Static Electricity: Creating Charge
Static electricity is defined as an electrical charge caused by an imbalance of electrons on the surface of a material. This imbalance of electrons produces an electric field that can be measured and that can influence other objects at a distance.
Electrostatic discharge is defined as the transfer of charge between bodies at different electrical potentials.
Electrostatic discharge can change the electrical characteristics of a semiconductor device, degrading or destroying it. Electrostatic discharge also may upset the normal operation of an electronic system, causing equipment malfunction or failure. Another problem caused by static electricity occurs in clean rooms. Charged surfaces can attract and hold contaminants, making removal from the environment difficult. When attracted to the surface of a silicon wafer or a device's electrical circuitry, these particulates can cause random wafer defects and reduce product yields.
Controlling electrostatic discharge begins with understanding how electrostatic charge occurs in the first place. Electrostatic charge is most commonly created by the contact and separation of two materials. For example, a person walking across the floor generates static electricity as shoe soles contact and then separate from the floor surface. An electronic device sliding into or out of a bag, magazine or tube generates an electrostatic charge as the device's housing and metal leads make multiple contacts and separations with the surface of the container. While the magnitude of electrostatic charge may be different in these examples, static electricity is indeed generated.
The age of electronics brought with it new problems associated with static electricity and electrostatic discharge. And, as electronic devices became faster and smaller, their sensitivity to ESD increased. Today, ESD impacts productivity and product reliability in virtually every aspect of today's electronics environment. Many aspects of electrostatic control in the electronics industry also apply in other industries such as clean room applications and graphic arts.
Despite a great deal of effort during the past decade, ESD still affects production yields, manufacturing costs, product quality, product reliability, and profitability. Industry experts have estimated average product losses due to static to range from 12-35%. Others estimate the actual cost of ESD damage to the electronics industry as running into the billions of Rupees annually. The cost of damaged devices themselves ranges from only a few rupees for a simple diode to several thousand rupees for complex hybrids. When associated costs of repair and rework, shipping, labor, and overhead are included, clearly the opportunities exist for significant improvements.
Perhaps the greatest threat facing system builders is electrostatic discharge (ESD). ESD is the sudden discharge of static electricity causing a momentary current flow that can weaken or permanently damage semiconductor components (such as the processor, memory, motherboard, and video card). Therefore it is imperative that the industry (Manufacturers, Integrators, Dealers, service centers, end-customers) follow the correct ESD storage and handling procedures.
Electrostatic Discharge (ESD) can damage a sensitive electronic component, resulting in failures, reduced reliability and increased rework costs, or latent component failures in equipment.
Namaguchi & Uchida (ESD Symposium 1998) found that 60-90% of defective devices were damaged by ESD, and 70% of these failures were attributed to damage by ungrounded people.
ESD Damage—How Devices Fail
Electrostatic damage to electronic devices can occur at any point from manufacture to field service. Damage results from handling the devices in uncontrolled surroundings or when poor ESD control practices are used. Generally damage is classified as either a catastrophic failure or a latent defect.
Catastrophic Failure
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 causing the device fail. Such failures usually can be detected when the device is tested before shipment. If the ESD event occurs after test, the damage will go undetected until the device fails in operation.
Latent Defect
A latent defect, on the other hand, is more difficult to identify. 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 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 relatively easy with the proper equipment to confirm that a device has experienced catastrophic failure. Basic performance tests will substantiate device damage. However, latent defects are extremely difficult to prove or detect using current technology, especially after the device is assembled into a finished product.
Basic ESD Events--What Causes Electronic Devices to Fail?
ESD damage is usually caused by one of three events: direct electrostatic discharge to the device, electrostatic discharge from the device or field-induced discharges. 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 voltage levels involved. This is known as the device's "ESD sensitivity".
Discharge to the Device
An ESD event can occur when any charged conductor (including the human body) discharges to an ESDS (electrostatic discharge sensitive) device. The most common cause of electrostatic damage is the direct transfer of electrostatic charge from the human body or 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). A similar discharge can occur from a charged conductive object, such as a metallic tool or fixture. The model used to characterize this event is known as the Machine Model.
Discharge from the Device
The transfer of charge from an ESDS device is also an ESD event. Static charge may accumulate on the ESDS device itself through handling or contact with packaging materials, worksurfaces, or machine surfaces. This frequently occurs when a device moves across a surface or vibrates in a package. The model used to simulate the transfer of charge from an ESDS device is referred to as the Charged Device Model (CDM). The capacitance and energies involved are different from those of a discharge to the ESDS device. In some cases, a CDM event can be more destructive than the HBM for some devices.
The trend towards automated assembly would seem to solve the problems of HBM ESD events. However, it has been shown that components may be more sensitive to damage when assembled by automated equipment. A device may become charged, for example, from sliding down the feeder. If it then contacts the insertion head or another conductive surface, a rapid discharge occurs from the device to the metal object.
Field Induced Discharges
Another event that can directly or indirectly damage devices is termed Field Induction. As noted earlier, whenever any object becomes electrostatically charged, there is an electrostatic field associated with that charge. If an ESDS device is placed in that electrostatic field, a charge may be induced on the device. If the device is then momentarily grounded while within the electrostatic field, a transfer of charge from the device occurs as a CDM event. If the device is removed from the region of the electrostatic field and grounded again, a second CDM event will occur as charge (of opposite polarity from the first event) is transferred from the device.
How Much Static Protection is Needed?
As noted earlier, 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 voltage levels involved—its ESD sensitivity. Defining the ESD sensitivity of electronic components is the first step in determining the degree of ESD protection required. Test procedures based on the models of ESD events help define the sensitivity of components to ESD. These procedures will be covered in a future article in this series.
Many electronic components are susceptible to ESD damage at relatively low voltage levels. Many are susceptible at less than 100 volts, and many disk drive components have sensitivities below 10 volts. Current trends in product design and development pack more circuitry onto these miniature devices, further increasing their sensitivity to ESD and making the potential problem even more acute.
Basic Principles of Static Control
Sometimes, controlling electrostatic discharge (ESD) in the electronics environment seems to be a formidable challenge. However, the task of designing and implementing ESD control programs becomes less complex if we focus on just six basic principles of control. In doing so, we also need to keep in mind the ESD corollary to Murphy's law, "no matter what we do, static charge will try to find a way to discharge."
1. Design In Immunity: by designing products and assemblies to be as immune as reasonable from the effects of ESD
2. Define the level of control needed in your environment: needed in your environment.
3. Identify and define the electrostatic protected areas (EPA): the areas in which you will be handling sensitive parts.
4. Eliminate and Reduce Generation: by reducing and eliminating static generating processes, keeping processes and materials at the same electrostatic potential, and by providing appropriate ground paths to reduce charge generation and accumulation.
5. Dissipate and Neutralize: by grounding, ionization, and the use of conductive and dissipative static control materials.
6. Protect Products: with proper grounding or shunting and the use of static control packaging and materials handling products.
Typical Facility Areas Requiring ESD Protection:
Static electricity is defined as an electrical charge caused by an imbalance of electrons on the surface of a material. This imbalance of electrons produces an electric field that can be measured and that can influence other objects at a distance.
Electrostatic discharge is defined as the transfer of charge between bodies at different electrical potentials.
Electrostatic discharge can change the electrical characteristics of a semiconductor device, degrading or destroying it. Electrostatic discharge also may upset the normal operation of an electronic system, causing equipment malfunction or failure. Another problem caused by static electricity occurs in clean rooms. Charged surfaces can attract and hold contaminants, making removal from the environment difficult. When attracted to the surface of a silicon wafer or a device's electrical circuitry, these particulates can cause random wafer defects and reduce product yields.
Controlling electrostatic discharge begins with understanding how electrostatic charge occurs in the first place. Electrostatic charge is most commonly created by the contact and separation of two materials. For example, a person walking across the floor generates static electricity as shoe soles contact and then separate from the floor surface. An electronic device sliding into or out of a bag, magazine or tube generates an electrostatic charge as the device's housing and metal leads make multiple contacts and separations with the surface of the container. While the magnitude of electrostatic charge may be different in these examples, static electricity is indeed generated.
The age of electronics brought with it new problems associated with static electricity and electrostatic discharge. And, as electronic devices became faster and smaller, their sensitivity to ESD increased. Today, ESD impacts productivity and product reliability in virtually every aspect of today's electronics environment. Many aspects of electrostatic control in the electronics industry also apply in other industries such as clean room applications and graphic arts.
Despite a great deal of effort during the past decade, ESD still affects production yields, manufacturing costs, product quality, product reliability, and profitability. Industry experts have estimated average product losses due to static to range from 12-35%. Others estimate the actual cost of ESD damage to the electronics industry as running into the billions of Rupees annually. The cost of damaged devices themselves ranges from only a few rupees for a simple diode to several thousand rupees for complex hybrids. When associated costs of repair and rework, shipping, labor, and overhead are included, clearly the opportunities exist for significant improvements.
Perhaps the greatest threat facing system builders is electrostatic discharge (ESD). ESD is the sudden discharge of static electricity causing a momentary current flow that can weaken or permanently damage semiconductor components (such as the processor, memory, motherboard, and video card). Therefore it is imperative that the industry (Manufacturers, Integrators, Dealers, service centers, end-customers) follow the correct ESD storage and handling procedures.
Electrostatic Discharge (ESD) can damage a sensitive electronic component, resulting in failures, reduced reliability and increased rework costs, or latent component failures in equipment.
Namaguchi & Uchida (ESD Symposium 1998) found that 60-90% of defective devices were damaged by ESD, and 70% of these failures were attributed to damage by ungrounded people.
ESD Damage—How Devices Fail
Electrostatic damage to electronic devices can occur at any point from manufacture to field service. Damage results from handling the devices in uncontrolled surroundings or when poor ESD control practices are used. Generally damage is classified as either a catastrophic failure or a latent defect.
Catastrophic Failure
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 causing the device fail. Such failures usually can be detected when the device is tested before shipment. If the ESD event occurs after test, the damage will go undetected until the device fails in operation.
Latent Defect
A latent defect, on the other hand, is more difficult to identify. 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 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 relatively easy with the proper equipment to confirm that a device has experienced catastrophic failure. Basic performance tests will substantiate device damage. However, latent defects are extremely difficult to prove or detect using current technology, especially after the device is assembled into a finished product.
Basic ESD Events--What Causes Electronic Devices to Fail?
ESD damage is usually caused by one of three events: direct electrostatic discharge to the device, electrostatic discharge from the device or field-induced discharges. 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 voltage levels involved. This is known as the device's "ESD sensitivity".
Discharge to the Device
An ESD event can occur when any charged conductor (including the human body) discharges to an ESDS (electrostatic discharge sensitive) device. The most common cause of electrostatic damage is the direct transfer of electrostatic charge from the human body or 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). A similar discharge can occur from a charged conductive object, such as a metallic tool or fixture. The model used to characterize this event is known as the Machine Model.
Discharge from the Device
The transfer of charge from an ESDS device is also an ESD event. Static charge may accumulate on the ESDS device itself through handling or contact with packaging materials, worksurfaces, or machine surfaces. This frequently occurs when a device moves across a surface or vibrates in a package. The model used to simulate the transfer of charge from an ESDS device is referred to as the Charged Device Model (CDM). The capacitance and energies involved are different from those of a discharge to the ESDS device. In some cases, a CDM event can be more destructive than the HBM for some devices.
The trend towards automated assembly would seem to solve the problems of HBM ESD events. However, it has been shown that components may be more sensitive to damage when assembled by automated equipment. A device may become charged, for example, from sliding down the feeder. If it then contacts the insertion head or another conductive surface, a rapid discharge occurs from the device to the metal object.
Field Induced Discharges
Another event that can directly or indirectly damage devices is termed Field Induction. As noted earlier, whenever any object becomes electrostatically charged, there is an electrostatic field associated with that charge. If an ESDS device is placed in that electrostatic field, a charge may be induced on the device. If the device is then momentarily grounded while within the electrostatic field, a transfer of charge from the device occurs as a CDM event. If the device is removed from the region of the electrostatic field and grounded again, a second CDM event will occur as charge (of opposite polarity from the first event) is transferred from the device.
How Much Static Protection is Needed?
As noted earlier, 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 voltage levels involved—its ESD sensitivity. Defining the ESD sensitivity of electronic components is the first step in determining the degree of ESD protection required. Test procedures based on the models of ESD events help define the sensitivity of components to ESD. These procedures will be covered in a future article in this series.
Many electronic components are susceptible to ESD damage at relatively low voltage levels. Many are susceptible at less than 100 volts, and many disk drive components have sensitivities below 10 volts. Current trends in product design and development pack more circuitry onto these miniature devices, further increasing their sensitivity to ESD and making the potential problem even more acute.
Basic Principles of Static Control
Sometimes, controlling electrostatic discharge (ESD) in the electronics environment seems to be a formidable challenge. However, the task of designing and implementing ESD control programs becomes less complex if we focus on just six basic principles of control. In doing so, we also need to keep in mind the ESD corollary to Murphy's law, "no matter what we do, static charge will try to find a way to discharge."
1. Design In Immunity: by designing products and assemblies to be as immune as reasonable from the effects of ESD
2. Define the level of control needed in your environment: needed in your environment.
3. Identify and define the electrostatic protected areas (EPA): the areas in which you will be handling sensitive parts.
4. Eliminate and Reduce Generation: by reducing and eliminating static generating processes, keeping processes and materials at the same electrostatic potential, and by providing appropriate ground paths to reduce charge generation and accumulation.
5. Dissipate and Neutralize: by grounding, ionization, and the use of conductive and dissipative static control materials.
6. Protect Products: with proper grounding or shunting and the use of static control packaging and materials handling products.
Typical Facility Areas Requiring ESD Protection:
- Receiving
- Inspection
- Stores and warehouses
- Assembly
- Test and inspection
- Research and development
- Packaging
- Field service repair
- Offices and laboratories
- Clean rooms
Grounding
Throughout this article, we have seen how important grounding is to effective ESD control. Effective ESD grounds are of critical importance in any operation, and ESD grounding should be clearly defined and regularly evaluated.
A primary means of protecting of ESD susceptible (ESDS) items is to provide a ground path to bring ESD protective materials and personnel to the same electrical potential. All conductors in the environment, including personnel, must be bonded or electrically connected and attached to a known ground or contrived ground, creating an equipotential balance between all items and personnel. Electrostatic protection can be maintained at a potential above a "zero" voltage ground reference as long as all items in the system are at the same potential. It is important to note that non-conductors in an Electrostatic Protected Area (EPA) cannot lose their electrostatic charge by attachment to ground.
ESD Association Standard ANSI EOS/ESD 6.1-Grounding recommends a two-step procedure for grounding ESD protective equipment.
The first step is to ground all components of the work area (worksurfaces, people, equipment, etc.) to the same electrical ground point called the "common point ground." This common point ground is defined as a "system or method for connecting two or more grounding conductors to the same electrical potential."
The second step is to connect the common point ground to the equipment ground or the third wire (green) electrical ground connection. This is the preferred ground connection because all electrical equipment at the workstation is already connected to this ground.
Connecting the ESD control materials or equipment to the equipment ground brings all components of the workstation to the same electrical potential. If a soldering iron used to repair an ESDS item were connected to the electrical ground and the surface containing the ESDS item were connected to an auxiliary ground, a difference in electrical potential could exist between the iron and the ESDS item. This difference in potential could cause damage to the item.
Controlling Static on Personnel and Moving Equipment
In many facilities, people are one of the prime generators of static electricity. The simple act of walking around or repairing a board can generate several thousand volts on the human body. If not properly controlled, this static charge can easily discharge into a static sensitive device—a human body model (HBM) discharge.
Even in highly automated assembly and test processes, people still handle static sensitive devices…in the warehouse, in repair, in the lab, in transport. For this reason, static control programs 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
Typically, wrist straps are the primary means of controlling static charge on personnel. When properly worn and connected to ground, a wrist strap keeps the person wearing it near ground potential. Because the person and other grounded objects in the work area are at or near the same potential, there can be no hazardous discharge between them. In addition, static charges are safely dissipated from the person to ground and do not accumulate.
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. Most wrist straps have a current limiting resistor molded into the ground cord head on the end that connects to the cuff. The resistor most commonly used is a one megohm, 1/4 watt with a working voltage rating of 250 volts.
Other methods of static control:
- Floors, Floor Mats, Floor Finishes
- Shoes, Grounders, Casters
- Clothing
- Workstations and Worksurfaces
- Production Equipment and Production Aids
- Packaging and Handling
- Ionization in Clean rooms: Ionizers are used when it is not possible to properly ground everything and as backup to other static control methods. In clean rooms, air ionization may be one of the few methods of static control available.
Your static control program is up and running. How do you determine whether it is effective? How do you make sure your employees follow it? We will focus on two more critical elements:
Training and Auditing:
Training:
The new ANSI/ESD S20.20 ESD Control Program standard cites training as a basic administrative requirement of an ESD control program. There is significant evidence to support the contribution of training to the success of the program. We would not send employees to the factory floor without the proper soldering skills or the knowledge to operate the automated insertion equipment. We should provide them with the same skill level regarding ESD control procedures.
Elements of Effective Training Programs:
1 -- Successful training programs cover all affected employees.
2 -- Effective training is comprehensive and consistent.
3 -- Use a variety of training tools and techniques.
4 -- Test, certify and retrain
5 -- Feedback, auditing, and measurement
Auditing
Developing and implementing an ESD control program itself is obvious. What might not be so obvious is the need to continually review, audit, analyze, feedback and improve. You will be asked to continually identify the program's return on investment and to justify the savings realized. Technological changes will dictate improvements and modifications. Feedback to employees and top management is essential. Management commitment will need reinforcement.
Like training, regular auditing becomes a key factor in the successful management of ESD control programs. The mere presence of the auditing process spurs compliance with program procedures. It helps strengthen management's commitment. Audit reports trigger corrective action and help foster continuous improvement.
The benefits to be gained from regular auditing of our ESD control procedures are numerous:
• They allow us to prevent problems before they occur rather than always fighting fires.
• They allow us to readily identify problems and take corrective action.
• They identify areas in which our programs may be weak and provide us with information required for continuous improvement.
• They allow us to leverage limited resources effectively.
• They help us determine when our employees need to be retrained.
• They help us improve yields, productivity, and capacity.
• They help us bind our ESD program together into a successful effort.
Requirements for Effective Auditing:
- Existence of written and well-defined standards and procedures.
- Taking of some measurements.
- Include all areas in which ESD control is required.
- Audit frequently and regularly.
- Maintain trend charts and detailed records and prepare reports.
- Implement corrective action
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for more information on ESD-DC and an effective static control program for your organization, contact:
Sam Communications
16-8-238/6/2
Ashraf Nagar
New Malakpet
Hyderabad-500024
email: memory.india@gmail.com
#: +91-9291523184
