What is electrostatic discharge and how can it be controlled to prevent damage to micro electronic components?
Electrostatic discharge (ESD) can cause damage to micro electronic equipment such as computer chips and other electronic components. How can you combat this problem? Implement an ESD control program. Such a program can increase profits by lowering component rejection rates, eliminating latent failures, and reducing the cost of field service.
But, investing in equipment and training represents just the first step in an ESD control program. To ensure success, the program must include a continual monitoring and testing effort. Let's look at the details.
What is ESD?
Electrostatic discharge is a phenomenon that occurs when two materials come together and then separate. At the time of separation, electrons are transferred from one material to the other; the material losing electrons is positively charged, and the material gaining electrons is negatively charged.
If a newly charged surface is conductive, its charge will generally move across its surface to ground and cause no problems, provided the surface itself is grounded. However if the surface is non-conductive, the charge will remain stationary (in other words, static) and retain its potential. It's important to remember that nonconductors cannot be grounded.
The human body is a good conductor; however, rubber-soled shoes insulate it from ground, and many clothing fabrics are poor conductors. Often, when repairing micro electronic equipment, a person can generate enough static electricity to damage several internal components.
The best way to counter ESD is to set up an ESD control program.
Setting up an ESD control program
Most assembly and repair on micro electronic boards are done at a workbench. In the past, many documents offered some information on how to set up and maintain an ESD-safe environment around a workbench. Now all of the information is available in a recently issued standard, EIA 625, Requirements for Handling Electrostatic Discharge Sensitive Devices. This 24-page document presents the methods and materials needed to protect electronic devices from static damage, up to a [+ or -]200V level. The following sections are of special importance.
* Section 5 discusses inspection frequency, testing equipment, and measurement methods. Items that must be checked and the frequency of the inspection are given in a table. Wrist straps and footwear must be checked daily. Weekly checks of work surfaces for cleanliness and ground wire connections are required. A monthly check for voltages greater than [+ or -]200V is called for. Work surfaces, flooring, and wiring must be tested quarterly. An annual audit of the entire system is also required. Records of checks should be kept for at least one year. Passing values can be recorded on a form, or a simple "pass" can be written down.
* Section 6 details the minimum requirements for an ESD protected work area. A static dissipative surface is preferred. Personnel must wear wrist straps or ESD footwear/heel grounds, and in some cases, a floor mat is also required. An object with a potential greater than [+ or -]200V should not be within 12 in. of an ESD-sensitive item.
* Section 7 describes how a grounding system should be set up for an ESD-safe area, using work surface mats, floor mats, and wrist straps, as shown in the diagram on page 72. An electrical system grounding conductor is preferred over the use of earth ground. If both are used, they should be bonded together as shown.
* Section 9 describes compliance to EIA 625 and verification of compliance, including the setting up of a site ESD coordinator or team; documentation of annual audits; and setting up of a procedure to deal with mishandled ESD-sensitive devices.
* Section 11 provides additional methods to reduce static charges: spraying anti-static solutions; establishing 40% relative humidity; using air ionizers; and using protective smocks, gloves, or finger cots. These methods must be used when unprotected devices are within 12 in. of static voltages at [+ or -]200V or greater.
ESD equipment details
In terms of test equipment, EIA 625 recommends a wrist-strap checker or constant monitors; a checker for footware or grounders; an electrostatic voltmeter; meters to test resistivity to ground and resistance top-to-top of the work surfaces; meters to verify ground connection integrity; and finally, a charge plate monitor to check air ionizers.
While an electrostatic voltmeter is more accurate (and expensive) than a field meter, the field meter may provide enough sensitivity in many cases. Field meters detect and measure the voltage and field strength of electrostatic charges on objects and materials.
Ionizers are available in two types: nuclear and electrical. A nuclear unit generates ions from an alpha-emitting isotope, usually Polonium 210. While highly effective, the unit must be leased and returned to the manufacturer each year for inspection and recharging. An electrical unit creates a high voltage potential at an air gap, thus ionizing the air around the gap. Since the ionizing points, or pins, tend to lose their sharpness slowly through erosion, the efficiency of the machine declines over time. A unit that compensates for this degradation is available, and this feature is useful for a company involved in ISO 9000 certification, because the ionizers have to be monitored regularly.
A number of new products have been developed for ESD control. One manufacturer has a disposable wrist strap that can be shipped with components to ensure protection at a receiving station or in the plant for short-term use. The manufacturer also has a wrist strap and footwear tester that checks the entire conductive path through a static control band, snap connector, built-in resistor, and the cord and plug of a wrist strap.
Anyone developing an ESD-control program should consider adapting EIA 625, or using it as a guide. The standard can be ordered by calling Global Engineering at 1-800-854-7179.