Summary of protection static electricity in electronic manufacturing
In the process of electronic manufacturing, electrostatic discharge often damages electronic components, and even causes electronic components to lose their function, resulting in losses caused by mistakes. Therefore, electrostatic protection in the electronics manufacturing industry is very important, mainly to understand, Analyze the principle of static electricity generation and electrostatic protection in electronic product manufacturing, and summarize some of the electrostatic protection technology foundations and corresponding measures in production.
Static electricity is an invisible and intangible electric energy. It remains on the surface of the object. It is the result of the imbalance of positive and negative charges in the local range. It is formed by the conversion of electrons or ions. The electrostatic phenomenon is the process of charge generation and disappearance. A general term for electrical phenomena produced in the process. For example: frictional electrification, human body walking and other phenomena.
The electrostatic voltage generated in the manufacture of electronic products and in the process of transportation, storage, etc. far exceeds the breakdown voltage of MOS devices, which often causes hard breakdown (partial damage of components), which causes them to fail or seriously affect the product. Reliability.
In order to control and eliminate ESD, some developed countries have established national, military, and corporate standards or regulations. From the design, manufacture, purchase, warehousing, inspection, warehousing, assembly, commissioning, packaging and transportation of semi-finished products and finished products, there are corresponding regulations on the manufacture, use and management of static protective equipment. Strict regulatory requirements,
Devices that are sensitive to electrostatic reactions are called electrostatically sensitive components (SSDs). Electrostatic sensitive components mainly refer to very large scale integrated circuits, especially metallized film semiconductors (MOS circuits). Different electrostatic protection measures are taken for different SSD devices.
Static power supply in the manufacture of electronic products:
(1) The activity of the human body, static electricity generated by friction, contact and separation between people and clothes, shoes, socks, etc. is one of the main static power sources in the manufacture of electronic products. Static electricity in the human body is the main cause of hard (soft) breakdown of the device. The electrostatic voltage generated by human activities is about 0.5-2KV. In addition, the humidity of the air has a great influence on the electrostatic voltage, and it is increased by one order of magnitude in a dry environment. Table 2 shows the relationship between relative humidity and human body activity.
When the human body is charged and touched to the ground wire, a discharge phenomenon will occur, and the human body will produce different degrees of electric shock response. The degree of reaction is called the electric shock sensitivity. Table 3 shows the shock sensitivity of the human body during different static voltage discharges.
(2) When the chemical fiber or cotton overalls are rubbed against the work surface and the seat, an electrostatic voltage of 6000V or more can be generated on the surface of the garment, and the human body is charged. When contact with the device, it will cause discharge and easily damage the device.
(3) The insulation resistance of the rubber or plastic sole is as high as 1013 Ω. When it rubs against the ground, it generates static electricity and charges the human body.
(4) When the resin, paint film, and plastic film packaged device are transported in a package, the surface of the device and the packaging material can generate an electrostatic voltage of several hundred volts to discharge the sensitive device.
(5) Various packaging, cartridges, and turnovers made of polymer materials such as PP (polypropylene), PE (polyethylene), PS (polyvinylidene), PVR (polyurethane), PVC, polyester, and resin. Boxes, PCB racks, etc. may generate 1-3.5KV electrostatic voltage due to friction and shock, and discharge sensitive parts.
(6) Ordinary work surface, generated by friction to generate static electricity.
(7) The insulation resistance of concrete, waxed polished floor, rubber sheet and other insulating floors is high, and the static charge on the human body is not easy to leak.
(8) Electronic production equipment and tools: High-voltage transformers and AC/blind flow circuits in equipment such as soldering irons, wave soldering machines, reflow soldering furnaces, placement machines, commissioning and inspection devices all induce static electricity in the equipment. If the equipment is not properly vented, it will cause the sensitive device to fail during the manufacturing process. The hot air circulating in the oven and the cabinet friction, and the CO2 vapor in the CO2 cryostat cooling box can generate a large amount of static charge.
Principle of electrostatic protection:
In the manufacture of electronic products, it is impossible to generate static electricity. The generation of static electricity is not a hazard, the danger lies in the accumulation of static electricity and the resulting electrostatic discharge. The core of electrostatic protection is “quiet elimination”.
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 method:
(1) Use antistatic material: Metal is a conductor, which will damage the device due to the large leakage current of the conductor. 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. (See GBJl79 or SJ/T10694-1996)
Electrostatic protective material grounding method: Connect the static protective material (such as countertop mat, floor mat, anti-static wrist strap, etc.) to the conductor 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 equation: UT = U0L1/RC. In the formula, the voltage at the UT-T time (V) U0 is the starting voltage (V) R-the 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.
Electrostatic protective equipment:
(1) The human body anti-static system includes anti-static wrist straps, overalls, caps, gloves, shoes, socks, etc.
(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: for table mats, anti-static bags, anti-static logistics trolleys, anti-static soldering irons and tools.
点击图标下载 App