Anti-Static Wrist Strap

[Moderator]

Static Electricity: Creating Charge

Static electricity is defined as an electrical charge caused by an
imbalance of electrons on the surface of a material. This imbalance of
electrons produces an electric field that can be measured and that can
influence other objects at a distance.

Electrostatic discharge is defined as the transfer of charge between
bodies at different electrical potentials.

Electrostatic discharge can change the electrical characteristics of a
semiconductor device, degrading or destroying it. Electrostatic
discharge also may upset the normal operation of an electronic system,
causing equipment malfunction or failure. Another problem caused by
static electricity occurs in clean rooms. Charged surfaces can attract
and hold contaminants, making removal from the environment difficult.
When attracted to the surface of a silicon wafer or a device's
electrical circuitry, these particulates can cause random wafer
defects and reduce product yields.

Controlling electrostatic discharge begins with understanding how
electrostatic charge occurs in the first place. Electrostatic charge
is most commonly created by the contact and separation of two
materials. For example, a person walking across the floor generates
static electricity as shoe soles contact and then separate from the
floor surface. An electronic device sliding into or out of a bag,
magazine or tube generates an electrostatic charge as the device's
housing and metal leads make multiple contacts and separations with
the surface of the container. While the magnitude of electrostatic
charge may be different in these examples, static electricity is
indeed generated.

The age of electronics brought with it new problems associated with
static electricity and electrostatic discharge. And, as electronic
devices became faster and smaller, their sensitivity to ESD increased.
Today, ESD impacts productivity and product reliability in virtually
every aspect of today's electronics environment. Many aspects of
electrostatic control in the electronics industry also apply in other
industries such as clean room applications and graphic arts.

Despite a great deal of effort during the past decade, ESD still
affects production yields, manufacturing costs, product quality,
product reliability, and profitability. Industry experts have
estimated average product losses due to static to range from 12-35%.
Others estimate the actual cost of ESD damage to the electronics
industry as running into the billions of Rupees annually. The cost of
damaged devices themselves ranges from only a few rupees for a simple
diode to several thousand rupees for complex hybrids. When associated
costs of repair and rework, shipping, labor, and overhead are
included, clearly the opportunities exist for significant
improvements.

Perhaps the greatest threat facing system builders is electrostatic
discharge (ESD). ESD is the sudden discharge of static electricity
causing a momentary current flow that can weaken or permanently damage
semiconductor components (such as the processor, memory, motherboard,
and video card). Therefore it is imperative that the industry
(Manufacturers, Integrators, Dealers, service centers, end-customers)
follow the correct ESD storage and handling procedures.

Electrostatic Discharge (ESD) can damage a sensitive electronic
component, resulting in failures, reduced reliability and increased
rework costs, or latent component failures in equipment.

Namaguchi & Uchida (ESD Symposium 1998) found that 60-90% of defective
devices were damaged by ESD, and 70% of these failures were attributed
to damage by ungrounded people.


ESD Damage—How Devices Fail

Electrostatic damage to electronic devices can occur at any point from
manufacture to field service. Damage results from handling the devices
in uncontrolled surroundings or when poor ESD control practices are
used. Generally damage is classified as either a catastrophic failure
or a latent defect.


Catastrophic Failure

When an electronic device is exposed to an ESD event, it may no longer
function. The ESD event may have caused a metal melt, junction
breakdown, or oxide failure. The device's circuitry is permanently
damaged causing the device fail. Such failures usually can be detected
when the device is tested before shipment. If the ESD event occurs
after test, the damage will go undetected until the device fails in
operation.


Latent Defect

A latent defect, on the other hand, is more difficult to identify. A
device that is exposed to an ESD event may be partially degraded, yet
continue to perform its intended function. However, the operating life
of the device may be reduced dramatically. A product or system
incorporating devices with latent defects may experience premature
failure after the user places them in service. Such failures are
usually costly to repair and in some applications may create personnel
hazards.
It is relatively easy with the proper equipment to confirm that a
device has experienced catastrophic failure. Basic performance tests
will substantiate device damage. However, latent defects are extremely
difficult to prove or detect using current technology, especially
after the device is assembled into a finished product.


Basic ESD Events--What Causes Electronic Devices to Fail?

ESD damage is usually caused by one of three events: direct
electrostatic discharge to the device, electrostatic discharge from
the device or field-induced discharges. Damage to an ESDS device by
the ESD event is determined by the device's ability to dissipate the
energy of the discharge or withstand the voltage levels involved. This
is known as the device's "ESD sensitivity.


Discharge to the Device

An ESD event can occur when any charged conductor (including the human
body) discharges to an ESDS (electrostatic discharge sensitive)
device. The most common cause of electrostatic damage is the direct
transfer of electrostatic charge from the human body or a charged
material to the electrostatic discharge sensitive (ESDS) device. When
one walks across a floor, an electrostatic charge accumulates on the
body. Simple contact of a finger to the leads of an ESDS device or
assembly allows the body to discharge, possibly causing device damage.
The model used to simulate this event is the Human Body Model (HBM). A
similar discharge can occur from a charged conductive object, such as
a metallic tool or fixture. The model used to characterize this event
is known as the Machine Model.


Discharge from the Device

The transfer of charge from an ESDS device is also an ESD event.
Static charge may accumulate on the ESDS device itself through
handling or contact with packaging materials, worksurfaces, or machine
surfaces. This frequently occurs when a device moves across a surface
or vibrates in a package. The model used to simulate the transfer of
charge from an ESDS device is referred to as the Charged Device Model
(CDM). The capacitance and energies involved are different from those
of a discharge to the ESDS device. In some cases, a CDM event can be
more destructive than the HBM for some devices.
The trend towards automated assembly would seem to solve the problems
of HBM ESD events. However, it has been shown that components may be
more sensitive to damage when assembled by automated equipment. A
device may become charged, for example, from sliding down the feeder.
If it then contacts the insertion head or another conductive surface,
a rapid discharge occurs from the device to the metal object.


Field Induced Discharges

Another event that can directly or indirectly damage devices is termed
Field Induction. As noted earlier, whenever any object becomes
electrostatically charged, there is an electrostatic field associated
with that charge. If an ESDS device is placed in that electrostatic
field, a charge may be induced on the device. If the device is then
momentarily grounded while within the electrostatic field, a transfer
of charge from the device occurs as a CDM event. If the device is
removed from the region of the electrostatic field and grounded again,
a second CDM event will occur as charge (of opposite polarity from the
first event) is transferred from the device.


How Much Static Protection is Needed?

As noted earlier, damage to an ESDS device by the ESD event is
determined by the device's ability to dissipate the energy of the
discharge or withstand the voltage levels involved—its ESD
sensitivity. Defining the ESD sensitivity of electronic components is
the first step in determining the degree of ESD protection required.
Test procedures based on the models of ESD events help define the
sensitivity of components to ESD. These procedures will be covered in
a future article in this series.
Many electronic components are susceptible to ESD damage at relatively
low voltage levels. Many are susceptible at less than 100 volts, and
many disk drive components have sensitivities below 10 volts. Current
trends in product design and development pack more circuitry onto
these miniature devices, further increasing their sensitivity to ESD
and making the potential problem even more acute.


Basic Principles of Static Control

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

1. Design In Immunity: by designing products and assemblies to be as
immune as reasonable from the effects of ESD
2. Define the level of control needed in your environment: needed in
your environment.
3. Identify and define the electrostatic protected areas (EPA): the
areas in which you will be handling sensitive parts.
4. Eliminate and Reduce Generation: by reducing and eliminating static
generating processes, keeping processes and materials at the same
electrostatic potential, and by providing appropriate ground paths to
reduce charge generation and accumulation.
5. Dissipate and Neutralize: by grounding, ionization, and the use of
conductive and dissipative static control materials.
6. Protect Products: with proper grounding or shunting and the use of
static control packaging and materials handling products.


Typical Facility Areas Requiring ESD Protection:
• Receiving
• Inspection
• Stores and warehouses
• Assembly
• Test and inspection
• Research and development
• Packaging
• Field service repair
• Offices and laboratories
• Clean rooms

Grounding

Throughout this article, we have seen how important grounding is to
effective ESD control. Effective ESD grounds are of critical
importance in any operation, and ESD grounding should be clearly
defined and regularly evaluated.
A primary means of protecting of ESD susceptible (ESDS) items is to
provide a ground path to bring ESD protective materials and personnel
to the same electrical potential. All conductors in the environment,
including personnel, must be bonded or electrically connected and
attached to a known ground or contrived ground, creating an
equipotential balance between all items and personnel. Electrostatic
protection can be maintained at a potential above a "zero" voltage
ground reference as long as all items in the system are at the same
potential. It is important to note that non-conductors in an
Electrostatic Protected Area (EPA) cannot lose their electrostatic
charge by attachment to ground.

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

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

The second step is to connect the common point ground to the equipment
ground or the third wire (green) electrical ground connection. This is
the preferred ground connection because all electrical equipment at
the workstation is already connected to this ground.

Connecting the ESD control materials or equipment to the equipment
ground brings all components of the workstation to the same electrical
potential. If a soldering iron used to repair an ESDS item were
connected to the electrical ground and the surface containing the ESDS
item were connected to an auxiliary ground, a difference in electrical
potential could exist between the iron and the ESDS item. This
difference in potential could cause damage to the item.


Controlling Static on Personnel and Moving Equipment

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

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


Wrist Straps

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

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


Other methods of static control:
• Floors, Floor Mats, Floor Finishes
• Shoes, Grounders, Casters
• Clothing
• Workstations and Worksurfaces
• Production Equipment and Production Aids
• Packaging and Handling
• Ionization in Clean rooms: Ionizers are used when it is not possible
to properly ground everything and as backup to other static control
methods. In clean rooms, air ionization may be one of the few methods
of static control available.


Your static control program is up and running. How do you determine
whether it is effective? How do you make sure your employees follow
it? We will focus on two more critical elements:

Training and Auditing:

Training:

The new ANSI/ESD S20.20 ESD Control Program standard cites training
as a basic administrative requirement of an ESD control program. There
is significant evidence to support the contribution of training to the
success of the program. We would not send employees to the factory
floor without the proper soldering skills or the knowledge to operate
the automated insertion equipment. We should provide them with the
same skill level regarding ESD control procedures.

Elements of Effective Training Programs:

1 -- Successful training programs cover all affected employees.
2 -- Effective training is comprehensive and consistent.
3 -- Use a variety of training tools and techniques.
4 -- Test, certify and retrain
5 -- Feedback, auditing, and measurement


Auditing

Developing and implementing an ESD control program itself is obvious.
What might not be so obvious is the need to continually review, audit,
analyze, feedback and improve. You will be asked to continually
identify the program's return on investment and to justify the savings
realized. Technological changes will dictate improvements and
modifications. Feedback to employees and top management is essential.
Management commitment will need reinforcement.
Like training, regular auditing becomes a key factor in the successful
management of ESD control programs. The mere presence of the auditing
process spurs compliance with program procedures. It helps strengthen
management's commitment. Audit reports trigger corrective action and
help foster continuous improvement.

The benefits to be gained from regular auditing of our ESD control
procedures are numerous:

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


Requirements for Effective Auditing:
• Existence of written and well-defined standards and procedures.
• Taking of some measurements.
• Include all areas in which ESD control is required.
• Audit frequently and regularly.
• Maintain trend charts and detailed records and prepare reports.
• Implement corrective action


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for more information on ESD and an effective static control program
for your organization, contact:

Khalid Hasan
Sam Communications
16-8-238/6/2
Ashraf Nagar
New Malakpet
Hyderabad-500024
INDIA
Tel #: 0939-135-6736
email: memory...@gmail.com