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 Complete facility evaluation
identifying ESD problems with a detailed, written report.
ESD surveys:
We divide surveys into hardware and systems.
Hardware surveys review the materials, controls devices, test equipment
and specifications, System surveys review : training, compliance, checks
and supplier control. Obviously a combined survey gives the best feedback
on the "goodness" of the total process.
The EIA-625 standard recommends a full annual check of the ESD system,
with daily, weekly, monthly and quarterly checks on high-risk areas. The
standard also outlines a schedule for reducing the number of checks based
on a continuos period of zero failures.
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Hardware
survey:
The ESDA test methods/advisors provide details of test equipment, processes
and limits.
It is critical that surveyors are trained as ESD test results are easily
miss-interpreted.
The following areas are surveyed:
- Raw material packaging.
- Signage/labels. Grounding.
- Flooring/mats. Work surfaces.
- Wrist straps.
- Footwear/heel straps.
- Seating.
- Ionization.
- Mobile equipment.
- Continuos monitors.
- Material handling,
- Packaging, totes,
- trolleys, shelving.
- AC power tools.
- Battery/pneumatic tools.
- Garments
- Shipping packaging.
- ESD test equipment.
- Specifications.
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System
survey:
The following areas are reviewed:
- Training at all levels.
- Retraining process.
- Compliance to process and procedures.
- Daily/weekly/monthly/quarterly checks.
- Supplier control.
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Developing an effective
ESD programme:
:With this basic understanding of ESD and its impact on your environment,
you can then begin to develop an effective ESD control programme. The
first Principle is to design products and assemblies to be as immune as
reasonable from the effects of ESD. This involves such steps as using
less static sensitive devices or providing appropriate input protection
on devices, boards, assemblies, and equipment. The second principle is
to eliminate as many static generating processes or materials, such as
the contact and separation of dissimilar materials and common plastics,
as possible from the work environment. Because we simply can't eliminate
all generation of static in the environment, our third Principle is to
safely dissipate or neutralize those electrostatic charges that do occur.
For some objects, such as common plastics and other insulators, grounding
does not remove an electrostatic charge because there is no conductive
pathway, so naturalizing using an ionizer is the only feasible alternative
. One way is to provide parts and assemblies with proper grounding or
shunting that will "dissipate" any discharge away from the product.
Based
on the excellent "Basics of Electrostatic Discharge" by the ESD Association
Rome, NY at www.esda.org
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Elements
of an affective ESD control programme.
Establishing a team and coordinator is critical to success as ESD cross
various departments and functions. Team composition includes line employees
as well as department heads or other management personnel. ESD teams or
committees help assure a variety of viewpoints, the availability of the
needed expertise, and commitment to success. The ESD coordinator is responsible
for developing, budgeting, and administering the programmeme.
Before seeking solutions to your problems, you will need to determine
the extent of your losses to ESD. These losses may be reflected in receiving
reports, QA and QC records, customer returns, in-plant yields, failure
analysis reports, and other data that you may already have or that you
need to gather. Armed with your loss and sensitivity data, you can evaluate
your facility, looking for areas and procedures that may be contributing
to your defined ESD problems. Be on the lookout for things such as static
generating materials and personnel handling procedures for ESD-sensitive
items. Then conduct a thorough facility survey or audit. Identify ESD
sensitive items and evaluate their sensitivity. Once you understand your
environment and the impact that ESD has on your products and services,
you then build justification for your ESD control programmeme.
To be successful, an ESD programme requires the support of your top management,
at the highest level possible. Develop and implement the appropriate procedures
that will solve the ESD problems you have identified. Train and retrain
your personnel in ESD and in your company's control procedures. Finally
you need to continually review, audit, analyze, feedback and improve.
Based
on the excellent "Basics of Electrostatic Discharge" by the ESD Association
Rome, NY at www.esda.org
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ESD
control procedures and materials.
Often you will find that there are more areas that require protection
than you originally thought, usually wherever ESDS devices are handled.
For this reason, static control programmemes 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 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. The combination of floor
materials and footwear provides a ground path for the dissipation of electrostatic
charge, thus reducing the charge accumulation on personnel and other objects
to safe levels.
Clothing is a consideration in some ESD protective areas, especially
in clean rooms and very dry environments. Grounded static control garments
are intended to minimize the effects of electrostatic fields or charges
that may be present on a person's clothing. Worksurfaces and workstations
are important parts of an ESD protective programmeme. An ESD protective
workstation refers to the work area of a single individual that is constructed
and equipped with materials and equipment to limit damage to ESD sensitive
items. Static protective worksurfaces with a resistance to ground of 10E6
to 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.
Production equipment and production aids can cause ESD problems. A device
can be charged sliding down a feeder, if the device contacts a grounded
test head a rapid discharge occurs -- a Charged Device Model (CDM) events.
Tools are grounded through the green wire of a 3 pin plug. Where insulators
can be charged, air ionization can neutralize the static charge on insulated
and isolated objects by charging the molecules of the gases of the surrounding
air.
The main ESD function of packaging is to limit the possible impact from
triboelectric charge generation, direct discharge and electrostatic fields.
The first step in grounding is to connect all the components of the work
area (work surfaces,people, equipment etc. ) to the same electrical point
called a common point ground. This ground is then common point ground
is connected to the electrical ground connection on the green wire to
ensure all points are at the same electrical potential. ESD Association
Standard ANSI ESD S8.1-1993 - ESD Awareness Symbols provides two symbols
for ESD identification. The ESD Susceptibility Symbol (Figure 3), consists
of a triangle, a reaching hand, and a slash through the reaching hand.
Based
on the excellent "Basics of Electrostatic Discharge" by the ESD Association
Rome, NY at www.esda.org
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Training
When was the last time your employees attended an ESD training programmeme?
We would not send employees to the factory floor without the proper soldering
skills or the knowledge to operate the automated insertion equipment.
Although individual requirements cause training programmemes to vary from
company to company, there are several common threads that run through
the successful programmemes. All line are trained but we also include
department heads, upper management, and executive personnel in the process.
Typically they are responsible for the day-to-day supervision and administration
of the programmeme or they provide leadership and support. Training not
only covers specific procedures, but also the physics of the problem and
the benefits of the programmeme as well.
Consistent content across various groups, plants, and even countries
(adjusted for cultural differences, of course) reduces confusion and helps
assure conformance. Effective training uses a variety of processes, live
instruction CD-ROM programmemes and employee involvement. Bulletin boards,
newsletters, and posters provide additional reminders and reinforcement.
Testing , certifying and retraining employees assure retention and build
employee pride in the process. Sharing yield or productivity data with
employees demonstrates the effectiveness of the programme and of their
efforts. Tracking these same numbers can indicate that it's time for retraining
or whether modifications are required in the training programmeme. A training
programmeme that is built on identifiable and measurable performance goals
helps assure employee understanding, implementation and success.
Based
on the excellent "Basics of Electrostatic Discharge" by the ESD Association
Rome, NY at www.esda.org
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Auditing.
There is a need to continually review, audit, analyze, feedback and improve.
Management commitment will need reinforcement. Like training, regular
auditing becomes a key ingredient in the successful management of ESD
control programmemes. Audit reports trigger corrective action and help
foster continuous improvement. Audit reports trigger corrective action
and help foster continuous improvement. Typically, we think of an ESD
audit as a periodic review and inspection of the ESD work area covering
use of the correct packaging materials, wearing of wrist straps, following
defined procedures, and similar items. programmeme management audits measure
how well a programmeme is managed and how strong the management commitment
is.
Quality process checking applies classical statistical quality control
procedures to the ESD process and is performed by operations personnel.
Visual and electrical checks of the procedures and materials, wrist strap
testing for example, are used to monitor the quality of the ESD control
process. Work area audits verify that programmeme procedures are followed
and that ESD control materials and equipment are within specification
or are functioning properly. Special instrumentation will be required
to conduct work area audits. However, as a minimum, you will need an electrostatic
field meter, a wide range resistance meter, a ground/circuit tester, and
appropriate electrodes and accessories. Additional instrumentation might
include a charge plate monitor, footwear and wrist strap testers, chart
recorders and timing devices, discharge simulators, and ESD event detectors.
If you are making measurements according to specific standards, be sure
the instrumentation meets the specifications of these standards.
Based
on the excellent "Basics of Electrostatic Discharge" by the ESD Association
Rome, NY at www.esda.org
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Device Sensitivety and testing.
Damage to an ESDS device by the ESD event is determined by the device's
ability to dissipate the energy of the discharge or withstand the current
levels involved. This is known as device "ESD sensitivity" or "ESD susceptibility".
Some devices may be more readily damaged by discharges occurring within
automated equipment, while others may be more prone to damage from handling
by personnel. These test procedures are based on the three primary models
of ESD events: Human Body Model (HBM), Machine Model (MM), and Charged
Device Model (CDM). These models have been proven to be successful in
reproducing over 95% of all ESD field failure signatures.
One of the most common causes of electrostatic damage is the direct transfer
of electrostatic charge through a significant series resistor from the
human body or from a charged material to the electrostatic discharge sensitive
(ESDS) device. When one walks across a floor, an electrostatic charge
accumulates on the body. Simple contact of a finger to the leads of an
ESDS device or assembly allows the body to discharge, possibly causing
device damage. The model used to simulate this event is the Human Body
Model (HBM). Originating in Japan as the result of trying to create a
worst-case HBM event, the model is known as the Machine Model. As a worst-case
human body model, the Machine Model may be over severe. However, there
are real-world situations that this model represents, for example the
rapid discharge from a charged board assembly or from the charged cables
of an automatic tester. A device may become charged, for example, from
sliding down the feeder in an automated assembler. If it then contacts
the insertion head or another conductive surface, a rapid discharge may
occur from the device to the metal object. This event is known as the
Charged Device Model (CDM) event, and can be more destructive than the
HBM for some devices. Although the duration of the discharge is very short,
often less than one nanosecond, the peak current can reach several tens
of amperes.These classification systems have a number of advantages.
Based
on the excellent "Basics of Electrostatic Discharge" by the ESD Association
Rome, NY at www.esda.org
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Why is ESD a problem in electronic
manufacturing?
When static electricity is discharged a short burst of very high current
flows. This high current can damage integrated circuits (IC's). IC's can
be fatally or partially damaged. Damaged parts can pass tests but fail
later. It is estimated that between 10% and 30% of all electrical failures
are caused by ESD.
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IC Technology trends.
A SEMATECH benchmarking study of integrated circuit suppliers indicated
that ESD will be one of the top three reliability concerns for at lest
the next five years. Increasing pad count. Higher density circuits Lower
supply voltages Repair and replacement costs: Cost to find, repair and
replace parts is increasing with device complexity. Every part lost to
ESD is more expensive as device complexity increases. Devices are more:
susceptible to ESD, expensive, difficult to find and replace.
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Standards.
There are a number of ESD standards. We have chosen three, which we think
are the basis for a successful ESD programme.
ANSI/ESDA S20.20 1999 Development of an Electrostatic Discharge
Control programmeme. Free download available from www.esda.org How to
establish, implement and maintain an ESD control programmeme for 100V
systems. Provides a reference to detailed test methods for each ESD control
process. Allows tailoring for different applications. Can be certified
by external auditors like ISO 9000.
ANSI EIA- 625-1994 Requirements for handling ESDS devices. Free
download available from www.jedec.org Requirements for ESD control for
200V systems. Audit frequency table defines weekly, monthly, quarterly
and annual checks. Provides detailed list of audit questions. Protection
of electrostatic sensitive devices.
CECC EN 100 015-1 1994.
Part 1 General requirements. CECC EN 100 015-2 1994.
Part 2 Requirements for low Humidity conditions (Basic specification)
CECC EN 100 015-3 1994.
Part 3 Requirements for clean room areas (Basic specification) CECC
EN 100 015-4 1994.
Part 4 Requirements for high voltage requirements (Basic specification)
Available from NSAI, Dublin 9, Ireland. www.nsai.ie Tel: 353 (0)1 807
3800. Fax: 353 (0) 1 807 3838. Requirements for ESD controls in Europe.
Note: Test parameters differ from S20.20.
S20.20. This standard requires a plan, a training programme and
a verification process.
The plan defines the scope, activities, tasks, procedures and owners
within the organisation. The most critical area here is the owner. When
standards are not clear, owners are understandably hard to find. When
ownership is not clear, programmes tend to fail.
It is critical that the ESD sensitivity of parts, assemblies and equipment
and their required protection levels are defined. A programme for MR heads
with sensitivity of five volts is radically different from a 100 Volt
programmeme. Simply having this information can provide an insight into
why some parts failed. Device sensitivity information is normally available
from manufacturers.
The training programme is required to define initial and refresher periods
and methods. An objective evaluation must be carried to ensure training
is effective. If people understand the risk to product quality and reliability
from ESD, the level of compliance increases.
The verification process is more complex. Unlike a standard quality audit,
you take electrical measurements to check compliance to the standard.
The measurements require inexpensive ESD audit test equipment. You can
integrate the S20.20 requirements into the document control system. This
allows the option of certification by external auditors.
The technical requirements are defined for the following areas:
- grounding/bonding, personal grounding,
- protected areas,
- packaging,
- marking,
- equipment and handling.
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About
Gannon & Associates ltd.
Gannon & Associates Ltd. is based in the West of Ireland. Mel Gannon
is a NARTE Qualified ESD Engineer with over 20 years electronic industry
experience. He has worked for major electronic multinationals in both
Europe and in the U.S. as an Engineer, Supervisor, Manager and Director.
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