Principle of electrostatic protection

Principle of electrostatic protection

There are many operations in which it is impossible not to produce static electricity, but the danger is not the accumulation of static electricity and the resulting electrostatic discharge. Therefore, it is particularly important to limit the accumulation of static electricity and take measures to make static electricity leakage (dissipation).

There are many electrostatic protection measures, lead-free welding platform manufacturers tell you the following points:

First, electrostatic dissipation and grounding

It is an effective way to prevent the danger of static electricity by dissipating the static electricity generated in the course of various operations. Electrostatic dissipation is accomplished by substituting antistatic materials for and grounding appliances (including various fixtures and products) prepared by various types of insulation that come into contact with the electronic production process.

Antistatic materials, also known as ESD protection materials, can be divided into three categories from the main aspect of the antistatic principle: electrostatic conduction, electrostatic dissipation, electrostatic shielding dissipation composite class. The three materials can be distinguished by their electrical conductivity. For example, which country GB12158-90 "General Guidelines for the Prevention of Electrostatic Accidents" and electrotechnics will be less than or equal to the volume resistivity and surface resistivity of 106 euros. Meters, 107 euros of material called electrostatic conductor, in 106~1010 euros. Meters, 107~1011 euros for electrostatic subconductors, at 1010 euros. Meters and 1011 ohms above are electrostatic non-conductors. According to the above provisions, the surface resistivity and volume resistivity can be 107 and 106 euros respectively. The material below the meter is defined as the electrostatic conductive material, the surface resistivity of 107~1011 euros is the electrostatic dissipative material, and the volume resistivity is less than 1*102 euros. The meter material is defined as electrostatic shielding material. The national military standard GJ3007-1997 "Technical requirements for Anti-static Work Area" also makes a clear division of electrostatic dissipative materials and electrostatic conductive materials, but it is different from GB12158-90.

For uniform provisions, the current international and domestic division of electrostatic dissipation and electrostatic conductive materials is based on the provisions of the International Electrotechnical Commission IEC1340-4-1. IEC1340-4-1 stipulates: surface resistance, volume resistance is equal to or less than 1*106 ohms of materials; 1*106~1*109 euros. Meter is electrostatic conductive material; Materials of 1*106~1*109 ohms are electrostatic dissipative materials.

Antistatic supplies (appliances) prepared by electrostatic dissipative materials are usually used in the working environment with 220 volts, 380 AC strong electricity or DC strong electricity, which can provide safety protection for operators. The electrostatic conductive material is used in the absence of strong electricity, fire and explosion-proof requirements.

1. Electrostatic dissipation and grounding

The static electricity generated during the production process can be leaked through ESD protection materials and grounding. If the area of ESD protection materials is large enough, the electrostatic voltage can also be reduced, for example, the charge of a static power supply is 0.6 microcoulomb, and its electrostatic voltage is 3000 volts when the capacitance to the ground is 20 picograms. When the leakage to the electrostatic conductive ground with a surface area of 10 square meters (ground capacitance 1000 picocoulomb method), the charge per square centimeter is only 0.6 picocoulomb, and the electrostatic voltage is only 60 volts. Therefore, it can be seen that when the area of the ESD protection material is large enough, and the capacitance to the ground is also large, when a small amount of electrostatic charge leaks on it, it will not cause damage to the ESD device. Therefore, in some cases, ESD protection materials with a very large area can be regarded as a suspended grounding. Even if it is not directly grounded, it will not cause adverse effects on ESD devices.

However, in actual production, a lot of static power is exposed to, and the amount of accumulated charge is also large. Often used ESD protection materials can not provide a larger area, then it is necessary to take grounding measures.

2, dissipation and grounding common occasions

Commonly referred to as electrostatic dissipation, is the electrostatic charge accumulated on ESD protection materials or conductors is leaked to the ground or a suspended ground body with a large enough surface area in a way that causes electrostatic conductivity. Because the electrostatic charge and leakage current are relatively small, it can be considered that the ESD protection material and the resistance between the conductive and the earth does not exceed about 1 megohm, which can be considered to meet the requirements. For electrostatic grounding resistance, the national standard GB12158-90 stipulates that the total leakage resistance between the electrostatic conductor and the earth is usually not greater than 1 megohm, and the grounding resistance value of each group of electrostatic grounding is generally not greater than 100 euros. In mountainous areas and other places, its grounding resistance should not be greater than 1000 euros.

YD/T754-1995 General Rules for ESD Prevention in Communication Rooms and SJ/T10553-1994 Technical Requirements for ESD Prevention of Electronic Equipment stipulate that the electrostatic grounding resistance is less than 10 OHM. This data has been widely used.

Electrostatic ground in the electronics industry is divided into soft ground and hard ground, soft ground refers to the ground wire is connected to a resistor with a higher resistance value and then connected to the earth. The purpose of soft grounding is to limit the current to the ground to a personal safety range (usually 5 MA). The resistance required for soft grounding depends on the AC and DC voltages that people near the ground point may be exposed to. Many standards set the soft ground resistance at 1 megohm. Hard grounding refers to grounding the ground wire directly or through a low resistance. Generally, hard grounding is used for electrostatic shielding or grounding of instruments, devices, and metal bodies.

2. Electrostatic neutralization

Static circuit sum is one of the important measures to eliminate static electricity. In some cases, when it is inconvenient to use ESD protection materials, or some highly insulated electrostatic supplies must be stored on the workbench or process line, in order to ensure product quality, electrostatic neutralization measures must be taken on the operating environment. Electrostatic neutralization is achieved by means of ionic electrostatic eliminator or inductive electrostatic brush.

Principle of electrostatic neutralization: neutralize the positive and negative ions with the positive and negative charges on the static power supply, thereby eliminating the static electricity accumulated on the static power supply.

The device that produces positive and negative ions is called ion electrostatic eliminator, which has many kinds, such as high pressure type, isotope type, ion flow and so on.

Electrostatic neutralization can also be achieved using a static brush (electrostatic comb), which is a row of conductive fibers attached to a metal rod. When the static brush is in contact with the insulation, the electrostatic charge on the insulation can be directly eliminated due to corona discharge, leakage, short circuit, etc.

In some special cases, anti-static agents (sprays, etc.) can also be used to achieve electrostatic neutralization. Such sprays are usually cationic or anionic surfactants.

The comparison of the above several electrostatic neutralization, ionic electrostatic elimination equipment does not affect the quality of the product, eliminate static electricity fast, easy to use, etc., the disadvantage is that the cost of the instrument is higher. Although the use of antistatic agents has the characteristics of simple operation, but from the effect of eliminating static electricity, in some cases, the effect is not as good as that of ionic electrostatic eliminator, and the durability is not good. Some antistatic agents are corrosive and toxic.

Third, electrostatic shielding and grounding

Electrostatic shielding and grounding are usually used to shield electrostatic fields generated by high-voltage sources, and some electrostatic electrode sensitive circuits, so as to avoid wide-band interference caused by electrostatic field induction and electrostatic discharge of ESDS devices and ESDS components.

The shielding function is achieved through a housing that encloses an area. The housing can be made into metal partition type, box type, and can also be made into cable shielding and connector shielding. The shielded shell generally has several types of solid core, non-solid core (such as metal mesh) and metal braided belt, the latter is mainly used as a shield for cables. The performance of various shields is expressed by the shielding effect of the shields. This value defines the intensity of the electrostatic field in a region of the space after shielding, and the decibels reduced by the electric field before shielding. The superior and inferior shielding effect is not only related to the performance of the shielding material, but also to the distance between the shielding and the static power supply, as well as various discontinuous holes and the number of holes that may exist on the shell.

The electrostatic shielding effect has a great relationship with the grounding, only the grounding of the shielding can ensure the leakage of the electrostatic charge induced on the shielding body, so that the circuit in the shielding body is not interfered with the electrostatic field.

Electrostatic shielding and grounding is one of the important measures to prevent electrostatic interference on many kinds of instrument circuits. At the same time, it is also used to prevent the interference of static power to the outside world.

Fourth, humidification

The increase in humidity increases the surface conductivity of the non-conducting material, so that the accumulated electrostatic charge of the object can leak faster. If the process conditions permit, air conditioning equipment and sprayers can be installed to increase the relative humidity of the air to remove static electricity. In general, the effect of eliminating static electricity with humidification is obvious. However, the city should point out that humidification is effective for insulators such as rubber that are easy to form a water film on the surface, that is, insulators that are easy to be moistened by water.