESD controlled area currently certified or escorted?

 

 

 

16. Are all personnel with access to the ESD controlled area trained?

 

 

 

17. Are ESD Control requirements imposed on visitors?

 

 

 

 

Reporting, Corrective Action
Upon completion of the auditing process, Reports should be prepared and distributed in a timely manner. Details of the audits need to be fully documented for ISO-9000. As with all audits, it is essential to implement corrective action if deficiencies are discovered. Trends need to be tracked and analyzed to help establish corrective action, which may include retraining of personnel, revision of requirement documents or processes, or modification of the existing facility.

 

Conclusion
Auditing and training are key elements in maintaining an effective ESD control program. They help assure that procedures are properly implemented and can provide a management tool to gauge program effectiveness and make continuous improvement.
 

Part 5---Device Sensitivity and Testing

 

In Part 2, we indicated that one of the key elements in a successful static control program was the identification of those items (components, assemblies, and finished products) that are sensitive to ESD and the level of their sensitivity. Damage to an ESDS device by the ESD event is determined by the device's ability to dissipate the energy of the discharge or withstand the current levels involved. This is known as device "ESD sensitivity" or "ESD susceptibility".

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. In this article we will cover the models and test procedures used to characterize, determine, and classify the sensitivity of components to ESD. These test procedures are based on the three primary models of ESD events: Human Body Model (HBM), Machine Model (MM), and Charged Device Model (CDM). The models used to perform component testing cannot replicate the full spectrum of all possible ESD events. Nevertheless, these models have been proven to be successful in reproducing over 95% of all ESD field failure signatures. With the use of standardized test procedures, the industry can

  1. Develop and measure suitable on-chip protection.
  2. Enable comparisons to be made between devices.
  3. Provide a system of ESD sensitivity classification to assist in the ESD design and monitoring requirements of the manufacturing and assembly environments.
  4. Have documented test procedures to ensure reliable and repeatable results.

Human Body Model

One of the most common causes of electrostatic damage is the direct transfer of electrostatic charge through a significant series resistor from the human body or from a charged material to the electrostatic discharge sensitive (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).

The Human Body Model is the oldest and most commonly used model for classifying device sensitivity to ESD. The HBM testing model represents the discharge from the fingertip of a standing individual delivered to the device. It is modeled by a 100 pF capacitor discharged through a switching component and a 1.5kW series resistor into the component. This model, which dates from the nineteenth century, was developed for investigating explosive gas mixtures in mines. It was adopted by the military in MIL-STD-883 Method 3015, and is also used in ESD Association standard ESD STM5.1-1998 Revised: Electrostatic Discharge Sensitivity Testing -- Human Body Model. The Human Body Model circuit is presented in Figure 1.

 

Figure 1: Typical Human Body Model Circuit

Testing for HBM sensitivity is typically performed using automated test systems. The device is placed in the test system and contacted through a relay matrix. ESD zaps are applied and the post stress I-V current traces are reviewed to see if the devices fail. The ESD Association HBM test standard was recently revised to include several technical changes. First, the number of zaps per stress level and polarity has been reduced from 3 to 1. Also, the minimum time interval between zaps has been reduced from 1 second to 300 milliseconds. The changes reduce the HBM qualification test time.

The second technical change is a revision in the HBM tester specifications. The maximum rise time for an HBM wave form measured through a 500 ohm load was increased from 20 to 25 nanoseconds. This will allow HBM test equipment manufacturers to build high pin count testers that typically have a higher parasitic test board capacitance that slows down the 500 ohm wave form.


Machine Model
A discharge similar to the HBM event also can occur from a charged conductive object, such as a metallic tool or fixture. Originating in Japan as the result of trying to create a worst-case HBM event, the model is known as the Machine Model. This ESD model consists of a 200 pF capacitor discharged directly into a component with no series resistor.

As a worst-case human body model, the Machine Model may be over severe. However, there are real-world situations that this model represents, for example the rapid discharge from a charged board assembly or from the charged cables of an automatic tester.

Testing of devices for MM sensitivity using ESD Association standard ESD STM5.2-1999 (Revised): Electrostatic Discharge Sensitivity Testing -- Machine Model is similar to HBM testing. The test equipment is the same, but the test head is slightly different. The MM version does not have a 1,500 ohm resistor, but otherwise the test board and the socket are the same as for HBM testing.

 

Figure 2: Typical Machine Model Circuit


Charged Device Model Testing
The transfer of charge from an ESDS device is also an ESD event. A device may become charged, for example, from sliding down the feeder in an automated assembler. If it then contacts the insertion head or another conductive surface, a rapid discharge may occur 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.

Several test methods have been explored to duplicate the real-world CDM event and provide a suitable test method that duplicates the types of failure that have been observed in CDM caused field failures. Current work in the area is concentrating on two separate CDM test methods. One is termed CDM and best replicates the real world charged device event. The other addresses devices that are inserted in a socket and then charged and discharged in the socket. It is termed the socketed discharge model (SDM).

A draft standard for CDM (ESD DS5.3.1-1996—Device Testing: Charged Device Model) was published in 1996. It is anticipated that the standard will be published as a full standard early in 2000. The test procedure involves placing the device on a field plate with its leads pointing up, then charging it and discharging the device. Figure 3 illustrates a typical CDM test circuit.

 

Figure 3: Typical Charged Device Model Test

 SDM testing is similar to testing for HBM and MM sensitivity. The device is placed in a socket, charged from a high-voltage source and then discharged. This procedure is still a work in process and still has a number of limitations including too great a dependency on the specific design of the SDM tester. A technical paper on SDM testing was presented by the ESD Association SDM Device Testing standards working group at the October 1998 EOS/ESD Symposium.

 

Device Sensitivity Classification
Each of the device testing methods includes a classification system for defining the component sensitivity to the specified model (See Tables 1, 2, and 3). These classification systems have a number of advantages. They allow easy grouping and comparing of components according to their ESD sensitivity and the classification gives you an indication of the level of ESD protection that is required for the component.

Class

Voltage Range

Class 0

<250 volts

Class 1A

250 volts to <500 volts

Class 1B

500 volts to < 1,000 volts

Class 1C

1000 volts to < 2,000 volts

Class 2

2000 volts to < 4,000 volts

Class 3A

4000 volts to < 8000 volts

Class 3B

>= 8000 volts

Table 1: ESDS Component Sensitivity Classification - Human Body Model

(Per ESD STM5.1-1998)

Class

Voltage Range

Class M1

<100 volts

Class M2

100 volts to <200 volts

Class M3

200 volts to <400 volts

Class M4

> or = 400 volts

Table 2: ESDS Component Sensitivity Classification - Machine Model
(Per ANSI/ESD-S5.2-1999)

Class

Voltage Range

Class C1

<125 volts

Class C2

125 volts to <250 volts

Class C3

250 volts to <500 volts

Class C4

500 volts to <1,000 volts

Class C5

1,000 volts to <1,500 volts

Class C6

1,500 volts to <2,000 volts

Class C7

=>2,000 volts

Table 3: ESDS Component Sensitivity Classification - Charged Device Model

(Per EOS/ESD-DS5.3-1996)

A fully characterized component should be classified using all three models: Human Body Model, Machine Model, and Charged Device Model. For example, a fully characterized component may have the following: Class 1B (500 volts to <1000 volts HBM), Class M1 (<100 volts MM), and Class C3 (500 volts to <1000 volts CDM). This would alert a potential user of the component to the need for a controlled environment, whether assembly and manufacturing operations are performed by human beings or machines.

A word of caution, however. These classification systems and component sensitivity test results function as guides, not necessarily as absolutes. The events defined by the test data produce narrowly restrictive data that must be carefully considered and judiciously used. The three ESD models represent discrete points used in an attempt to characterize ESD vulnerability. The data points are informative and useful, but to arbitrarily extrapolate the data into a real world scenario can be misleading. The true utility of the data is in comparing one device with another and to provide a starting point for developing your ESD control programs.

 

Summary
Device failure models and device test methods define the sensitivity of the electronic devices and assemblies to be protected from the effects of ESD. With this key information, you can design more effective ESD control programs.

 Part 6---ESD Standards

In a world that is characterized by change, the ESD industry seems to have jumped into the swirling eddy head-first. Control programs have mushroomed. Black has been replaced by green, blue and gold. Shielding bags dominate the warehouse. Ionizers exist along side wrist straps and ground cords. An early history of "smoke and mirrors," magic and lofty claims of performance is rapidly and safely being relegated to the past.

Today, meeting the complex challenge of reducing ESD losses requires more than reliance on faith alone. Users require a way to legitimately evaluate and compare competing brands and types of products. They need objective confirmation that their ESD control program provides effective solutions to their unique ESD problems.

That’s where standards come into play. They help define the sensitivity of the products manufactured and used. They help define the performance requirements for various ESD control instruments and tools. Standards are playing an ever increasing role in reducing marketplace confusion in the manufacture, evaluation, and selection of ESD control products and programs.

 

The Who and Why of Standards
Who uses ESD standards? Manufacturers of ESD control products; users of ESD control products; and third party testers of ESD control products.

Why use ESD standards? They help assure consistency of ESD sensitive products and consistency of ESD control products and services. They provide a means of objective evaluation and comparison among competitive ESD control products. They help reduce conflicts between users and suppliers of ESD control products. They help in auditing ESD control programs. And, they help reduce confusion in the marketplace.

In the United States, the use of standards is voluntary, although their use can be written into contracts or purchasing agreements between buyer and seller. In most of the rest of the world, the use of standards, where they exist, is compulsory.

 

Key Standards and Organizations
Just 20 years ago, there were relatively few reliable ESD standards and few ESD standards development organizations. Today’s ESD standards landscape is not only witnessing an increase in the number of standards, but also increasing cooperation among the organizations that develop them.

Today’s standards fall into three main groups. First, there are those that provide ESD program guidance or requirements. These include documents such as ANSI ESD S20.20-1999--Standard for the Development of an ESD Control Program, ANSI/ESD S8.1-ESD Awareness Symbols; ESD ADV2.0-ESD Handbook; EIA 625-Requirements for Handling Electrostatic Discharge Sensitive Devices; or EN100015-Protection of Electrostatic Sensitive Devices.

A second group covers requirements for specific products or procedures such as packaging or grounding. Typical standards in this group are ANSI/ESD S6.1-Grounding; or EIA 541-Packaging of Electronic Products for Shipment.

A third group of documents covers the standardized test methods used to evaluate products and materials. Historically, the electronics industry has relied largely on test methods established for other industries or even other materials (ASTM-257-DC Resistance or Conductance of Insulating Materials). Today, however, specific test method standards focus on ESD in the electronics environment, largely as a result of the ESD Association’s activity. These include standards such as ESD S5.1-Device Testing, Human Body Model and ANSI/ESD S7.1: Floor Materials -- Resistive Characterization to cite just a few.

 

Who Develops Standards?
Standards development and usage is a cooperative effort among all organizations and individuals affected by standards. There are several key ESD standards development organizations.

 

Military Standards
Traditionally the U.S. military developed and issued specific standards and specifications with regard to ESD control. Once a primary developer of ESD standards, US military agencies are taking a less proactive approach, relying on commercially developed standards rather than developing standards themselves. For example, the ESD Association has received the assignment from the Department of Defense to convert MIL-STD-1686 into a commercial standard. Among current military standards, MIL-STD-1686 - Electrostatic Discharge Control Program and MIL-HDBK-263 - Electrostatic Discharge Control Handbook are the predominant documents. The 1686 standard requires vendors to classify the ESD susceptibility of their components and have an ESD control program in place. The accompanying 263 Handbook provides guidelines and suggestions for ESD control programs and procedures.

 

Electronic Industries Alliance/JEDEC
The Electronic Industry Alliance (EIA) and EIA’s Joint Electronics Development Engineering Council (JEDEC) are also active in standards development. EIA 541 -- Packaging Material Standards for ESD Sensitive Items specifies test methods for the evaluation of packaging materials for protection of ESDS parts during shipment. JEDEC’s EIA 625 Requirements for Handling Electrostatic-Discharge-Sensitive (ESDS) Devices establishes methods for semiconductor manufacturers, distributors, processing and testing facilities and users to use in protecting components that have human body model sensitivities of 200 volts or greater.

 

ESD Association
The ESD Association has been a focal point for the development of ESD standards in recent years. An ANSI-Accredited Standards Development Organization, the Association is charged with the development of ESD standards and test methods. It also represents the US on the International Electrotechnical Commission Technical Committee 101-Electrostatics.

The ESD Association has published 27 standards documents and 4 technical reports. These voluntary standards cover the areas of material requirements, electrostatic sensitivity, and test methodology for evaluating ESD control materials and products. In addition to standards documents, the Association also publishes a number of informational advisories.

 

ESD Association Standards Classifications and Definitions
There are four types of ESD Association standards documents with specific clarity of definition. The four document categories are consistent with other standards development organizations. These four categories are defined below.

Standard: A precise statement of a set of requirements to be satisfied by a material, product, system or process that also specifies the procedures for determining whether each of the requirements is satisfied.

Standard Test Method: A definitive procedure for the identification, measurement and evaluation of one or more qualities, characteristics or properties of a material, product, system or process that yields a reproducible test result.

Standard Practice: A procedure for performing one or more operations or functions that may or may not yield a test result. Note: If a test result is obtained, it may not be reproducible between labs.

Technical Report: A collection of technical data or test results published as an informational reference on a specific material, product, system, or process.

As new documents are approved and issued, they will be designated into one of these four new categories. Existing documents are being reviewed and will be reclassified as appropriate.

 

International Standards
The international community, led by the European-based International Electrotechnical Commission, has also climbed on board the standards express. Europe’s CENELEC has issued a European electrostatic standard EN100015 - Protection of Electrostatic Sensitive Devices that was adopted as a European Norm. Additional work by the IEC to will result in a comprehensive series of standards that may someday be the successor to EN100015.

Japan also has released its proposed version of a national electrostatic Standard, which also shares many aspects of the European and U.S. documents.

 

Organizational Cooperation
Perhaps one of the more intriguing changes in ESD standards has been the organizational cooperation developing between various groups. In revising EIA541, the EIA and the ESD Association have implemented a cooperative effort to update the standard.

Another cooperative effort is underway between the ESD Association and the U.S. Department of Defense. The ESD Association prepared a commercial version of MIL-STD-1686. It is anticipated that defense agencies will use that commercial version as a reference.

Internationally, European standards development organizations and the ESD Association have developed working relationships that result in an expanded review of proposed documents, greater input, and closer harmonization of standards that impact the international electronics community.

For users of ESD standards, this increased cooperation will have a significant impact. First, we should see standards that are technically improved due to broader input. Second, we should see fewer conflicts between different standards. Finally, we should see less duplication of effort.

 

Summary
For the electronics community, the rapid propagation of ESD standards and continuing change in the standards environment mean greater availability of the technical references that will help improve ESD control programs. There will be recommendations to help set up effective programs. There will be test methods and specifications to help users of ESD control materials evaluate and select products that are applicable to their specific needs. And there will be guidelines for vendors of ESD products and materials to help them develop products that meet the real needs of their customers.

Standards will continue to fuel change in the international ESD community.

 

Principle ESD Standards

U.S. Military/Department of Defense
MIL-STD-1686C : Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices)
This military standard establishes requirements for ESD Control Programs. It applies to U.S. military agencies, contractors, subcontractors, suppliers and vendors. It requires the establishment, implementation and documentation of ESD control programs for static sensitive devices, but does NOT mandate or preclude the use of any specific ESD control materials, products, or procedures. It is being updated and converted to a commercial standard by the ESD Association.

MIL-HBDK-263B: Electrostatic Discharge Control Handbook for Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices)
This document provides guidance, but NOT mandatory requirements, for the establishment and implementation of an electrostatic discharge control program in accordance with the requirements of MIL-STD-1686.

MIL-PRF 87893—Workstation, Electrostatic Discharge (ESD) Control
This document defines the requirements for ESD protective workstations.

MIL-B-81705—Barrier Materials, Flexible, Electrostatic Protective, Heat Sealable
This documents defines requirements for ESD protective flexible packaging materials.

MIL-STD-129—Marking for Shipment and Storage
Covers procedures for marketing and labeling ESD sensitive items.

 

EIA/JEDEC
EIA541—Packaging Material Standards for ESD Sensitive Items
This document covers packaging materials and definitions of material types for ESD sensitive items.

EIA-625: Requirements for Handling Electrostatic-Discharge-Sensitive (ESDS) Devices
This voluntary standard establishes minimum requirements for ESD control methods and materials used to protect electronic devices that have human body model sensitivities of 200 volts or greater. It is applicable to semiconductor manufacturers, semiconductor distributors, semiconductor processing and testing facilities, and semiconductor users. It is currently being revised and updated with a reduced scope.

 

International/European
EN100015: Protection of Electrostatic Sensitive Devices
Adopted in 1992 and 1993, this European Norm covers ESD handling practices for electronic devices.


ESD Association

Standards Documents
Standards, Standard Test Methods and Standard Practices
have completed the industry comment and review process, and the document has been approved by the Standards Committee as a Final Standard Document.

ESD S1.1—1998: Evaluation, Acceptance, and Functional Testing of Wrist Straps.
A successor to the EOS/ESD S1.0, this document establishes test methods for evaluating the electrical and mechanical characteristics of wrist straps. It includes improved test methods and performance limits for evaluation, acceptance, and functional testing of wrist straps.

ESD STM2.1—1997: Resistance Test Method for Electrostatic Discharge Protective Garments
This Standard Test Method provides test methods for measuring the electrical resistance of garments used to control electrostatic discharge. It covers procedures for measuring sleeve-to-sleeve and point-to-point resistance.

ANSI EOS/ESD S3.1-1991: Ionization
Test methods and procedures for evaluating and selecting air ionization equipment and systems are covered in this standard. The document establishes measurement techniques to determine ion balance and charge neutralization time for ionizers.

ESD S4.1- 1997(Revised): Worksurfaces – Resistance Measurements
This Standard establishes test methods for measuring the electrical resistance of worksurface materials used at workstations for protection of ESD susceptible items. It includes methods for evaluating and selecting materials, and testing new worksurface installations and previously installed worksurfaces.

ESD STM4.2-1998: Worksurfaces - Charge Dissipation Characteristics
This Standard Test Method provides a test method to measure the electrostatic charge dissipation characteristics of worksurfaces used for ESD control. The procedure is designed for use in a laboratory environment for qualification, evaluation or acceptance of worksurfaces.

ESD STM5.1-1998 Revised: Electrostatic Discharge Sensitivity Testing -- Human Body Model
This Standard Test Method updates and revises an existing Standard. It establishes a procedure for testing, evaluating and classifying the ESD sensitivity of components to the defined Human Body Model (HBM).

ESD STM5.2-1999 (Revised): Electrostatic Discharge Sensitivity Testing -- Machine Model
This Standard establishes a test procedure for evaluating the ESD sensitivity of components to a defined Machine Model. It also provides a system of classifying the sensitivity of these components. The component damage caused by the Machine Model is often similar to that caused by the Human Body Model, but it occurs at a significantly lower voltage.

ESD S6.1-1999: Grounding -- Recommended Practice
This Standard recommends the parameters, procedures, and types of materials needed to establish an ESD grounding system for the protection of electronic hardware from ESD damage. This system is used for personnel grounding devices, worksurfaces, chairs, carts, floors, and other related equipment.

ANSI ESD S7.1-1994: Floor Materials -- Resistive Characterization of Materials
Measurement of the electrical resistance of various floor materials such as floor coverings, mats, and floor finishes is covered in this document. It provides test methods for qualifying floor materials before installation or application and for evaluating and monitoring materials after installation or application.

ANSI ESD S8.1-1993: ESD Awareness Symbols
Three types of ESD awareness symbols are established by this document. The first one is to be used on a device or assembly to indicate that it is susceptible to electrostatic charge. The second is to be used on items and materials intended to provide electrostatic protection. The third symbol indicates the common point ground

ESD S9.1-1995: Resistive Characterization of Footwear
This Standard defines a test method for measuring the electrical resistance of shoes used for ESD control in the electronics environment.

ANSI ESD S11.11-1993: Surface Resistance Measurement of Static Dissipative Planar Materials
This Standard defines a direct current test method for measuring electrical resistance. The Standard is designed specifically for static dissipative planar materials used in packaging of ESD sensitive devices and components.

ANSI ESD S11.31-1994: Evaluating the Performance of Electrostatic Discharge Shielding Bags
This Standard provides a method for testing and determining the shielding capabilities of electrostatic shielding bags.

ESD STM12.1-1997: Seating-Resistive Characterization
This Standard provides test methods for measuring the electrical resistance of seating used to control ESD. The test methods can be used for qualification testing as well as for evaluating and monitoring seating after installation. It covers all types of seating, including chairs and stools.

ANSI ESD S20.20-1999: Standard for the development of an ESD Control Program
This Standard provides administrative, technical requirements and guidance for establishing, implementing and maintaining an ESD Control Program.

ESD STM97.1-1999: Floor Materials and Footwear – Resistance Measurement in Combination with a Person.
This Standard Test Method provides for measuring the electrical resistance of floor materials, footwear and personnel together, as a system.

ESD STM97.2-1999: Floor Materials and Footwear – Voltage Measurement in Combination with a Person.
This Standard Test Method provides for measuring the electrostatic voltage on a person in combination with floor materials and footwear, as a system.

 

Draft Documents
Draft
documents have not yet completed an industry comment and review stage necessary before being issued as full standards documents. Draft documents are subject to revision before being approved as full standards documents.

ESD DSP3.3—1997: Periodic Verification of Air Ionizers.
This Draft Standard practice provides a measurement technique to periodically determine ion balance and charge neutralization time for ionizers in actual use locations.

EOS/ESD DS5.3-1996: Electrostatic Discharge Sensitivity Testing - Charged Device Model
This Draft Standard establishes a test method for evaluating the ESD sensitivity of active and passive components to a defined charged device model.

ESD DSP10.1-1999: Automated Handling Equipment
This draft Standard Practice provides test methods for evaluating the ESD ground integrity; charge generation and charge accumulation on devices in automated handling equipment

ESD DS11.12-1996: Volume Resistance Measurement of Static Dissipative Planar Materials
This Draft Standard provides test methods for measuring the volume resistance of static dissipative, planar materials.

ESD DS13.1-1999: Measuring Electrical Potential from Electrical Soldering & Desoldering Hand Tools
This Draft Standard provides Test Methods for measuring the electrical leakage and tip to ground reference point resistance of electric soldering/desoldering hand tolls. It also provides parameters for EOS safe soldering operations.

 

Advisory Documents
Advisory
Documents are not Standards, but provide general information for the industry or additional information to aid in better understanding of Association Standards.

ESD ADV1.0-1994: Glossary of Terms
Definitions and explanations of various terms used in Association Standards and documents are covered in this Advisory. It also includes other terms commonly used in the ESD industry.

ESD ADV2.0-1994: ESD Handbook
The ESD Handbook is a complete guide to static control in the work place. Nineteen chapters cover ESD basics, control procedures, auditing, symbols, device testing, standards.

ESD ADV3.2-1995: Selection and Acceptance of Air Ionizers
This Advisory document provides end users with guidelines for creating a performance specification for selecting air ionization systems. It reviews four types of air ionizers and discusses applications, test method references, and general design, performance and safety requirements.

ESD ADV11.2-1995: Triboelectric Charge Accumulation Testing
The complex phenomenon of triboelectric charging is discussed in this Advisory. It covers the theory and effects of tribocharging. It reviews procedures and problems associated with various test methods that are often used to evaluate triboelectrification characteristics. The test methods reviewed indicate gross levels of charge and polarity, but are unrepeatable if used to give more exact values in real world situations.

ESD ADV53.1-1995: ESD Protective Workstations
This Advisory document defines the minimum requirements for a basic ESD protective workstation used in ESD sensitive areas. It provides a test method for evaluating and monitoring workstations. It defines workstations as having the following components: support structure, static dissipative worksurface, a means of grounding personnel, and any attached shelving or drawers.

Sources of Standards

  • ESD Association, 7900 Turin Road, Building 3, Suite 2, Rome, NY 13440. Phone: 315-339-6937. Fax: +1 315-339-6793. Web Site: http://www.esda.org/
  • Electronic Industries Alliance/JEDEC, 2001 Pennsylvania Ave., NW, Washington, DC 20006. Phone: 202-457-4900. Fax: 202-457-4985. Web Site: http://www.eia.org/
  • HIS Global Engineering Documents, 15 Inverness Way East, Englewood, CO 80112. Phone: 800-854-7179. Fax: 303-397-2740. Web Site: http://global.ihs.com/
  • International Electrotechnical Commission, 3, rue de Varembe, Case postale 131, 1211 Geneva 20, Switzerland. Fax: 41-22-919-0300. Web Site: http://www.iec.ch/
  • Military Standards, Naval Publications and Forms Center, 5801 Tabor Avenue, Philadelphia, PA 19120
» Gửi ý kiến của Bạn
Các tin / bài viết cùng loại:
Copyright © 2011-2018 Lắp Ráp Điện Tử . All rights reserved.
® Lắp Ráp Điện Tử giữ bản quyền nội dung trên website này
Vui lòng ghi "nguồn bởi Lắp Ráp Điện Tử" khi trích nội dung từ website này
Địa chỉ: 377 Tân hương Tp.HCM
Hotline: +84 (0)938041068 - Email: admin@laprapdientu.vn

 
Smartit Web7Mau - Website: www.web7mau.com - Email: developers.web7mau@gmail.com
Basics of Electrostatic Discharge (update on Jun 11 2013) Rating: 5 out of 10 166830.
Core Version: 1.6.6.0