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The fundamental purpose of PCBA electrostatic protection is to prevent or minimize the hazards caused by the mechanical and discharge effects of static electricity through various protective measures during the manufacturing and use of electronic components, components and equipment, so as to ensure that the design performance and use performance of components, components and equipment are not damaged by electrostatic effects.
The main form of electrostatic hazard in the electronic industry is the sudden failure and potential failure of components caused by electrostatic discharge, and then the degradation or failure of the whole machine performance. Therefore, the main purpose of electrostatic protection and control should be to control electrostatic discharge, that is, to 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; The main way to control the dissipation of static electricity is to discharge and neutralize the static electricity quickly and safely. As a result of the joint action of the two, it is possible to make the electrostatic level not exceed the safety limit and achieve the purpose of electrostatic 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 to minimize the damage to components. The basic ideas for electrostatic protection and control of electrostatic sensitive devices are as follows:
1)To prevent the accumulation of static electricity in places where grounding may occur, take certain measures to avoid or reduce the generation of electrostatic discharge, or take the method of "producing while leaking" to eliminate the accumulation of static charge, and control the static charge to the extent that it will not cause harm.
2)Eliminate the existing charge accumulation quickly and reliably.
In the PCBA production process, the core of electrostatic protection is "electrostatic elimination". To this end, a complete static electricity working area can be established, that is, by using various anti-static products and equipment, taking various anti-static measures to keep the potential electrostatic voltage in the area under the safe threshold for the most sensitive devices. The basic ways are:
1)Process control method. The process control method aims to minimize the generation of static charges during the trial production process. Therefore, 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, inhibit the electrostatic potential and the ability of electrostatic discharge, so that it does not exceed the degree of harm. For example, in the semiconductor manufacturing process, when the shallow junction formation process of high-speed PCB devices is completed, the resistivity of deionized water for washing must be controlled. Although the higher the resistivity, the better the cleaning effect, the higher the resistivity, the better the insulation, and the higher the static electricity generated on the chip. Therefore, it should be controlled at a level slightly higher than 8MΩ, rather than 16-17MΩ for the initial process. In terms of material selection, anti-static materials should be used as packaging materials, and untreated polymer materials should be avoided as far as possible.
2)Leakage method. The purpose of leakage method is to eliminate static electricity through leakage. Electrostatic grounding is usually used to leak electric charges to the ground, or to increase the conductivity of the object to make the grounding leak along the surface or through the interior, such as adding electrostatic agent or humidification. The most common ones are the anti-static wrist strap and electrostatic grounding pole worn by the staff.
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 separate the charged body from other objects with a grounded shield, so that the electric field of the charged body will not affect other objects around (infield shield); Sometimes the shielded case is also used to enclose the isolated object to protect it from the influence of external electric field (external field shielding). For example, GaAs devices are usually packaged with metal boxes or metal films.
4)Compound neutralization method. The purpose of compound neutralization method is to eliminate static charges by means of compound neutralization. Generally, the ground eliminator is used to generate ions with different charges and recombine the charges on the charged body to achieve the purpose of neutralization. Generally speaking, when the charged body is an insulator, because the charge cannot flow on the insulator, the grounding method cannot be used to leak the charge. At this time, the electrostatic eliminator must be used to generate different ions to neutralize. For example, this method is used to eliminate the static charge generated on the conveyor belt of the production line.
5)Cleaning measures. The cleaning measures are designed to avoid tip discharge. Therefore, the surface of charged body and surrounding objects should be kept smooth and clean as far as possible to reduce the PCBA possibility of tip discharge.
