In short, ESD is charge fast neutralization, electronic industrial spend on it costs have billions of dollars more of. We know that all the material are made up of atoms, atom of electrons and protons. When matter to gain or lose electron, it will lose electric balance and become negatively charged or positive, positive or negative charge in material surface accumulation can make the object with electrostatic. Charge accumulation usually due to material mutual contact separation and produce, also can cause by friction, called friction electric.
There are many factors affect the accumulation of charge, including the contact pressure, the coefficient of friction and separation speed, etc. Electrostatic charge will continue to accumulate, until the cause of charge generation role stop, charge is relief or achieve enough strength can breakdown the surrounding material so far. Dielectric be breakdown, electrostatic charge will soon get balance, this kind of charge fast and is called electrostatic discharge. Because in the small resistance on the quick release voltage, drain discharge abandoned is very big, may be more than 20 ampere, if the discharge through the integrated circuit or other electrostatic sensitive element, such a big current will be to design for only conducting microampere or milliamperes current circuit caused serious damage.
Have a variety of model can be used to express device how to hurt, like the human body model (HBM), machine model (MM), charged device model (CDM) and the influence of electric field on the device. For automatic assembly in terms of equipment, mainly considered three kinds of damage model (model), we discuss in the following respectively.
Machine model/model automatic assembly equipment use guide, belt, slide, element conveyor and other devices to mobile device according to the technological requirements of the direction of the movement, if equipment design undeserved, belt and transport system may accumulate a large amount of charge, and the charge will be in process through the device relief. Equipment unit through the device discharge is called machine model/mode.
Charged device model/mode if a device for some reason and accumulated charge with a charged less surface phase contact, charge will be through the device on the conductive part relief. When the device to other material discharge, is called charged device model, with charged device model said.
The electric field effect in electric field induction IC impedance line the generation between potential difference, cause insulator dielectric breakdown. The failure cause of another reason is the charge on a device in the field will be polarization, which causes the potential difference and anisotropic charge discharge, forming double discharge or neutral. In the ESD control used with different resistance characteristics of the materials, the materials used in the automatic assembly equipment can get ideal effect. Describe material resistance characteristic usually use surface resistivity or body resistivity.
Common concept and application
Surface resistivity simply surface resistivity is the same on the surface between two electrode measured resistance, electrode shape and resistance together through the calculation can get unit area of resistance. On market now could get the readings for the unit area resistance measuring instrument.
Volume resistivity volume resistivity is through the material thickness of the resistance, the unit is Ω · cm.
Conductive material conductive material refers to the surface resistivity and volume resistivity were less than 106 Ω and 106 Ω · cm material.
Dissipative material dissipative material refers to the surface resistivity and volume resistivity were less than 1012 Ω or 1012 Ω · cm material.
Anti-static materials "static" refers to the can inhibit charge accumulation, can be in materials manufacturing process add or local to join a substance to get this kind of characteristic. Anti-static materials need not use surface or body resistivity said.
Conductive additives and film if the cost or other design reasons can only use plastic material or composite material, can use additives to improve electrostatic properties, will additive interfuse plastic materials, according to the different additives and resin percentage can obtain the conductivity or dissipation sex.
In the resin added fiber can make them get conductivity or dissipation sex and enhance strength, this kind of fiber may itself has conductivity or adopted surface plating process. Although add fibre can get these benefits, but it also changed the shrinkage and toughness. Filling agent can provide electrical conductivity and dissipation sex, increase strength, but often will reduce the hardness of matrix resin.
Belt conveyor is used to transport components, PCB and other devices, material general for plastics, fiber products or rubber. If the conveyor belt to receive other parts from the machine from the device, it should be the dissipation sexual material. When the belt surface resistivity for 1 ~ 106 Ω, it can electricize device discharge speed fast, damage to the device, When the surface resistance in 106 ~ 109 Ω, as long as the conveyor belt through the wheel pulley and frame good ground, the conveyor belt won't charged.