Five principles form the basis for material selection and program decisions.
Over the years, the ESDA has regularly advised applying this principle to static control: keep it simple and keep it focused. Although technology, processes and materials have continued to evolve, the design and implementation of effective static control programs continue to be based on the following five concepts:
- Design static protection into devices and products.
- Eliminate static generating materials and processes.
- Dissipate or neutralize static charges on process materials, equipment and personnel.
- Provide physical protection from discharge events.
- Measure processes and environment frequently to make sure everything is working correctly and make necessary corrections and improvements.
Design in protection. Our first principle is to design devices, products and assemblies to be as robust as is reasonable to the effects of ESD. Whenever possible, use less-static-sensitive devices or provide appropriate input protection on those ESD-sensitive (ESDS) devices that are used. The paradox here is that advancing product technology means further miniaturization and higher speed of operation that often are more susceptible to ESD and preclude the installation of protection devices at the chip level. However, the more ESD control that is built in to product design, the fewer problems later.
Eliminate and reduce generation. Obviously, product design isn’t the whole answer. You cannot get away from ESDS devices and other products that are sensitive to some form of electrostatic charge and discharge, but the generation and accumulation of electrostatic charge can be reduced to below the threshold of problems in most processes.
Start by reducing or eliminating static generating processes or materials, such as common plastics and other static-generating materials, from the work environment. ANSI/ESD S20.20-2007, “Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Device)” and IEC 61340-5-1-2007, “Electrostatics – Part 5-1: Protection of electronic devices from electrostatic phenomena – General requirements,” both require a careful review of the process areas to remove any unnecessary insulators. Because ESD does not occur between materials kept at the same potential or at zero potential, processes and materials in the work environment should be kept at the same electrostatic potential. Typically, these conductive or dissipative materials should be electrically bonded to the same common point ground, such as the verified utility or earth ground. In addition, provide ground paths with wrist straps, flooring, or work surfaces to safely reduce charge generation and accumulation on all personnel and conductive or dissipative items that enter the defined electrostatic protected area (EPA).
Dissipate and neutralize. Because all generations of static cannot be totally eliminated, our third 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 by attaching a grounded wrist strap or by stepping on a grounded ESD floor mat while wearing ESD control footwear. The charge goes to ground rather than being discharged into a sensitive part.
For some objects, such as common plastics and other insulators, grounding cannot remove an electrostatic charge. Typically, ionization is used to neutralize charges on these insulating materials. The ionization process generates negative and positive ions that are attracted to opposite charges on the surface of a charged object, thereby effectively neutralizing the charge. Use of dissipative materials for work surfaces and at the point of contact to a device can also be effective.
Protect products. Our fourth principle is to prevent discharges that do occur from reaching susceptible parts and assemblies. One way is to provide parts and assemblies with proper grounding or shunting that will dissipate any discharge away from the product. A second method is to package and store or transport susceptible devices in proper packaging and materials handling products. These materials may effectively shield the product from charge and discharges and reduce the generation of charge caused by any product movement within the container.
Proper packaging and materials handling not only provide protection within our own facilities and process, but may also provide protection when the final product reaches the consumer.
Measure and improve. Finally, measure your processes and environment. Don’t ask whether a process will create an ESD hazard and then guess at the answer; measure it. For example, use a fieldmeter to detect the presence of an electrostatic field that could pose an ESD threat. Measure the resistance to ground of personnel and process equipment. Measure the voltage accumulation on personnel and components in the work environment. Measurement is a win/win situation. Identify and quantify those areas that really do need static protection, so focus can be on those areas that are of the greatest concern. In addition, you can identify areas that do not pose an ESD hazard, saving you the expense of providing unneeded protection.
These five principles form the basis for effective static control programs. They aid in the selection of appropriate materials and procedures to use in effectively controlling ESD. In most circumstances, effective programs will involve all these concepts. No single procedure or product will do the whole job. In developing control programs, identify the devices that are susceptible, their level of susceptibility, and those operations that pose an ESD hazard to them. Then ask which of these concepts will protect these devices. Finally, select and implement the combination of procedures and materials that accomplishes the task.
References
1. ESD Association, 7900 Turin Road, Bldg. 3, Rome, NY 13440, 315-339-6937, esda.org.
2. IEC – International Engineering Consortium, iec.org.
This column is written by The ESD Association (esda.org); This email address is being protected from spambots. You need JavaScript enabled to view it..
