ESD protective equipment, equipment and its use knowledge
1. Human ESD protection products
There are roughly 6 kinds of ESD protection products for human body.
(1) ESD protective work clothes (also called anti-static work clothes)
ESD protective clothing is made of conductive fibers. When rubbing against other objects, the static electricity generated will leak through the contact between conductive fibers and the human body or through corona discharge and diffusion between conductive fibers. So as to prevent the accumulation of static electricity.
There are three types of materials and conductive fibers used in ESD protective clothing:
A. Metal fiber clothing: clothing made of blended stainless steel wire and cotton with a diameter of 8-50 microns.
B, carburizing (or metal) clothing: spraying and electroplating conductive materials (conductive carbon black, metal powder) on the surface of synthetic fibers.
C. Conductive synthetic fiber clothing: Conductive materials are mixed into synthetic fibers.
There are the following requirements for the design and use of ESD protective clothing:
Non-anti-static fabric is generally not used as lining. When it must be used, the lining used shall not exceed 20% of the total lining area.
The whole garment shall not use any metal accessories. When it must be used, it should not be exposed directly to the surface of the garment, and the metal should discharge on the same side.
The detection charge of each piece of ESD protective clothing should be lower than 0.6 microcoulomb per piece specified in the national GB12014-89 "Anti-static Work". The electric quantity should not exceed 0.01 microcoulomb/piece, the level II area should not exceed 0.1 microcoulomb/piece, and the level III area should be less than 0.4 microcoulomb/piece.
When wearing ESD protective clothing, it must be used in conjunction with ESD protective shoes (anti-static shoes).
The ESD protective clothing passed by the national standard shall be re-inspected every other year. When the charged amount exceeds the above-mentioned specified value, it shall not be used.
Pure cotton clothing cannot prevent static electricity from accumulating in an environment where the relative humidity is lower than 40%, so it is not suitable for ESD protective clothing.
(2) ESD protective shoes (anti-static shoes)
The sole of the shoe is made of synthetic rubber with conductive carbon black to leak the electrostatic charge carried by the ESDS device operator to the ground and prevent the accumulation of static electricity on the human body.
The national standard GB4386-84 "Safety Technical Conditions for Anti-static Rubber-soled Shoes and Conductive Rubber-soled Shoes" stipulates that the resistance of anti-static soles is 5×104～108 ohms, and the resistance of conductive shoes is not more than 1.5×105 ohms. The difference in use between anti-static shoes and conductive shoes is that anti-static shoes can be used in an environment with electrical equipment and can prevent electric shocks to the human body caused by a voltage of 250 volts. Conductive shoes cannot be used in environments where there is a risk of electric shock.
In order to ensure the elimination of human body static electricity, the resistance value between the ground and the earth when the shoes are put on should be no more than 108 ohms and 0.5×105 ohms respectively. During the transfer process, no insulation should be stained on the bottom of the shoe and the transfer of insulating socks is prohibited.
(3) Anti-static wrist strap and foot strap
Anyone who handles ESDS devices should use anti-static wrist straps and foot straps. The role of wrist straps and foot straps is to quickly leak the static charge of the human body to the ground.
A general anti-static wrist strap is composed of a buckle, a belt and a ground connection wire. The straps, buckles and ground connection wires have good electrical contact. In order to ensure the safety of operators, there is usually a 1 megohm resistor in series on the ground connection line of the wrist strap to limit the current flowing through when the human body gets an electric shock to no more than 5 mA.
There are two types of wrist straps: one is used in the operating environment of general ESDS devices, and its resistance to ground in series is 1 megohm. The other is used for the operation of ESDS devices in special environments, and its resistance to ground in series is about 50 kilohms (see ETS).
When using the wrist strap, make sure that the wrist strap is in good contact with the human wrist, the ground connection wire, and the ground wire.
(4) ESD protection finger cots
The finger cots used by people who operate ESDS devices are made of latex with antistatic agents to prevent damage to ESDS devices due to unequal potential. The surface resistance of the finger cot should be less than 108 ohms.
(5) Anti-static socks
Anti-static socks are made by implanting conductive electric wires into the sole material to achieve electrical contact between the human body and conductive shoes. The resistance of the sole should not be greater than 108 ohms.
ESD protection equipment in the production environment of the electronics industry:
(1) ESD protection workbench
The workbench made of ESD protection material and grounded is called ESD protection workbench. The table top is made of static conductive or static dissipative materials (table mats made of ESD protective materials are also used). The table top is in good contact with the ground wire. The connection between the ESD protection workbench surface and the ground wire adopts soft grounding (with 1 MΩ resistance in series). The system resistance of the general ESD protection workbench surface should be in the range of 105-109 ohms, and the system resistance of the workbench surface in the level I ESD protection area should not be higher than 108 ohms.
(2) Shunt rod, wire clamp, conductive foam material
In order to ensure that the ESD device does not fail, it is necessary to use metal shunt bars, metal wires or non-corrosive conductive foam materials to short the leads of the ESDS device or its components when preventing ESD devices. In order to function as a shunt, the resistance of the above-mentioned materials should be at least one order of magnitude lower than the smallest resistance between any two pins of the ESDS device.
(3) ESD protection floor
The protective floor is made of static conductive or static dissipative materials, such as carpet, ethylene floor, terrazzo, rubber, PVC, and raised floor. For safety and good static leakage, ESD protection floors should be grounded. The system resistance should be within 105～1010 ohm.
(4) ESD protection electric soldering iron, solder suction device
The electric soldering iron and tin sucker used for ESD device operation should be connected with the soldering iron tip and the shell to the ground to prevent the electric soldering iron and the shell from being charged by induction.
(5) ESD fixtures, maintenance kits, maintenance tools used by storage racks in the ESD protection zone, and ESD protection fixtures, etc. should be made of static dissipative materials or take electrostatic leakage measures.
(6) There are many types of ESD protection packaging and containers. According to preliminary statistics, there are 20 types. It can be summarized into two categories: one is antistatic hard plastic products, which are mainly composed of synthetic resin plastics and conductive materials. Like synthetic plastics, there are thermoplastics such as polyethylene (PE), polypropylene (PP), ABS, and polycarbonate (PC). Thermosetting resins include polyester, phenolic and other types. As antistatic fillers, there are carbon black, metal fibers, antistatic agents and so on.
The above-mentioned materials have good mechanical strength and are mostly used to make component boxes, turnover boxes, pallet tooling racks, pallet integrated circuit packaging tubes, etc. In order to protect ESDS devices, the volume resistance of these materials should not be greater than 108 ohms or the surface resistivity of the materials should not be greater than 109 ohms. The other is soft ESD protective packaging materials. These materials are also made by modifying the above-mentioned materials. Soft ESD protective packaging materials are different in terms of conductivity, so the scope of use is also different. There are three types of flexible packaging: static electricity, static electricity dissipation, and shielding and dissipation composite types.
The protection provided by all kinds of ESD protective packaging is: provide equipotential and shunt the pins of various ESDS devices; leak and diffuse the static charge generated by the friction of the insulation surface of the ESDS device in time; shielding materials provide shielding against electrostatic induction. Protect ESDS devices.
(7) ESD protection transfer vehicle, seat
In the ESD protection zone, all the seats of the wedding car used by operators when transporting ESDS devices should take anti-static measures. The structure of the running car is a metal frame, on which is a box made of static dissipative materials. The box body and the metal frame have good electrical contact. The metal frame and the conductive rubber wheels form an electrical connection. Under normal circumstances, it is required that the system resistance of the metal frame of the moving vehicle to the static conductive ground of the protective area is not more than 108 ohms.
The electrostatic leakage principle of the seat used in the ESD protection area is the same as that of a running car. The cushion used should be made of anti-static fabrics and other static dissipative materials and keep good contact with the metal body.
(8) Ionizing static eliminators (ionizers) can use ionized air to discharge static charges in places where they cannot be effectively grounded, or to dissipate static charges on insulating objects that cannot be effectively grounded. Ionizers are also useful for dissipating electric charges in areas where spraying processes such as sandblasting and painting have been awakened. The three methods commonly used for air ionization are radioactivity, dot and electrostatic combs. The radioactive material provides A particles that ionize the air. The electrical method uses a high-voltage square wave signal that is air ionization. The principle of an electrostatic comb is similar to that of a lightning rod. It uses the concentrated points on the tip of the needle to ionize the air.
This method is based on the principle of the point from the earth's surface and self-repulsion, so that the charge is deposited on the surface with the smallest radius of curvature. The air from the ionizer should contain approximately equal amounts of positive and negative ions. The imbalance of positive and negative ions can cause residual voltage across the ionization area. The placement of the ionizer should be determined according to the manufacturer’s recommendations or through experience. The manufacturer's specifications usually provide information on the relationship between the decay time and the distance and angle between the ionizer and the charged area. It may take a few seconds or even a few minutes for the ionizer to dissipate the charge, depending on the amount of charge and the distance of the charge from the source. In order to neutralize the charge on the charged area, the ionizer should work for at least 2 to 3 minutes. Some ionizers have extremely high residual voltage, which can damage some ESDS products. In order to adaptively control ESD, when selecting and placing an ionizer, it is necessary to measure the residual voltage on the protected area and compare it with the voltage sensitivity level of the first ESDS product.