What is the difference between antistatic, conductive and static dissipative?

The terms conductive and static dissipative typically refer to resistance or resistivity ranges used in the evaluation of ESD control materials and products. By definition, a conductive material has a surface resistivity of less than 1 x 10E5 ohms per square or a volume resistivity less than 1 x 10E4 ohm-cm. A static dissipative material has a surface resistivity of 1 x 10E5 to 1 x 10E12 ohms per square or a volume resistivity of 1 x 10E4 to 1 x 10E11 ohm-cm. These definitions appear in the ESD Association Glossary as well as in various other static control standards documents.

For some materials, surface resistance rather than surface resistivity is often used to define these terms. In this case, a simple conversion factor is applied, dividing the resistivity ranges by 10. Thus conductive becomes less than 1 x 10E4 ohms and static dissipative becomes 1 x 10E4 to 1 x 10E11 ohms, provided that the appropriate electrodes with the correct geometric conversions are used. ANSI/ESD S11.11 provides additional information on this issue.

The term antistatic, however, does not refer to resistance or resistivity. By definition, the term refers to a material that resists tribocharging. At one time, the term referenced a resistance value, but it was severely misused and today no longer represents any resistance range.

We have regular vinyl tile on our manufacturing floor. If our employees wear ESD shoes or foot straps, will we control the generation of static electricity on the body?

Laboratory testing has demonstrated that relatively high levels of body voltage generation and poor body voltage decay rates can occur when wearing ESD footwear on uncontrolled floors such as ordinary vinyl composition tile, or when wearing ordinary shoes with insulated or leather soles on various ESD floor materials. Voltage generation can be as high as several hundred volts and body voltage decay rates can exceed 1/2 second.

When controlled footwear and controlled floor materials are used together, voltage generation and decay rates are significantly improved, often to only a few volts with decay rates less than 1/4 second. Such performance indicates that floor materials and footwear depend upon each other and should be thought of as a system for best performance.

Footwear and floor materials create a ground path from the wearer to ground. Insulative flooring such as standard vinyl tile, and standard street shoes or industrial footwear with rubber, crepe, or polyurethane soles insulate the wearer from the floor. Body charges cannot readily flow from the body through the shoes through the floor to ground.

If a person is sitting at a grounded work station -- table mat and wrist straps -- can that person wear thin cotton gloves and still be grounded while handling static sensitive devices?

Yes, the person can be grounded even though wearing the cotton gloves that prevent oils from getting onto sensitive devices and boards. Grounding is through the wrist strap. As long as the wrist strap is properly connected and contacts the skin of the wearer, that person is grounded. The wrist strap should contact the skin directly and NOT be worn over the gloves.

Table mats, if properly grounded provide a ground path for objects placed on the mat. This ground path would not be affected by wearing cotton gloves.

A related matter is whether the cotton gloves tribocharge, thus creating an ESD hazard. Cotton absorbs moisture from hands and air and does not tribocharge readily. If you're handling highly sensitive devices of 100 volts or less, thoroughly test all materials, such as cotton gloves, for tribocharging characteristics.

I've been reading a lot about EOS/ESD Association Standards. Where do I obtain copies?

Association Standards can be purchased from Association headquarters, 7900 Turin Road, Bldg. 3, Suite 2, Rome, NY 13440. Call 315-339-6937; Fax 315-339-6793; Email: eosesd@aol.com. The Midwest Chapter library also has copies of all Association Standards that you can borrow to review.

If we don't use ESD control procedures when designing and handling prototypes, will we damage static sensitive parts used in these prototypes? Could these prototypes have a design flaw when they reach the production floor?

Yes, you definitely could damage static sensitive parts if you were not using proper protective measures. Because ESD events are statistical events and you are handling very few parts in contrast to the production line, it's possible to think that design and prototyping are immune. But the law of averages will catch up with you.

Whether your prototypes might contain a design flaw due to an ESD event is more conjectural. If your design was based on the actual operating parameters of a component whose characteristics had shifted due to an ESD event, you might end up with a flaw when you use good components in the final product.

Basic precautions such as wrist straps, proper work surfaces and protective handling and packaging materials will reduce your exposure to ESD damage in design and prototyping. Implementing these precautions need not be either expensive or complicated.

For years, we've been wearing foot grounders on only one shoe. We seldom measure a very high charge on our personnel Now it's been suggested that we wear foot grounders on both shoes. Why is this necessary?

Shoe grounders, and shoes, reduce charge generation and accumulation by providing an electrical path from the body to a static control floor material. This electrical path is maintained only when the grounder is in contact with the floor material. If you wear a grounder on only one shoe, you lose this contact every time you take a step with this foot or when you're sitting at a work station and this foot leaves the floor. Because there's no path for the charges to flow to ground, the charges will accumulate on your body. Grounders on both feet increase the frequency of contact and create a better chance for contact when you are seated. The result is less charge generated, less charge accumulated.

Although the accumulated charge might appear rather small, the relationship between capacitance and charge may result in a higher charge level than one might think. Capacitance is directly related to whether both feet are in contact with the floor surface. Capacitance is highest with both feet on the floor and lowest with only one foot on the floor. If we apply the formula Q=CV and Q/C=V, where Q=Charge, C=Capacitance, and V=Voltage, we see that charge decreases with both feet on the floor and increases with only one foot on the floor. Because we often measure body voltages with both feet on the floor, it's easy to think we're safe, forgetting that in many instances only one foot is in contact with the floor.

Grounders on both feet=lower charge=reduced risk to susceptible devices.

We just bought new ESD control shoes. The manufacturer assures us that the resistance of the shoes is between 1 x 10E6 and 1 x 10E8 ohms. Whenever we test them with the same equipment we use to test wrist straps, the tester tells us that the shoes are out of specification. What's wrong?

The problem is probably with your tester, not your shoes. Most wrist strap testers are set for resistance ranges that are different than those of your shoes. Also, they simply indicate whether the wrist strap is out of this range rather than giving a specific resistance measurement. For example, the tester may have a range of 7.5 x 10E5 to 1.0 x 10E7ohms which may be appropriate for wrist straps. Anything outside of this range will cause an indicating light to appear. Because your shoes may have a higher resistance than the high range of the tester, the tester will indicate that the shoes are out of specification.

There are three solutions to this problem. First, you can obtain a tester that is specifically designed for testing shoes. Second, you can obtain a wrist strap tester that can be adjusted for different resistance ranges or one that will indicate the actual resistance being recorded rather than simply indicating whether the resistance is within a specified range. Finally, you can use a wide range ohmmeter for measuring the resistance of your shoes as well as your wrist straps.

The objective is to make sure that you have the proper equipment to do the job rather than simply trying to adapt existing equipment that may not work for the intended application. If you use the right equipment, you can be sure that your shoes meet both your specifications and the manufacturer's claims.

Why do manufacturing facilities for munitions or explosives use conductive materials instead of static dissipative materials. Why do they use wrist straps without a 1-megohm resistor?

There are several reasons that explosive environments use more conductive materials than static dissipative.

One reason is historical. Many of the specifications for these environments were developed when conductive materials were more common than dissipative. Some of these specifications may not have been updated to consider the availability of other materials or procedures.

A second consideration is that the reason for ESD control in these environments is different from that in our more familiar electronics environments. An electrostatic discharge in explosive environments may result in fire or explosion, causing considerable physical damage, human injury, or loss of life.

When reducing the risk of fire or explosion is the main reason for ESD control, the theoretical faster discharge rates through conductive materials or through wrist straps without the one megohm resistor is often given precedence over the electrical hazards that may be presented by the use of these materials.

In the electronics environment, ESD controls are used to protect components. Our concerns for personnel safety are in the area of potential shock hazards.

Where can I find information about static control for high voltage areas? (For example, areas in which electrical voltages exceed 250V)

High voltage environments tend to create some unique situations for ESD control, because many of our control procedures have the possibility of compromising personnel safety in these environments. A key procedure is to be sure to follow all electrical codes and safety codes. You can find information in the National Electrical Code(ANSI/NFPA 70) and the IEEE Green Book (ANSI/IEEE Std. 142).

Another source of information is your own safety department. Be sure to enlist them in your activities so that you can provide both safety as well as ESD control.

If unsealed concrete has a resistance in the 1.0 x 10E8 range and dusting is not a problem, can this be considered a static control floor?

There are several potential problems in considering this concrete floor a static protective floor, mostly problems of lack of adequate control over the floor's properties.

First, resistance alone doesn’t always indicate the charge that will be generated on people moving across the floor. Tests for body voltage generation are better indicators of performance.

Second, the resistance of a concrete floor can vary considerably over time. Because the resistance of concrete is highly dependent upon its moisture content, the resistance can change depending upon the amount of moisture present in the concrete. Newly poured concrete will normally show a significantly lower resistance than concrete that has "dried out" over a couple of years. Concrete can also absorb moisture from the ground so that changes in the water table that can occur during rainy or dry seasons will affect the concrete.

Third, it is difficult to provide a proper ESD ground with a concrete floor because of its size, the metal reinforcing, and the moisture content. Items in direct contact with the concrete, such as grounded work benches may not be properly isolated from the concrete. The concrete thus could provide an alternate path to ground, bypassing the ESD ground on the work bench.

I've heard that layered static control mats could have an alternate path to ground. What does that mean? Is this a problem?

Layered floor mats are generally composed of two or more layers of materials, with each layer having a different level of conductivity. For example, the top layer might be in the dissipative range, the bottom layer in the insulative range, and the middle layer in the conductive range.

The ground path is generally created by a ground snap attached to a metal screw that penetrates the mat and contacts the more conductive layer. If there is another means of contact with that conductive layer and that contact is grounded, then an alternate path to ground is created, bypassing the resistor in the ground cord that is attached to the ground snap. This could create a safety problem or could allow a charge from a charged device to dissipate through the mat too rapidly, potentially damaging the device.

This additional contact with the more conductive layer could occur due to wear on the surface of the material, cuts through the surface or bottom layer, or perhaps even excessive moisture that penetrates the mat. Someone might have even bolted down the mat to a metal surface, also creating an alternate contact.

Problems can be avoided by knowing the physical structure of the mat, installing it properly, taking care in its use, and disposing of damaged mats.

Paper, cotton and wood tend to absorb moisture and at times can read around 1 x 10E9 ohms. With this reading, can these items be grounded.

If the moisture content of these materials remains high enough to keep the resistance low enough, then it is possible to ground these materials and remove any static charge from them by grounding. However, the moisture content can be highly variable. Different materials absorb moisture at different rates than others meaning that some wood items may be conductive, others insulative.

Moisture content also is often affected by factors such as the humidity in the air. Materials that are sufficiently conductive in the humid midwest summers may dry out in drier midwest winters.

Testing and caution are the main recommendations here. You don't want to blindly trust conductive properties to such highly variable factors as moisture. If charges on these materials are of concern in your environment, you may want to implement procedures other than grounding to control the problem. Ionization, for example, may be a better alternative in your environment.

Do I need to take any ESD precautions when I service or repair products in the field?

If you’re servicing ESD sensitive products in the field, you need to take the same ESD control precautions that you would take in the manufacturing environment. This includes using wrist straps, protective worksurfaces, and even placing products in protective packaging. These items should be part of your ESD field service kit.

The challenge that you face in field service is that you are working in someone else's environment, one over which you have little control. You may face low relative humidity, or an office with static generating carpet, or a lack of work space. Your customer may not know whether the products you work on are ESD sensitive or not.

You need to be adaptable to the environments in which you are working. For example, in addition to your wrist strap, carry extra alligator clips, jacks, and extension cords.

You should assume that all the products you work on are ESD sensitive and handle them accordingly. Take the extra time to clear a work area large enough to place your protective work surface.

Follow good ESD protection procedures at all times. Your responsibility is to help solve your customers' problems, not contribute to them.

We are installing some new automated IC handling equipment in our facility. Because people won't be handling the sensitive parts in these operations, do we still need to be concerned with ESD control?

Even though we most frequently associate ESD problems with personnel handling of our sensitive parts and assemblies, similar problems can also occur with automated IC handling equipment. They often occur less frequently, but when problems do occur, they can be quite serious for two reasons. First, the automated processes are repetitive and are carried out at high speeds. Thus a large number of parts can very quickly be exposed to the ESD problem.

Second, there are two types of ESD events that can occur. One is a Machine Model discharge delivered from a sharp edge or point on a metal part of the equipment and the effective voltage levels of the discharge are often higher than those from the human body. The other is a Charged Device Model (CDM) event, which can be more destructive than the HBM for some devices. In the CDM event, the device itself becomes charged. For example, this can occur as a device slides down a feeder or is picked and placed in an automatic insertion machine. The device is then discharged by contact with a conductive surface, such as an insertion head.

Static charges can be generated by moving conveyors, rotating shafts, sliding parts, pick and place arms, and even flowing air.

Controlling the problem requires a number of methods. These include the following:

1. Eliminating as many static generating materials from the process as possible.

2. Properly grounding the equipment, especially the metal parts that are close proximity to the electrostatic discharge sensitive (ESDS) products being processed.

3. Shielding of static generative materials, such as glass window panes, gaskets, and thermal insulating spacers that are in proximity to ESDS products

4. Using antistatic solutions where possible.

5. Providing localized air ionization to neutralize charges on insulative surfaces such as device packages.

6. Frequent monitoring of the process to assure that the procedures are working.

On the plus side, once the source of a problem has been identified and a remedy applied, there tends to be less recurrence of the problem in equipment. Frequent monitoring and auditing of your production equipment and processes will help assure that the any potential problems remain under control.

We need to label parts as being ESD sensitive (ESDS), but we’re confused over which symbols to use. We also need a method to easily identify ESD control materials. There seem to be a lot of choices. How do we properly identify ESDS parts and ESD control materials?

The most common symbols traditionally used to identify ESDS parts or ESD control materials have been replaced with newer, more appropriate symbols. The lightning bolt inside a circle is no longer used in ESD control because internationally it symbolizes an electrical hazard to persons. The circle with three arrows is a generic symbol for electrostatic, electromagnetic, magnetic, and radioactive fields. Using this symbol for ESD control purposes would not properly alert people to the reason for its presence.

The ESD Association Standard ESD S8.1 -- ESD Awareness Symbols provides two symbols for ESD identification.

Figure 1--ESD Susceptibility Symbol

The ESD Susceptibility Symbol (Figure 1), 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 2-- ESD Protective Symbol

The ESD Protective Symbol (Figure 2), 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 specially designed or modified to provide ESD control.

As an example of the proper use of these symbols, we'll use an ESD control bag containing a circuit board that is static sensitive. The circuit board inside has the ESD Susceptibility Symbol applied to it. Because the bag offers protection against ESD, it would have the ESD Protective Symbol affixed to it. The bag also would have the ESD Susceptibility Symbol to indicate its contents were ESD sensitive. Now we can easily see that the bag is an ESD protective material and that its contents are ESD sensitive.

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

Grounding cords for table mats and floor mats are available with or without a 1-megohm resistor. Is there any standard recommending when to use or not use the resistor?

Specific guidelines in this area are somewhat limited. Some older standards require the use of the 1 megohm resistor in all cases. Recent ESD Association standards do not directly address the issue. However, they do provide some information.

EOS/ESD 6.1 - Grounding recommends the following regarding worksurfaces and floor mats:

• Where provisions are not made and not required for this resistor, a direct connection is fully acceptable and recommended.
• Provisions may be made to include a resistor where it may be required for a purpose other than ESD.
• Conductive worksurfaces such as stainless steel shall be hard ground connected.
• Ground fault circuit interrupters (GFCI) and other safety protection should be considered wherever personnel may come into contact with electrical sources.

If a test card has no active components on it, do we need to use ESD procedures when handling the card? What procedures should we follow?

There are many types of circuit boards used for system testing. Sometimes they are called "streaker" cards or extender cards that are designed without active electronic components. While these test cards themselves may not be ESD sensitive, the systems in which they are used probably are. Other boards in the vicinity of the test card likely may contain ESDS components. Therefore, when using test cards, the following procedures help assure reliability and reduce the incidents of ESD to other ESDS devices in the system.

1. Wear a grounded wrist strap while inserting, working with, or removing a test card from a system. Test the wrist strap to assure that it is functioning properly.

2. When not in use, store test cards in static protective packaging to protect them from dust and other contaminants. The use of static protective packaging reduces the likelihood of non-conforming materials being brought into a protective environment.

3. Observe all other procedures required in your facility regarding the handling of sensitive circuit boards.

These three simple procedures will help assure that ESD is controlled, reduce contaminants from test cards and assure reliable service.

Which ground is better for ESD purposes, a direct earth ground or an electrical ground?

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

First, 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."

Second, 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 were connected to the electrical ground and if 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) at the workstation must be bonded to the equipment ground to minimize differences in potential between the two grounds.

Remember to follow requirements of the National Electrical Code as well as any local code requirements. You also may need ground fault circuit interrupters or other safety protection wherever personnel may come into contact with electrical sources.

For additional information, see:

ESD-S4.1-1997, Worksurfaces - Resistance Measurements, ESD Association

ANSI/ESD-S6.1-1991, Grounding - Recommended Practice, ESD Association

ANSI/NFPA 70, National Electrical Code, National Fire Protection Association.

EIA 625, Requirements for Handling Electrostatic Discharge Devices, Electronic Industries Alliance

ESD-ADV-2.0-1994, Electrostatic Discharge Control Handbook, ESD Association

ESD-ADV53.1-1995, ESD Protective Workstations, ESD Association

Rather than resorting to static control products such as wrist straps, floors, and packaging materials, why can't we control the problem with humidity? Isn’t static build-up likely to be lower when the humidity is high.

Certainly, relative humidity can help reduce static generation, particularly on those materials on which moisture can condense on the surface or those that can absorb moisture. However, relative humidity needs to be in the 40-50% or higher range to have a significant impact. As RH goes below 40%, static build up becomes more readily noticeable.

A relative humidity above 60% can lead to corrosion on product, tool and equipment problems or personnel discomfort.

In our dry northern winter climates, increasing the humidity can become quite costly and may not be very cost effective. Also, humidity levels in various parts of the facility can vary significantly, even when controlled. For example, the heat of burn-in operations can dry out the air that our humidity control just added moisture to.

Remember, too, that when the product goes out the door, we no longer have control over its environment. It can be subjected to a variety of humidity levels before it reaches the final customer. You will still need static control packaging materials for your products in shipment if they will be exposed to static potentials.

Humidity helps, but doesn't substitute for other ESD control procedures.

We are just getting started. How often should I audit my facility?

The actual frequency of audits can be variable depending upon your facility and the ESD problems that you have. Following initial audit, some experts recommend auditing each department once a month if possible and probably a minimum of six times per year. If this seems like a high frequency level, remember that these regular audits are based upon a sampling of work areas in each department, not necessarily every work station.

Once you've gotten your program underway, your frequency of audit will be based on your experience. If your audits regularly show acceptable levels of conformance and performance, you can reduce the frequency of auditing. If, on the other hand, your audits regularly uncover continuing problems, you may need to increase the frequency.

In addition, you may need to adjust the frequency depending upon the critical nature of the functions performed in specific areas.

Can you explain the 3 ESD failure modes in simple terms? Give some examples.

Understanding how ESD damages a device is essential to implementing an effective ESD control program. For each of the three major sources of ESD, there are models that represent the ways in which ESD can damage a sensitive device. These models are used to describe the ESD event itself and also to classify the ESD sensitivities of the devices.

Because human beings are a principal source of ESD, the Human Body Model (HBM) is the most commonly used model to describe an ESD event. This model represents the discharge from the fingertip of a standing individual to the device. This type of discharge can occur from the simple act of physically picking up a sensitive component or inserting a printed circuit board into a PC.

The move towards automated assembly seems, at first, to avoid the ESD problems of charged people represented by the human body model. However, components may also be damaged when assembled by machine. A device can be charged, for example, when 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. Similarly, a charged device placed on a conductive work surface will discharge rapidly through the worksurface, possible damaging the device.

This occurrence of a charged device discharging when it comes in contact with a conductive material, is known as the Charged Device Model (CDM) event. It can be more destructive than the HBM for some devices. Although the duration of the discharge is very short--often less than one nanosecond--the peak current can reach several tens of amperes.

A third model that has been used to characterize ESD events is known as the Machine Model (MM). The model is representative of a worst-case human body model event. Rather than the discharge occurring from the human body to the component, the Machine Model represents a discharge from an object to the component. The object could be a hand tool or production equipment. Because the magnitude of the discharge increases as the capacitance of the charged body decreases, this type of discharge can be quite damaging if delivered from a small capacitance object such as a pointed tweezers.

Protecting your products from the effects of static damage begins by knowing how they can be damaged and what their level of sensitivity is. Once you have this key information, you can begin to design your control programs. For example, you may use wrist straps to help prevent electrostatic discharge from people to devices. You may use dissipative rather than conductive work surfaces to reduce exposure to charge device events.

For additional information on these various failure models, consult the following resources:

Renninger, R. G., "Mechanisms of Charged Device Electrostatic Discharges," EOS/ESD Symposium Proceedings, 1991

Avery, L. R., "Charged Device Model Testing: Trying to Duplicate Reality," EOS/ESD Symposium Proceedings, 1987

Avery, L. R.., "Beyond MIL HBM Testing - How to Evaluate the Real Capability of Protection Structures," EOS/ESD Symposium Proceedings, 1991

ESD S5.1-1998--Electrostatic Discharge Sensitivity Testing - Human Body Model, ESD Association

ANSI ESD S5.2-1994--Electrostatic Discharge Sensitivity Testing - Machine Model, ESD Association

EOS/ESD DS5.3-1993--Electrostatic Discharge Sensitivity Testing - Charged Device Model, ESD Association

ESD ADV-2.0-1994--ESD Handbook, ESD Association

Do conductive floor mats remove charges from personnel not wearing special footwear? Can static dissipative floor polish reduce charges generated by personnel not wearing special footwear?

In general, conductive floor mats perform best when personnel are wearing special footwear, either static control shoes or foot straps. The combination of mats and flooring with appropriate footwear creates a conductive pathway to ground to dissipate charges from the body. Most ordinary footwear is insulative, and when used with the mats or flooring, there is no conductive pathway between the body and the floor. Thus, charges on the body would not be dissipated satisfactorily.

Floor finishes tend to work differently from mats and flooring materials. They function partially by affecting the tribocharging characteristics of the interaction between flooring material and footwear. Thus, they may reduce the level of static charge generated on the body with special foot-wear. However, the actual performance can be affected by a number of factors such as humidity.

Street shoes sometimes work on ESD personal testers. How do you explain this to a non-ESD auditor?

The operative word in your question is "sometimes." Leather soled shoes may accumulate enough moisture in the soles that lowers the resistance of the footwear below the normal insulative range. If it’s a rainy day and the wearer has walked through a water puddle, the resistance is likely to be lower. If the shoes haven’t been worn for a while, then the soles will dry out and the resistance will be higher and likely insufficient for static control. Perspiration can have a similar effect on the resistance of leather soled shoes.

The explanation for the auditor is that ordinary footwear is highly variable and cannot be depended upon for static control purposes.

Some experts feel the inside layer of a shielding bag should be "antistatic" meaning it does not generate a charge. Yet bags are measured on surface resistivity and are often specified as being in the dissipative or conductive. So what should I demand?

There really is no conflict in having an ESD protective bag that has an antistatic inside layer, but also has a surface resistivity in the dissipative range. The term antistatic refers to the material’s ability to resist triboelectric charge generation. For example, the sliding of a board or component in a bag could generate a static charge. A material's antistatic properties are not necessarily predicted by its resistance or resistivity.

Surface or volume resistivity measurements help define the bag’s ability to provide charge dissipation or electrostatic shielding.

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.

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.

The important factor to remember is that antistatic is not necessarily defined by resistance or resistivity. However, shielding and charge dissipation can be defined by resistance and resistivity. Thus, you can specify a bag that is antistatic as well as static shielding or static dissipative.

For additional information, see:

ESD-ADV 1.0 1994, Glossary, ESD Association

EOS/ESD S11.11-1994, Surface Resistance Measurement of Static Dissipative Planar Materials, ESD Association

EIA-541, Packaging of Electronic Products for Shipment, Electronic Industries Alliance

We are setting up work stations for handling static sensitive devices. What are some guidelines we should follow to be sure we are adequately protecting our components?

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 basic concept of the ESD protective workstation is to keep all materials and personnel at the same electrical potential. Electrostatic discharge occurs when two objects at different potentials come into contact with or in the proximity of each other. If the potentials are equal, no discharge occurs.

The key ESD control elements comprising most workstations are a static dissipative work surface, a means of grounding personnel (usually a wrist strap), a common grounding connection, and appropriate signage and labeling.

Figure 1-Typical ESD Workstation

Dissipative worksurfaces with a resistance to ground of 1.0 x10E6 to 1.0 x 10E9 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 is connected to the common point ground.

Personnel grounding devices, such as wrist straps, also are essential elements of an ESD protective workstation. They are connected to the common point ground to keep personnel at the same potential with other equipment and materials in the area.

Two types of signage are recommended. The first identifies the common point ground for easy reference and assurance of the proper connection. The second designates the area as an ESD protective work area or the workstation as an ESD protective workstation. These designations alert personnel that appropriate ESD control procedures must be followed. ESD Association standard ANSI/ESD-S8.1-1993, Symbols - ESD Awareness identifies the symbols to be used in marking these areas.

Depending on the work being performed, the structure of the workstations, and the ESD sensitivity of the components, your workstations may require additional ESD control materials or procedures. For example the metal structure of a task table should be connected to the common point ground. Shelves, bins, and drawers may require protective worksurface materials that should be grounded. The presence of insulative materials may require the use of ionization. If personnel working in the area must be mobile, then static protective flooring and footwear may be needed.

What about grounding of workstations?

At a protective workstation, grounding is a primary mechanism for equalizing potentials. The common point ground is bonded to the electrical equipment ground (green wire) as recommended in ANSI/ESD-S6.1-1991, Grounding - Recommended Practice.

All ESD protective materials and equipment used at the workstation, such as wrist straps and worksurfaces, are then connected to the common point ground. Because all electrical equipment at the workstation is already connected to this ground, connecting the ESD protective materials to the same ground brings all components of the workstation to the same electrical potential.

ESD grounding of cabinets and shelves is the same as that for ESD grounding of equipment, work surfaces, floors and even wrist straps: be certain that you have good electrical continuity from and between all parts of the item being grounded. If the cabinets and shelves are painted, you may need to sand away some paint at the point where you are making the ground connection. Drawers and shelves may be insolated from the frame rest of the cabinet with insulating glides or supports. These may need replacement. If you rely on contact with a static control floor or mat for grounding of the cabinet, there is the possibility that the bottom of the cabinet has casters or floor protectors that may insulate the cabinet from the floor. And be certain that the grounding procedure being used doesn't compromise personnel safety in the area.

When establishing your workstation grounding system, remember to follow requirements of the National Electrical Code as well as any local code requirements. You also may need to consider ground fault circuit interrupters or other safety protection wherever personnel may come into contact with electrical sources.

Your specific needs will determine the composition of your workstations. However, the most effective workstations utilize a well-coordinated combination of the individual ESD control materials and devices. For additional detailed information refer to the publications in the reference list below.

References:

ANSI/ESD-S6.1, Grounding - Recommended Practice, ESD Association

ANSI/ESD-S7.1, Floor Materials - Resistive Characterization , ESD Association

ANSI/ESD-S8.1-1993, Symbols - ESD Awareness, ESD Association

ANSI/NFPA 70, National Electrical Code, National Fire Protection Association

EIA 625, Requirements for Handling Electrostatic Discharge Devices, Electronic Industries Alliance, 1994

ESD TR 20.20: ESD Handbook, ESD Association

ESD-ADV53.1, ESD Protective Workstations, ESD Association

ESD-S1.1, Evaluation, Acceptance and Functional Testing of Wrist Straps, ESD Association

ESD-S4.1, Worksurfaces - Resistance Measurements, ESD Association

ESD-STM12.1, Seating - Resistive Characterization, ESD Association

Do you believe as devices become more static sensitive, will the human body model failures be the main culprit or will it be the charged device model failure?

For years, we have focused on the human body model (HBM) as the main model for characterizing ESD events. This model represents the discharge from the fingertip of a standing individual to the device. This type of discharge can occur from the simple act of physically picking up a sensitive component or inserting a printed circuit board into a PC.

However, as manufacturers continue to automate their processes, the charged device model (CDM) is playing an increasingly significant role in explaining ESD events. In automated processes, the CDM represents an event that occurs when a device becomes charged as it slides down an integrated circuit carrier, or is picked up and placed by an automatic insertion machine, or experiences random motion in a tape-and-reel carrier tape. Once the device is grounded, for example by contact with a conductive surface such as insertion head, a rapid discharge occurs from the device to the metal object. Although the duration of the discharge is very short, often less than a nanosecond, the peak current can reach several amperes.

The ESD event characterized by the CDM can be much more destructive than that of the human body model. Typically the damage occurs at a much lower voltage than that produced by HBM.

Research has indicated that many factors contribute to charged device failures: interfacing materials, speed of movement of the device, package configuration, material makeup and orientation. Controlling the problem in automated processes involves unique challenges. Solutions involve proper equipment grounding; replacement of insulated equipment parts with grounded, conductive ones whenever possible; shielding of static generative materials; and localized air ionization.

For more detailed discussion of the problems of ESD in automated processes, be sure to attend the October meeting of the Midwest Chapter. Wayne Tan from AMD will speak on the topic of ESD and Production Equipment, discussing not only charged device model problems, but also machine model problems.

With the increasing use of automated equipment in manufacturing and and test processes, the importance of models other than the human body model will continue to grow.

References:

ESD-ADV-2.0-1994, Electrostatic Discharge Control Handbook, ESD Association.

ESD-DS5.3-1996, Electrostatic Discharge Sensitivity Testing-Charged Device Model, ESD Association

"Standards Define ESD Control," John H. Mayer, Test and Measurement World, September 1996.

"ESD and Automated Processes," Wayne Tan and Michael T. Brandt, Circuits Assembly, August 1995.

"Evaluating and Qualifying Automated Test Handlers in a Semiconductor Company," Leng-Leng Ow and Wayne Tan, EOS/ESD Symposium Proceedings, 1994.

"A Combined Socketed and Non-Socketed CDM Test Approach for Eliminating Real-World CDM Failures," Andrew Olney, EOS/ESD Symposium Proceedings, 1996.

Some static generative items (i.e., monitors, key boards or special items) must be used in a static controlled area, how do you provide static protection under these circumstances?

Ideally, we should try to keep these static generating materials to a minimum in any ESD-protective environment. However, this is not always possible. If you have to use them, there are several steps you can take to reduce their possible impact on your ESD-sensitive products.

First, keep the static generators as far away from your ESDS products as possible, preferably several feet. Second, be sure to keep ESDS products from entering any field that may be produced by the static generator. Third, in the static generator is a conductor, ground it if possible to remove any charge that may be generated on it. Fourth, treat the surface of the static generator with a topical antistat to reduce the charge generated on the material. Fifth, ionize the material to remove any generated charges. Sixth, if the charge generator is employee clothing, use static protective garments to help suppress any fields from the clothing.

You may need to implement several of these steps to provide adequate static protection.

If you are using a roll of table or floor mat material that is 10 to 100 feet long, is one ground connection sufficient?

There are no specific guidelines to follow in response to this question.It really depends on the construction of the mat material as well as the environment in which the material is used. In many cases, the single ground connection should be sufficient. However, they may be some circumstances in which you may want to use multiple ground connections.

First, if the resistance point to point or point to ground increases significantly with the distance between electrodes, then additional ground connections may be required. To test this, use your normal megger you use for testing floors or work surfaces. Connect one ground lead to the ground snap and the other to the 5 lb electrode placed at varying distances from the ground snap. If the resistance increases as you move the electrode further from the snap, you may need additional ground connections.

Second, if there is a significant risk that the mechanical integrity of the electrical continuity can be interrupted between the ground connection and points on the mat, then additional ground connections may help reduce this risk.

Most floor coverings, such as vinyl, rubber, or polymerics, usually require multiple connections as well.

In all cases, be sure to check the manufacturer's instructions for grounding the product. In the absence of specific recommendations, you may have to do your own testing to determine the grounding requirements for your specific application.

Recently, I have seen some heel straps with 2 megohm resistors. I am used to seeing them with 1 megohm resistors. What is the reason for using a 2 megohm resistor?

The 1 megohm resistor traditionally has been considered as providing a degree of safety in protection personnel from the hazards of electrical shock. The function of the resistor is to limit the current that personnel may be exposed to if they contact live power.

Because the grounders are work on both feet, and both feet are often in contact with the floor at the same time, parallel ground paths are created and the 1 megohm resistor now provides an equivalent resistance of 0.5 megohms. Grounders with 2 megohm resistors would provide an equivalent resistance of 1 megohm because of the parallel ground path.

Remember, too, that the floor material on which you are standing is also part of the ground path. Because the resistance of the floor material is in series with the footwear, the resistance to ground of the combination of the floor and grounder typically is higher than the resistance of the individual components.

We are having occurrences of high levels of static charge on plastic parts dropped into bins. How do we control this problem? Are there special bins we can use?

Part of your problem may require ionization to neutralize the charge on the plastic parts that are dumped into and out of the bin(s). The plastic parts that you described tribocharge quite easily and are good insulators that cannot be grounded to remove the static charge. The parts charge as they are dumped into the bin and as they slide against each other in the bin.

You also want to use the proper type of bin for holding these parts. When charged insulative materials are dumped into a conductive bin, the bins suppress the field associated with the static charge but will drain very little charge from the plastic parts. If the conductive bin is on an insulative floor or other insulative surface, the bin may become charged by field induction and could zap personnel who touch it. If the bin is placed on a conductive or dissipative surface, the induced charge on the bin can be drained away, but the plastic parts will retain a charge.

Bins made of insulative materials do not suppress the electrostatic field, but they tend to charge and retain a charge. Intense electrostatic fields from the charged parts and charged bin can cause field induced zaps to people or other conductive items that are brought within close proximity to the bin and then grounded. For example a person dumping charged parts into an insulative bin may feel an electrostatic discharge (ESD) after touching a metallic surface while in the electrostatic field generated by the charged bin and parts.

Static dissipative or conductive bins may be a better choice than insulative bins. They should be placed on a dissipative floor or grounded mat to drain static charges that may be induced on them.

Why are we experiencing electrostatic charges on metal fixtures and equipment in our facility? I thought that conductive materials did not charge.

It is not unusual for conductive materials like metals to become electrostatically charged.

Even though a material is conductive, it is not necessarily antistatic. Insulative materials tend to retain an electrostatic charge because there is no electrical path to allow the charge to dissipate to ground, even if the material is grounded.

Because electrons flow more freely across the surface of conductive materials, any accumulated charge tends to dissipate to ground if the material is properly grounded. The rate of dissipation is determined by the material's resistance and the resistance to ground. In high humidity environments, this charge can even dissipate through the "slightly conductive" humid air.

If you are experiencing unexpected electrostatic charges on metals or other conductors in your environment, it is often due to a grounding problem. The resistance to ground may be too high or there may be no pathway to ground at all. For example, on assembly equipment, the equipment may be grounded, but various parts of the equipment may be isolated from ground by plastic parts or insulating lubicrants. Fixtures on worksurfaces may have rubber pads that insulate them from the static control worksurface.

If these metal or conductive surfaces are presenting you with ESD control problems, thoroughly check the ground paths and correct as necessary. Replace or remove insulating rubber pads on the bottom of fixtures. Replace insulating plastic parts with conductive ones. Use conductive lubricants. Be sure that any connections to ground are complete and not broken.

When taking these corrective actions, be careful not to compromise personnel safety. In many instances, the use of insulative materials protects workers from the hazards of electrical shock. In these instances, the use of ionization to remove electrostatic charges may be a better, and safer, solution than grounding of the conductive objects.

What is the best method to ground painted metal shelving and cabinets?

This question seems pretty straight forward with an obvious answer. However, as is often the case in ESD, the obvious sometimes covers up hidden dangers.

The challenge here is not in making that basic electrical connection from the cabinet to a common point ground. Something as basic as a clamp or bolt connection would seem to fit the bill. The challenge is in assuring that the cabinet or shelving is indeed grounded and that all parts are grounded.

Why such a challenge? First, the question stated painted metal cabinets. Although the metal underneath is obviously a conductor, the paint on the surface is usually an insulator. If your ground connection only makes contact with the paint and not the metal, you may not be adequately grounded. You may need to sand away some paint at the point where you are making the ground connection.

Second, what is the electrical continuity between the drawers or the shelves to the frame? Do the drawers contact the metal frame via insulating glides? Do the shelves sit on insulating supports. If so, they are not grounded if the ground is connected to the frame. You may need to replace these insulated glides and supports with conductive ones, or you may need to electrically connect the drawers and shelves to the frame in some other fashion.

Some companies may rely for grounding on the cabinet being in contact with a static control floor or mat. Again, there is the possibility that the bottom of the cabinet has casters or floor protectors that insulate the cabinet from the floor.

The guide to ESD grounding of cabinets and shelves is the same as that for ESD grounding of equipment, work surfaces, floors and even wrist straps: be certain that you have good electrical continuity from and between all parts of the item being grounded. And be certain that the grounding procedure being used doesn’t compromise personnel safety in the area.

Why does the resistance of the floor material measure high?

First, if your measurements disagree with those claimed by the manufacturer, be sure that you and the manufacturer are using the same measurement procedures. Typically, the measurements should be perANSI EOS/ESD 7.1 - Floor Materials.

Second, check your test equipment. If you're using a surface resistivity meter with parallel bars or three prong probes, you will get a high reading. You should be using a megohmmeter with 2-1/2" diameter, 5 pound electrodes to measure the resistance. Be sure that your meter is actually applying 100 volts to the sample. Some instruments may actually apply much less voltage than you think. Also, the instrument should be properly calibrated and the various cables in satisfactory condition.

Third, be sure that newly installed floors are properly cured before making your measurements. Some adhesives and some epoxy type floors take several hours or days to fully cure and provide the proper resistance readings.

Fourth, be sure that the floor surface is free of contaminants. Dirt and standard floor finishes can create an insulating layer between the floor and your electrode.

Fifth, if your resistance to ground measurements are high, but your resistance between electrodes is OK, check your grounds. They may have become disconnected or may not even exist.

Sixth, check your humidity. Some materials are humidity dependent. If you are measuring at low humidity in the winter, your resistance measurements may be significantly higher than those you took at higher humidity last summer.

Finally, if you still have problems, call the manufacturer for help.

We're experiencing high static voltage levels on the carts in our plant. We have a static control floor. What's going on?

Your problem may have several different causes.

First, your carts may not be making electrical contact with the static control floor. Most likely you have insulating wheels or casters on your carts. This is similar to the problems of people wearing insulating footwear on static control floors.

You should replace these insulating wheels or casters with conductive or dissipative ones, making sure that they also have electrical continuity with the frame of the cart. Be sure that you use them for all wheels on the cart to improve consistency of electrical contact with the floor.

Remember, that even these wheels and casters can become dirty creating an insulating layer between them and the floor. Clean them regularly to maintain the proper electrical contact.

An alternative to wheels and casters is the use of ground straps or ground chains that attach to the cart and then drag on the floor. However, chains and straps have only intermittent contact with the floor and are not as effective from a static control point. In addition, the chains and straps can catch on expansion joints, mats, cords, or other items on the floor.

An additional problem may be a lack of electrical continuity between shelves and the frame of the cart or between the frame and the wheels. If the cart hasn't been specifically made for static control purposes, you may find that the shelves rest on insulating supports or that the frames are insulated from the wheels. Use your ohmmeter to check the electrical continuity between the various parts of the cart. Then replace the insulating supports or fittings with conductive ones. If you can't replace the insulators, you may be able to jerryrig an electrical connection from one part to another.

I am new to ESD auditing and am trying to learn everything as I go. My company tests already packaged DRAM, SDRAM, and Modules. When using a field meter to measure items found in an ESD safe area, I have been told that anything above 200V is a discrepancy. I have heard that another group that uses 1000V as its discrepancy number. What is the standard voltage that would be viewed as too high?

It is difficult to generalize about what the maximum allowable voltage should be because it is a function of the sensitivity of the components and devices with which you are working. There are several recommended courses of action.

Someone in your company should establish the internal specifications for maximum allowable voltage to which you as the auditor should audit. Usually, this is defined in terms of human body model voltage, although in some circumstances, it could be defined in machine model terms. Often, this information can be obtained from the supplier of the components. If the information is not available from the supplier of the devices, a source of generic information on sensitivity is the Reliability Analysis Center in Rome, NY. Phone: 319-339-7036.

If the sensitivity information is not readily available from either of the above, you or your customer may need to arrange to have the items tested for sensitivity or you may need to simply assume a maximum allowable voltage that you are comfortable with.

We are receiving components in packaging that appear to violate basic principles of ESD protection. These components include resistors and capacitors shipped in common plastic bags or non-charging charging bags rather than static shielding bags. We are told that these components are not static sensitive. Are they sensitive and should they be packaged in ESD protective packaging?

One of the common misconceptions in static control is that only CMOS or the newer GaAS type devices are static-sensitive. However, there is a very broad range of electronic components that have varying degrees of sensitivity to electro-static discharge. These include semiconductors, bi-polar devices, diodes, thin-film resistors, JFETs, and the like.

Because of the vast number of different components and their varying sensitivities, it is difficult to make a blanket statement about the specific types of items. The supplier of these parts should be your most reliable source of information on the ESD sensitivity of the items. A good source of generic information is the Reliability Analysis Center in Rome, NY. Phone: 319-339-7036.

If the sensitivity information is not readily available from either of the above, you or your supplier may need to arrange to have the items tested for sensitivity. On a practical note, if you have not been experiencing problems, then it is likely that the specific items are not highly sensitive to ESD.

If you do determine that the items are ESD sensitive, then you will need to work with your supplier to make sure that they are properly packaged.

What is the resistance range for conductive, static dissipative, electrostatic shielding, insulative, and anti-static materials?

For ESD purposes, many materials are classified by their resistance or resistivity characteristics. These definitions are found in ESD Association or EIA standards publications.

Conductive Materials are defined as those having a surface resistivity less than 1 x 10E5 ohms/square or a volume resistivity less than 1 x 10E4 ohm-cm(1). With a low electrical resistance, electrons flow easily across the surface or through the bulk. Charges will go to ground or another conductive object the material comes in close proximity to or contacts.

Dissipative Materials are defined as those having a surface resistivity equal to or greater than 1 x 10E5 ohms/square but less than 1 x 10E12 ohms/square or a volume resistivity equal to or greater than 1 x 10E4 ohm-cm but less than 1 x 10E11(4). Charges will flow to ground more slowly and in a somewhat more controlled manner than with conductive materials.

Electrostatic Shielding Materials have a conductive layer with a surface resistivity of less than 1 x 104 ohms/square, or a volume resistivity of less than 1.0 x 10E3 ohm-cm per millimeter of thickness(4). These materials provide Faraday cage protection from energy transfer to electrostatic discharge sensitive devices.

Insulative Materials are defined as those having a surface resistivity of at least 1 x 10E12 ohms/square or a volume resistivity of at least 1 x 1011 ohm-cm(1). Insulative materials prevent or limit the flow of electrons across its surface or through its volume. Insulative materials have a high electrical resistance and are difficult to ground. Static charges remain in place on these materials for a very long time.

Antistatic Materials are not defined by resistance or resistivity. Antistatic refers to the property of a material that inhibits triboelectric charging. A material's antistatic characteristic is not necessarily correlated with its resistivity or resistance.(1)

References

1ESD-ADV1.0-1994, Glossary

2EOS/ESD-S11.11-1993, Surface Resistance Measurement of Static Dissipative Planar Materials

3ESD-STM11.12-2000, Volume Resistance Measurement of Static Dissipative Planar Materials

4EIA-541, Packaging of Electronic Products for Shipment.

Does the fan at the end of our production line create a static charge? If we insert work sheets inside our static control packaging, are our parts at risk from ESD?

These are typical questions that we're asked every day. Will a certain process or activity generate a charge or put our sensitive parts at risk. Since we haven't visited the facility to observe the actual process and there are so many parameters involved, it's difficult to give a concrete answer. What might cause an ESD problem in one environment might not cause a problem in another. Our suggestion is "measure it."

Generally, your initial evaluation of a potential situation is not too precise or detailed. A basic fieldmeter can provide an indication of whether an electrostatic field or potential exists, as well as the general magnitude of the field. If you identify a potential problem, you can use more sophisticated equipment for measurements.

If you want to know whether a problem exists, you can start by simply measuring.

Our static control garments that show variability in sleeve to sleeve resistance when tested according to ESD STM2.1-Garments. Some fail at 100 volts, but pass at 500 volts. Some pass when we test at 100 volts, others won't. Some fail after only one or two washes. Is it the test method that's the problem?

Inconsistent sleeve to sleeve resistance results can occur when measuring static control garments. These inconsistencies are not, however, due to the test method. The inconsistency usually stems from the ability, or the inability, of the current to flow across sewn joints or seams of the garments. Usually the problem is more pronounced in the shoulders. Laundering simply aggravates the problem. Higher test voltages may allow current to flow between conductive fibers that are close to one another, but a lower test voltage would not allow the current flow. You want a good solid electrical connection between garment pieces to allow charge to flow between them. Testing with a higher test voltage only disguises the problem.

Make some additional tests on the garments, but this time test from point-to-point on the same panel. If your test results are good here, then you will have a pretty good indication that you don't have good electrical contact at the seams. This point-to-point test is also part of ESD STM2.1: Garments. By using both the sleeve-to-sleeve and the point-to-point data, you will improve your evaluation of the garment.

What types of instruments do I need to audit and evaluate my ESD control program?

Most material and procedure evaluations involve charge or voltage generation, resistance or resistivity, or ground connections,. To make these measurements, the typical minimum instrumentation requirements include an electrostatic field meter, a charge plate monitor, a wide range resistance meter, a ground/circuit tester, and appropriate electrodes and accessories.

Because resistance and resistivity are key parameters in evaluating many ESD control materials, a wide range resistance meter is one of our most critical instruments. The equipment you choose should be capable of applying these both 100 volts and 10 volts to the materials being tested. Also, the meter should be capable of measuring resistance ranges of 10E3 to 10E12 ohms depending on the resistance range of materials you typically use.

Many standard test methods specify test instruments with open circuit voltages. The actual applied voltage of these instruments may vary with the resistance of the material depending upon the short circuit current of the instrument. Be sure you know what the actual applied voltage of the instrument is.

For measuring electrostatic charge or voltages, you will need a hand held electrostatic field meter or a charge plate monitor. Many field meters simply measure the gross level of electrostatic charge and are used as general indicators of the presence of a charge and the approximate level of this charge. 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.

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

Your specific needs are determined by what you are trying the measure, the required precision, and the sophistication of your program. Instrumentation requirements for laboratory evaluation of materials usually are different from the requirements for auditing or monitoring your program on the factory floor. Some meters are designed for very precise measurements and typically would be used for laboratory evaluations. Others are less precise, usually designed for portability and used for auditing and monitoring.

In selecting your instrumentation, remember, you want the right tool for job.

When packaging sensitive devices in ESD protective bags, is it necessary to use special labels to close the bag? Can I print my own stickers? Do I have to use special paper?

Unless required by agreement with another company, distributor, or end user, the choice of label or sticker used to close the bag is pretty much open to the supplier of the product. However, here are some label characteristics and application suggestions that I would recommend whether purchasing pre

1. Select a label material that is low charge generating. Most paper labels are low charge generating after they are applied. The greatest charge generation occurs when labels are removed from their backing or release liner.

2. Always perform the packaging operation at a fully functional ESD protective workstation. Make sure that the device is placed well inside the bag before the label is removed from its backing. Untreated Mylar and plastic coated labels can retain significant charge after separation from their release liners and they also tend to tribocharge during shipping and handling. There are some static dissipative plastic labels and dissipative paper materials available on the market.

3. Incorporate an ESD warning symbol on the label. There are several versions of ESD warning symbols available on pre-printed paper labels. I suggest using the warning symbol shown in EOS/ESD Standard S8.1.

4. Wear a grounded wrist strap when peeling the label from its backing. This helps to drain charge from the label before it is applied to the bag.

5. Keep unpackaged devices away from the separated label and backing. During the removal of the label from its release liner, make sure that other unpackaged components and modules are kept at least one foot away during the bag closing operation. High charges may occur on both the label and its release.

6. Use an ionizer during the packaging operation. An ionizer provides an extra measure of protection. It will neutralize the charges on the label and its backing. It is especially important to use an ionizer when labels are removed from their liners within one foot of exposed components.

7. Provide bold ESD handling precautions. I would also recommend that both your label and installation instructions provide bold warnings that ESD precautions must be observed before opening the bag. By following these recommendations, you will have done a good job of protecting the memory module up to the point when the customer opens the bag. As long as the customer follows proper ESD prevention procedures, the memory module should have a long life expectancy and operate as designed.

Be careful about using too large of a label. In low humidity environments, labels can become more insulative and if too large, may interfere with the removal of any charge that may be present on the bag's exterior surface.

What gauge wire is required for grounding workbenches. It is not mentioned in ESD S6.1?

When making ground connections, you should follow the National Electrical Code or your local electrical codes as far as the appropriate gauge of the ground wire. In fact, any ESD grounding must conform to the requirements of the National Electrical Code or local electrical codes.

What is the difference between EOS and ESD?

By definition, electrical overstress (EOS) is "the exposure of an item (an electronic component for example) to a current or voltage beyond its maximum ratings. This exposure may or may not result in a catastrophic failure of the item."(1) Electrostatic discharge (ESD) is a specific type of EOS. ESD is "the rapid, spontaneous transfer of electrostatic charge induced by a high electrostatic field. Usually the charge flows through a spark between two bodies at different electrostatic potentials as they approach one another."(2)

Typically, you can infer that an overstress has occurred when an item fails to meet its electrical characteristics. Determining whether the failure was caused by an ESD event or some other type of overstress is often more difficult. In addition, overstress may result in latent damage to an item, damage that is not immediately detected in its electrical properties, but which may later result in a failure of the item.

References:

(1, 2) ESD-ADV1.0, 1994, Glossary, ESD Association, Rome, NY

Where can I find the ESD sensitivity information for specific microhybrid electronic components, (i.e.; ICs, Resistors, Capacitors, CMOS ICs, etc.)?

The first source would be the manufacturer or supplier of the component itself. An additional source is ITT Research Institute/Reliability Analysis Center in Rome, NY. The organization issues a publication VZAP-95, Electrostatic Discharge Susceptibility Data. It contains ESD susceptibility data for 22,000 devices, including microcircuits, IIT Research Institute / Reliability Analysis Center, 201 Mill Street, Rome, NY 13440-6916. Tel: (888) 722-8737. Fax: (315) 337-9932. Web site: http://rac.iitri.org

How do you decide what is the proper packaging for sensitive products: conductive, dissipative, or antistatic?

ESD protective packaging accomplishes two major goals in controlling electrostatic discharge: eliminating or reducing charge generation and accumulation, and preventing discharges from reaching susceptible parts and assemblies.

Packaging materials are generally classified in one of three categories depending upon their electrical resistance. However, these definitions do not necessarily indicate a material's ability to provide static protection. Conductive materials have a surface resistance of equal to or less than 1 x 10E4 ohms per EOS/ESD Standard S11.11, or volume resistivity or equal to or less than 1 x 10E4 ohm-cm per EIA Standard 541. Conductive materials act as a Faraday cage, helping prevent direct discharges from reaching the components inside the package.

Shielding (Electrostatic) materials have a surface resistance equal to or less than 1 x 10E3 ohms per EOS/ESD S11.11, or a volume resistivity of equal to or less than 1 x 103 ohm-cm per EIA 541. Shielding attenuates electrostatic fields on the packaging material's surface to prevent a difference in electrical potential from existing inside the package. Some materials also may protect from a direct discharge.

Dissipative materials have a surface resistance of greater than 1 x 10E4 ohms but less than or equal to 1 x 10E11 ohms per EOS/ESD S11.11 or a volume resistivity of greater than 1 x 10E5 ohm-cm but less than 1 x 10E12 ohm-cm per EIA 541. These materials drain charges and dissipate charges across the entire surface of the packaging material.

The term antistatic is no longer used to classify materials. It is sometimes used to describe materials that resist triboelectric charge generation caused by the material contacting and separating from itself or from other materials. The capability of a material to resist triboelectric charge is not necessarily indicated by resistance or resistivity measurements.

There are several factors that influence the selection of the proper package. Your first step will be to determine First, you'll want to select a material You'll need to determine whether the item being packaged is to be protected from triboelectric charge generation, direct electrostatic discharge, electrostatic fields, or a combination of any of the three. The packaging decision should consider the ESD sensitivity of the item and a package design determined accordingly. Most companies today utilize packaging materials that provide all three benefits. For example, you may require conductive or shielding materials to prevent direct discharges from reaching the product, but may need to combine these materials with a dissipative material in order to reduce the possibility of a charged device model discharge from the product.

A second consideration is the entire shipping cycle of the item; from the moment it is first packaged to internal handling, actual shipment and final handling by the receiver of the part. Longer cycles and more handling increase the chances for exposure to ESD events and influence the level of protection required of the packaging material.

A related factor is reuse of the package by the manufacturer or customer. Will it be reused several times by the manufacturer or the purchaser? What is the shipping cycle from the time the product is packaged until the customer removes it from the package for use? Both reuse and longer shipping cycles require greater material integrity. Materials designed for reuse or for long life cycles must retain their ESD properties and provide physical protection for the anticipated useful life of the material.

Another factor is the physical protection that needs to be given to the item. Some items may require the use of foam or cushioning in order to protect component leads or to prevent physical damage to the item. In some instances flexible bags may be quite suitable, in others, corrugated or thermoformed packaging may be required.

Other factors in evaluating ESD packaging materials go beyond ESD protection. These include contamination and cleanliness, chemical and physical compatibility of the material with the product being packaged, transparency, tear strength, water or moisture vapor transmission, printability, bar code reading, the effects of humidity and temperature on the material, disposability and recyclability.

Perhaps the final factor in selecting the proper packaging material is the cost/value consideration. The cost analysis should take into consideration the value received, not simply the out of pocket costs. A more expensive package may be less expensive in the long run because of it can be reused. Or you may be tempted to use a multitude of different packaging types in order to lower costs for the less expensive parts. However, you may find that the added costs for carrying multiple inventories may negate the cost savings.