Static electricity protection method
In the manufacture of electronic products, it is impossible not to generate static electricity. The generation of static electricity is not the hazard, but the hazard lies in the accumulation of static electricity and the resulting electrostatic discharge. The core of static electricity protection is "Meditation Elimination".
Principle of electrostatic protection:
(1) Prevent static electricity from accumulating in places where static electricity may occur. Take measures within a safe range.
(2) The existing static electricity accumulation is quickly eliminated and released immediately.
Electrostatic protection method:
(1) Use anti-static materials: Metal is a conductor, and the leakage current of the conductor is large, which will damage the device. In addition, since insulating materials are prone to triboelectricity, metals and insulating materials cannot be used as antistatic materials. Instead, a so-called electrostatic conductor with a surface resistance of 1×105Ω·cm or less and an electrostatic subconductor with a surface resistance of 1×105-1×108Ω·cm are used as antistatic materials. For example, the commonly used electrostatic protection materials are realized by mixing conductive carbon black in rubber, and the surface resistance is controlled below 1×106Ω·cm.
(2) Leakage and grounding: ground the parts that may or have generated static electricity, and provide static discharge channels. Use the method of burying the ground wire to establish an "independent" ground wire. Make the resistance between the ground wire and the ground <10Ω. (See GBJl79 or SJ/T10694—1996)
Grounding method of electrostatic protection materials: connect electrostatic protection materials (such as countertop mats, floor mats, anti-static wrist straps, etc.) to the conductor leading to the independent earth wire through a resistance of 1MΩ (see SJ/T10630-1995). The 1MΩ resistor is connected in series to ensure a current of less than 5mA to the ground, which is called soft grounding. The equipment shell and electrostatic shield are usually directly grounded, which is called hard grounding.
(3) Elimination of the static electricity of the conductor: the static electricity on the conductor can be grounded to make the static electricity leak to the ground. The voltage and release time of the discharge body can be expressed by the following formula: UT=U0L1/RC. In the formula, the voltage (V) at the moment of UT-T (V) U0-the initial 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 drops to a safe zone below 100V within 1 second. This can prevent damage to the SSD caused by excessive leakage speed and excessive leakage current. If U0=500V, C=200pf, and want to make UT reach 100V within 1 second, R=1.28×109Ω is required. Therefore, 1MΩ current-limiting resistors are usually used in electrostatic protection systems to limit the discharge current to less than 5mA. This is designed for operational safety. If the operator accidentally touches the 220V industrial voltage in the electrostatic protection system, it will not bring danger.
(4) Elimination of static electricity charged by non-conductors: As for the static electricity on the insulator, since the charge cannot flow on the insulator, the grounding method cannot be used to eliminate the static electricity. The following measures can be adopted:
(a) Use ion blower-ion blower generates positive and negative ions, which can neutralize the static electricity of static electricity. It can be installed in the space and near the placement head of the placement machine.
(b) Use static eliminators-static eliminators are surfactants. Use static eliminator sassafras to wash the surface of the instrument and the object, which can quickly eliminate the static electricity on the surface of the object.
(c) Control the humidity of the environment—increasing humidity can increase the surface conductivity of non-conducting materials and make the surface of the object less likely to accumulate static electricity. For example, humidification and ventilation measures can be adopted in the dry environment in the north.
(d) Use electrostatic shielding—shielding cover (cage) can be used for equipment prone to static electricity, and the shielding cover (cage) can be effectively grounded.
(5) Process control method: In order to generate as little static electricity as possible in the manufacture of electronic products, control the accumulation of static charge, quickly eliminate the existing static accumulation, and release it immediately. It should be from the aspects of plant design, equipment installation, operation, management system, etc. take effective action.