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  • Static Electricity and ESD explained

  • Technical information on anti-static bags
Technical information on anti-static bags
 #41  by imported_admin
 Tue Aug 24, 2004 11:55 pm
Overview Static electricity has always existed in the electronics workplace. It has become a major concern only since the incorporation of static-sensitive integrated circuits and other microelectronic components into low and high end products. 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 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 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. Each 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 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.