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Static electricity is now viewed as a source of industrial contamination. It is a significant problem and managers at most facilities that manufacture electronic devices like cell phones and personal music players know that they must be exceptionally vigilant about electrostatic discharge (ESD).

On average, says the ESD Association, stray electrostatic discharges destroy about 16 to 22 percent of electronic components before they are installed into an assembly. After assembly, anywhere from 33 to 70 percent of digital devices fail soon after customers purchase them because ESD may only damage a component, enabling it to function for a brief time before total failure.

The costs of these losses can reach into the billions of dollars annually. Not only does the cost include the loss of the damaged product, it also includes all the repair, rework, shipping, labor and overhead costs associated with the damage. As engineers at companies like IBM, Jabil, Flextronics, Selectron and Sanmina-SCI develop electronic chips and components that can be destroyed by as few as 5 volts, the costs could reach even higher.

Most contract manufacturing facilities can control static electric discharges as low as 100V. As mentioned earlier, however, that level will not be sufficient for components sensitive to 50 or even 5 volts. Increasingly, these facilities will need to prove that they have eliminated all potential opportunity for electrostatic discharge.

The Development of Static
Exactly how static electricity occurs and how to control it are subjects that are not as well understood as they need to be. There are several “myths” or misconceptions about the discharge of electrostatic energy (often referred to as ESD events), that lead to its continued occurrence in allegedly correctly designed facilities.

In an ESD event, a static charge transfers to something. Preferably, that something is the ground, but often the energy moves to a sensitive component.

People are one of the more common sources of static. They acquire a static charge from handling insulating material, movement of clothing, sitting, standing, changing position, or walking across a floor, a phenomenon known as tribocharging.

Tribocharging most commonly arises from rapid contact and separation of two materials, such as shoes walking across a floor. The shoes and the floor have charges at different strengths and polarity. During contact, the materials form a slight chemical bond where electrons are exchanged to equalize their electrochemical potential. When the materials separate, some of the bonded atoms keep the extra electrons, while others give them away, resulting in a potential charge.

The factors that influence tribocharging include how intimately two materials make contact, how quickly they separate, the materials’ conductivity levels, and relative humidity levels.

The most common ways to provide a safe path to ground for static electricity are the use of ground wires to benches, wrist and heel straps, and ESD type flooring. The goal should be to have as near zero potential for an ESD event as possible.

Ten Myths, Demystified
Even through use of the above methods, there are still opportunities for ESD to occur because people tend to make assumptions about ESD. Here are some of the more prevalent myths, and the reality behind them.

1. “Did you see that?”
Some people think that an electrostatic discharge is something you can see, like the lightning-like zap that can happen when you walk across a carpeted floor and reach for a metal door knob. The familiar spark seen at these ESD events is only felt when a body is discharging at least 3500V. Discharges of 100, 50, and even 5V will not be seen by the human eye, even those these discharges can still damage components.

2. “I didn’t touch anything, honest!”
Some think that static discharge only occurs when you touch a component. Touching a sensitive component is one way for static charge to move, but it is not the only way. Because of the tribocharging effect, we constantly build a field of static electricity charge around us, which can discharge to a ground source even from a distance of several feet. Thus, an individual walking from the door to a position near the assembly bench can build a field of electrostatic charge around their body. Before he has had a chance to put on wrist and heel straps, the field around his body can discharge, travel to sensitive components, and destroy them.

3. “Spread the heel straps around, so everyone gets one.”
Some people think one heel strap is sufficient to provide a ground. Not true. Instrument readings on a person’s electrical charge show that, when a protected foot touches ground, there is no charge, but when the other non-strapped foot touches ground, the instrument records a voltage. Thus, two heel straps are recommended. And they need to be in good working order. Heel straps wear out and need frequent replacement to help guarantee zero potential for ESD. Bench wiring can fail too, so that must be checked often as well.

4. “I used my designer sneakers.”
Some people think that wearing sneakers or shoes made from polyurethane will protect against ESD events. The fact is that these types of shoes can themselves be a problem. The goal in any ESD-rated facility is to provide a clear path to ground for static discharge. Ideally, this path should be one of least resistance. If the ground connection is through the floor, then employees need to wear shoes that help their personal static buildup discharge through those shoes to the ground plane in or underneath the flooring. Insulative shoes break that path to ground, preventing safe static discharge. Meanwhile, the individual is still building up static charge on and around their body, a charge that will find the path of least resistance to ground, even if that path leads through sensitive components.

If the facility does not have a floor designed to handle ESD events (i.e., an ESD floor), no type of shoe will help safely discharge static. If the facility has an ESD floor which provides a ground, then ESD shoes or heel straps are needed to ensure a clear path to ground.

Ground Issues and #5, “Polymers conduct just fine!”
Polymer flooring systems have been used to protect sensitive parts against static discharge for years. Caution is needed, though, when evaluating different polymer systems. First, polymers are poor conductors of electricity. They are often used as insulative materials in electrical and electronic applications. Second, formulation differences can give misleading information depending on the test method used. In general, epoxies have high mechanical strength, and are durable and abrasion resistant. Their chemical composition can be varied to provide resistance to a range of aggressive chemicals.

6. “Resistance testing is futile.”
The most common method used to characterize a floor’s ability to control static is to measure electrical resistance. Resistance, however, is an indication of how easily a charge will move through that object or material. Tests that measure resistance, by themselves, do not adequately predict the static-generating or static-dissipative properties of the flooring material.

However, many installers and contract manufacturing organizations insist on using resistance as an indicator of a floor’s ability to cope with ESD events. They will refer to standard protocols from such organizations as ASTM, EOS/ESD and ANSI, which include ASTM F-150, NWC TP 5786 (Mil spec), EOS/ESD 7.1, ESD STM 97.2, and ANSI/ESD Association Standard S20.20-1999. These standards, which were adapted for use within the microelectronics industry to address concerns with ESD, provide instructions on how to test for resistance.

Floors tend to come in two ranges of measured resistance. In the “conductive range,” the measured resistance will fall between 2.5 x 10⁴ and 1 x 10⁶ ohms. A floor within this range is considered conductive, that is, it will conduct an electrostatic discharge rather quickly to a ground.

The second range is referred to as dissipative. Measured resistance falls between 10⁶ and 10⁹ ohms. Floors with measured resistance within this range will also move charge, but at a slower rate than floors with measured resistance in the conductive range. Measurements higher than this range indicate that it will be too difficult to use the floor as a means of moving a charge to ground.

Having a floor with a specific resistance level, however, does not eliminate the risk of ESD. Resistance does not eliminate charge. Floor resistance levels only indicate how easily a charge will move to ground.

7. “Ohms vs. Volts.”
It is not the number of ohms that damage sensitive components but, rather, the number of volts building up in an environment looking for a source to ground. The focus on ohms, plus the inaccurate use of the terms resistance, conductive, and dissipative, have helped promote confusion and multiple myths about electrostatic discharge.

A facility is likely to have multiple sources of electrostatic buildup. It is prudent to have multiple methods for controlling this buildup. People, footwear and floors need to be evaluated together.

If the floor is or will be your primary method for grounding, though, ANSI/ESD Association Standard S20.20-2007 recommends that you conduct two tests to determine the floor’s ability to move charge to ground. These tests are the human body model (HBM) test and the body voltage generation (BVG) test.

HBM measures the resistance of a floor with a person as part of the electrical circuit. BVG measures how many volts a body has built up, that is, the charge of a person. (If the floor is not your primary method of grounding, then you can remain with ESD protocol 7.1 found in the standard ANSI/ESD Association Standard S20.20-1999.)

Equipment for the BVG test includes a voltage meter and a charged plate monitor. The equipment measures how large a charge a person carries, and how quickly that charge moves off of the person. The BVG test specifies that a 5000V charge be dissipated in 1/10^th of a second. Theoretically, BVG should be tested under laboratory conditions.

8. “Zero potential not possible”
It is possible to achieve an environment with near zero potential for static charge through the following steps. At minimum, all equipment, tools, and employees must be grounded. Wrist and heel straps and wiring benches to ground are musts. And employees must also wear ESD shoes in conjunction with ESD flooring.

The floor, though, can be a major tool in the control of ESD. One choice is tiles. Tiles last for a while, but then they age and become damaged, needing repair. Tiles cannot be refurbished. Therefore, the only option is replacement of worn tiles. The result is often an unattractive mix of old and shiny new tiles.

The next flooring choice is based on two kinds of technology and two kinds of delivery vehicles. The technologies are fiber systems or particulate systems. The vehicle choices are epoxy or polyurethane.

For a time, fiber floor technology was a manufacturer’s only option. With fiber floors, the basecoat is electrically conductive. The topcoat is modified with extremely fine electroconductive fibers. These fibers must orient in such a way that they create an electrical pathway from the flooring surface down through the topcoat to the underlying conductive basecoat. Ideally, when a person walks on these floors, static charges should travel from their body through their shoes to the conductive fibers in the floor and down to the groundling plane basecoat.

Installation consists of several steps. First an insulative primer is rolled onto the concrete floor to get it to a baseline. Concrete tends to be inconsistent in its ability to conduct a charge, so a primer smoothes out these inconsistencies. The next steps are to apply the ground plane primer, then the floor coating. This coating, which is usually mixed on site, is typically installed to a depth of between 60 to 120 mils to give the carbon fibers enough room to stand vertically.

9. “Just match the fibers with a floor patch.”
Fiber floors look great, but they are hard to install, hard to make uniform, and hard to repair. When worn, the fibers show through, for a salt and pepper effect. When the floor is damaged, its measured resistance levels can change.

You cannot cut out the damaged portion and put in a new patch because it’s too difficult to match the old and new fibers. Lastly, this type of floor needs some humidity to function.

Particulate ESD floors do not contain fibers or carbon. Instead they contain a special subclass polymer specifically designed for ESD events. The material is a type of spongy pigment, often blue-gray in color. It is mixed with polymer resins to a homogenous mix at a factory under controlled conditions. Its homogeneity ensures uniform readings.

Like the fiber floor installation, the first coat is an insulative primer that is rolled over a concrete floor. Then the particulate ESD coating is installed and usually spread to a thickness of 10 to 15 mils, and the installation is complete. The particulate system is its own ground plane.

For additional leveling, you can install an intermediate coat of epoxy to achieve greater thickness to make it appear similar to fiber floors. The particulate polymer coating is available in conductive and dissipative ranges.

10. “There’s no hope for worn fiber floors.”
Beyond providing an excellent ground, the advantages of an ESD particulate floor are that: 1) it wears well; 2) it has uniform resistance readings; 3) it is easy to repair; 4) it is economical; and 5) it is easy to maintain. In addition, it can be used as a top coat for fiber floors that have lost their uniform resistance readings due to wear. From personal experience, we know of a number of fiber floors that have been repaired simply by installing these floor systems over existing fiber floors and ESD tile floors.

Summary
Static electricity and both the latent (down the road) and catastrophic (immediately damaged) nature of its impact on manufacturing cannot be understated. There are countless ways manufacturers can both anticipate and dissipate this silent enemy. An honest look at your facility’s procedures (both written and those actually followed) is one major step towards improvement. Consider your environment, the habits of those directly involved, and how you can educate them to the dangers. By dispelling myths in both their day-to-day activities as well as the facility design, you should see dramatic improvement in your ability to control this silent enemy.

Don Schutz is with ICS Building Technologies Corporation, and can be reached at dschutz@icshome.com.