Effects of Static Electricity The effects of ESD are not recognized. Failures due to ESD are often misanalyzed as being caused by electrical overstress due to transients other than static. Many failures, often classified as other, random, unknown, etc., are actually caused by ESD. Misclassification of the defect is often caused by not performing failure analysis to the proper depth. COMPONENT SUSCEPTIBILITY All solid-state devices, except for various power transistors and diodes, are susceptible to damage by discharging electrostatic voltages. The discharge may occur across their terminals or through subjection of these devices to electrostatic fields. LATENT FAILURE MECHANISMS The ESD overstress can produce a dielectric breakdown of a self-healing nature when the current is unlimited. When this occurs, the device may retest good, but contain a hole in the gate oxide. With use, metal will eventually migrate through the puncture, resulting in a shorting of this oxide layer. Another structure mechanism involves highly limited current dielectric breakdown from which no apparent damage is done. However, this reduces the voltage at which subsequent breakdown occurs to as low as one-third of the original breakdown value. ESD damage can result in a lowered damage threshold at which a subsequent lower voltage ESD will cause further degradation or a functional failure. ESD ELIMINATION The heart of an ESD control program is the ESD-protected work area and ESD grounded work station. When you handle an ESD-sensitive device outside of its ESD protective packaging, you need to provide a means to reduce generated electrostatic voltages below the levels at which the item is sensitive. The greater the margin between the level at which the generated voltages are limited and the ESDS item sensitivity level, the greater the probability of protecting that item. PRIME GENERATORS All common plastics and other generators should be prohibited in the ESD protected work area. Carpeting should also be prohibited. If you must use carpet, it should be of a permanently antistatic type. Perform weekly static voltage monitoring where carpeting is in use. PERSONAL APPAREL AND GROUNDING An essential part of the ESD program is grounding personnel and their apparel when handling ESDS material. Means of doing this are described in this section. Smocks Personnel handling ESDS items should wear long sleeve, ESD-protective smocks, short sleeve shirts or blouses, and ESD-protective gauntlets banded to the bare wrist and extending toward the elbow. If these items are not available, use other antistatic material (such as cotton) that will cover sections of the body that could contact an ESDS item during handling. Personnel Ground Straps Personnel ground straps should have a minimum resistance of 250,000 ohms. Based upon limiting leakage currents to personnel to 5 milliamperes, this resistance will protect personnel from shock from voltages up to 125 volts RMS. The wrist, leg, or ankle bracelet end of the ground strap should have some metal contact with the skin. Bracelets made completely of carbon-impregnated plastic may burnish around the area in contact with the skin, resulting in too high an impedance to ground. ESD-PROTECTIVE MATERIALS There are two basic types of ESD-protective material—conductive and antistatic. Conductive materials protect ESD devices from static discharges and electromagnetic fields. Antistatic material is a nonstatic generating material. Other than not generating static, antistatic material offers no other protection to an ESD device. Conductive ESD-Protective Materials Conductive ESD-protective materials consist of metal, metal-coated, and metal-impregnated materials. The most common conductive materials used for ESD protection are steel, aluminum, and carbon-impregnated polyethylene and nylon. The latter two are opaque, black, flexible, heat sealable, electrically conductive plastics. These plastics are 10-17