Complete set of static grounding methods in electronics factory
Anti-static ground wire embedding:
(1). The lightning rod of the factory building is generally welded to the reinforced concrete of the building to be properly grounded. When a lightning strike occurs, the grounding point and the ground of the entire building will become a discharge point for high voltage and large current. There will be a "step voltage" in the 20M range of the site, that is, it is no longer an ideal zero potential in this range. In addition, the zero line of the three-phase power supply may also have an unbalanced current and flow into the zero line because it cannot be absolutely balanced. The grounding point of the anti-static ground wire should be 20 meters away from the building and equipment ground.
(2). Buried method: In order to ensure the reliability of the grounding, there should be more than three points grounding, that is, digging more than 1.5M pits every 5M, and driving iron pipes or angle irons above 2M into the pit (that is, the angle iron is inserted into the underground more than 2M ), And then use 3MM thick copper bars to weld these three places together, and use 16M㎡ insulated copper core wire welding to introduce the indoor as the main line.
(3). Appropriate amount of charcoal powder and industrial salt are applied in the pit to increase the conductivity of the soil. After the landfill is measured with a grounding resistance tester, the grounding resistance should be <4ω. (see="" figure="" 2)="" and="" tested="" at="" least="" once="" a="">
2. Laying and testing of anti-static ground wire:
(1). All anti-static ground wires use 6M㎡ multi-strand copper core insulated wires, with copper bars or switches above 40A for each floor or appropriate section, and the knife is connected to the main line to facilitate inspection and maintenance.
(2). The anti-static ground cable should be well insulated from the equipment shell, workbench iron frame, work light frame, etc. to prevent short circuit, lap or broken skin connection.
(3). On the "trunk line end" of the segmented copper bar or switch, lay another inspection line. (1.5 ~ 2M㎡ is enough), each workshop has 2 ~ 3 inspection points, fixed and marked clearly.
(4). Measurement: use pointer multimeter, resistance file.
A). The resistance between each antistatic test point and the antistatic ground wire is 5 ~ 15Ω, ideally 0Ω. But the actual measurement is 2M㎡ wire resistance from the test point to the summary point + 6M㎡, the wire from the summary point to the measured The sum of the point resistance, this value is about 5-15Ω and basically unchanged, if the measurement result tends to infinity, it is because the anti-static ground wire or the measurement wire has a broken wire, and it should be repaired in time.
B). The resistance between the anti-static ground and the equipment ground, this resistance value is composed of the anti-static ground wire's own wire resistance + the equipment ground wire's own wire resistance + the ground resistance between the two ground wires. , The influence of ground current is very complicated, especially the current, the frequency of the direction of the size changes every moment, and mainly determines the measurement result, so it can only be measured with a pointer meter, and its value ranges from more than ten euros to hundreds of K It is normal, only that there is no short circuit or open circuit between the two places.
3. Anti-static floor
The most standardized anti-static floor is a composite structure similar to anti-static rubber. The lower layer is a conductive layer and an anti-static ground, and the upper layer is an insulating anti-static generating layer, which will not generate static electricity due to friction during walking. The mat is separated from the building ground and walls to prevent static electricity from the floor during lightning strikes, and the conductive layer is connected to the anti-static ground through a 1MΩ 20W resistor. It plays the role of electrostatic shielding and electromagnetic shielding. This kind of floor is too expensive, but it can be effective Prevent various hazards of lightning and the generation of static electricity.
General electronic factories mostly use simple anti-static floors (only the insulating anti-static generation layer is mostly paint or floor adhesive), which is directly laid on the building floor, which greatly reduces the cost and can also play a role in preventing static electricity generated by walking. The ultra-high voltage electrostatic induction and strong electromagnetic induction generated by the lightning strike have poor protection.
4. Anti-static work surface:
The green surface of the anti-static rubber is the anti-static generation layer, which has a large resistance and a surface resistance of 108 ~ 1010Ω.
The black surface of the anti-static rubber has a small resistance, the surface resistance is 104 ~ 106, and it is well connected with the green surface, which can be properly grounded. It plays the role of electrostatic shielding and discharge. It can be connected by a button, and the special electrostatic bracelet wire 1M resistor) ground. Or put 0.2MM thick iron plate or copper foil on the insulating table, solder the wire and connect to the static ground through the 1MΩ resistor, and then flatten the antistatic rubber (black side down, close to the conductive sheet) The 1MΩ resistor also serves to provide an electrostatic discharge path to prevent over-speed discharge and ignition and isolation.
Even chairs (stools) should be paid attention to. Most of the production lines use ordinary plastic stools, which are very easy to rub against clothes and generate static electricity. If possible, anti-static chairs should be used, and the static ground should be connected through a 1MΩ resistor. Put on static cloth.
5. Grounding and testing of electric equipment such as electric soldering irons, small tin furnaces, test instruments:
Electric soldering irons, small tin furnace test instruments, etc. must be properly connected to the equipment ground with a three-terminal plug. Ground) disconnection and other phenomena, so it should be detected every shift, and can be tested with a self-made simple on-off indicator, and if the problem is found, it should be replaced immediately.
6. Anti-static clothing (clothes, shoes, gloves, etc.):
The so-called anti-static clothing is made of special synthetic fibers woven into cloth. Generally, rubbing and rubbing will not generate static electricity. But it is not an electrostatic shielding clothing. It cannot eliminate the static electricity generated by other clothing on the body. A piece of shirt or underwear, with anti-static clothing on the outside. It is useless to wear multiple pieces of chemical fiber in winter, and anti-static clothing for woolen clothing. Therefore, it is more important to control the ambient temperature and humidity, and wear an electrostatic bracelet than an electrostatic clothing. Anti-static gloves are used to prevent the generation of static electricity; isolate the hand from the product (insulation); prevent sweat from contaminating the product and other multiple functions, it is necessary.
7. Anti-static wristband:
The anti-static wristband is made of a stainless steel case that is close to the wrist, grounded by a wire and an iron clip through the 1MΩ resistance in the wire. Causes damage and plays a role in isolation. Broken wires or poor contact will make the electrostatic hand ring dummy. The so-called wireless bracelet does not actually discharge the static charge carried by the human body.
(1). Standardize the wearing and holding method of electrostatic bracelet:
A. The stainless steel shell of the electrostatic bracelet should be worn on the inside of the left wrist, where the contact resistance is the smallest.
B. Keep in close contact with the skin, do not relax, and do not wear clothing.
C. The crocodile clip should use the root to clamp the bare part of the electrostatic ground wire, instead of using the front teeth.
D. When leaving work or walking, the operator can take off the bracelet. Mobile personnel (cadres, quality control) should remove the clip and wrap the line around the wrist for mobile use.
(2). The static ring should be measured and recorded once in the morning and in the afternoon of each shift. The tightness is subject to the test, and if it fails, it should be adjusted or replaced immediately.
(3). Do not wear wireless bracelet.
8. Install ion fan:
The temperature of the preheating part of the wave furnace is 80 ~ 120 ℃, and it is very easy to generate static electricity under such high-temperature dry hot air. The ion fan is ionized by high pressure into positive and negative ions, and the fan blows air containing a large amount of positive and negative ions into the furnace In order to neutralize the static electricity generated by high temperature hot air on the PCB and components. Therefore, an ion fan should be installed at the entrance of the wave furnace. At the entrance of the anti-static work area, the ionizer can also be installed at the beginning or above the conveyor belt according to actual needs.
9. The conveyor belt is equipped with anti-static cleaning roller:
Self-made simple device: wrap a hard plastic tube with a length slightly shorter than the width of the conveyor belt onto a towel cloth (should be flat), wet it with a steel rod (for shaft) in the middle and fix it to both ends of the conveyor belt, and fill it with cola bottles to imitate a hospital The bottle method is constantly humidified. When the conveyor belt starts, the roller rotates with the conveyor belt under its own weight, which plays a role in cleaning, humidification and anti-static. This simple self-made device can function as a conveyor belt fan under certain circumstances.
10.PCB is on the assembly line (mainly refers to the DIP post-welding, testing, assembly and other processes), that is, anti-static sponge pads should be placed to prevent static electricity and board scratches. Anti-static board card storage and transportation vehicles should be used for transfer between processes Or card box. (Its surface resistance is less than 106Ω), properly connect the static ground through a 1MΩ resistor.
It is important to distinguish between static shielding materials (bags) and anti-static materials (bags) .Anti-static materials (bags, pads) are mostly pink.They are only used as inexpensive pads and intermediary packaging for static-sensitive devices. Static electricity is generated, if static discharge occurs, it can pass through these anti-static materials to cause harm. Static shielding packaging is mostly silver, black, gray, there are aluminum foil-shaped opaque and black, gray translucent materials, and now there are grid-like The basic principle of all transparent materials, etc. is to vacuum-coat a layer of aluminum outside the anti-static material as a conductive electrostatic shielding layer. It can prevent the discharge of high electrostatic potential difference at local points, and also has a good shielding effect on high-frequency strong electromagnetic fields. With different protection levels, the conductive layer of the electrostatic shielding packaging material is also divided into: outer layer resistance layer-insulating film-aluminum foil layer-insulating film; insulating film ─Aluminum foil layer─Insulating film; Vacuum coating layer─Insulating film; Printed conductive grid layer─Insulating film and other grades. A little attention can be found when IC is shipped except for packaging In addition to 2 ~ 3 layers of electrostatic shielding materials, the internal support materials (such as push plates and feed belts) are black high-resistance conductive materials, about 106MΩ, which also only shields and neutralizes the electrostatic potential of each pin to provide electrostatic discharge Pathway effect. When static-sensitive components (such as ICs) and products are shipped, they must be packaged with static shielding materials instead of using anti-static bags.
11. Regulation of temperature and relative humidity:
Electronic operations, especially SMT, have high requirements on temperature and humidity.Generally, the temperature is controlled at 18 ~ 28 ℃, too high or too low will affect the normal operation and accuracy of the equipment; relative humidity should be 50% ~ 85%, If it is too low, it is easy to generate static electricity (see Table 1). Equipment that is too high is prone to condensation and the water content of the solder paste increases. Therefore, monitoring and regulation should be strengthened. Local method.
12. Other
(1) The electric soldering iron should use anti-static low-voltage constant-temperature soldering iron as much as possible, and be well grounded.
(2) Use a low-voltage DC electric screwdriver (electric screwdriver) with a ground wire.
(3) Anti-static brushes should be used for PCB small batch cleaning operations, and ordinary plastic brushes cannot be used.
(4) On some occasions, anti-static materials should be used for ceilings and walls.Generally speaking, even ordinary gypsum boards and lime-painted walls can be used, but the use of plastic products for ceilings and ordinary wallpapers and plastic wallpapers is prohibited.
13. Static Tester:
If the conditions are met, consider adding a "handheld non-contact electrostatic tester", so as to monitor the generation and size of static electricity in real time, track the defects caused by static electricity, and understand the improvement effect. Because its price is too expensive, most companies dare not pay attention.
14. Monitoring and recording:
The anti-static measures must be implemented by a special person and form a system before they can be truly implemented. Otherwise, all hardware investment may not be effective.
(1). Personnel: Two persons should manage, test, and record part-time. In most cases, two persons are required to cooperate and prevent the movement of personnel.
(2) Tests and records: In summary, the following tests and records should be completed every day.
A. Electrostatic test point -------- Electrostatic ground Analog multimeter
Static ground ----------- Equipment ground Resistance measurement +
B. Grounding of the soldering iron tip / temperature measurement of the soldering iron tip.
C. Small tin furnace grounding / tin furnace temperature measurement.
D. Test instrument ground measurement.
E. Electrostatic wristband grounding test. Electrostatic wristband tester
F. Indoor temperature / relative humidity measurement and regulation. Temperature / hygrometer.
(3). Check the implementation of the regulations on the dress and anti-static of the staff in the anti-static work area.
(4). If possible, use static tester to measure the static voltage in various situations on the work site and assembly line.
The static voltage should generally be less than 100V, and in special cases should be less than 25V.
15. Training and literacy: Anti-static knowledge / measures should be taken as an important part of the training for all employees so that every employee can understand it and form good professional habits. For example:
Clothing, drawings, documents, etc. must not touch components, and drawings should be placed in anti-static file bags and hung;
Plastic boxes, such as leather, cardboard, glass and other debris that are easy to generate static electricity are not allowed to be stacked on the static safety workbench;
The electrostatic ring and gloves must be loaded before touching the components. Hold the edge of the PCB or sensitive device as much as possible to avoid touching its leads and lugs.