Principle of electrostatic protection:
(1) Prevent static electricity from accumulating in places where static electricity may be generated. Take measures within the safe range.
(2) Quickly eliminate the existing static electricity accumulation and release it immediately.
Electrostatic protection methods or means
(1) Use anti-static materials: Metal is a conductor, because the leakage current of the conductor is large and the speed is high, which will damage the device. In addition, since the insulating material is prone to triboelectric charging, metal and insulating materials cannot be used as the antistatic material. Instead, a so-called electrostatic conductor having a surface resistance of 1 × 10 5 Ω·cm or less and an electrostatic sub-conductor having a surface resistance of 1 × 10 5 -1 × 10 8 Ω·cm were used as an antistatic material. For example, a commonly used electrostatic protection material is realized by mixing conductive carbon black in rubber, and the surface resistance is controlled to be 1×10 6 Ω·cm or less.
(2) Leakage and grounding: Ground the part where static electricity may or may have occurred, and provide an electrostatic discharge channel. The method of burying the ground wire establishes an "independent" ground line. Make the resistance between ground and earth <10Ω.
Grounding method for static protective materials: Connect static protective materials (such as countertop mats, floor mats, antistatic wrist straps, etc.) to the conductors leading to the independent ground wire through a 1MΩ resistor (see SJ/T10630-1995). The 1MΩ resistor is connected in series to ensure that the current is <5mA to ground, called soft ground. The device housing and electrostatic shield are usually grounded directly, called hard ground.
(3) Elimination of static electricity on the conductor: Static electricity on the conductor can be grounded to cause static electricity to leak to the earth. The voltage and release time of the discharge body can be expressed by the following formula:
UT=U0L1/RC type UT-T voltage (V) U0-starting voltage (V) R-equivalent resistance (Ω) C-conductor equivalent capacitance (pf)
It is generally required to leak static electricity within 1 second. That is, the voltage is reduced to a safe area below 1OOV in 1 second. This will prevent the leakage speed from being too fast and the leakage current to be too large to damage the SSD. If U0=500V, C=200pf, if you want to make the UT reach 100V in 1 second, you need R=1.28×109Ω. Therefore, the ESD protection system usually uses a 1MΩ current limiting resistor to limit the bleeder current to less than 5mA. This is designed for operational safety. If the operator is in the ESD protection system, accidentally touching the 220V industrial voltage will not pose a danger.
(4) Elimination of static electricity with non-conductors: For static electricity on insulators, since electric charges cannot flow on the insulator, static electricity cannot be removed by grounding. The following measures can be taken:
(a) Using an ion fan-ion fan to generate positive and negative ions, which can neutralize the static electricity of the static power source. Can be placed near the space and the placement head of the placement machine.
(b) Use of a static eliminator - A static eliminator is a surfactant. The static elimination agent can be used to wash the surface of the instrument and the object, and the static electricity on the surface of the object can be quickly eliminated.
(c) Controlling the humidity of the environment - Increasing the humidity can increase the surface conductivity of the non-conducting material, making it difficult for the surface of the object to accumulate static electricity. For example, in a dry environment in the north, humidification and ventilation measures can be taken.
(d) Electrostatic shielding—Use a shield (cage) for equipment that is prone to static electricity and effectively ground the shield (cage).
(5) Process control method: In order to generate static electricity as little as possible in the manufacture of electronic products, control the accumulation of static charge, quickly eliminate the existing static electricity accumulation, and release immediately, from the plant design, equipment installation, operation, management system, etc. take effective action.
Six: electrostatic protection supplies
(1) The human body anti-static system includes anti-static wrist straps, overalls, caps, gloves, shoes, socks, and the like.
(2) Anti-static floor includes anti-static terrazzo floor, anti-static rubber floor, PVC anti-static plastic floor, anti-static carpet, anti-static floor.
(3) Anti-static operation series: including anti-static: I: work table pad, anti-static bag, anti-static logistics car, anti-static turnover container, anti-static soldering iron and tools.
Detection of electrostatic measuring instruments
(1) Electrostatic field tester: used to measure the working environment voltage value, the voltage measuring instrument is inductive and does not need to touch the surface of the object. SIMCO FMX-004
(2) Wrist strap tester: Measure whether the wrist strap is effective.
(3) Human body comprehensive tester: It is used to measure the anti-static wrist strap with the same amount of test anti-static shoes. It is easy to operate. Users can set the upper and lower limits of IEC or ANSI specifications, and have the electronic switch for dry contact output.
(4) Megohmmeter: Used to measure the impedance or resistance of all conductive, antistatic and electrostatic discharge surfaces.
Anti-static technical indicators in the production process of electronic products
(1) Anti-static grounding resistance of grounding resistance <10Ω, the condition is allowed to do static grounding pile resistance as low as possible
(2) Anti-static floor or floor mat: surface resistance value 105-1010Ω; friction voltage <100V.
(3) Wall: The resistance value is 5×104-109Ω.
(4) Work surface or pad: surface resistance value 106-109 Ω; friction voltage <100V; ground system resistance 106-108 Ω.
(5) The work chair faces the caster resistance of 106-108 Ω.
(6) Workwear, cap, gloves friction voltage <300V; sole friction voltage <100V.
(7) The wristband connection cable resistance is 1MΩ; when the wristband is worn, the system resistance is 1-1OMΩ. The heel strap (shoe bundle) system resistance is 0.5×105-108Ω.
(8) The logistics platform faces the wheel system resistance of 106-109 Ω.
(9) The material transfer device such as the material box, the turnover box and the PCB frame has a surface resistance value of 103-108 Ω; the friction voltage is <100V.
(10) Packaging generation, box one friction voltage <100V.
(11) Human body comprehensive resistance 106-108Ω.
Anti-static measures and protection requirements for electrostatic operating areas
The SMT production equipment must be well grounded, and the placement machine should be grounded by a three-phase wireless grounding method. The ground, work surface mats, and chairs of the production site shall comply with the anti-static requirements. Maintain a constant temperature and constant humidity environment in the workshop. It should be equipped with anti-static box, turnover box, PCB frame, logistics trolley, anti-static packaging tape, anti-static wrist strap, anti-static soldering iron and tools.
(1) Set an anti-static area according to anti-static requirements, and have obvious anti-static warning signs. According to the electrostatic sensitivity of the devices used in the work area, it is divided into 1, 2, and 3 levels, and different protective measures are formulated according to different levels.
Level 1 electrostatic sensitivity range: 0-1999V
Level 2 electrostatic sensitivity range: 2000-3999V
Level 3 electrostatic sensitivity range: 4000-15999V
More than 16000V is a non-electrostatic sensitive product.
(2) The room temperature of the electrostatic safety zone (point) is 23 ± 3 ° C, and the relative humidity is 45-70% RH. It is forbidden to operate SSD (electrostatic sensitive components) in less than 30% of the environment.
(3) Regularly measure the surface resistance values of the ground, table top, and turnover box.
(4) Non-productive items such as tableware, tea sets, bags, woolen fabrics, newspapers, rubber gloves, etc. are prohibited on the workbench in the electrostatic safety zone (point).
(5) The staff enters the anti-static area and needs to be discharged. Work clothes and anti-static shoes and socks must be worn by the operator. The static electricity safety test must be carried out before each operation, and it can only be produced after passing the test.
(6) Wear an anti-static wrist strap during operation to measure the effectiveness of the wrist strap every day.
(7) When testing the SSD, take one piece from the box, tube, and disk, measure one piece, put one piece, and do not pile it on the table. Devices that fail the test shall be returned to the library.
(8) The power-on test must follow the sequence of power-on and power-off: low voltage → high voltage → signal voltage. The order of power-off is reversed. Also note that the polarity of the power supply must not be reversed and the supply voltage must not exceed the rated value.
(9) The inspector should be familiar with the model, variety and testing knowledge of the SSD and understand the basic knowledge of electrostatic protection.
Requirements for storage and movement of static sensitive components
(1) Sensitive components shall not fall to the ground during transportation, and shall not be arbitrarily removed from the packaging.
(2) Relative humidity of warehouses storing sensitive components: 30-40% RH.
(3) Keep the original packaging during the storage of sensitive components. If you need to replace the packaging, use a container with anti-static properties.
(4) In the warehouse, the anti-static special label should be attached to the location where the sensitive components are placed.
(5) Apply the visual method when distributing sensitive component devices, and count the quantity in the original package of the sensitive component device.
(6) When writing, wiping and information protection operations on the EPROM, the writer/erase device should be fully grounded, with an anti-static bracelet.
(7) Operators such as assembly, welding, repairing, and debugging must strictly operate in accordance with the requirements for electrostatic protection.
(8) Tested and inspected printed circuit boards are sprayed once with an ion gun before packaging to eliminate static charges that may accumulate.
Anti-static work area management and maintenance work
(1) Formulate an anti-static management system and have an ESD person in charge.
(2) Standby anti-static overalls, shoes, bracelets and other personal items for use by outsiders.
(3) Regularly maintain and check the effectiveness of anti-static facilities.
(4) Check the wrist strap once a week (or day) and install the wrist strap online monitor.
(5) The grounding of the table mat and floor mat and the performance of the static eliminator are checked once a month.
(6) Anti-static component frame, printed board frame and turnover box; the anti-static performance of transport vehicles, table mats and floor mats is checked every six months.