Part Two --- Principles of ESD Control
| In our previous article, Introduction to Electrostatic Discharge, we discussed the basics of
electrostatic charge, discharge, types of failures, ESD events, and device sensitivity. We
concluded our discussion with the following summary: 1. Virtually all materials, even conductors, can be triboelectrically charged. 2. The level of charge is affected by material type, speed of contact and separation, humidity, and several other factors. 3. Electrostatic fields are associated with charged objects. 4. Electrostatic discharge can damage devices so they fail immediately, or ESD may result in latent damage that may escape immediate attention, but cause the device to fail prematurely once in service. 5. Electrostatic discharge can occur throughout the manufacturing, test, shipping, handling, or operational processes. 6. Component damage can occur as the result of a discharge to the device, from the device, or from charge transfers resulting from electrostatic fields. Devices vary significantly in their sensitivity to ESD. |
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| Basic Principles of Static Control | ||
| 1. Design In Immunity |
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| The first Principle is to design products and assemblies to be as immune as reasonable from the effects of ESD. This involves such steps as using less static sensitive devices or providing appropriate input protection on devices, boards, assemblies, and equipment. For engineers and designers, the paradox is that advancing product technology requires smaller and more complex geometries that often are more susceptible to ESD. | ||
| 2. Define the level of control needed in your environment. |
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| What is the sensitivity level of the parts you are using and the products that you are manufacturing and shipping? ANSI/ESD S 20.20 defines a control program for items that are sensitive to 100 volts Human Body Model (HBM). Your environment may be different. | ||
| 3. Identify and define the electrostatic protected areas (EPA). |
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| These are the areas in which you will be handling sensitive parts and the areas in which you will need to bond or electrically connect all conductive and dissipative materials, including personnel, to a known ground | ||
| 4. Eliminate and Reduce Generation |
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| Obviously, product design isn't the whole answer. The fourth Principle of control is to eliminate or reduce the generation and accumulation of electrostatic charge in the first place. It's fairly basic: no charge -- no discharge. We begin by reducing as many static generating processes or materials, such as the contact and separation of dissimilar materials and common plastics, as possible from the work environment. We keep other processes and materials at the same electrostatic potential. Electrostatic discharge does not occur between materials kept at the same potential or at zero potential. We provide ground paths, such as wrist straps, flooring and work surfaces, to reduce charge generation and accumulation. | ||
| 5. Dissipate and Neutralize |
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| Because we simply can"t eliminate all generation of static in the environment, our fifth Principle is to safely dissipate or neutralize those electrostatic charges that do occur. Proper grounding and the use of conductive or dissipative materials play major roles. For example, workers who "carry" a charge into the work environment can rid themselves of that charge when they attach a wrist strap or when they step on an ESD floor mat while wearing ESD control footwear. The charge goes to ground rather than being discharged into a sensitive part. To prevent damaging a charged device, the rate of discharge can be controlled with static dissipative materials. | ||
| 6. Protect Products |
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| Our final ESD control Principle is to prevent discharges that do occur from reaching susceptible parts and assemblies. One way is to provide our parts and assemblies with proper grounding or shunting that will dissipate any discharge away from the product. A second method is to package and transport susceptible devices in proper packaging and materials handling products. These materials may effectively shield the product from charge, as well as reduce the generation of charge caused by any movement of product within the container. | ||
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