Grounding the Sparks
Most electronic components are susceptible to electrostatic discharge (ESD) damage at relatively low voltage levels. Many devices are susceptible at less than 100 volts, while disk drive components have sensitivities below 10 volts. Medical devices and fiber optic components also are extremely vulnerable to the effects of static.
Damage can occur at almost any time before, during and after assembly, rework, automated handling or transport. Despite ongoing efforts to curtail ESD with new technology, the age-old problem continues to affect production yields, manufacturing costs and product quality.
Electrostatic discharge refers to the sudden transfer of free electrons from a positively charged object to a negatively charged object. The Electrostatic Discharge Association (ESDA, Rome, NY), defines static electricity 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."
The most common way to create an electrostatic charge is through friction, or triboelectric charging. When two dissimilar materials are rubbed together and separated, a transfer of electrons from one material to the other may occur. This exchange of energy constitutes the electrostatic discharge, which takes place within microseconds or nanoseconds.
Electrostatic charge is often created by the contact and separation of two materials. For example, a person walking across a floor generates static electricity as shoe soles contact and then separate from the floor surface. An electronic component 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. A significant electrostatic charge can also be generated simply by sliding a plastic-encased screwdriver across a metallic work surface.
Static SourcesThe amount of static electricity that is generated depends on the material, the amount of friction and the relative humidity of the environment. Plastic generally creates the greatest static charge. For instance, thin-film plastic commonly used to wrap parts or package components, can pose numerous ESD headaches.
Low humidity conditions, such as when indoor air is heated during the winter months, also generates significant amounts of static. When the air is humid, a thin moisture layer forms on the surface of many insulating materials, which helps electrostatic charges dissipate. Generally, anything above 30 percent relative humidity will help prevent static charges from building up.
Steve Halperin, ESDA president, says electrostatic discharge can wreak havoc on electronic components. For instance, it can change the electrical characteristics of a semiconductor device, degrading or destroying it. It 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.
According to Halperin, who also runs a Bensenville, IL-based consulting company, ESD occurs far more frequently than most people realize. In fact, many ESD events go unnoticed because static electricity is invisible.
Indeed, most electrostatic discharges pass unnoticed by assemblers. Halperin says ESD damage is generally not visible as it occurs. It may be latent or not show up in functional testing of electronic devices.
More than 75 percent of all ESD is generated by the human body. The routine movement of an assembler sitting at a workbench can generate as many as 6,000 volts of static. But, in order for humans to feel static electricity, the discharge typically must be 3,000 to 4,000 volts.
"While you can feel electrostatic discharges of 3,000 volts, smaller charges are below the threshold of human sensation," says Dave Bermani, corporate marketing coordinator at Desco Industries Inc. (Chino, CA). "Unfortunately, smaller charges can and do damage semiconductor devices. Many CMOS [complementary metal oxide semiconductor] components can be damaged by charges of less than 1,000 volts. Some of the more sophisticated components can be damaged by charges as low as 10 volts."
Expensive DilemmaThe cost of ignoring ESD can be staggering. One little "shock" can destroy a component—or worse, significantly shorten its life and result in a field failure.
Up to 25 percent of all electronic product failures can be attributed to ESD. Electronics manufacturers lose millions of dollars annually due to ESD damage.
"The cost of damaged devices themselves ranges from only a few cents for a simple diode to several hundred dollars for complex hybrids," claims Halperin. Those numbers multiply when costs associated with repair and rework, labor, supplies, equipment and lost production time are factored in.
The damage done by ESD takes two forms: catastrophic failure and latent failure. Catastrophic failure renders a component or circuit card instantly defective. An ESD event may cause a metal melt, junction breakdown or oxide failure. Even at ESD voltages of less than 200 volts, gate oxide destruction can occur in a semiconductor, completely changing its electrical characteristics. Low ESD voltages can also cause junction failure where the bonding wire attaches to the package leads.
Basic quality and performance tests usually detect these failures long before product shipment. In this case, swapping out the damaged part or card immediately rectifies the problem. However, if damage occurs after testing, it will often go undetected until the device fails in operation.
Latent failures are less obvious and are more difficult to identify. A device exposed to ESD may be partially degraded, yet continue to perform its intended function throughout the testing process. Latent defects occur when ESD weakens or wounds a component to the point where it will still function correctly during test and inspection. But, the component may cause poor system performance or complete system failure once the product gets shipped to the customer and placed in the field. Latent defects are difficult to detect by any process other than examining parts under an electron microscope.
There are two basic types of ESD models: a human body model (HBM) and a charge device model (CDM). The human body is one of the most common and most damaging sources of electrostatic discharge. The HBM simulates when a discharge occurs between a human body part, such as a hand or a finger, and a conductor, such as a metal rail. The HBM typically occurs when somebody touches a part and zaps it. These types of zaps occur pin-to-pin from the package pins and are protected against via the input-output cells. All parts should be able to tolerate 2,000 volts of HBM zaps.
To test for HBM static in electronic components, "a charged 100pF capacitor is discharged into the device via a 1,500 watt resistor," says Jeremy Smallwood, Ph.D., president of Electrostatic Solutions Ltd. (Southampton, UK). "The 100pF capacitor simulates charges stored on the average human body, and the resistor simulates the resistance of the human body and skin."
The CDM-type zaps occur more commonly in automated assembly lines. When a part sits on a charged plate, it builds a charge and then gets zapped when a machine, such as a robotic arm, handles it. Because the device builds up a charge internally, all internal structures are at risk. It is more difficult to protect against. Components should be capable of tolerating 200 to 500 volts.
A third type of model, the machine model (MM) simulates ESD events occurring during automatic handling operations. It originated in Japan as a worst-case HBM.
Smaller PackagingAlthough today’s electronic devices are faster, cheaper and more powerful than just a few years ago, the risk of ESD is greater than ever. The drive for miniaturization has reduced the width of electronic device structures, but increased ESD susceptibility.
"The number of ESD-related incidents has gone up considerably in the last 5 years," claims Phil Baratti, manager of applications engineering for factory automation and robotics at Epson America Inc. (Carson, CA). "It’s definitely increasing. Components are becoming more and more susceptible to static."
According to Baratti and other observers, the ongoing trend toward smaller and smaller packaging is the culprit. "When you make things smaller, they’re inherently more delicate," notes Baratti.
"The increasing sophistication of electronic devices has continued to make them more and more susceptible to ESD-related damage," adds Desco’s Bermani. "As the size of the components is reduced, so is the microscopic spacing of insulators and circuits within them, increasing their sensitivity to ESD.
"Typically, surface-mount devices have much smaller architecture, making them more susceptible to ESD than through-hole packaged devices," explains Bermani. "The width of the circuitry conductors is as small as 0.1 micrometer. To pack more and more circuitry into small packages, the spacing isolating circuitry has been reduced and can be as little as 300 micrometers.
"For integrated circuit packaging, the I/O count has climbed from 600 to more than 1,000," Bermani points out. "The spacing between the I/Os has decreased dramatically. Where wire bonding is used, the air gap becomes that much smaller, making the neighboring I/Os even more susceptible to ESD."
Today’s operating voltages of as little as 1.5 volts and chip-set traces measuring only 400 angstroms in width contribute to this vulnerability. "As component technology progresses, internal device sizes reduce and become more ESD sensitive," says Electrostatic Solutions’ Smallwood. "Many modern components are protected by on-chip protection circuits, without which they would be extremely sensitive."
In most cases, the design goal is to increase the amount of ESD voltage that the device can withstand. In some cases, this goal cannot be met for various reasons. "There is often a tradeoff between ESD protection and device performance," Smallwood points out.
Control ToolsIn addition to the trend toward smaller architectures, lean manufacturing initiatives and ISO 9000 certification have sparked growing concern over ESD. Fortunately, the pesky problem of electrostatic discharge can be controlled and monitored. Manufacturing engineers have numerous tools to choose from, ranging from inexpensive wrist straps to expensive ionizers. "However, that range of options someArial causes engineers to do an overkill, instead of identifying hot spots," says Baratti.
Some engineers make the mistake of not controlling ESD from start to finish. It’s important to examine the entire manufacturing cycle, from receiving and storage to parts kitting and assembly to testing and packaging. "Assembly lines should be as diligent with their ESD control program as hospital operating rooms are in implementing sterilization procedures," warns an electronics industry veteran.
A wide variety of products are available to control ESD in production environments. Preventative measures include the use of wrist and heel straps, floor and table mats, static-dissipative lab coats and ESD-shielding bags. Static-dissipative chemicals, such as hand lotion and floor finish, are also available.
Products that control ESD work by charge prevention, grounding, shielding and neutralization. All static control products function by: reducing charge accumulation; providing a path for the static charge to move away from sensitive components; or shielding components from static fields or charges.
The word "antistatic" is often used in a generic sense to describe the full range of static control materials and products. But, this term has been misused and misapplied, resulting in a great deal of confusion between suppliers and end users.
A new term, "low charging," has replaced the word antistatic. It refers to the low static charge generation between surfaces that contact and separate. A material that inhibits the generation of static charges from triboelectric generation is classified as low charging. A low charging material can be conductive, dissipative or insulative. Only conductive or dissipative materials should be used in ESD safe areas.
Three types of materials can be used for static control: conductive, dissipative and insulative. These material properties govern what happens after the material is charged. Conductive materials allow electrons to move freely across their surface or through their volume. Dissipative materials allow electrons to move more slowly. Insulative materials do not allow electrons to move.
A conductive material has a surface resistivity of less than 1x105 ohms per square centimeter (ohms/cm2). A dissipative material typically has a surface resistivity greater than 1x105 ohms/cm2, but less than 1x1012 ohms/cm2. Anything with a surface resistivity greater than 1x1012 ohms/cm2 is considered insulative.
One common misconception is that conductive materials do not generate charges. This is because the dissipation of static charges from grounded conductive material tends to be complete and rapid. Ungrounded conductors can generate and hold static charges.
Human InterventionStatic electricity can be conducted to an assembly through the human body or a machine. However, Smallwood says the most common cause of damage usually occurs through the direct transfer of electrostatic charge from the human body to the ESD-sensitive device. Because many problems arise from ungrounded operators, most preventative measures, such as wrist straps, focus on this channel of transference.
A wrist strap connects the wearer to the ground. Wrist bans are typically made from elastic nylon fabric that has conductive fibers on the inside surface. These conductive fibers connect to the skin with a coiled cord. One end of the cord snaps to the wrist band; the other end plugs into a ground point.
Special ESD chairs and workstations are available. Every workbench should have a dissipative-grounded work surface, a common point ground or continuous monitor with banana jacks for grounding wrist straps and a ground cord connected to the common point ground or continuous monitor.
Electrically conductive or dissipative floor mats conduct a charge when grounded. Mats are typically either made from vinyl or rubber material and can be homogeneous or multilayered. Rubber mats have good chemical and heat resistance, but vinyl tends to be more cost-effective.
Ionizers allow assemblers to dissipate static charges from any insulating materials quickly and easily. They neutralize a static charge on the surface of nonconductive materials by blowing air filled with an equal number of negative and positive ions across the material. Ionizers are available in many different sizes and configurations, including blowers and static bars.
Monitors test wrist straps, floor mats and other protective products continuously. They sound an alarm if there is a problem.
Resistance meters check a material’s ability to move a charge across the surface. They can be used to check packaging, flooring and work surfaces.
Despite environmental efforts to minimize static on assembly lines, if screwdrivers and other production tools lack sufficient grounding, electronic products will still be at risk. For instance, an operator may be totally grounded with wrist and shoe straps, but if he brings a sensitive component or circuit card in contact with an ungrounded electric screwdriver, ESD damage can occur.
"Electrostatic charges can build up on most fastening tools," claims Gordon Wall, Ph.D., development manager for electronic products at Mountz Inc. (San Jose, CA). "Unless those charges are dissipated or prevented, damage to the components can result."
Screwdrivers and other electric fastening tools should have an uninterrupted ground path from the bit to the power outlet. To maximize the path for dissipating static charges, Wall says the resistance between the part to be fastened and earth-ground should be less than 1 ohm. To achieve that, the tool housing should be made of nylon or other conductive material. The transformer should also be fully grounded.
Industrial robots and other automated assembly equipment also are susceptible to electrostatic discharge. However, Epson’s Baratti claims that these devices have a finite amount of variables that can generate static. By comparison, he says a human operator has an infinite amount of variables that must be studied and treated.
According to Baratti, manufacturing engineers should check for any equipment hot spots that generate excessive static. "Any plastic components that are constantly moving, such as cable ducting, can generate unwanted static," he points out.
The motor assembly should be built to dissipate charges. "A key identifier for an ESD-compliant robot is its chrome cover," says Baratti. "A special coating allows it to ground out." Baratti also recommends using grippers that are nickel-plated. "Anodized grippers don’t have the correct ground path," he claims.
Industry StandardsUntil recently, no standards existed to help manufacturing engineers determine acceptable ESD levels. "Each customer had their own limits for determining how much static was acceptable, and their own methods for testing static discharge," notes Baratti. Every manufacturer had a different set of requirements.
But, that lack of consistency is beginning to change. With the recent release of the ANSI/ESD S 20.20 standard, the electronics industry now has a consistent set of rules for the development of all ESD control programs.
The standard was developed by ESDA in response to a request from the U.S. Department of Defense to prepare an ESD process standard to replace MIL-STD-1686. The document covers all elements of a static control program, rather than concentrating on individual components such as work surfaces.
It has two major sections. An administrative section outlines all the documentation, training and process requirements. A technical section outlines the electrical and mechanical requirements, such as grounding, protected areas and packaging. Both administrative and technical provisions can be tailored to specific applications and requirements.
The standard is flexible enough to allow companies with established ESD programs to adopt it. And, to reduce duplication of process controls, it has requirements acceptable to both OEMs and contract manufacturers.
The 20.20 standard provides a consistent set of rules for everyone in the industry to follow. They help alleviate disagreements about test methods, performance attributes and material specifications.
"It levels the playing field between all OEMs and contract manufacturers," says David Swenson, technical service specialist for the electronic handling and protection division of 3M Co. (Austin, TX). "Now everyone has one agreed-to static control plan. And that’s very good for the industry because it’s going to save everyone a lot of time and money."