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DESCO INDUSTRIES ESD AWARENESS BOOKLET PLEASE READ THOROUGHLY BEFORE WORKING ON OR HANDLING ESD SUSCEPTIBLE COMPONENTS OR ASSEMBLIES Use as part of Training Plan required by ANSI/ESD S20.20 ITEM #06821 / #36010 ESD CONTROL PROGRAM PLAN “The Organization shall prepare an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include: • Training • Compliance Verification • Grounding / Equipotential Bonding Systems • Personnel Grounding • EPA Requirements • Packaging Systems • Marking” [ANSI/ESD S20.20-2007 section 7.1] “The selection of specific ESD control procedures or materials is at the option of the ESD Control Program Plan preparer and should be based on risk assessment and the established electrostatic discharge sensitivities of parts, assemblies, and equipment.” [ANSI/ESD S20.20-2007 Annex B] TRAINING PLAN “Initial and recurrent ESD awareness and prevention training shall be provided to all personnel who handle or otherwise come into contact with any ESDS [ESD sensitive] items. Initial training shall be provided before personnel handle ESDS items. The type and frequency of ESD training for personnel shall be defined in the Training Plan. The Training Plan shall include a requirement for maintaining employee training records and shall document where the records are stored. Training methods and the use of specific techniques are at the Organization’s discretion. The training plan shall include the methods used by the Organization to verify trainee comprehension and training adequacy.” [ANSI/ESD S20.20-2007 section 7.2] Desco Industries does not provide any express warranties and disclaims all statutory and implied warranties (including implied warranty of merchantabililty and fitness for a particular purpose) in the use of this booklet. In no event shall Desco Industries be liable for any damages with respect to its furnishing of this written document. ESD training should be repeated as specified in the company’s written ESD Control Plan. The quizzes included in this booklet may be determined as possible “objective evaluation technique to ensure trainee comprehension and training adequacy.” The written ESD Control Plan should be in accordance with ANSI/ESD S20.20-2007 (a copy can be downloaded at no charge from the ESD Association website ESDA.org which can be accessed via the Desco Industries website, DescoIndustries.com. Also, other documents such as the ESD Handbook ESD TR20.20 can be purchased). "The 100 volt HBM limit was selected for ANSI/ESD S20.20 as the baseline susceptibility threshold since a large majority of the ESD susceptible products on the market have a sensitivity of greater than 100 volts." [ESD Handbook ESD TR20.20-2008 section 1.1 Overview] For more sensitive items such as HBM (Human Body Model) Class 0, see page 15 for suggested practices. "ANSI/ESD S20.20 is the culmination of the ESD Association's response to the request of the US Department of Defense to write a commercial version of MIL-STD-1686, Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices)." [ESD Handbook ESD TR20.20-2008 section 1.1 Overview] ESD BASICS FOR ESD PROGRAMS PER ANSI/ESD S20.20 Electrostatic Discharge [ESD] can damage components and products containing electronics. It is the hidden enemy in many high-tech factories. Often this damage cannot be detected by quality control inspections, and can be very frustrating; adversely impacting productivity, quality, product reliability and thus a company's reputation and profitability. The prerequisites of ESD control are: • Identify ESD Protected Area [EPA] • Identify ESD sensitive items [ESDS] • Provide ESD control training The ESD protected area should have signage, often including aisle tape, to clearly identify where it is. You need to understand and follow the basics of ESD control to limit the generation of ElectroStatic charges, limit and slow discharges in the EPA. ESD BASICS Protect your work following these guidelines: • Only handle unpackaged ESD sensitive items [ESDS] in the ESD protected area [EPA] when grounded • Only allow trained or escorted people in the EPA • Ground all conductors including people in the EPA • Use continuous monitors or test wrist straps at least daily • If ESD footwear is used, test at least daily • Visually check that grounding cords are connected • Keep wristband snug, foot grounder grounding tab in shoe, and ESD smocks covering all clothing on torso • Keep work area clean and clear of all non essential insulators, or neutralize essential insulators with ionizers with the airflow directed towards the work area • Use packaging with shielding properties to store or transport ESDS outside the EPA GROUNDING AND PERSONNEL SAFETY This booklet will deal with ESD basics. ElectroStatic charges or static electricity can be everywhere, however conductors can be effectively grounded and charges removed to ground. A fundamental rule in ESD control is to ground all conductors, including people. However, while ESD control is important, it is of secondary importance to employee safety. Personnel should not be grounded in situations where they could come into contact with voltage over 250 volts AC. UL Caution, Underwriters Laboratories states regarding listed personnel grounding items: "Note: This product is not recommended for use on equipment with operating voltage exceeding 250 VAC. CAUTION: The ESD Series is for electrostatic control. It will not reduce or increase your risk of receiving electric shock when using or working on electrical equipment. Follow the same precautions you would use without [the personnel grounding item], including: • Make certain that equipment having a grounding type plug is properly grounded. • Make certain that you are not in contact with grounded objects other than through the ESD Series." 1 STATIC ELECTRICITY, ELCTROSTATIC CHARGE All materials can tribocharge or generate ElectroStatic charges. This is static electricity which is an electrical charge at rest. When an electrical charge is not at rest, but discharges, problems can occur and we will discuss ESD [ElectroStatic Discharge] in some detail later. • When two surfaces contact then separate • Some atom electrons move from one surface to the other, causing imbalance One surface has a positive charge and one surface has a negative charge. CHARGE GENERATION OR TRIBOCHARGING EXAMPLES The simple separation of two surfaces, as when tape is pulled off a roll, can cause the transfer of electrons between surfaces, generating an ElectroStatic charge. • Unwinding a roll of tape Static electricity can be a very hair-raising event. Remember that ElectroStatic charges and ElectroStatic discharges are different. All matter is constructed from atoms which have negatively charged electrons circling the atom’s nucleus which includes positively charged protons. The atom having an equal number of electrons and protons balances out having no charge. • Gas or liquid moving through a hose or pipe • A person walking across a floor with heels and soles contacting and separating from the floor ElectroStatic charges are most commonly created by contact and separation: The amount of static electricity generated varies,and is affected by materials,friction, area of contact, and the relative humidity of the "For most people, static electricity is represented by the noise or crackle heard on a radio that interferes with good reception or the shock experienced when touching a metal object after walking across a carpeted room or sliding across a car seat. Static electricity is also observed as static cling when clothes are stuck together after coming out of a clothes dryer. Most of the time, people observe static electricity when the weather is cold and dry." "While many people tend to think of static electricity as being at rest or not moving, static electricity causes the most concern when it ceases to be stationary." [ESD Handbook ESD TR20.20 section 2.1 Basics of Static Electricity, Introduction] "Electrostatic charge is most commonly formed by the contact and separation of two materials. The materials may be similar or dissimilar although dissimilar materials tend to liberate higher levels of static charge. An example is a person walking across the floor. Static electricity is produced when the person's shoe soles make contact, then separate from the floor surface. Another example is an electronic device sliding into or out of a bag, magazine or tube." [ESD Handbook ESD TR20.20-2008 section 2.3 Nature of Static Electricity] 2 environment. At lower relative humidity, charge generation will increase as the environment is drier. Common plastics generally create the greatest static charges. Typical Electrostatic Voltages Many common activities may generate charges on a person’s body that are potentially harmful to electronics components. (A higher charge is generated at low humidity, in a dry environment) However if two items are at different levels of ElectroStatic charge, they will want to come into balance. If they are in close enough proximity, there can be a rapid, spontaneous transfer of electrostatic charge. This is called discharge or ElectroStatic Discharge (ESD). Examples in daily life: • Lightning, creating lots of heat and light • Walking across carpet, 1,500 to 35,000 volts • Walking over untreated vinyl floor, 250 to 12,500 volts • Vinyl envelope used for work instructions, 600 to 7,000 volts • The occasional zap felt when reaching for a door knob • Worker at bench, 700 to 6,000 volts • Picking up a common plastic bag from a bench, 1,200 to 20,000 volts ELECTROSTATIC DISCHARGE (ESD) If two items are at the same electrostatic charge or equipotential, no discharge will occur. • The occasional zap felt when sliding out of an automobile and touching the door handle "The phenomenon of forming an electrostatic charge by contact and separation of materials is known as "triboelectric charging", from the Greek "tribos" meaning "to rub", and "elektros" meaning "amber", as mentioned above. It involves the transfer of electrons between materials. The atoms of a material with no static charge will have an equal number of positive (+) protons in their nucleus and negative (-) electrons orbiting the nucleus … and is electrically neutral. … When the two materials are placed in contact, negatively charged electrons will transfer from the surface of one material to the other material. Electrons are carried off upon separation of the two materials. The material that loses electrons becomes positively charged, while the material that gains electrons is negatively charged. The amount of charge created by triboelectric charging is affected by the area of contact, the speed of separation, relative humidity, material work function, surface energy and other factors. Once the charge is formed on a material, it becomes an "electrostatic" charge (if it remains on the material). This charge may be transferred from the material, creating an ESD event." [ESD Handbook ESD TR20.20 section 2.3 Nature of Static Electricity] 3 In a normal environment like your home, there are innumerable ESD events occurring, most of which you do not see or feel. It takes a discharge of about 2,000 volts for a person to feel the “zap”. It requires a much larger ESD event to arc and be seen. While a discharge may be a nuisance in the home, ESD is the hidden enemy in a high tech manufacturing environment. Modern electronic circuitry can be literally burned or melted from these miniature lightning bolts. ESD control is necessary to reduce and limit these ESD events. TYPES OF ESD DEVICE DAMAGE ESD damage to electronic components can be: • Catastrophic Failures • Latent Defects EVEN LESS THAN 100 VOLTS MIGHT DAMAGE A COMPONENT Direct catastrophic failures, meaning completely failed or dead components. A latent defect can occur when an ESD sensitive item is exposed to an ESD event and is partially degraded. It may continue to perform its intended function, so may not be detected by normal inspection. However, intermittent or permant failures may occur at a later time. Latent defects, meaning degraded or wounded components. COSTLY EFFECTS OF ESD Catastrophic failure causes a failure in an ESD sensitive item that is permanent. The ESD event may have caused a metal melt, junction breakdown or oxide failure. Normal inspection is able to detect a catastrophic failure. A catastrophic failure of an electronic component can be the least costly type of ESD damage as it may be detected and repaired at an early manufacturing stage. Per ESD Handbook ESD TR20.20-2008 section 2.7 Device Damage - Types and Causes "Electrostatic damage to electronic devices can occur at any point, from the manufacture of the device to field service of systems. Damage results from handling the devices in uncontrolled surroundings or when poor ESD control practices are used. Generally damage can manifest itself as a catastrophic failure, parametric change or undetected parametric change (latent defect)." 2.7.1 Catastrophic Failures "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, resulting in a catastrophic failure." 2.7.2 Latent Defects "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 a 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 easy with the proper equipment to confirm that a device has experienced catastrophic failure or that a part is degraded or fails test parameters. Basic performance tests will substantiate device damage. However, latent defects are virtually impossible to prove or detect using current technology, especially after the device is assembled into a finished product. Some studies claim that the number of devices shipped to users with latent defects exceeds the number that fail catastrophically due to ESD in manufacturing." 4 Latent damage caused by ESD is potentially more costly since damage occurs that cannot be felt, seen, or detected through normal inspection procedures. Latent defects can be very expensive as the product passes all inspection steps, and the product is completed and shipped. Latent defects can severely impact the reputation of a company’s product. Intermittent failures after shipping a product can be frustrating, particularly when the customer returns a product, reporting a problem which the factory again fails to detect. It consequently passes inspection and the product is returned to the customer with the problem unresolved. The worst event is when the product is installed in a customer’s system, and performs for a while and then performs erratically. It can be very expensive to troubleshoot and provide repairs in this situation. Catastrophic failures are detected during inspection but components with latent defects pass as good. One study indicated the cost to be: • $10 Device • $10 Device in board - $100 • $10 Device in board and in system - $1,000 • $10 Device and system fails - $10,000 Industry experts have estimated average electronics product losses due to static discharge to range from 8 to 33%. Others estimate the actual cost of ESD damage to the electronics industry as running into the billions of dollars annually. ESD CONTROL Many firms consider all electronic components ESD sensitive. It is critical to be aware of the most sensitive item being handled in your factory. As electronic technology advances, electronic circuitry gets progressively smaller. As the size of components is reduced, so is the microscopic spacing of insulators and circuits within them, increasing their sensitivity to ESD. As you can predict, the need for proper ESD protection increases every day. Any ESD sensitive item should be indentified with the ESD susceptibility symbol, either on itself or its container. The ESD Susceptibility Symbol (also called Sensitivity or Warning Symbol) identifies items that can be damaged by ESD and should be unpackaged and handled while grounded at an ESD protected workstation. Per ESD Handbook ESD TR20.20-2008 section 2.2 "Electronic items continued to become smaller, faster and their susceptibility to static damage increased...all electronic devices required some form of electrostatic control to assure continued operation and product reliability." Per ESD Handbook ESD TR20.20-2008 section 5.3.2.1 "Static electricity is a natural phenomenon that occurs in all climates and at all levels of relative humidity year round. Most people cannot feel an electrostatic discharge unless the static voltage is greater than 2,000 volts. However, some electronic circuitry can be damaged by ESD that is less than 2,000 volts. The damage can be done without people having any sensation of the ESD event. When a wrist strap is worn properly and connected to ground, the person wearing it will stay near ground potential." 5 Most firms use the ANSI/ESD S20.20 document to construct their ESD control plan which is based on handling ESD sensitive items having a Human Body Model withstand voltage of 100 volts or greater. The Human Body Model simulates discharges from a person and increasingly tests an electronic device at higher and higher discharges until it fails, thus establishing the device’s withstand voltage. OHM’S LAW At this point we should consider some basic physics: Resistance to ground (RTG) is a measurement that indicates the capability of an item to conduct an electrical charge (current flow) to an attached ground connection. The measurement may be shown in various ways. Most commonly: • 1 kilohm • 1 k • 1 x 103 ohm • 1 x 10E3 ohm • 10^3 ohm Prefix Symbol Scientific Notation 1 x 103 or 10E3 1,000 or one thousand 1 x 106 or 10E6 1,000,000 or one million 1 x 109 or 10E9 1,000,000,000 or one billion Ohm’s law is an extremely useful equation in the field of electrical/electronic engineering because it describes how voltage, current and resistance are interrelated. kilo- K mega- M giga- G Ohm’s law states that, in an electrical circuit, the current passing through a conductor between two points is directly proportional to the potential difference (i.e. voltage drop or voltage) across the two points, and inversely proportional to the resistance between them. TYPES OF MATERIALS Resistance determines how much current will flow through a component. A very high resistance allows a small amount of current to flow. A very low resistance allows a large amount of current to flow. Resistance is measured in ohms. Common Usage Conductors • Electrical current flows easily • Can be grounded Materials that easily transfer electrons (or charge) are called conductors and are said to have “free” electrons. Some examples of conductors are metals, carbon, and the human body’s sweat layer. Grounding works effectively to remove ElectroStatic charges from conductors to ground. However, the item grounded must be Per ESD Handbook ESD TR20.20-2008 section 5.3.6.1.2 ESD Effects "In the electronics industry, smaller device geometries have resulted in higher density devices. ESD events can cause device damage such as metal vaporization, punch-through of thin oxide layers, or other failure modes. As mentioned earlier ESD events may or may not weaken devices so that they still pass production testing, but fail when installed in a customer’s equipment." Per ESD Handbook ESD TR20.20-2008 section 4.1.1 Determining Part ESD Sensitivity "The first step in developing an ESD Control Program plan is to determine the part, assembly or equipment sensitivity level under which the plan is to be developed. The organization can use one of several methods to determine the ESD sensitivity of the products that are to be handled. Some of the various methods are: 1) Assumption that all ESD products have a HBM sensitivtiy of 100 volts; 2) Actual testing of products using accepted test methods." 6 conductive. The other term often used in ESD control is dissipative which is 1 x 10E4 to less than 1 x 10E11 ohms and is sufficiently conductive to remove ElectroStatic charges when grounded. Insulators like this plastic cup will hold the charge and cannot be grounded and “conduct” the charge away. When a conductor is charged, the ability to transfer electrons gives it the ability to be grounded. Insulators • Electrical current does not flow easily • Cannot be grounded Materials that do not easily transfer electrons are called insulators, by definition non-conductors. Some well known insulators are common plastics, and glass. An insulator will hold the charge and cannot be grounded and “conduct” the charge away. Both conductors and insulators may become charged with static electricity and discharge. Grounding is a very effective ESD control tool, however, only conductors (conductive or dissipative) can be grounded. THINK OF STATIC ELECTRICITY AS GERMS AND CONTAMINATION! Daily life has other examples of hidden enemies where careful procedures must be followed to regularly obtain positive results. One example is sterilization, which combats germs and contamination in hospitals. Damage caused by invisible and undetectable events can be understood by comparing ESD damage to medical contamination of the human body by viruses or bacteria. Although invisible, they can cause severe damage. In hospitals, the defense against this invisble threat is extensive contamination control procedures including sterilization. Per ESD Handbook ESD TR20.20-2008 section 2.5 Material Electrical Characteristics - Insulative, Conductive and Static Dissipative "Virtually all materials, including water and dirt particles in the air, can be triboelectrically charged. An insulator is a material that prevents or limits the flow of electrons across or through its volume is called an insulator. A considerable amount of charge can be formed on the surface of an insulator. A conductive material allows electrons to flow easily across its surface. Conductive materials have low electrical resistance. If the charged conductive material makes contact with another conductive material, the electrons will transfer between the materials quite easily. If the second conductor is a wire lead to an earth grounding point, the electrons will flow to or from ground and the excess charge on the conductor will be "neutralized". Static dissipative material will allow the transfer of charge to ground or to other conductive objects. The transfer of charge from a static dissipative material will generally take longer than from a conductive material of equivalent size." There is no correlation between resistance measurements and the ability of a material to be low charging. Static dissipative material shall have a surface resistance of greater than or equal to 1.0 x 10^4 ohms but less than 1.0 x 10^11 ohms. Conductor less than 1.0 x 10^4, and non-Conductor or Insulator 1 x 10^11 ohms or higher. (Reference ANSI/ESD S541 section 7.2) 7 OPERATOR’S PART IN ESD CONTROL Just as you would never consider having surgery in a contaminated operating room, you should never handle, assemble, or repair electronic assemblies without taking adequate protective measures against ESD. As an employee, the invisible threat of ESD should be of great concern to you. ESD damage can significantly reduce your company’s profitability. This may affect your company’s ability to compete in the marketplace, your profit sharing, and even your employment. Everyone likes to take pride in their work, but without proper ESD controls, your best efforts may be destroyed by ElectroStatic discharges that you can neither feel nor see. People in the high-tech manufacturing environment are still a major source of ElectroStatic charges and discharges. Operators need training and to be vigilant that ESD control procedures are followed. In order for the ESD control program to be effective, operators must be aware of the threat of ESD, and understand and adhere to the rules of controlling static electricity, and how to properly use EPA ESD control items. We are aware of the benefits of sterilization in medicine. We must develop the same attitude towards ESD control and “sterilize” against its contamination. Just as you would never consider having surgery in a contaminated operating room, you should never handle, assemble, or repair electronic assemblies without taking adequate measures against ESD. For the hospital to sterilize most of the instruments is not acceptable; actually it may waste money. Each and every instrument needs to be sterilized. Likewise, it is not acceptable to protect the ESDS most of the time. Effective ESD control must occur at each and every step where ESDS is manufactured, processed, assembled, installed, packaged, labeled, serviced, tested, inspected, transported, or otherwise handled. 8 EPA ESD control items are ESD protective products that have been specially formulated to possess at least one of the ESD control properties: 1) low charging (antistatic) 2) resistance (conductive or dissipative, able to be grounded) 3) shielding. These products should be identified by the ESD Protective Symbol. Note: the ESD Protective Symbol has an arc which the ESD Susceptibility Symbol does not. The ESD Protective Symbol identifies products designed to provide ESD control protection. FUNDAMENTAL ESD CONTROL PRINCIPLES • Ground all conductors including people • Remove insulators, substitute with ESD protective versions, or neutralize with ionizers • ESDS outside the EPA to be in packaging having ESD shielding property PERSONNEL GROUNDING A fundamental principle of ESD control is to ground conductors including people at ESD protected workstations. Wrist straps are the first line of defense against ESD, the most common personnel grounding device used, and are required to be used if the operator is sitting. The wristband should be worn snug to the skin with its coil cord connected to a common point ground which is connected to ground, preferably equipment ground. If you are not using a continuous or a constant monitor, a wrist strap should be tested while being worn at least daily. This quick check can determine that no break in the path-to-ground has occurred. Part of the path-to-groud is the perspiration layer on the person; an operator with dry skin may inhibit the removal of static charges and may cause a test failure. Specially formulated lotion can solve this problem. Failures may also be caused by dirty or loose wristbands which should be cleaned or tightened. When a wrist strap fails a test, the supervisor should be contacted, and the failure effectively addressed or the wrist strap replaced. A Flooring / Footwear system is an alternative for personnel grounding for standing or mobile workers. Foot grounders or other types of ESD footwear are worn while standing or walking on an ESD floor. ESD footwear is to be worn on both feet and should be tested independently at least daily while being worn. Unless the tester has a split footplate, each foot should be tested independently, typically with the other foot raised in the air. Per ANSI/ESD S20.20-2007 Foreward "The fundamental ESD control principles are: • All conductors in the environment, including personnel, must be attached to a known ground • Necessary non-conductors in the environment cannot lose their electrostatic charge by attachment to ground. Ionization systems provide neutralization of charges on these necessary non-conductive items (circuit board materials and some device packages are examples of necessary non-conductors). • Transportation of ESDS items outside of an ESD Protected Area requires enclosure in static protective materials... Outside an EPA, low charging and static discharge shielding materials are recommended." 9 Both ESD footwear and ESD floor are required. Wearing ESD footwear on a regular, insulative floor is a waste of time and money. Part of the path-to-ground is the perspiration in the person’s shoes. Foot grounder conductive tabs or ribbons should be placed inside the shoe under the foot with the excess length tucked into the shoe. Thanks to the perspiration in the shoe, direct contact with the skin is normally not necessary. If an operator leaves the EPA and walks outside wearing ESD footwear, care should be taken not to get the ESD footwear soiled. Dirt is typically insulative, and the best practice is to re-test the ESD footwear while being worn each time when re-entering the EPA. WORKSTATION GROUNDING DEVICES ESD worksurfaces, such as mats, are typically an integral part of the ESD workstation, particularly in areas where hand assembly occurs. The purpose of the ESD worksurface is two-fold. (1) To provide a surface with little to no charge on it. (2) To provide a surface that will remove ElectroStatic charges from conductors (including ESDS devices and assemblies) that are placed on the surface. ESD mats need to be grounded. A ground wire from the mat should connect to the common point ground which is connected to ground, preferably equipment ground. For electronics manufacturing a worksurface resistance to ground (RTG) of 1 x 10E6 to less than 1 x 10E9 ohms is recommended. "The wrist strap system should be tested daily to ensure proper electrical value. Nominally, the upper resistance reading should be