Complete Electrostatic Grounding Method for Electronics Factory
1. Installation of Anti-static Grounding Wire: (1). The lightning rod of the factory building is usually welded to the reinforced concrete of the building and properly grounded. When lightning strikes, the grounding point and even the ground of the entire building will become the discharge point of high voltage and high current. It is generally believed that "step voltage" will be generated within 20M of the discharge grounding point, that is, it is no longer an ideal zero potential within this range. In addition, the neutral wire of the three-phase power supply will also have unbalanced current generated and flow into the grounding point of the neutral wire because it is impossible to be absolutely balanced. Therefore, the installation point of the anti-static grounding wire should be far from the building. (1) The grounding depth should be at least 20 meters away from the object and equipment.
(2) Burial method: To ensure reliable grounding, there should be at least three grounding points. This involves digging a pit at least 1.5 meters deep every 5 meters, driving an iron pipe or angle iron at least 2 meters deep into the pit (i.e., the angle iron should be inserted at least 2 meters underground), and then welding these three points together with a 3mm thick copper busbar. A 16mm² insulated copper core wire should be welded on and introduced into the room as the main line.
(3) Apply an appropriate amount of charcoal powder and industrial salt to the pit to increase soil conductivity. After backfilling, measure the grounding resistance with a grounding resistance tester. The grounding resistance should be <4Ω (see Figure 2) and tested at least once a year.
2. Installation and Testing of Anti-static Grounding Wires:
(1) All anti-static grounding wires should use 6mm² multi-strand copper core insulated wire. Each floor or appropriate section should use copper busbars or switches of 40A or higher connected to the main line for easy inspection and maintenance.
(2) The anti-static grounding cable should be well insulated from equipment casings, workbench frames, work light stands, etc., to prevent short circuits, bridging, or damaged connections. (3) Lay an additional inspection wire (1.5~2 mm²) at the "main line end" of the segmented copper busbar or switch. Set up 2~3 inspection points in each workshop, fix them securely, and clearly label them.
(4) Measurement: Use a pointer-type multimeter in resistance mode.
a) The resistance between each anti-static test point and the anti-static ground wire is 5~15Ω, ideally it should be 0Ω. However, the actual measured resistance is the sum of the resistance of the 2 mm² wire from the test point to the grounding point + 6 mm², and the resistance of the wire from the grounding point to the test point. This value is approximately 5-15Ω and remains basically constant. If the measurement result tends towards infinity, it indicates… If any of the anti-static grounding wires or measuring wires is broken, it should be repaired promptly.
b) The resistance between the anti-static ground and the equipment ground is calculated as: the resistance of the anti-static grounding wire itself + the resistance of the equipment grounding wire itself + the resistance between the two grounding wires. However, the resistance between the two grounding wires is very complex due to factors such as the dryness or wetness of the ground and the influence of ground current. Ground current, in particular, changes constantly in magnitude, direction, and frequency, and it primarily determines the measurement result. Therefore, only an analog multimeter can be used for measurement, and values ranging from tens of ohms to hundreds of kilohms are considered normal, simply indicating that there is no short circuit or open circuit between the two grounds.
3. Anti-static Flooring
The most standard anti-static flooring has a composite structure similar to anti-static rubber. The lower layer is a conductive layer connected to the anti-static ground, and the upper layer is an insulating anti-static layer, preventing static electricity generation from friction during walking. During installation, the conductive layer should be separated from the building's floor and walls by insulating pads to prevent the floor from becoming statically charged during lightning strikes. The conductive layer is then connected to the anti-static ground through a 1MΩ 20W resistor, providing both electrostatic and electromagnetic shielding. This type of flooring is very expensive, but it effectively prevents various damages from lightning and the generation of static electricity.
Generally, electronics factories often use simple anti-static floors (with only insulation). Antistatic layers (often paint or floor adhesive) are laid directly on building floors, significantly reducing costs and providing protection against static electricity generated by walking. However, they offer poor protection against ultra-high voltage electrostatic induction and strong electromagnetic induction from lightning strikes.
4. Antistatic Workbench:
The green side of the antistatic rubber is the antistatic layer, with a relatively high resistance (10⁸~10¹⁰ Ω).
The black side of the antistatic rubber has a lower resistance (10⁴~10⁶ Ω) and connects well with the green side, ensuring proper grounding. It serves as electrostatic shielding and discharge. Grounding can be achieved via a snap-fit connection using a dedicated anti-static wrist strap lead (containing a 1MΩ resistor). Alternatively, a 0.2mm thick iron plate or copper foil can be placed on an insulated surface, and the lead wire can be soldered to connect to the anti-static ground wire through a 1MΩ resistor. Then, anti-static rubber (black side down, tightly against the conductive sheet) can be laid flat. This 1MΩ resistor also provides a path for static discharge, preventing excessive discharge and arcing, and serves as isolation.
Even chairs (stools) should be considered. Many production lines use ordinary plastic stools, which easily generate static electricity through friction with clothing. Anti-static materials should be used where possible. Chairs should be grounded via a 1MΩ resistor, and at least the plastic stool should be covered with an anti-static cloth.
5. Grounding and testing of electrical equipment such as soldering irons, small soldering pots, and testing instruments:
Soldering irons, small soldering pots, and testing instruments must be properly grounded using three-terminal plugs. This is not difficult to do, but due to frequent occurrences such as loose grounding terminals, broken wires, and soldering iron tips detaching from the casing (grounding point) due to oxidation, they should be checked every shift. A simple homemade continuity indicator light can be used for testing, and any problems found should be addressed by replacing the faulty component immediately.
6. Antistatic Clothing (Clothing, Shoes, Gloves, etc.):
Antistatic clothing is made of special synthetic fibers that generally do not generate static electricity when rubbed or polished. However, it is not static-shielding clothing and cannot eliminate static electricity generated by other clothing. Therefore, the correct way to wear it is to wear only a shirt or underwear underneath, with the antistatic clothing on top. Wearing multiple layers of synthetic or wool clothing underneath in winter is not very effective with antistatic clothing. Therefore, controlling the ambient temperature and humidity, and wearing antistatic wrist straps are more important than wearing antistatic clothing. Antistatic gloves are used to prevent the generation of static electricity. It serves multiple functions, including isolating the hand from the product (insulation) and preventing sweat stains from contaminating the product, making it essential.
7. Antistatic Wristband:
Antistatic wristbands consist of a stainless steel shell that fits snugly against the wrist, connected to a wire with a 1MΩ resistance inside, and grounded by an iron clip. Its purpose is to discharge static electricity from the body while preventing sparks from rapid discharge that could damage static-sensitive devices, and to provide isolation. A broken wire or poor contact renders the antistatic wristband ineffective. So-called wireless wristbands fail to discharge static charges carried by the body.
(1). Proper wearing and clamping method for antistatic wristbands:
a. The stainless steel shell of the antistatic wristband should be worn on the inside of the left wrist, where the contact resistance is lowest.
b. It must be in close contact with the skin, not loose, and not separated by clothing.
c. The alligator clip should be used to clamp the exposed part of the static ground wire at the base, not the front teeth.
d. When leaving get off work or walking, operators can remove their wristbands. Mobile personnel (cadres, quality control) should remove the clips and wrap the wristband around their wrists for mobile use.
(2) Static wristbands should be tested and recorded twice a day, morning and afternoon, with the tightness determined by passing the test. Those that fail should be adjusted or replaced immediately.
(3) Wireless wristbands are not allowed.
8. Install an ion fan:
The preheating temperature of the wave soldering oven is 80~120℃. Static electricity is easily generated under such high temperature and dry hot air. An ion fan uses high voltage to ionize air into positive and negative ions, which are then blown into the oven to neutralize the static electricity generated on the PCB and components by the high temperature and hot air. Therefore, an ion fan should be installed at the entrance of the wave soldering oven. At the entrance of the anti-static work area, a fan should be installed... An ion fan can be added to the starting point or above the conveyor belt as needed.
9. Install anti-static cleaning rollers on the conveyor belt:
Homemade simple device: Wrap a piece of hard plastic tubing (slightly shorter than the width of the conveyor belt) with a towel (it should be relatively flat), wet it, and then insert an iron rod (as a shaft) through the middle to fix it to both ends of the conveyor belt. Use a cola bottle filled with water to continuously humidify it, similar to a hospital IV drip. When the conveyor belt starts, the roller rotates with its own weight, providing cleaning, humidification, and anti-static effects. This simple homemade device can function as an ion fan for the conveyor belt in certain situations.
10. PCBs should be placed under anti-static sponge pads on the assembly line (mainly referring to DIP post-soldering, testing, assembly, etc.) to prevent static electricity and board surface scratches. Anti-static board storage carts or boxes should be used for inter-process transfers. (Its surface resistance is below 10⁶ Ω), and it should be properly grounded using a 1 MΩ resistor.
It is important to distinguish between electrostatic shielding materials (bags) and antistatic materials (bags). Antistatic materials (bags, pads) are mostly pink and are used only as inexpensive padding and intermediate packaging for electrostatic sensitive devices. They are only less prone to generating static electricity themselves; if electrostatic discharge occurs, it can penetrate these antistatic materials and cause damage. Electrostatic shielding packaging is mostly silver, black, or gray, and includes opaque aluminum foil-shaped materials and black or gray semi-transparent materials. There are also grid-shaped fully transparent materials now available. The basic principle is to vacuum-plate an aluminum layer on top of the antistatic material as a conductive electrostatic shielding layer. When electrostatic potential is generated, the shielding layer will evenly distribute the induced electrostatic potential across the entire packaging surface, reducing the surface potential difference and preventing localized high potential difference discharges. It also provides good shielding against high-frequency, strong electromagnetic fields. Depending on the protection level, the conductive layer of electrostatic shielding packaging materials is also divided into: outer resistive layer – insulating film – aluminum foil layer – insulating film; There are various grades of insulating film – aluminum foil layer – insulating film; vacuum-plated layer – insulating film; printed conductive grid layer – insulating film, etc. A closer look reveals that, besides the 2-3 layers of electrostatic shielding material in the outer packaging, the internal support materials (such as push trays and feed tapes) are all black high-resistivity conductive material, approximately 106MΩ. Their sole function is to shield and neutralize the electrostatic potential of the pins, providing a path for electrostatic discharge. Electrostatic sensitive components (such as ICs) and finished products must be packaged with electrostatic shielding material, not anti-static bags, when shipped.
11. Temperature and Relative Humidity Control: Electronic operations, especially SMT, have high requirements for temperature and humidity. Generally, the temperature should be controlled between 18 and 28°C. Too high or too low a temperature will affect the normal operation and accuracy of the equipment. The relative humidity should be between 50% and 85%. Too low a humidity will easily generate static electricity (see Table 1). Too high a humidity will easily cause condensation on the equipment, increasing the water content of the solder paste. Therefore, monitoring and control should be strengthened. For anti-static purposes, in autumn and winter when the relative humidity is low, a humidifier or a damp cloth can be used to solve the problem.
12. Other
(1) Soldering irons should be anti-static low-voltage constant-temperature soldering irons as much as possible, and properly grounded.
(2) Use low-voltage DC electric screwdrivers (electric screwdrivers) with grounding wires.
(3) For small-batch PCB cleaning operations, anti-static brushes should be used; ordinary plastic brushes should not be used.
(4) In some situations, anti-static materials should be used for ceilings and walls. Generally speaking, even ordinary gypsum board and lime-coated walls are acceptable, but plastic ceilings and ordinary or plastic wallpaper are prohibited.
13. Static Electricity Tester:
If conditions permit, consider acquiring a handheld non-contact static electricity tester. This allows for real-time monitoring of static electricity generation and magnitude, tracking of negative effects caused by static electricity, and understanding the effectiveness of improvements. However, due to its high price, most companies are hesitant to purchase one.
14. Monitoring and Recording:
Anti-static measures must be implemented by designated personnel and formalized into a system to ensure effective implementation. Otherwise, all hardware investment may be ineffective.
(1) Personnel: Two people should be responsible for management, testing, and recording. In most cases, two people should work together to prevent staff turnover and disruptions.
(2) Testing and Recording: In summary, the following tests and records should be completed daily:
a. Static electricity test point -------- Static electricity ground Pointer multimeter
Static electricity ground ----------- Equipment ground Resistance measurement
b. Soldering iron tip grounding/soldering iron tip temperature measurement.
c. Small solder pot grounding/solder pot temperature measurement.
d. Testing instrument grounding measurement.
e. Static electricity wristband grounding test. Static electricity wristband tester.
f. Indoor temperature/relative humidity measurement and control. Thermometer/hygrometer.
(3) Check the attire of personnel in the anti-static work area and the compliance with all anti-static regulations.
(4) If possible, use a static electricity tester to measure the static electricity voltage under various conditions at the work site and on the production line.
The electrostatic voltage should generally be less than 100V, and in special cases, less than 25V.
15. Training and Etiquette:
Anti-static knowledge and measures should be an important part of all-staff training to ensure that every employee understands and comprehends them, forming good professional habits. For example: Clothing, drawings, and other materials should not come into contact with components; drawings and materials should be placed in anti-static document bags and hung up; plastic boxes, leather, cardboard, glass, and other items that easily generate static electricity should not be piled on the anti-static safety workbench; anti-static wrist straps and gloves must be worn before handling components; when handling PCBs or sensitive components, hold them by the edges to avoid contact with leads and connectors.