|
Information
contained in this application note is from the extensive experience of Plug-in Storage® Systems, Inc. and other sources believed reliable. While every precaution has been taken in
the preparation of this ESD article and our ESD Guidelines, the publisher and authors assume no responsibility for errors or omissions. Plug-in Storage® Systems, Inc. does not
guarantee the accuracy, adequacy, or completeness of any information and is not responsible for any results obtained from use of this information. ©2000 Plug-in Storage®
Systems, Inc. All rights reserved.
Overview
Static electricity has always existed in the telecommunications workplace. It has become a major concern in the industry only since the
incorporation of static-sensitive integrated circuits and other microelectronic components into telephone switching and transmission systems. Technology advances dramatically increase
component density on a circuit chip and significantly decrease power requirements for circuit operation. Therefore these components and the printed circuit boards that house them became
increasingly susceptible to impairment or destruction from the effects of electrostatic discharge (ESD).
Past static control procedures and ESD guidelines do not provide effective protection for more sophisticated and vulnerable electronics in
widespread use today. With increasing static sensitivity and with significantly increasing individual circuit board costs, a more consistent and higher level of static control is
required. Doing otherwise risks increased degradation in customer service, higher maintenance and repair costs, and excessive capital investment in circuit board inventory.
This brief set of ESD guidelines provides a common understanding of: static electricity, the basis and magnitude of static charge generation,
electrostatic discharge and its impact, and the rules for effective static control in the workplace. Every work group handling circuit boards should insure that they have a defined static
control program in place. A static control program should provide for: ongoing employee awareness, full protection of both in-service and spare circuit boards and for periodic evaluation
of the effectiveness of the local program itself.
What Is Static Electricity?
Static electricity is a stationary charge of electricity resting on the object's surface. The location of this positive or
negative charge on the object's surface generally depends on conductivity of the surface material. Static charges on non-conductive surfaces (polystyrene foam, rubber, plastic, etc.) are
generally localized. Charges on conductive surface (ungrounded metal, human skin, etc.) are generally evenly distributed across the surface.
How And When Are Static Charges Generated?
A static charge is caused by the transfer of electrons between two objects. The charge is generated either by contact and
separation of objects (known as direct charging by friction or triboelectric charging) or by passing one object through a charged electrostatic field surrounding another object (inductive
charging). In both cases, objects may acquire and retain a static charge on their surfaces ranging from 5 to more than 50,000 electrostatic volts.
Generally, non-conductive materials are more prone to charging by friction. Conductive materials are more likely to be charged by induction. For
example, friction between a carpet and a shoe generates a charge on and an electrostatic field surrounding, the shoe sole as you walk across the carpeted floor. The charge is then immediately passed by induction to the conductive moisture layer on your skin surface. As described below, people are the primary cause of static charging in the workplace, and non-conductive materials are the prime source of most static charges.
Many materials in today's world generate static charges through the heat of friction with another material, and some much more so than others. Table
A, "Triboelectric Series," indicates a variety of materials and the type of relative degree of static charge they will generate by friction. Generally, the farther apart materials are
located in the table, the greater the magnitude of the charge that may be generated. A material acquires a positive charge with respect to any material below it on the scale.
Table A — Triboelectric Series
|
Materials
|
Polarity (+ or -)
|
|
Human Hands, Asbestos, Acetate, Glass, Human Hair, Nylon, Wool, Fur, Lead, Silk, Aluminum, Paper, Cotton, Wood,
Steel,
|
Acquires a more
positive charge
|
|
Sealing Wax, Hard Rubber, Acetate Fiber, Epoxy Glass, Nickel, Copper, Silver, Brass, Gold, Platinum, Stainless Steel, Synthetic
Rubber, Acrylic, Polystyrene Foam, Polyurethane Foam, Polyester, Orlon, Polyethylene, Polypropylene, PVC (Vinyl), Silicon, Teflon
|
Acquires a more
negative charge |
Table B
shows "Typical Charge Generators" in the workplace. The table includes many common items that are encountered with
each day, such as synthetic clothing, vinyl floors and chair coverings. This listing demonstrates that static electric generators are all around us.
Table B — Typical Charge Generators
|
Items
|
Type
|
|
Work Surfaces
|
Formica, Finished Wood, Synthetic Mats
Ungrounded Metal, Glass or Fiberglass
|
|
Chairs
|
Fiberglass, Vinyl, Other Plastics,
Ungrounded Metal, Finished Wood
|
|
Clothes
|
Synthetics, Gloves, Wool, Shoes and Boots
|
|
Floors
|
Carpeted, Vinyl, Waxed
|
|
Packaging Materials
|
Polyethylene Bags, Bubble Pack Material, Foam
Packaging Pellets, Plastic Trays and Boxes
|
Table C
, "Typical electrostatic Voltages", show how many volts of static electricity are generated by some typical activities
involving people in relationship to percent of relative humidity. Many people do not realize that a short walk across a carpet on a dry
day can charge an individual with up to 35,000 volts of static electricity – or that the simple act of standing up from a standard vinyl
chair can generate a 5,000 volt charge, even on a somewhat humid day. These charges are of significant magnitude. However, they
do not become a concern in the workplace until the damaging effects of an electrostatic discharge event has disabled or destroyed a circuit board.
Table C — Typical Electrostatic Voltages
|
Means of Generation
|
Relative Humidity
|
|
10%
|
40%
|
55%
|
|
Walking Across Carpet
|
35,000
|
15,000
|
7,500
|
|
Walking Across Vinyl Floor
|
12,000
|
5,000
|
3,000
|
|
Motions of Worker at Bench
|
6,000
|
500
|
400
|
|
Vinyl Envelopes for Work Instructions
|
7,000
|
1,500
|
750
|
|
Common Poly Bag Picked Up From Bench
|
20,000
|
6,500
|
3,000
|
|
Work Chair Padded With Polyurethane Foam
|
18,000
|
5,000
|
3,000
|
|
Removing Circuit Boards From Standard Bubble Wrap
|
26,000
|
20,000
|
7,000
|
|
Packaging Circuit Boards in Standard Foam-Lined Box
|
21,000
|
11,000
|
5,500
|
What is Electrostatic Discharge (ESD)?
Electrostatic discharge in the sudden transfer of a static charge between two objects – not necessarily with respect to ground –
just to each other. The extreme example of ESD is lightning. The massive charge generated by the friction between cold and warm
air masses is discharged through the air to ground. A more common example is the sight, sound and feel of the spark that "zaps"
the person or other conductive surface that is touched after walking across a carpeted floor. The spark of an ESD event, and
particularly the heat created by the sudden transfer of energy, is what damages sensitive electronic components.
The Impact of ESD
Human senses can only perceive an electrostatic discharge of about 3,500 volts or higher. That is, we do not see, feel or hear
static discharges below 3,500 volts. This threshold becomes very important in relationship to the sensitivities of today's electronic components.
Table D, "Microelectronic Device Sensitivity," shows the known ESD sensitivity range of a number of electronic components
commonly used in today's telecommunications industry. It is important to recognize the range of voltages within which an ESD event will likely impair or destroy the component. Such damage often requires replacement of the entire circuit board. Most of the
devices listed have an upper limit of 3,000 or fewer volts. This means that they can be destroyed by a person's electrostatic discharge — without the person ever knowing it. This "invisibility" makes ESD the significant hazard that it is and makes its
prevention vital.
Uncontrolled ESD causes premature, unnecessary failure of circuit boards through its destructive impact on electronic
components. These failures result in degraded or lost service to customers, increased maintenance and repair expenses and a
higher amount of capital investment for the larger inventory of circuit board required to maintain desired service levels. All these
factors directly impact a company's profitability and competitiveness in today's communications marketplace. Effective static control helps to deminish these requirements.
Table D — Microelectronic Device Sensitivity
|
Device Type
|
Range of ESD
Sensitivity Volts
|
|
VMOS
|
30 – 1,800
|
|
MOSFET
|
100 – 200
|
|
GaAsFET
|
100 – 300
|
|
EPROM
|
100 – 2,500
|
|
JFET
|
140 – 10,000
|
|
OP AMP
|
190 – 2,500
|
|
CMOS
|
250 – 2,000
|
|
Schottky Diodes, TTL
|
300 – 2,500
|
|
Film Resistors (Thick, Thin)
|
300 – 3,000
|
|
BIPOLAR Translators
|
300 – 7,000
|
|
ECL (PC Board Level)
|
500 – 1,500
|
|
SCR
|
600 – 1,000
|
|
Schottky TTL
|
1,000 – 2,500
|
General Rules For Controlling ESD
Specific requirements for effective static control depend upon the nature of each work environment. But there are some general rules
that may be applied to protect circuit boards throughout their life cycle against three types of static problems: ELECTROSTATIC
CHARGING BY FRICTION, INDUCED CHARGING FROM ELECTROSTATIC FIELDS, AND ELECTROSTATIC DISCHARGE (ESD).
1. Assume all circuit boards contain static-sensitive components. Some circuit boards are virtually not static-sensitive.
However, it is procedurally easier to treat all circuit boards as static sensitive. Treating circuit board differently may risk making a mistake.
2. Do Not handle, transport or store a circuit board except in a static safe environment. Three corrective steps are required
to create such an environment:
First, control static build-up wherever possible. Remove all unnecessary non-conductive materials from the workplace.
Common items such as polystyrene foam cups and plastic sheet protectors generate and hold significant static
charges. These charges will not generally drain to an available ground.
Second, eliminate charges wherever they exist. How these charges are removed depends upon whether the charged
object is a conductor or a non-conductor. Charges on a non-conductor must be either shielded from the sensitive
device or neutralized by use of an air ionizer, which safely showers the work area with both positive and negative ions.
Third, ground all conductors, both people and equipment, in the workplace. A ground safely drains away a charge
from a conductor. Work surfaces upon which circuit boards are handled should be static-dissipative to drain any
remaining charges away to ground at a safe, controlled rate. Ground people either by wrist straps to ground, or by
footwear grounds to a grounded dissipative or conductive floor surface.
3. Do Not
touch a circuit board unless the board and individual are properly grounded. Proper grounding before handling a
unit insures that any static charge one may be carrying is safely drained away before one touches the circuit board.
Therefore static charges are not discharged through the circuit, causing a damaging ESD event.
4. Handle all circuit boards only by the faceplate or latch and by the top and bottom outermost edges. Do Not touch the
components, conductors or connector pins.
5. Do Not
place an unprotected circuit board on top of another unprotected unit or onto any surface not constructed of either dissipative or anti-static material.
Proper Circuit Board Handling Procedures
1. All packaging materials used in connection with circuit board shipment or transport must be static-safe from three types
of static problems: ELECTROSTATIC CHARGING BY FRICTION, INDUCED CHARGING FROM ELECTROSTATIC FIELDS, AND ELECTROSTATIC DISCHARGE (ESD). Anti-static materials must not be used beyond their effective design or shelf
life. Do not assume that manufacturers' packaging for bulk shipment is static-safe for individual circuit board handling.
2. Unpack incoming circuit boards for identification, inspection, testing or inventory control only at a static safe workstation,
at which:
A. The workstation has a static dissipative work surface, and is properly grounded to allow drainage of charges at a
safe, controlled rate.
B. Personnel are grounded to the grounded dissipative floor mat, floor or workstation.
C. The workstation chair or stool is static dissipative and conductively grounded to the grounded dissipative or
conductive floor mat or floor.
D. There are no static-generating materials, such as food wrappers, plastic sheet protectors, polystyrene foam
containers or other untreated nonconductors.
3. Transport circuit boards in shielded, static-safe transport devices such as barrier bags, static-safe pouches, cases or
containers. The transport device must protect against all three types of static problems. Do not overuse shielding bags. Do not transport or carry circuit boards in anti-static packaging only.
4. Keep circuit boards protectively shielded at all times during transport to avoid contact with ungrounded personnel or other
conductors, or with the charged fields of non-conductors.
5. Remove circuit boards from their protective device at the equipment bay, storage cabinet or static-safe workstation only
after properly grounding both transport device and person.
6. Store spare circuit boards in a conductive, grounded cabinet that provides physical separation and high-density, vertical
storage on grounded permanently static-dissipative shelving. Remove all flammable packaging materials from the workplace.
Keep cabinet doors closed to shield from any static charges or fields in the vicinity. Place or remove circuit boards from the storage cabinet only after properly grounding both personnel and the transport device.
7. Static control procedures apply to all circuit boards in the workplace, including those in service. ESD can damage or
destroy an in-service circuit board which is grounded even more easily than one not in service.
8. Use ESD precautions when handling cords, wiring connectors and plugs which are directly connected to peripherals or
the common equipment.
9. Handle defective or replaced circuit boards with the same static precautions as with working units. Since mishandling can
damage additional components, repairable circuit boards may be destroyed.
10. Do not attach instructions, customer orders or repair tags to the circuit boards or place them inside protective bags,
pouches, cases and containers.
11. Static control procedures apply to everyone in the work environment, including installation and maintenance personnel,
supervisors and visitors.
Summary
Effective static control is imperative in today's telecommunications marketplace. Major steps are taken in circuit board assembly
operations to assure circuit boards produced today are virtually void of any defects when they leave the manufacturer's shipping
dock. Without effective awareness and control throughout intermediate warehouse and operational work sites, premature failure of
circuit boards will continue to have a significant negative effect on the companys customers and on its bottom line.
The most critical point to remember for effective static control is that it must be an integral part of ongoing service and support
operations. It must be an integrated process, providing protection during all phases of a circuit board's life cycle: shipping, packing
and unpacking, testing, maintaining, transporting and storing. Any single missing piece, like the proverbial weak link in the chain, jeopardizes the entire process and the life of the involved circuit boards.
Nine Reminders To Save The Boards
1. A static control wrist strap MUST be worn when handling circuit boards.
2.Treat ALL circuit boards as ESD-sensitive.
3. Transport all circuit boards in static-shielding bags, pouches, cases or containers.
4. Handle all circuit boards at static-safe workstations only.
5. Store all circuit boards in static-protective cabinets.
6. Do not use torn or punctured static-shielding bags.
7. Do not place instructions, customer orders or repair tags inside the protective
packaging with the circuit boards.
8. Do not allow chargeable plastics within three feet of unprotected circuit boards.
9. Use ESD precautions when handling cords, wiring, connectors, and plugs which are
directly connected to peripherals or the common equipment.
Electrostatic Discharge Awareness and Control Training
ESD damages billions of dollars worth of micro-electronics each year. Failed printed circuit boards, system outages,
downtime - does this describe your business? ESD eats profits! Your company's claim to quality and reliability are at stake.
Client training is critical to the success of any ESD, EOS and/or EMI program. Proper training effects a change in
behavior through a goal oriented, positive reinforcement program. Our consultants can help you improve your bottom
line. We specialize in ESD Awareness and Control Training, auditing and evaluations.
Our training programs focus on the basics of ESD, how to recognize the threats and how to mitigate them, covering
specific procedures and techniques proven to be successful in reducing the harmful effects of ESD. The programs can be customized to fit the needs of your specific group.
PSSI's Technical Service Department will be glad to assist you in answering any static control questions and help you to
select the best ESD cabinet, static safe case, static safe transporter, conductive container, computer security cabinet,
customizing accessories and static safe products for your particular application.
Are you concerned about quality, reliability? Then call PSSI, your ESD control experts, at 800 231 5952 or email us at info@pluginstorage.com for more information.
|
