PCB Technical - The purpose and principle of electrostatic protection during SMT patch processing

The fundamental purpose of electrostatic protection is to prevent or minimize the hazards caused by the mechanics and discharge effects of static electricity during the manufacturing and use of electronic components, components,and equipment.To ensure that the design performance and use performance of components, components and equipment are not damaged by static electricity. The main form of electrostatic hazard in the electronics industry is the sudden failure and potential failure of components caused by electrostatic discharge, which in turn causes the degradation or failure of the entire machine.Therefore,the main purpose of electrostatic protection and control should be to control electrostatic discharge, that is, prevent the occurrence of electrostatic discharge or reduce the energy of electrostatic discharge below the damage threshold of all sensitive devices. In principle. Electrostatic protection should be carried out from two aspects: controlling the generation of static electricity and controlling the dissipation of static electricity. Controlling the generation of static electricity is mainly to control the process and the selection of materials in the process; controlling the dissipation of static electricity is mainly to discharge and neutralize static electricity quickly and safely . As a result of the combined action of the two, it is possible that the static electricity level does not exceed the safety limit and achieve the purpose of static electricity protection.

Electrostatic discharge can cause damage to the device.However,by taking correct and appropriate electrostatic protection and control measures and establishing an electrostatic protection system, the occurrence of electrostatic discharge can be eliminated or controlled, and the damage to components can be minimized.

There are two basic ideas for electrostatic protection and control of electrostatic sensitive devices:

1.Prevent the accumulation of static electricity in the place where grounding may occur, take certain measures to avoid or reduce the generation of electrostatic discharge, or adopt the method of "generating and leaking" to eliminate the accumulation of electric charge, and control the static charge to prevent it. The degree of harm caused;

2.The existing charge accumulation can be quickly and reliably eliminated.

In the production process, the core of electrostatic protection is "static elimination". To this end, an electrostatic complete work area can be established, that is, through the use of various anti-static products and equipment, and various anti-static measures, the electrostatic voltage that may be generated in the area is kept below the threshold value that is safe for the most sensitive devices.

The basic ways are:

1.Process control method. The process control method aims to generate as little static charge as possible during the trial production process. For this reason, measures should be taken from the aspects of process flow, material selection, equipment installation and operation management to control the generation and accumulation of static electricity, and suppress the ability of electrostatic potential and electrostatic discharge, so that it does not exceed the level of harm. For example, in the semiconductor manufacturing process, after the shallow junction formation process of the high-speed device is completed, the resistivity of the deionized water used for flushing must be controlled. Although the higher the resistivity, the better the cleaning effect, but the higher the resistivity and the better the insulation, the higher the static electricity generated on the chip. Therefore, it should generally be controlled at a level slightly higher than eight MΩ, rather than the 16-17 MΩ used in the initial process. Also in terms of material selection, anti-static materials should be used for packaging materials, and untreated polymer materials should be avoided as much as possible.

2.Leakage method. Leakage method aims to eliminate static electricity through leakage. Electrostatic grounding is usually used to leak electric charges to the ground. There are also methods to increase the conductance of the object to make the ground leak along the surface of the object or through the interior, such as adding electrostatic agents or humidifying. The most common ones are anti-static wrist straps and electrostatic grounding posts worn by workers.

3.Electrostatic shielding method. According to the principle of electrostatic shielding, it can be divided into internal field shielding and external field shielding. The specific measure is to isolate the charged body from other objects with a grounded shield, so that the electric field of the charged body will not affect other surrounding objects (infield shielding); sometimes the shielded cover is also used to surround the isolated object to make it To avoid the influence of external electric field (external field shielding), such as GaAs device packaging mostly adopts metal box or metal film.

4.Compound neutralization method. The compound neutralization method aims to eliminate static charges through compound neutralization. Usually the grounding eliminator is used to generate ions with different charges to recombine with the charges on the charged body to achieve the purpose of neutralization. Generally speaking, when the charged body is an insulator, since the electric charge cannot flow on the insulator, the grounding method cannot be used to leak the electric charge. At this time, a static eliminator must be used to generate the opposite-sign ion to neutralize it. For example, this method is used to eliminate the static charge generated on the conveyor belt of the production line.

5.Clean up measures. The cleaning measures are designed to avoid the phenomenon of tip discharge. For this reason, the surface of charged objects and surrounding objects should be kept as smooth and clean as possible to reduce the possibility of tip discharge.

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