What are the requirements for the composition of a standard anti-static work area?
The protective wax applied to the surface of the anti-static floor should be electrostatically conductive. For EPA, the following equipment should also be equipped when possible: Wristband monitor: In order to ensure the electrostatic leakage effect of the wristband, a wristband monitor should be set up in the EPA area. When the resistance value of the wristband system worn by the operator of the ESDS device exceeds or falls below the set resistance range of the instrument, the instrument alarms. At this time, the wristband should be replaced in time. Electrostatic voltmeter, ohmmeter, high resistance meter: The use of these instruments can determine whether the products and equipment used in the EPA area meet the anti-static requirements. Ground monitor: The use of this instrument can monitor the integrity of the ground system. It can be used to detect when the resistance of the grounding system changes or when an open circuit occurs.
1． EPA internal grounding precautions
The EPA grounding wire can be bare wire or non-bare wire. If non-bare wire is used, electrical continuity needs to be checked frequently. The ground wire used at all times should have corrosion resistance and certain mechanical strength. The connection between the ground connection point and the network must be firm and reliable. Use a ground clamp or welding to ensure a small ground resistance. Ground Potential of Electronic Test Equipment and Tools: EPA areas and workbenches should be designed and constructed so that all exteriors, surfaces, and shields within electronic test equipment and power tools are at a common ground potential at all times during operation. Design should include consideration of ground faults and established voltage ranges where hazardous voltages or currents are present. Plugs and convenience sockets for portable tools and equipment with metal casings should have measures to automatically ground the metal frames or casings of the tools and equipment when they are mated, and the grounding pin should be grounded first and disconnected last. Hard grounding of such tool and test equipment enclosures shunts the protective resistors within the workbench ground cable. As an additional protective measure for personal safety, a ground fault circuit interrupter should be used with the test equipment. Ground fault circuit breakers detect leakage currents caused by faulty test equipment and immediately interrupt the circuit when the current reaches dangerous levels. When using a parallel path, special attention should be paid to the fact that the parallel path can reduce the equivalent resistance of the human body to the ground to a dangerous level. This parallel pathway can be caused by movement of the human body along with wrist straps, workbench tops, and floors. Grounding of workbenches and other protective equipment: Workbench surfaces should be covered with ESD protective materials and grounded. How to connect the ground cable is as follows. Use steel screws and place a steel flat washer with a diameter of about 2.5CM under the screw head to install the ESD protective material on the workbench surface, or use a method like installing double-sided felt tape and use a steel washer under the steel screw head to install the ESD protective material. Connect one end of the resistive ground cable between the screw head and the metal washer. Connect the terminal on the other end of the resistive ground cable to the static ground wire. If a laminate is used as the workbench top surface, the screws for mounting the ground cable should extend into this layer of material, as the inner layer of material is more conductive than the finished surface. Cables and resistors should have sufficient current carrying capacity. Since the workbench ground wire is used to discharge static charges, a 0.5W resistor is usually sufficient; the ground cable connection should be continuous and permanent. Taking into account all parallel paths, the resistance to ground should be large enough to limit the leakage current to a maximum of 5mA based on the highest voltage source accessible to the grounding person. This voltage source includes the power supply and test equipment; the materials for ground cables and wiring should have sufficient mechanical strength to avoid accidental ground disconnection. Where problems may be caused by reflection action, protective measures can be taken to keep the current below 5mA; the top surface of the workbench, floor mats, grounding straps and other ESDS device protection areas used to dissipate static electricity should all be connected to the ground through current-limiting resistors. The wrist strap should be connected to ground through the countertop ground point. Workbenches should not be connected in series with each other because series resistance can cause additional ESD dissipation times. In addition, the disconnection of one ground cable will cause the disconnection of the ground cable of other workstations.
2. Other requirements for anti-static work areas
① Electrical equipment, tools, soldering irons, solder boxes, wave soldering equipment, soldering irons, and soldering equipment should be hard grounded. The resistance value from the hot soldering iron tip to the ground should be less than 2Ω so that the voltage increase is less than 15V. Other electrical equipment in contact with ESDS devices must also be grounded. Solder straws should be protected using ESDS devices like metal spraying or anti-static. The insulated handles of hand tools should be inspected for static electricity generation and, if necessary, treated with anti-static agents regularly. Small hand tools that are often held by hand are often ESD-resistant by collecting moisture from the skin.
② The test equipment should have all exposed metal surfaces, and its electrical connection is through a ground plug to the power system of the test equipment, or other hard grounding. For personal safety from electrical shock, test equipment should not be placed on a conductive work surface as it may cause this surface to be hard grounded. Depending on the size of the nearby voltage source, the experimental equipment can be placed on a high-resistance anti-static material. Ground fault circuit interrupters should be used as an additional personal safety measure in electrical outlets used to power experimental equipment.
③The thermostat should be equipped with a grounding deflector to dissipate the charge in the circulating air. On the other hand, ionized air should be used inside the chamber to dissipate static charges due to air movement, or shielding can be used to divert the charged air away from the ESDS device. It should be noted that in a cooling box using CO2, CO2 vapor can generate a large amount of electrostatic charge. The parts under test in the temperature box should be placed in an ESDS device protection transport bucket, or placed in a pallet on a grounded metal frame in the temperature box. The thermal stability of the ESDS device protection materials used in the thermostat should be adapted to the entire test temperature range.
④Spraying, cleaning, painting and sandblasting equipment When spraying, cleaning, painting and sandblasting ESDS devices, ionizing air compressors, conductive solvents or ionizing nozzles suitable for preventing the increase of static charges in the working area should be used. Wet spray conductive or antistatic treatment should be used. Clean mortar with a maximum volume resistivity of 500Ω·cm is used instead of dry sandblasting. Low-resistivity solvents like ethanol mixed with normal cleaning solvents can reduce static electricity.
⑤ Humid air increases surface conductivity by keeping the surface moist, helping to dissipate static charges. As relative humidity decreases, significant electrostatic voltage can accumulate. Obviously, a relative humidity between 60% and 70% is desirable as long as it does not lead to increased corrosion, or other detrimental effects such as PWB delamination during welding. Where high relative humidity levels cannot be maintained, ionized air is used to dissipate electrostatic charges. For humidity measurement, a wet and dry bulb thermometer should be used, and a hair disc hygrometer is often not accurate enough.
⑥It is very important to maintain a high level of air cleanliness in the EPA area. In order to ensure product quality, it is very necessary to take some air purification measures