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Employees must understand what ESD is and be trained in various methods of controlling it.
Floyd H. Bertagnolli
Walk across the carpet, reach for the doorknob and a spark is created. That phenomenon is known as electrostatic discharge (ESD). An amount of static far below a human`s level of feeling can kill many of today`s sensitive electronic parts. It is crucial that anyone who handles electronic parts and assemblies has a serious awareness of these potential problems, and is taught proper techniques to avoid costly losses of hardware and reputation. ESD training is guaranteed to give a high return on investment.
Everyone agrees that lightning hitting a house is a great tragedy. Equally devastating is the miniature lightning bolt of ESD that penetrates the delicate structure of an electronic component. Some integrated circuits (IC) contain up to four million individual components, with a single part measuring only 0.18 µ. Lightning and ESD are both forms of electricity, which is always trying to get to ground. The ESD "zap" will be happy to go through an electronic device to get there if it can, causing destruction on its way.
Humans can generate up to 25,000 V in static charge through normal daily activities. The nerves in a human hand can feel a static discharge down to about 3,000 V. It only requires 10 V of ESD to damage some of the extremely small parts and traces within today`s ICs. The result is an ESD range that cannot be seen, heard or felt, but can significantly damage or destroy the operation of present electronics.
What Causes ESD?
When two materials are rubbed together, one gives up electrons while the other gathers them. The former is positively charged; the latter, negatively. Static electricity is a non-moving electrical charge on an object. Given a chance, the object will give up its charge to be neutral again (as in the case of the spark to the doorknob). This charge release is ESD. Insulators tend to keep their static charge concentrated in a localized area where it does not move, thus the voltage may be very high. Conductors tend to equalize their charge and transport it to grounded objects that they are in contact with. Methods of generating ESD include walking on carpet, combing hair, dispensing adhesive (plastic) tape, opening a plastic bag where the sides had been touching, etc. Typical sources of ESD include personnel, processing equipment and some common plastics.1 ESD-sensitive devices (parts that can be easily damaged by ESD) include: semiconductor devices, thick- and thin-film resistors, chip components, hybrid devices and crystals.
Entrance, Exit and Fields
A printed circuit assembly (PCA) or individual component can be damaged from ESD in several ways:
- Direct contact from hand to component lead.
- Indirect contact, such as a hand touching a connector or gold fingers. The ESD "zap" travels through the traces of a PCA until it finds a weak IC and destroys it.2
- A PCA or individual component is set on a charged surface and "picks up" a similar static charge via induction. Thereafter, it is set on a metal surface and quickly discharges (a slow discharge is safe).
- The PCA or individual component is exposed to strong external fields that induce excessive internal currents.
Healthy, Dead and Walking Wounded
A PCA or individual component may exist in one of three basic conditions:
Healthy. This part was handled with proper ESD techniques and will operate successfully for years.
Dead. This part suffered severe damage by ESD, resulting from improper parts handling, and is inoperable.
Walking wounded. This part suffered partial ESD damage and contains latent defects. Although a signal passed successfully through it during final test, the stresses of regular use will cause a premature and unwanted field failure. This condition cannot normally be detected - prevention is the best defense.
Some types of components are at high risk, but it is not wise to limit care and precautions to only a limited group of parts. It is best to maintain good habits and be ESD aware with all electronics.
Insulators, Conductors and In Between
Insulators are materials such as plastic, rubber, wood, cloth and paper. When any of these items are rubbed together, they become static charge generators. If this charge can find a path to ground, it becomes ESD.
Conductors are materials such as copper, aluminum, silver, gold, steel and carbon. When connected to the earth, they provide a rapid drain path for static charges. If a PCA or single component becomes part of the rapid drain path, damage to electronic components may result.
Dissipative materials are in the conductor family but when connected to the earth, they provide a slow-discharge path for static charges. If a PCA or component becomes part of the slow-discharge path, any existing charge on the PCA will be harmlessly routed to ground.
After a charge is built up on an insulator (e.g., following a rag wiping a plastic table), it cannot be "drained" off. The only way to eliminate it is with the use of an ionizer, which combines a source of extra positive and negative ions with a fan to disperse them. As the ions meet the charges on the insulator surface, they combine and neutralize, eliminating the potential problem (Table 1).
Transporting ESD-sensitive Devices
A PCA or individual component should only be placed on a properly equipped ESD-approved assembly line or workbench, or within a fully sealed ESD-approved container and nowhere else. Common transportation materials/items include:
Antistatic. This category includes the classic pink special-formula plastic bags and opaque IC shipping tubes. These have the characteristic of virtually not generating any new static as the parts move within them. But it is important to note they are not shielding containers and cannot block strong electric fields (e.g., an IC shipping tube within a box of highly charged foam pellets exposes the parts to ESD).
Shielding. This category includes the classic metalized shipping bags and carbon-lined cardboard boxes. The metalized bag began as Mylar, which was then sprayed with a continuous layer of aluminum. This type of bag creates a faraday cage for the contents and can effectively block electric fields. The aluminum-coated bag is excellent for shipping any distance but must be fully closed to be effective. It should be ziplocked (if provided), or folded and taped. Do not staple the bag closed because deadly electric fields can be conducted from the outside to the inside via the staple. Do not leave the bag open because the contents will be exposed to strong electric fields. Do not carry the PCA "potholder style," with the empty bag wrapped around it. Do not rest PCAs on empty bags.
The carbon-lined cardboard box started out as cardboard that was thoroughly coated inside and out with conductive carbon paint. This box is excellent for shipping and is only effective when the lid is fully closed. Because PCAs move around within the box, they are not always provided a ground path. They may be damaged from strong electric fields if the lid is partially off and the contents exposed.
Conductive plastic such as trays and tote boxes (made of a special recipe plastic). It is okay to place PCAs or components atop a sheet of dissipative pad that is then placed within conductive plastic trays, as long as pins or leads consistently contact the pad, and the human holding the tray is wearing heel straps and walking only on specially coated ESD-dissipative waxed/ tiled floor. With this specific arrangement, the parts are constantly grounded.
Tote boxes are also made with special recipe plastic, making them conductive and shielding. These can be used for local and long-distance shipment, but the lid must be fully closed and latched.
Metal carts. These carts are an effective means of inner-factory transportation, provided there is a dissipative pad on the top (to prevent rapid discharging) and that they are grounded with a chain to a static-dissipative waxed floor.
The goal is zero volts.
The perfect worker. The person who will be the most productive and the least likely to damage parts via ESD will be the one who is equipped with a good attitude, knowledge, two foot straps (heel or toe type) and a wrist strap (if sitting is ever part of the job).
Wrist strap details:
1.The wrist strap and workbench pads provide a well-defined path for the ESD to go to ground, leaving the owner at near zero volts charge. Both strap and pad use a built-in, current-limiting 1 M resistor that creates a safety system, preventing shock, for assembly or manufacturing workers, from accidental contact to 110 VAC power.
2.Some individuals have naturally dry skin, and repeatedly fail at the wrist-strap tester each day when they begin work. To solve this, do not use commercially available home hand lotions, which contain silicon and could negatively affect soldering connections. Managers should ensure ESD-approved lotion is available.
3.Some individuals can develop a skin irritation caused by the metal portion of the wrist strap. (This is because of a small percentage of nickel that is blended into the stainless steel.) Because the entire wrist strap has conductive material woven into it, this problem can be easily overcome by putting a small portion of white first-aid tape over the metal area where it touches the skin.
Warning: Personnel working on a regular basis with high voltages (e.g., 250 VAC and above, such as system repairs/maintenance) should not use wrist or heel straps, or lean on workbench pads, because they provide a path from the body to ground. A worker should perform PCA handling or machine maintenance, not both. They are two different jobs and require different procedures and equipment.
The perfect workstation. To reach the goal of zero volts static charge, all nonessential materials should be removed from the station, especially common plastics, foams and paper. Care should be given to keep computer monitors at least 24" (60 cm) from the PCAs. An approved (look for the ESD triangle symbol) grounded pad should cover the top of the workbench. A quick-release plug-in should be provided for wrist straps. Antistatic floor mats are beneficial as long as PCAs or parts are kept at that station and do not have to be hand carried into an area that is not ESD safe (e.g., plain cement or carpet). When possible, antistatic tile or dissipative floor wax should be used as a floor covering to provide a good ground path for heel straps.
The Humidity Myth
As humidity rises (lots of moisture in the air), the risk of ESD goes away. This statement is wrong. It is true that the risk is lessened, but it does not leave. All workers handling parts must always follow good ESD practices.
ESD education must exist in all locations of a part`s life cycle: the manufacturer, vendor, contract manufacturer (CM), installer, customer, etc. Educate vendors, employees and customers on proper ESD practices to protect all common interests. If a CM has careful methods but the party before and after does not, parts will die or suffer damage from ESD. Mutual support, teamwork and partnership will get a lot accomplished via prevention.
Do`s and Don`ts
Do`s. Hold components and PCAs on sides. Transport parts and PCAs in proper containers. Test heel/wrist straps each time before entering the work area and record it on a log sheet per company policy. Wear heel straps on both feet. Ensure all visitors wear heel straps. When cleaning printed circuit boards (PCB), use only ESD-approved natural bristle (no plastic) brushes and solvents. Keep parts grounded during cleaning because solid brush against board and liquid solvent against board can generate ESD. Keep the PCA "pins down" on the ESD mat as much as possible. Mark the manufacturing area with ESD zone boundaries using yellow warning tape. Minimize PCA motion as much as possible during all operations and inspections. Minimize handling whenever possible. Keep parts in their original containers until ready to use and only store in completely closed shielded containers, or pin down on a dissipative grounded mat. Keep parts 24" (60 cm) from any computers or CRT machine displays.
Don`ts. Rub hair. Use non-ESD-approved bags/tote boxes. Allow clothing/hair/ties to rub on PCAs. Use hand lotion from home. Stack boards. Carry boards "pot holder" style with shielding bags. Rest PCA atop shielding bags. Use grounding wrist straps, heel straps or table pads when working on power circuits (e.g., machine maintenance where 250 VAC or more is present). Slide static-sensitive parts over any surface.
Plan to Achieve an ESD Compliant Workplace
2.Remove static generators from the work area, receiving, manufacturing, shipping and installation.
3.Provide discharge path for personnel, tables, floors and tools.
4.Implement safe techniques for handling, transporting and storing ESD-sensitive components/assemblies.
ESD - Aggressively erase a white board for 20 seconds, then hold a key close to a metal table (with the lights out).
Conductivity - Using parts from a flashlight, replace the switch with various common objects to create a list of conductors and insulators (the conductors will enable the light to shine).
Charged field - a) Hold two pieces of plastic adhesive tape in one hand, and see how tape has a charge and how like charges repel; b) Hold one piece of adhesive tape 4" above some cigarette ashes and see how the ashes are attracted by the charge on the tape.
Thanks to Tom Champion of Jabil Circuit and Bill Pellegrin of 3M for providing information and critique.
1 Jabil Circuit operations procedure 1-OP20-1000-000B 4-29-98.
2 Videotape VT-19, "Electrostatic Discharge ESD Control," IPC, Northbrook, IL 60062, 1994.
FLOYD H. BERTAGNOLLI may be contacted at Jabil Circuit, 30 Great Oaks Blvd., San Jose, CA 95119; (408) 361-3283; E-mail: email@example.com; Web site: www .jabil.com.
Conductor: A material with the ability to transfer an electric charge.
Electrostatic field: The area near an electrically charged object.
Grounded: To connect a material to the earth, via a conductor.
Induction: One item with an electrical charge influences another item to have a similar charge without direct contact.
Insulator: A material without the ability to transfer an electric charge.
Resistance: The characteristic to restrict or limit the flow of electrical charge or current.
Resistor: A device that exhibits resistance.
Zap: An electrostatic discharge that often implies resulting damage to components.