ESD FAQFrequently Asked Questions NewsHow do we test garments?What types of instruments do I need to audit and evaluate my ESD control program?Is it necessary to use special labels to close ESD protective bags? What gauge wire is required for grounding workbenches?What is the difference between EOS and ESD?Where can I find the ESD sensitivity information for specific microhybrid electronic components? How do you decide what is the proper packaging for sensitive products?General Are prototypes subject to ESD damage?Why do explosive environments use different materials and procedures?Where do I find information on static control in high voltage areas? Can you ground insulators that have high moisture content? What ESD precautions are required in field service?Explain the ESD failure models?Do we need ESD control with automated handling equipment? Can we use humidity to control static?Do we need to use ESD control procedures with cards that have no active components?Which will be a greater problem in the future, HBM failures or CDM failures? How do we control static on items such as monitors and key boards?How do we control static on items in bins? Do conductive materials charge?How do I label ESDS products and static control materials? What is the difference between antistatic, conductive and static dissipative? What is the maximum allowable voltage in an ESD safe work area? What is the resistance range for conductive, static dissipative, electrostatic shielding, insulative, and anti-static materials?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?What is the difference between EOS and ESD?Device SensitivityAre components such as resistors and capacitors sensitive to ESD?Where can I find the ESD sensitivity information for specific microhybrid electronic components? • • •Auditing, Testing, Evaluation How often should I audit my facility?What types of instruments do I need to audit and evaluate my ESD control program? • • •Garments How do we test garments?Flooring/Footwear Will ESD control footwear work on regular vinyl tile?
How do we test garments? What types of instruments do I need to audit and evaluate my ESD control program? Is it necessary to use special labels to close ESD protective bags? What gauge wire is required for grounding workbenches? What is the difference between EOS and ESD? Where can I find the ESD sensitivity information for specific microhybrid electronic components? How do you decide what is the proper packaging for sensitive products?
General
Are prototypes subject to ESD damage? Why do explosive environments use different materials and procedures?
Where do I find information on static control in high voltage areas? Can you ground insulators that have high moisture content? What ESD precautions are required in field service? Explain the ESD failure models? Do we need ESD control with automated handling equipment? Can we use humidity to control static? Do we need to use ESD control procedures with cards that have no active components? Which will be a greater problem in the future, HBM failures or CDM failures? How do we control static on items such as monitors and key boards? How do we control static on items in bins? Do conductive materials charge? How do I label ESDS products and static control materials? What is the difference between antistatic, conductive and static dissipative?
What is the maximum allowable voltage in an ESD safe work area? What is the resistance range for conductive, static dissipative, electrostatic shielding, insulative, and anti-staticmaterials? Does the fan at the end of our production line create a static charge? If we insert work sheets inside our staticcontrol packaging, are our parts at risk from ESD? What is the difference between EOS and ESD?
Device Sensitivity
Are components such as resistors and capacitors sensitive to ESD? Where can I find the ESD sensitivity information for specific microhybrid electronic components?
•••• Auditing, Testing, Evaluation
How often should I audit my facility? What types of instruments do I need to audit and evaluate my ESD control program?
•••• Garments
How do we test garments?
Flooring/Footwear
Will ESD control footwear work on regular vinyl tile?
Why wear 2 shoe grounders instead of one? Why do our shoes test out of specification? Will unsealed concrete work as a static control floor? Do layered floor mats have an alternate path to ground? Do floor mats or dissipative floor finishes require special footwear?
Why do street shoes sometimes work for static control? How frequently do you ground floor mats? Why do some shoe grounders have 2 megohm resistors? Why does our floor resistance measure high? Why do we have high voltage levels on carts? When do you use a resistor in floor and table mats?
Standards
Where do I obtain ESD Association Standards?
Grounding
Which ground is better, a direct earth ground or an electrical ground? How do we ground painted shelves and cabinets? Can a person wearing cotton gloves be grounded? How do we ground work stations? Can you ground insulators that have high moisture content? What gauge wire is required for grounding workbenches?
Packaging
Why should shielding bags be both antistatic and dissipative or conductive? Is it necessary to use special labels to close ESD protective bags?
How do you decide what is the proper packaging for sensitive products?
Work Stations
What are some guidelines for setting up work stations? How do we ground work stations?
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 than1 x 105 ohms per square or a volume resistivity less than 1 x 10 4 ohm-cm. A static dissipative material has asurface resistivity of 1 x 105 to 1 x 1012 ohms per square or a volume resistivity of 1 x 10 4 to 1 x 1011 ohm-cm.These definitions appear in the ESD Association Glossary as well as in various other static control standardsdocuments.
For some materials, surface resistance rather than surface resistivity is often used to define these terms. In thiscase, a simple conversion factor is applied, dividing the resistivity ranges by 10. Thus conductive becomes lessthan1 x 104 ohms and static dissipative becomes 1 x 104 to 1 x 1011 ohms, provided that the appropriateelectrodes with the correct geometric conversions are used. ANSI/ESD S11.11 provides additional information onthis 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 andtoday 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 wecontrol the generation of static electricity on the body?
Laboratory testing has demonstrated that relatively high levels of body voltage generation and poor body voltagedecay 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. Voltagegeneration 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 ratesare significantly improved, often to only a few volts with decay rates less than 1/4 second. Such performanceindicates that floor materials and footwear depend upon each other and should be thought of as a system for bestperformance.
Footwear and floor materials create a ground path from the wearer to ground. Insulative flooring such as standardvinyl tile, and standard street shoes or industrial footwear with rubber, crepe, or polyurethane soles insulate thewearer from the floor. Body charges cannot readily flow from the body through the shoes through the floor toground.
If a person is sitting at a grounded work station -- ESD table mat and ESD wrist straps -- can that person wearthin 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 ontosensitive devices and boards. Grounding is through theESD wrist strap. As long as the wrist strap is properly
connected through a common point ground cord 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.
ESD Table mats, if properly grounded provide a ground path for objects placed on the mat. This ground pathwould not be affected by wearing cotton gloves.
A related matter is whether the cotton gloves tribocharge, thus creating an ESD hazard. Cotton absorbs moisturefrom hands and air and does not tribocharge readily. If you're handling highly sensitive devices of 100 volts orless, 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: [email protected]. The Midwest Chapter libraryalso 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 sensitiveparts 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 wasbased 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. Please refer to an
example of an ESD Workstation Layout or take a look at ESD Control Procedures.
Basic precautions such as ESD wrist straps, proper work surfaces and protective handling such as the useof ESD Finger Cots, ESD Jackets or Smocks, ESD Gloves and packaging materials such as ESD StaticShielding Bags, will reduce your exposure to ESD damage in design and prototyping. Implementing theseprecautions 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 ourpersonnel. 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 thebody to a static control floor material. This electrical path is maintained only when the grounder is in contact withthe ESD Floor Mat. If you wear an ESD heel grounder on only one shoe, you lose this contact every time youtake a step with this foot or when you're sitting at a work station and this foot leaves the floor. Because there's nopath for the charges to flow to ground, the charges will accumulate on your body. ESD Heel Grounders on bothfeet increase the frequency of contact and create a better chance for contact when you are seated. The result isless charge generated, less charge accumulated.
Although the accumulated charge might appear rather small, the relationship between capacitance and chargemay result in a higher charge level than one might think. Capacitance is directly related to whether both feet are incontact with the floor surface. Capacitance is highest with both feet on the floor and lowest with only one foot onthe floor. If we apply the formula Q=CV and Q/C=V, where Q=Charge, C=Capacitance, and V=Voltage, we seethat 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 instancesonly one foot is in contact with the floor.
Heel 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 between1 x 10E6 and 1 x 10E8 ohms. Whenever we test them with the same equipment we use to test wrist straps, thetester tells us that the shoes are out of specification. What's wrong?
The problem is probably with your ESD tester, not your shoes. Most ESD wrist strap testers are set for resistance
ranges that are different than those of your shoes. Also, they simply indicate whether the ESD wrist strap is outof this range rather than giving a specific resistance measurement. For example, the tester may have a range of7.5 x 10E5 to 1.0 x 10E7ohms which may be appropriate for Anti static wrist straps. Anything outside of thisrange will cause an indicating light to appear. Because your shoes may have a higher resistance than the highrange 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 testingshoes. Second, you can obtain an ESD wrist strap tester that can be adjusted for different resistance ranges orone that will indicate the actual resistance being recorded rather than simply indicating whether the resistance iswithin a specified range. Finally, you can use a wide range ohmmeter for measuring the resistance of your shoes
as well as your wrist straps. Two great options for ESD wrist strap testers are our ESD Wrist Strap Test
Meter (ESDWSTM-1) or ESD Combination Test Meter (ESDCT-1) that will check wrist straps, heel
The objective is to make sure that you have the proper equipment to do the job rather than simply trying to adaptexisting equipment that may not work for the intended application. If you use the right equipment, you can be surethat 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 dissipativematerials. Why do they use ESD 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 conductivematerials were more common than dissipative. Some of these specifications may not have been updated toconsider 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 morefamiliar 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 dischargerates through conductive materials or through anti static wrist straps without the one megohm resistor is oftengiven 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 safetyare in the area of potential shock hazards.
Where can I find information about static control for high voltage areas? (For example, areas in which electricalvoltages exceed 250V)
High voltage environments tend to create some unique situations for ESD control, because many of our controlprocedures have the possibility of compromising personnel safety in these environments. A key procedure is to besure to follow all electrical codes and safety codes. You can find information in the National ElectricalCode (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 youcan 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 oflack 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 thefloor. 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 ishighly dependent upon its moisture content, the resistance can change depending upon the amount of moisturepresent in the concrete. Newly poured concrete will normally show a significantly lower resistance than concretethat 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 beproperly isolated from the concrete. The concrete thus could provide an alternate path to ground, bypassing the
ESD ground on the work bench. Some suggested solutions include an ESD Floor Mat Kit such as an ESD
Anti Fatigue Mat which provides cushioning for added comfort or our ESD PVC Floor Runner Mat.
If you decide to purchase an entire ESD Floor Mat roll, you may want to look at some of ourgroundingaccessories to help you setup a proper ESD safe work area.
I've heard that layered static control mats could have an alternate path to ground. What does that mean? Is this aproblem?
Layered floor mats are generally composed of ESD Two Layer mats also known as ESD Rubber Mats or threelayers of material such as our ESD Vinyl Mat orESD Three Layer Mat, with each layer having a different level ofconductivity. 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 (ESDSSO) 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 andthat contact is grounded, then an alternate path to ground is created, bypassing the resistor in the common pointground cord (ESDCGC) that is attached to the ground snap. This could create a safety problem or could allow acharge 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, cutsthrough the surface or bottom layer, or perhaps even excessive moisture that penetrates the mat. Someone mighthave 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 ispossible to ground these materials and remove any static charge from them by grounding. However, themoisture content can be highly variable. Different materials absorb moisture at different rates than others meaningthat 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 tosuch highly variable factors as moisture. If charges on these materials are of concern in your environment, youmay want to implement ESD control procedures other than grounding to control the problem. Ionization, forexample, 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 ESD wrist straps, ESD Ground
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 alsoneed a method to easily identify ESD control materials. There seem to be a lot of choices. How do we properlyidentify ESDS parts and ESD control materials?
The most common symbols traditionally used to identify ESDS parts or ESD control materials have been replacedwith newer, more appropriate symbols. The lightning bolt inside a circle is no longer used in ESD control becauseinternationally it symbolizes an electrical hazard to persons. The circle with three arrows is a generic symbol forelectrostatic, electromagnetic, magnetic, and radioactive fields. Using this symbol for ESD control purposes would
not properly alert people to the reason for its presence. Please take a look at the ESD labels we offer as a
possible solution.
The ESD Association Standard ESD S8.1 -- ESD Awareness Symbols provides two symbols for ESDidentification.
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 slashthrough the reaching hand means "Don't touch." Because of its broad usage, thehand in the triangle has become associated with ESD and the symbol literallytranslates to "ESD sensitive stuff, don't touch."
The ESD Susceptibility Symbol is applied directly to integrated circuits, boards, andassemblies that are static sensitive. It indicates that handling or use of this itemmay result in damage from ESD if proper precautions are not taken. If desired, thesensitivity 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 thetriangle. An arc around the triangle replaces the slash. This "umbrella" meansprotection. The symbol indicates ESD protective material. It is applied to mats,chairs, wrist straps, garments, packaging, and other items that provide ESDprotection. It also may be used on equipment such as hand tools, conveyor belts,or automated handlers that is specially designed or modified to provide ESDcontrol.
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 theESD Susceptibility Symbol applied to it. Because the bag offers protection againstESD, it would have the ESD Protective Symbol affixed to it. The bag also would have the ESD SusceptibilitySymbol to indicate its contents were ESD sensitive. Now we can easily see that the bag is an ESD protectivematerial and that its contents are ESD sensitive.
Neither symbol is applied to ESD test equipment, footwear checkers, wrist strap testers, resistance or resistivitymeters or similar items used for ESD purposes, but which do not provide actual protection.
Grounding cords for ESD table mats and ESD floor mats are available with or without a 1-megohm resistor. Isthere 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 megohmresistor in all cases. Recent ESD Association standards do not directly address the issue. However, they doprovide 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 andrecommended. 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 maycome 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? Whatprocedures should we follow?
There are many types of circuit boards used for system testing. Sometimes they are called "streaker" cards orextender cards that are designed without active electronic components. While these test cards themselves maynot be ESD sensitive, the systems in which they are used probably are. Other boards in the vicinity of the testcard likely may contain ESDS components. Therefore, when using test cards, the following procedures helpassure 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 thewrist strap to assure that it is functioning properly.
2. When not in use, store test cards in static protective packaging such as ESD Bags to protect them from dustand other contaminants. The use of static protective packaging reduces the likelihood of non-conformingmaterials 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 andassure 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 ESDprotective equipment.
First, ground all components of the work area (worksurfaces, people, equipment, etc.) to the same electricalground point called the "common point ground." This common point ground is defined as a "system or method forconnecting 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 groundconnection. This is the preferred ground connection because all electrical equipment at the ESD workstation isalready connected to this ground. Connecting the ESD control materials or equipment to the equipment groundbrings 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 wereconnected to an auxiliary ground, a difference in electrical potential could exist between the iron and the ESDSitem. 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 theequipment 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. Youalso 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.
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 , ESD floor mats , 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 moisturecan 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 personneldiscomfort.
In our dry northern winter climates, increasing the humidity can become quite costly and may not be very costeffective. Also, humidity levels in various parts of the facility can vary significantly, even when controlled. Forexample, 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 canbe subjected to a variety of humidity levels before it reaches the final customer. You will still need static controlpackaging 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 ESD audit, some experts recommend auditing each department once a month if possible andprobably a minimum of six times per year. If this seems like a high frequency level, remember that these regularaudits 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 ofauditing. If, on the other hand, your audits regularly uncover continuing problems, you may need to increase thefrequency.
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. Foreach of the three major sources of ESD, there are models that represent the ways in which ESD can damage asensitive device. These models are used to describe the ESD event itself and also to classify the ESDsensitivities 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 individualto the device. This type of discharge can occur from the simple act of physically picking up a sensitive componentor inserting a printed circuit board into a PC.
The move towards automated assembly seems, at first, to avoid the ESD problems of charged peoplerepresented by the human body model. However, components may also be damaged when assembled bymachine. A device can be charged, for example, when sliding down the feeder. If the device then contacts theinsertion 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 asthe 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 tensof amperes.
A third model that has been used to characterize ESD events is known as the Machine Model (MM). The model isrepresentative of a worst-case human body model event. Rather than the discharge occurring from the humanbody to the component, the Machine Model represents a discharge from an object to the component. The objectcould be a hand tool or production equipment. Because the magnitude of the discharge increases as thecapacitance of the charged body decreases, this type of discharge can be quite damaging if delivered from asmall capacitance object such as a pointed tweezers.
Protecting your products from the effects of static damage begins by knowing how they can be damaged andwhat their level of sensitivity is. Once you have this key information, you can begin to design your controlprograms. 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 SymposiumProceedings, 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 ProtectionStructures," 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 dissipativefloor polish reduce charges generated by personnel not wearing special footwear?
In general, conductive floor mats perform best when personnel are wearing special footwear, either staticcontrol shoes, ESD heel grounders or foot straps. The combination of esd floor mats such as ESD anti fatiguefloor mats or ESD PVC floor runner mats and flooring with appropriate footwear creates a conductive pathwayto 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 notbe dissipated satisfactorily.
Floor finishes tend to work differently from mats and flooring materials. They function partially by affecting thetribocharging characteristics of the interaction between flooring material and footwear. Thus, they may reduce thelevel of static charge generated on the body with special foot-wear. However, the actual performance can beaffected 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 thesoles that lowers the resistance of the footwear below the normal insulative range. If it’s a rainy day and thewearer has walked through a water puddle, the resistance is likely to be lower. If the shoes haven’t been worn fora 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 staticcontrol purposes.
Some experts feel the inside layer of a static shielding bag should be "antistatic" meaning it does not generate acharge. 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 asurface resistivity in the dissipative range. The term antistatic refers to the material’s ability to resist triboelectriccharge generation. For example, the sliding of a board or component in a bag could generate a static charge. Amaterial'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 orelectrostatic shielding.
Electrostatic shielding attenuates electrostatic fields on the surface of a package in order to prevent a differencein 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 resistancegreater than 1.0 x 10E4 but less than or equal to 1.0 x 10E11 when tested according to EOS/ESD-S11.11 or avolume 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 isantistatic as well as static shielding or static dissipative.
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 followto 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 equippedwith materials and equipment to limit damage to ESD sensitive items. It may be a stand-alone station in astockroom, warehouse, or assembly area, or in a field location such as a computer bay in commercial aircraft. Aworkstation 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 sameelectrical potential. Electrostatic discharge occurs when two objects at different potentials come into contact withor 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 ofgrounding personnel (usually a wrist strap), acommon grounding connection, and appropriate signage andlabeling.
Dissipative worksurfaces with a resistance to ground of 1.0 x10E6 to 1.0 x 10E9 provide a surface that is at thesame electrical potential as other ESD protective items in the workstation. They also provide an electrical path toground for the controlled dissipation of any static potentials on materials that contact the surface. The worksurfaceis connected to the common point ground.
Personnel grounding devices, such as anti static wrist straps, also are essential elements of an ESD protectiveworkstation. They are connected to the common point ground to keep personnel at the same potential with otherequipment and materials in the area.
Two types of signage are recommended. The first identifies the common point ground for easy reference andassurance of the proper connection. The second designates the area as an ESD protective work area or theworkstation as an ESD protective workstation. These designations alert personnel that appropriate ESD controlprocedures must be followed. ESD Association standard ANSI/ESD-S8.1-1993, Symbols - ESDAwareness 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 thecomponents, your workstations may require additional ESD control materials such as ESD table mats or ESDfloor mats 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 bemobile, 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 pointground 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 cord. Because all electrical equipment at the workstation isalready connected to this ground, connecting the ESD protective materials to the same ground brings allcomponents 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, esd
table mats, esd floor mats, floors and even esd wrist straps: be certain that you have good electrical continuityfrom and between all parts of the item being grounded. If the cabinets and shelves are painted, you may need tosand away some paint at the point where you are making the ground connection. Drawers and shelves may beinsolated from the frame rest of the cabinet with insulating glides or supports. These may need replacement. Ifyou rely on contact with a static control floor or mat for grounding of the cabinet, there is the possibility that thebottom of the cabinet has casters or floor protectors that may insulate the cabinet from the floor. And be certainthat 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 ElectricalCode as well as any local code requirements. You also may need to consider ground fault circuit interrupters orother 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 detailedinformation 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
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 orwill 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 ofdischarge can occur from the simple act of physically picking up a sensitive component or inserting a printedcircuit 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
If you are using a roll of esd table mat or esd floor mat material that is 10 to 100 feet long, is one groundconnection sufficient?
There are no specific guidelines to follow in response to this question. It really depends on the construction of themat material as well as the environment in which the material is used. In many cases, the single groundconnection should be sufficient. However, they may be some circumstances in which you may want to usemultiple ground connections.
First, if the resistance point to point or point to ground increases significantly with the distance betweenelectrodes, then additional ground connections may be required. To test this, use your normal megger you use fortesting floors or work surfaces. Connect one ground lead to the ground snap and the other to the 5 lb electrodeplaced at varying distances from the ground snap. If the resistance increases as you move the electrode furtherfrom 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 thisrisk.
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 specificrecommendations, you may have to do your own testing to determine the grounding requirements for yourspecific application.
Recently, I have seen some ESD heel straps with 2 megohm resistors. I am used to seeing them with 1 megohmresistors. 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 personnelfrom the hazards of electrical shock. The function of the resistor is to limit the current that personnel may beexposed to if they contact live power.
Because the grounders are work on both feet, and both feet are often in contact with the esd floor mat at thesame time, parallel ground paths are created and the 1 megohm resistor now provides an equivalent resistance of0.5 megohms. Grounders with 2 megohm resistors would provide an equivalent resistance of 1 megohm becauseof the parallel ground path.
Remember, too, that the floor material on which you are standing is also part of the ground path. Because theresistance 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 controlthis 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 andout of the bin(s). The plastic parts that you described tribocharge quite easily and are good insulators that cannotbe grounded to remove the static charge. The parts charge as they are dumped into the bin and as they slideagainst 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 littlecharge from the plastic parts. If the conductive bin is on an insulative floor or other insulative surface, the bin maybecome charged by field induction and could zap personnel who touch it. If the bin is placed on a conductive ordissipative 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 acharge. Intense electrostatic fields from the charged parts and charged bin can cause field induced zaps topeople or other conductive items that are brought within close proximity to the bin and then grounded. Forexample a person dumping charged parts into an insulative bin may feel an electrostatic discharge (ESD) aftertouching 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 adissipative 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 anelectrostatic charge because there is no electrical path to allow the charge to dissipate to ground, even if thematerial is grounded.
Because electrons flow more freely across the surface of conductive materials, any accumulated charge tends todissipate to ground if the material is properly grounded. The rate of dissipation is determined by the material'sresistance and the resistance to ground. In high humidity environments, this charge can even dissipate throughthe "slightly conductive" humid air.
If you are experiencing unexpected electrostatic charges on metals or other conductors in your environment, it isoften due to a grounding problem. The resistance to ground may be too high or there may be no pathway toground at all. For example, on assembly equipment, the equipment may be grounded, but various parts of theequipment may be isolated from ground by plastic parts or insulating lubicrants. Fixtures on worksurfaces mayhave 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 groundpaths and correct as necessary. Replace or remove insulating rubber pads on the bottom of fixtures. Replaceinsulating plastic parts with conductive ones. Use conductive lubricants. Be sure that any connections to groundare complete and not broken.
When taking these corrective actions, be careful not to compromise personnel safety. In many instances, the useof 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 conductiveobjects.
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, theobvious 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 isobviously a conductor, the paint on the surface is usually an insulator. If your ground connection only makescontact 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 contactthe metal frame via insulating glides? Do the shelves sit on insulating supports. If so, they are not grounded if theground is connected to the frame. You may need to replace these insulated glides and supports with conductiveones, 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 fromthe floor.
The guide to ESD grounding of cabinets and shelves is the same as that for ESD grounding of equipment, worksurfaces, floors and even wrist straps: be certain that you have good electrical continuity from and between allparts of the item being grounded. And be certain that the grounding procedure being used doesn’t compromisepersonnel safety in the area.
Why does the resistance of the esd floor mat measure high?
First, if your measurements disagree with those claimed by the manufacturer, be sure that you and themanufacturer are using the same measurement procedures. Typically, the measurements should be per ANSIEOS/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 poundelectrodes to measure the resistance. Be sure that your meter is actually applying 100 volts to the sample. Someinstruments may actually apply much less voltage than you think. Also, the instrument should be properlycalibrated and the various cables in satisfactory condition.
Third, be sure that newly installed floors are properly cured before making your measurements. Some adhesivesand 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 aninsulating 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 thewinter, your resistance measurements may be significantly higher than those you took at higher humidity lastsummer.
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 goingon?
First, your carts may not be making electrical contact with the static control floor. Most likely you have insulatingwheels or casters on your carts. This is similar to the problems of people wearing insulating footwear on staticcontrol floors.
You should replace these insulating wheels or casters with conductive or dissipative ones, making sure that theyalso have electrical continuity with the frame of the cart. Be sure that you use them for all wheels on the cart toimprove consistency of electrical contact with the floor.
Remember, that even these wheels and casters can become dirty creating an insulating layer between them andthe 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 thendrag on the esd floor mat. However, chains and straps have only intermittent contact with the floor and are not aseffective 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 betweenthe frame and the wheels. If the cart hasn't been specifically made for static control purposes, you may find thatthe shelves rest on insulating supports or that the frames are insulated from the wheels. Use your ohmmeter tocheck the electrical continuity between the various parts of the cart. Then replace the insulating supports orfittings with conductive ones. If you can't replace the insulators, you may be able to jerryrig an electricalconnection 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 toldthat 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 thesensitivity of the components and devices with which you are working. There are several recommended coursesof action.
Someone in your company should establish the internal specifications for maximum allowable voltage to whichyou as the auditor should audit. Usually, this is defined in terms of human body model voltage, although in somecircumstances, it could be defined in machine model terms. Often, this information can be obtained from thesupplier of the components. If the information is not available from the supplier of the devices, a source of genericinformation 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 voltagethat you are comfortable with.
We are receiving components in packaging that appear to violate basic principles of ESD protection. Thesecomponents include resistors and capacitors shipped in common plastic bags or non-charging charging bagsrather than static shielding bags. We are told that these components are not static sensitive. Are they sensitiveand 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 arestatic-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,
Because of the vast number of different components and their varying sensitivities, it is difficult to make a blanketstatement about the specific types of items. The supplier of these parts should be your most reliable source ofinformation 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 toarrange 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 surethat 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. Thesedefinitions 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 avolume resistivity less than 1 x 10E4 ohm-cm(1). With a low electrical resistance, electrons flow easily across thesurface or through the bulk. Charges will go to ground or another conductive object the material comes in closeproximity to or contacts.
Dissipative Materials are defined as those having a surface resistivity equal to or greater than 1 x 10E5ohms/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 104ohms/square, or a volume resistivity of less than 1.0 x 10E3 ohm-cm per millimeter of thickness(4). Thesematerials 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 avolume resistivity of at least 1 x 1011 ohm-cm(1). Insulative materials prevent or limit the flow of electrons acrossits 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.
Anti static Materials are not defined by resistance or resistivity. Anti static refers to the property of a material that
inhibits triboelectric charging. A material's anti static characteristic is not necessarily correlated with its resistivityor 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 staticcontrol 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 putour sensitive parts at risk. Since we haven't visited the facility to observe the actual process and there are somany parameters involved, it's difficult to give a concrete answer. What might cause an ESD problem in oneenvironment 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 canprovide an indication of whether an electrostatic field or potential exists, as well as the general magnitude of thefield. 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 ESDSTM2.1-Garments. Some fail at 100 volts, but pass at 500 volts. Some pass when we test at 100 volts, otherswon'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 jackets or static controlsmocks. These inconsistencies are not, however, due to the test method. The inconsistency usually stems fromthe ability, or the inability, of the current to flow across sewn joints or seams of the garments. Usually the problemis 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 allowthe current flow. You want a good solid electrical connection between garment pieces to allow charge to flowbetween them. Testing with a higher test voltage only disguises the problem.
Make some additional tests on the ESD garments, but this time test from point-to-point on the same panel. If yourtest results are good here, then you will have a pretty good indication that you don't have good electrical contactat the seams. This point-to-point test is also part of ESD STM2.1: Garments. By using both the sleeve-to-sleeveand 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 groundconnections,. To make these measurements, the typical minimum instrumentation requirements include anelectrostatic field meter, a charge plate monitor, a wide range resistance meter, a ground/circuit tester, andappropriate electrodes and accessories.
Because resistance and resistivity are key parameters in evaluating many ESD control materials, a wide rangeresistance meter is one of our most critical instruments. The equipment you choose should be capable ofapplying these both 100 volts and 10 volts to the materials being tested. Also, the meter should be capable ofmeasuring resistance ranges of 10E3 to 10E12 ohms depending on the resistance range of materials youtypically use.
Many standard test methods specify ESD test meters 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 theinstrument. 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 chargeplate 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 facilitymeasurements or for laboratory evaluation, a charge plate monitor can be attached to some field meters orconnected 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 ESDgrounds 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 thesophistication of your program. Instrumentation requirements for laboratory evaluation of materials usually aredifferent from the requirements for auditing or monitoring your program on the factory floor. Some meters aredesigned for very precise measurements and typically would be used for laboratory evaluations. Others are lessprecise, 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 ESD labels to closethe 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 usedto close the esd bag is pretty much open to the supplier of the product. However, here are some labelcharacteristics 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 theyare 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 thedevice is placed well inside the bag before the label is removed from its backing. Untreated Mylar and plasticcoated labels can retain significant charge after separation from their release liners and they also tend totribocharge during shipping and handling. There are some static dissipative plastic labels and dissipative papermaterials available on the market.
3. Incorporate an ESD warning symbol on the label. There are several versions of ESD warning symbols availableon pre-printed paper labels. I suggest using the warning symbol shown in EOS/ESD Standard S8.1.
4. Wear a grounded ESD wrist strap when peeling the label from its backing. This helps to drain charge fromthe esd label before it is applied to the static shielding bag.
5. Keep unpackaged devices away from the separated label and backing. During the removal of the label from itsrelease liner, make sure that other unpackaged components and modules are kept at least one foot away duringthe 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 willneutralize the charges on the label and its backing. It is especially important to use an ionizer when labels areremoved from their liners within one foot of exposed components.
7. Provide bold ESD handling precautions. I would also recommend that both your label and installationinstructions 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 thecustomer opens the bag. As long as the customer follows proper ESD prevention procedures, the memorymodule 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 asfar as the appropriate gauge of the ground wire. In fact, any ESD grounding must conform to the requirements ofthe 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 acurrent or voltage beyond its maximum ratings. This exposure may or may not result in a catastrophic failure ofthe item."(1) Electrostatic discharge (ESD) is a specific type of EOS. ESD is "the rapid, spontaneous transfer ofelectrostatic charge induced by a high electrostatic field. Usually the charge flows through a spark between twobodies 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 inits 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 ITTResearch 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, includingmicrocircuits, IIT Research Institute / Reliability Analysis Center, 201 Mill Street, Rome, NY 13440-6916. Tel:(888) 722-8737. Fax: (315) 337-9932.
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 andassemblies.
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, orvolume resistivity or equal to or less than 1 x 10E4 ohm-cm per EIA Standard 541. Conductive materials act as aFaraday 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/ESDS11.11, or a volume resistivity of equal to or less than 1 x 103 ohm-cm per EIA 541. Shielding attenuateselectrostatic fields on the packaging material's surface to prevent a difference in electrical potential from existinginside 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 10E11ohms per EOS/ESD S11.11 or a volume resistivity of greater than 1 x 10E5 ohm-cm but less than 1 x 10E12ohm-cm per EIA 541. These materials drain charges and dissipate charges across the entire surface of thepackaging material.
The term antistatic is no longer used to classify materials. It is sometimes used to describe materials that resisttriboelectric charge generation caused by the material contacting and separating from itself or from othermaterials. The capability of a material to resist triboelectric charge is not necessarily indicated by resistance orresistivity measurements.
There are several factors that influence the selection of the proper package. Your first step will be to determineFirst, you'll want to select a material You'll need to determine whether the item being packaged is to be protectedfrom triboelectric charge generation, direct electrostatic discharge, electrostatic fields, or a combination of any ofthe three. The packaging decision should consider the ESD sensitivity of the item and a package designdetermined accordingly. Most companies today utilize packaging materials that provide all three benefits. Forexample, you may require conductive or shielding materials to prevent direct discharges from reaching theproduct, but may need to combine these materials with a dissipative material in order to reduce the possibility of acharged 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 internalhandling, actual shipment and final handling by the receiver of the part. Longer cycles and more handlingincrease the chances for exposure to ESD events and influence the level of protection required of the packagingmaterial.
A related factor is reuse of the package by the manufacturer or customer. Will it be reused several times by themanufacturer or the purchaser? What is the shipping cycle from the time the product is packaged until thecustomer removes it from the package for use? Both reuse and longer shipping cycles require greater materialintegrity. Materials designed for reuse or for long life cycles must retain their ESD properties and provide physicalprotection 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 someinstances 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 andcleanliness, 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 andtemperature on the material, disposability and recyclability.
Perhaps the final factor in selecting the proper packaging material is the cost/value consideration. The costanalysis should take into consideration the value received, not simply the out of pocket costs. A more expensivepackage 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 findthat the added costs for carrying multiple inventories may negate the cost savings.
