What are the standard requirements for an anti-static work area?
The protective wax applied to the anti-static floor surface should be conductive. For EPAs, the following equipment should also be provided when possible: Wristband monitor: To ensure the electrostatic leakage effect of wristbands, wristband monitors should be installed in EPA areas. The instrument will alarm when the resistance value of the wristband system worn by the operator of EDS devices exceeds or falls below the instrument's set resistance range. The wristband should be replaced immediately. Electrostatic voltmeter, ohmmeter, high-resistance meter: These instruments can be used to determine whether the products and equipment used in the EPA area meet the anti-static requirements. Grounding monitor: This instrument can monitor the integrity of the grounding system. It can detect changes in grounding system resistance or open circuits.
1. Grounding in EPAs: EPA grounding wires can be bare or non-bare. If non-bare wires are used, their electrical conductivity must be checked frequently. Grounding wires used at all times should be corrosion-resistant and have sufficient mechanical strength. Grounding connections and network connections must be secure and reliable. Using grounding clamps or welding can ensure very low grounding resistance. Grounding potential of electronic testing equipment and tools: The design and construction of EPA areas and workbenches should ensure that all external surfaces and shielding of electronic testing equipment and power tools are at a common ground potential throughout the entire working period. The design should include consideration of grounding faults and voltage ranges established for locations with hazardous voltages or currents. Plugs and convenient sockets for portable tools and equipment with metal housings should have automatic grounding mechanisms for the metal frame or housing of the tool or equipment upon mating, with the grounding pin grounded first and disconnected last. Hard grounding of the grounded housing of such tools and testing equipment can shunt the protective resistance within the grounding cable of the workbench. As an additional safety measure, ground fault circuit interrupters (GFCIs) should be used with the testing equipment. GFCIs detect leakage current caused by faulty testing equipment and immediately disconnect the circuit when the current reaches a hazardous potential. Special care should be taken when using parallel circuits, as parallel circuits can reduce the equivalent resistance of the human body to ground to a dangerous level. Movement of the human body along with wrist straps, the top of the workbench, and the floor can cause such parallel circuits. Grounding of workbenches and other protective equipment: The surface of the workbench should be covered and grounded with ESD protection material. The method for connecting the grounding cable is as follows: Use a steel screw with a steel flat washer approximately 2.5 cm in diameter placed under the screw head to install the ESD protection material on the workbench surface, or install the ESD protection material using a method similar to installing double-sided felt tape, with a steel washer placed under the screw head. Connect one end of the resistive grounding cable between the screw head and the metal washer. Connect the other end of the resistive grounding cable to the static grounding wire. If a laminate is used as the top surface of the workbench, the screw for installing the grounding cable should extend into the material layer, as the inner layer of the material has better conductivity than the finished surface. The cable and resistor should have sufficient current-carrying capacity. Since the workbench ground wire is for discharging static charge, a 0.5W resistor is usually sufficient; the grounding cable connection should be continuous and permanent. Considering all parallel paths, the resistance to ground should be large enough to limit leakage current to a maximum of 5mA, based on the highest voltage source accessible to grounding personnel. This voltage source includes the power supply and testing equipment; the materials of the grounding cable and wiring should have sufficient mechanical strength to prevent accidental ground wire disconnection. Where problems may arise due to reflective action, protective measures should be taken to keep the current below 5mA; the top surface of the workbench, floor mats, grounding straps, and other ESD devices used to dissipate static electricity should all be connected to ground through current-limiting resistors. Wrist straps should be connected to ground through the workbench grounding point. Workbenches should not be connected in series, as series resistance can add to and increase ESD dissipation time. Furthermore, a break in one grounding cable will cause the grounding cable of another workbench to break.
2. Other requirements for the anti-static work area: ① Electrical equipment, tools, soldering irons, solder boxes, wave soldering equipment, soldering irons, and soldering equipment should be hard grounded. The resistance from the hot soldering iron tip to ground should be less than 2Ω to ensure the voltage increase is less than 15V. Other electrical equipment in contact with ESD devices should also be grounded. Solder suction tubes should be protected with ESD devices such as metal-plated or anti-static devices. For the insulated handles of hand tools, an electrostatic discharge (ESD) test should be performed, and if necessary, they should be treated periodically with an antistatic agent. Small hand tools that are frequently handled often become ESD-resistant due to the accumulation of moisture from the skin.
② The test equipment should have all exposed metal surfaces, and its electrical connection should be via a grounding plug to the power supply system of the test equipment or other hard grounding. For personal safety from electric shock, the test equipment should not be placed on a conductive workbench surface, as this could cause the surface to become hard grounded. Depending on the magnitude of the nearby voltage source, the test equipment can be placed on a high-resistance antistatic material. A ground fault circuit interrupter (GFCI) should be used in the electrical outlet used to power the test equipment as an additional personal safety measure.
③ The temperature chamber should be equipped with a grounding plate to dissipate the charge in the circulating air. Alternatively, ionized air should be used inside the temperature chamber to dissipate the static charge generated by airflow, or shielding can be used to redirect the charged air towards the ESD devices. It should be noted that in cooling chambers using CO2, CO2 vapor can generate a large amount of electrostatic charge. Test parts should be placed in the ESDS device protection transport container or on a tray on a grounded metal frame inside the chamber. The thermal stability of the ESDS device protection material used in the chamber should be suitable for the entire test temperature range.