is the difference between antistatic, conductive and static dissipative?
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 105 ohms per square or a volume resistivity less than
1 x 104 ohm-cm. A static dissipative material has a surface resistivity
of 1 x 105 to 1 x 1012 ohms per square or a volume resistivity of
1 x 104 to 1 x 1011 ohm-cm. These definitions appear in the ESD Association Glossary as well as in various other static control standards
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 than1 x 104 ohms and static dissipative becomes
1 x 104 to 1 x 1011 ohms, provided that the appropriate electrodes with the
correct geometric conversions are used. ANSI/ESD S11.11 provides additional
information on this issue.
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misused and today no longer represents any resistance range.
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?
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.
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.
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.
a person is sitting at a grounded work station --ESD table mat and ESD wrist
straps -- can that person wear thin cotton gloves and still be grounded
while handling static sensitive devices?
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 ESD 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.
mats, if properly grounded provide a ground path for objects placed
on the mat. This ground path would not be affected by wearing cotton
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.
been reading a lot about EOS/ESD Association Standards. Where do I obtain
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@example.com. The Midwest Chapter library also has copies of
all Association Standards that you can borrow to review.
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?
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.
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. Please refer to an example of an ESD Workstation Layout or take a look at ESD Control Procedures.
precautions such as ESD wrist straps, proper work surfaces and protective
handling such as the use of ESD Finger Cots, ESD Jackets or Smocks, ESD Gloves and packaging materials such as ESD Static Shielding Bags, will reduce your exposure to ESD damage
in design and prototyping. Implementing these precautions need not be
either expensive or complicated.
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?
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 ESD Floor Mat. If you wear an ESD heel 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. ESD Heel 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.
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.
on both feet=lower charge=reduced risk to susceptible devices.
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?
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
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 Anti static 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.
are three solutions to this problem. First, you can obtain a tester
that is specifically designed for testing shoes. Second, you can obtain
an ESD 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. 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 groudners, and ESD Shoes.
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
do manufacturing facilities for munitions or explosives use conductive
materials instead of static dissipative materials. Why do they use ESD wrist
straps without a 1-megohm resistor?
are several reasons that explosive environments use more conductive
materials than static dissipative.
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.
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.
reducing the risk of fire or explosion is the main reason for ESD control,
the theoretical faster discharge rates through conductive materials
or through anti static 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.
the electronics environment, ESD controls are used to protect components.
Our concerns for personnel safety are in the area of potential shock
can I find information about static control for high voltage areas?
(For example, areas in which electrical voltages exceed 250V)
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).
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.
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?
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.
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.
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.
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. 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 our grounding accessories to help you setup a proper ESD safe work area.
heard that layered static control mats could have an alternate path
to ground. What does that mean? Is this a problem?
floor mats are generally composed of ESD Two Layer mats also known as ESD Rubber Mats or three layers of material such as our ESD Vinyl Mat or ESD Three Layer Mat,
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.
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 and
that contact is grounded, then an alternate path to ground is created,
bypassing the resistor in the common point ground cord (ESDCGC) 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.
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.
can be avoided by knowing the physical structure of the mat, installing
it properly, taking care in its use, and disposing of damaged mats.
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.
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,
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.
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 ESD control procedures other than grounding to control the
problem. Ionization, for example, may be a better alternative in your
I need to take any ESD precautions when I service or repair products
in the field?
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 Cord, protective ESD Mats, and even placing products in ESD protective packaging such as ESD Static Shielding Bags. These
items should be part of your ESD field service kit.
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.
need to be adaptable to the environments in which you are working. For
example, in addition to your anti static wrist strap, carry extra alligator clips,
jacks, and extension cords.
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.
good ESD protection procedures at all times. Your responsibility is
to help solve your customers' problems, not contribute to them.
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?
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.
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.
charges can be generated by moving conveyors, rotating shafts, sliding
parts, pick and place arms, and even flowing air.
the problem requires a number of methods. These include the following:
Eliminating as many static generating materials from the process as
Properly grounding the equipment, especially the metal parts that are
close proximity to the electrostatic discharge sensitive (ESDS) products
Shielding of static generative materials, such as glass window panes,
gaskets, and thermal insulating spacers that are in proximity to ESDS
Using antistatic solutions where possible.
Providing localized air ionization to neutralize charges on insulative
surfaces such as device packages.
Frequent monitoring of the process to assure that the procedures are
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
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?
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. Please take a look at the ESD labels we offer as a possible solution.
ESD Association Standard ESD S8.1 -- ESD Awareness Symbols provides
two symbols for ESD identification.
1--ESD Susceptibility Symbol
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."
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.
2-- ESD Protective Symbol
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.
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.
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.
cords for ESD table mats and ESD floor mats are available with or without a
1-megohm resistor. Is there any standard recommending when to use or
not use the resistor?
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
6.1 - Grounding recommends the following regarding worksurfaces
and floor mats:
provisions are not made and not required for this resistor, a direct
connection is fully acceptable and recommended.
may be made to include a resistor where it may be required for a purpose
other than ESD.
worksurfaces such as stainless steel shall be hard ground connected.
fault circuit interrupters (GFCI) and other safety protection should
be considered wherever personnel may come into contact with electrical
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?
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.
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
When not in use, store test cards in static protective packaging such as ESD Bags 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.
Observe all other procedures required in your facility regarding the
handling of sensitive circuit boards.
three simple procedures will help assure that ESD is controlled, reduce
contaminants from test cards and assure reliable service.
ground is better for ESD purposes, a direct earth ground or an electrical
Association Standard ANSI EOS/ESD 6.1-Grounding recommends
a two step procedure for grounding ESD protective equipment.
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
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 ESD 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.
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
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.
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.
additional information, see:
Worksurfaces - Resistance Measurements, ESD Association
Grounding - Recommended Practice, ESD Association
70, National Electrical Code, National Fire Protection Association.
625, Requirements for Handling Electrostatic Discharge Devices,
Electronic Industries Alliance
Electrostatic Discharge Control Handbook, ESD Association
ESD Protective Workstations, ESD Association
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.
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.
relative humidity above 60% can lead to corrosion on product, tool and
equipment problems or personnel discomfort.
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.
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.
helps, but doesn't substitute for other ESD control procedures.
are just getting started. How often should I audit my facility?
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 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.
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.
addition, you may need to adjust the frequency depending upon the critical
nature of the functions performed in specific areas.
you explain the 3 ESD failure modes in simple terms? Give some examples.
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.
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.
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.
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.
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.
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.
additional information on these various failure models, consult the
R. G., "Mechanisms of Charged Device Electrostatic Discharges," EOS/ESD Symposium Proceedings, 1991
L. R., "Charged Device Model Testing: Trying to Duplicate Reality," EOS/ESD Symposium Proceedings, 1987
L. R.., "Beyond MIL HBM Testing - How to Evaluate the Real Capability
of Protection Structures," EOS/ESD Symposium Proceedings,
S5.1-1998--Electrostatic Discharge Sensitivity Testing - Human Body
Model, ESD Association
ESD S5.2-1994--Electrostatic Discharge Sensitivity Testing - Machine
Model, ESD Association
DS5.3-1993--Electrostatic Discharge Sensitivity Testing - Charged Device
Model, ESD Association
ADV-2.0-1994--ESD Handbook, ESD Association
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?
general, conductive floor mats perform best when personnel are wearing
special footwear, either static control shoes, ESD heel grounders or foot straps. The combination
of esd floor mats such as ESD anti fatigue floor mats or ESD PVC floor runner 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.
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.
shoes sometimes work on ESD personal testers. How do you explain this
to a non-ESD auditor?
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.
explanation for the auditor is that ordinary footwear is highly variable
and cannot be depended upon for static control purposes.
experts feel the inside layer of a static 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?
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 volume resistivity measurements help define the bag’s ability
to provide charge dissipation or electrostatic shielding.
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.
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.
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.
additional information, see:
1.0 1994, Glossary, ESD Association
S11.11-1994, Surface Resistance Measurement of Static Dissipative Planar
Materials, ESD Association
Packaging of Electronic Products for Shipment, Electronic Industries
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?
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.
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
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.
1-Typical ESD Workstation
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.
grounding devices, such as anti static 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.
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.
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 such as ESD table mats or ESD floor 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 be mobile,
then static protective flooring and footwear may be needed.
about grounding of workstations?
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.
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 is already
connected to this ground, connecting the ESD protective materials to
the same ground brings all components of the workstation to the same
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 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.
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
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
Grounding - Recommended Practice, ESD Association
Floor Materials - Resistive Characterization , ESD Association
Symbols - ESD Awareness, ESD Association
70, National Electrical Code, National Fire Protection Association
625, Requirements for Handling Electrostatic Discharge Devices,
Electronic Industries Alliance, 1994
TR 20.20: ESD Handbook, ESD Association
ESD Protective Workstations, ESD Association
Evaluation, Acceptance and Functional Testing of Wrist Straps,
Worksurfaces - Resistance Measurements, ESD Association
Seating - Resistive Characterization, ESD Association
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
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.
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
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.
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.
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
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.
Electrostatic Discharge Control Handbook, ESD Association.
Electrostatic Discharge Sensitivity Testing-Charged Device Model,
Define ESD Control," John H. Mayer, Test and Measurement World,
and Automated Processes," Wayne Tan and Michael T. Brandt, Circuits
Assembly, August 1995.
and Qualifying Automated Test Handlers in a Semiconductor Company,"
Leng-Leng Ow and Wayne Tan, EOS/ESD Symposium Proceedings,
Combined Socketed and Non-Socketed CDM Test Approach for Eliminating
Real-World CDM Failures," Andrew Olney, EOS/ESD Symposium Proceedings,
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?
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.
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.
may need to implement several of these steps to provide adequate static
you are using a roll of esd table mat or esd floor mat material that is 10 to 100
feet long, is one ground connection sufficient?
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.
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.
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
floor coverings, such as vinyl, rubber, or polymerics, usually require
multiple connections as well.
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
I have seen some ESD 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
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.
the grounders are work on both feet, and both feet are often in contact
with the esd floor mat 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.
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.
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?
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.
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
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.
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.
are we experiencing electrostatic charges on metal fixtures and equipment
in our facility? I thought that conductive materials did not charge.
is not unusual for conductive materials like metals to become electrostatically
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.
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"
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.
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
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.
is the best method to ground painted metal shelving and cabinets?
question seems pretty straight forward with an obvious answer. However,
as is often the case in ESD, the obvious sometimes covers up hidden
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
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.
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.
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.
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
does the resistance of the esd floor mat measure high?
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.
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.
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.
be sure that the floor surface is free of contaminants. Dirt and standard
floor finishes can create an insulating layer between the floor and
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.
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
if you still have problems, call the manufacturer for help.
experiencing high static voltage levels on the carts in our plant. We
have a static control floor. What's going on?
problem may have several different causes.
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.
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.
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.
alternative to wheels and casters is the use of ground straps or ground
chains that attach to the cart and then drag on the esd floor mat. 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.
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.
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?
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.
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.
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.
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?
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.
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.
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
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.
is the resistance range for conductive, static dissipative, electrostatic
shielding, insulative, and anti-static materials?
ESD purposes, many materials are classified by their resistance or resistivity
characteristics. These definitions are found in ESD Association or EIA
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.
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.
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
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.
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 resistivity or resistance.(1)
Surface Resistance Measurement of Static Dissipative Planar Materials
Volume Resistance Measurement of Static Dissipative Planar Materials
Packaging of Electronic Products for Shipment.
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?
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."
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
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
sleeve to sleeve resistance results can occur when measuring static
control jackets or static control smocks. 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
some additional tests on the ESD 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.
types of instruments do I need to audit and evaluate my ESD control
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.
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
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 the instrument.
Be sure you know what the actual applied voltage of the instrument is.
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.
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.
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
In selecting your instrumentation, remember, you want the right tool
packaging sensitive devices in ESD protective bags, is it necessary
to use special ESD labels to close the bag? Can I print my own stickers?
Do I have to use special paper?
required by agreement with another company, distributor, or end user,
the choice of label or sticker used to close the esd 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
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
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.
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.
Wear a grounded ESD wrist strap when peeling the label from its backing.
This helps to drain charge from the esd label before it is applied to the
static shielding bag.
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.
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.
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.
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
gauge wire is required for grounding workbenches. It is not mentioned
in ESD S6.1?
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.
is the difference between EOS and ESD?
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)
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.
2) ESD-ADV1.0, 1994, Glossary, ESD Association,
can I find the ESD sensitivity information for specific microhybrid
electronic components, (i.e.; ICs, Resistors, Capacitors, CMOS ICs,
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.
do you decide what is the proper packaging for sensitive products: conductive,
dissipative, or antistatic?
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.
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.
(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.
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.
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.
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.
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.
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.
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
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.