Precautions for the composition of anti-static work area
The protective wax coated on the surface of the anti-static floor should have electrostatic conductivity. For the EPA, the following equipment should be equipped when possible: Wristband monitor: In order to ensure the electrostatic leakage effect of the wristband, a wristband monitor should be installed in the EPA area. When the resistance value of the wrist strap system worn by the ESDS device operator exceeds or falls below the set resistance range of the instrument, the instrument alarms. At this time, the wristband should be changed 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. Grounding monitor: The use of this instrument can monitor the integrity of the grounding system. It can be found when the resistance of the grounding system changes or there is an open circuit.
1. EPA internal grounding attention
The EPA grounding wire can be either a bare wire or a non-naked wire. If you use non-bare wires, check their electrical continuity frequently. The grounding wire used at any time should have corrosion resistance and a certain degree of mechanical strength. The connection between the grounding connection point and the network must be firm and reliable. The use of grounding clamps or welding can ensure a small grounding resistance. Ground potential of electronic test equipment and tools: The design and structure of the EPA area and workbench should ensure that all exteriors, surfaces and shields in the electronic test equipment and electric tools are at a common ground potential during the entire working time. The design should include consideration of ground faults and voltage ranges established in places with dangerous voltages or currents. Plugs and convenient sockets for portable tools and equipment with metal casings should have automatic grounding measures for the metal frame or casing of the tools and equipment when they are mated. Moreover, the grounding pin must be grounded first, and then disconnected. The hard grounding of this tool and the grounding shell of the test equipment can shunt the protective resistance in the grounding cable of the workbench. As an additional protective measure for personal safety, ground fault circuit breakers should be used with test equipment. The ground fault circuit breaker can detect the leakage current caused by the faulty test equipment. When the current reaches a dangerous potential, the circuit breaker immediately cuts off the circuit. When using the 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 degree. The movement of the human body along with the wrist buckle, the top of the workbench and the floor can cause this parallel path. Grounding of the workbench and other protective equipment: The surface of the workbench should be covered with ESD protection materials and grounded. The method of connecting the grounding cable is as follows. Using steel screws, place a steel flat washer with a diameter of about 2.5cm under the screw head, and install the ESD protection material on the surface of the workbench, or use a method like installing a double-sided felt belt to place a steel washer under the head of the steel screw Come to install ESD protection materials. Connect one end of the resistive grounding cable between the screw head and the metal washer. Connect the terminal on the other end of the resistive grounding cable to the electrostatic grounding wire. If the stack is used as the top surface of the workbench, the screw for installing the grounding cable should extend into the material layer, because the inner layer of the material has better conductivity than the finished surface. Cables and resistors should have sufficient current-carrying capacity. Since the ground wire of the workbench is used to discharge static charges, a 0.5W resistor is usually sufficient; the grounding cable connection should be continuous and long-lasting. Considering all parallel paths, the resistance to ground should be large enough to limit the leakage current to a maximum of 5 mA according to the highest voltage source accessible to the grounding personnel. This voltage source includes power supply and test equipment; grounding cables and wiring materials should have sufficient mechanical strength to avoid accidental ground wire disconnection. Where there may be problems due to reflection action, protective measures can be used to make the current less than 5mA; the top surface of the workbench, floor mats, grounding straps and other ESDS devices used to dissipate static electricity should be connected through current limiting resistors. arrived. The wrist strap should be connected to the ground through the grounding point on the work surface. Workbenches should not be connected in series, because series resistance can additionally cause a longer ESD dissipation time. In addition, a disconnection of a grounding cable will cause the grounding cable of another workbench to be disconnected.
2. Other requirements for anti-static work area
①Electrical equipment, tools, soldering iron, solder box, wave soldering equipment, soldering iron, 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 should also be grounded. An ESDS device like metal spraying or anti-static should be used to protect the solder pipette. The insulating handles of hand tools should be tested for static electricity generation, and if necessary, they should be treated with antistatic agents on a regular basis. Small hand tools that are often held in hand are often protected against ESD due to the accumulation of moisture in the skin.
②The test equipment should have all metal surfaces exposed to the outside, and its electrical connection should be through the grounding plug to the power system of the test equipment, or other hard grounding. For personal safety from electrical shocks, the test equipment should not be placed on the surface of the conductive workbench, as it may cause this surface to be hard grounded. According to the size of the nearby voltage source, the experimental equipment can be placed on a high-resistance anti-static material. Ground fault circuit breakers should be used as an additional personal safety measure in the electrical sockets used to supply power to the experimental equipment.
③The incubator should be equipped with a grounding deflector to dissipate the electric charge in the circulating air. On the other hand, ionized air should be used in the incubator to dissipate the static charge generated by the air flow, or a shield can be used to divert the charged air to the ESDS device. It should be noted that in the cooling box using CO2, CO2 vapor can generate a large amount of electrostatic charge, and the tested parts in the incubator should be placed in the ESDS device protection transport hopper, or placed in the tray on the grounded metal frame in the incubator . The thermal stability of the ESDS device protection material used in the incubator should be adapted to the entire test temperature range.
④Spray, cleaning, painting and sandblasting equipment When spraying, cleaning, painting and sandblasting ESDS devices, ionized air compressors, conductive solvents or ionization nozzles suitable for preventing the increase of static charge in the working area should be used. Should be wet spray conductive or antistatic treatment. *Clean mortar with large volume resistivity of 500Ω·cm instead of dry sandblasting. Solvents with low resistivity, such as ethanol mixed with normal cleaning solvents, can reduce static electricity generation.
⑤ Humid air increases surface conductivity by keeping the surface moist and helps dissipate static charges. As the relative humidity decreases, a 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 harmful 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 the precision of a disc hygrometer is often insufficient.
⑥ It is important to maintain high air cleanliness in the EPA area. In order to ensure product quality, it is necessary to take some air purification measures.