SMT production line electrostatic protection
1. The concept of static electricity and its protection
The generation of electrostatic charge is inevitable, such as mutual friction and movement between objects, when two materials are separated, or the flow of liquid and gas will generate charges. Electrons move on the contact surfaces of the two materials, and when they separate, both surfaces are charged. The electrostatic charge on the surface of the object can be directly transferred to other surfaces or very sensitive objects through electrostatic discharge. The charged surface will also charge the surrounding objects through induction, and there is no need to contact these objects.
In the process of assembling electronic products on the SMT production line, the generation of electric charges is also unavoidable, and the transfer of electric charges cannot be avoided. The static electricity of about 8000V will produce sparks when discharging, the static electricity of about 3000V can be felt during the discharge, but the electrostatic discharge of 250V will damage many kinds of semiconductor devices. Especially in large-scale and ultra-large-scale integrated circuits, these devices have short lead spacing, high integration, low power consumption, low voltage, high input impedance, and are very sensitive to static electricity. The phenomenon of zone breakdown and oxide film breakdown can cause semiconductor devices to fail and degrade performance, or cause electronic products to produce a “soft breakdown” phenomenon that is difficult to be discovered, leaving potential hidden dangers for electronic products, which directly affects the performance of electronic products. Quality and service life, so active measures should be taken in SMT production to ensure that the impact of static electricity on electronic products is reduced to zero, so that the factory can produce high-quality and cheap electronic products.
The protection of static electricity is mainly to take corresponding protective measures to prevent static electricity from causing failures of the electronic product, including hard and soft failures. The protection of static electricity involves the entire process of electronic products from development to mass production, including the procurement, testing, storage, transportation, and configuration of static sensitive devices. It is very difficult to completely eliminate static electricity in terms of static electricity protection. , But it is completely achievable to take necessary anti-static measures to control the generation and accumulation of static electricity within a safe value. The basic idea of electrostatic protection is to give priority to prevention. Prevent the accumulation of electrostatic charges in the place where static electricity may be generated, and try to leak the generated electrostatic charges. Therefore, the process of “generating and leaking” is adopted in the process. , The accumulated charge can be leaked quickly and reliably to avoid or reduce the generation of electrostatic hazards.
2. Measures for electrostatic protection
Electrostatic protection technology involves a wide range of work methods. It can be roughly divided into: the use of anti-static materials, tools and equipment on the production line, the so-called hardware measures, and the strengthening of anti-static technical management, the so-called Software measures.
(1) Hardware measures-establish an electrostatic safe working area
The circuit board is assembled on the SMT production line, and some of the components in the circuit board assembly are electrostatic sensitive devices, that is, electronic devices that are sensitive to static electricity. This type of device mainly refers to high-density integrated circuits, especially metal oxide semiconductor (MOS) devices. General MOSIC has an antistatic charge energy of 5MJ~100MJ, and bipolar ICs are in the range of 5/J~500 mJ. The breakdown voltage is between 100V-500V. Therefore, in the area of the circuit board assembly SMT production line, the rated value of electrostatic charge accumulation must be controlled within this range. This area is a special area-the electrostatic work safety zone, which is defined as: the work area, including all conductive objects, must not have enough charge to damage the electrostatic sensitive devices. The specific indicators are:
The resistivity of the electrostatic conductor material is less than 1 x 106Q, and its safe electrostatic voltage is C100V;
The resistivity of electrostatic subconductor material is 1 x 106Q~1x, and its safe electrostatic voltage is C500V;
The resistivity of electrostatic non-conductor materials is above 1 x iolon, and its safe electrostatic voltage is less than 5oova. There is an “equipotential working system” in the electrostatic safe working area that can quickly and effectively leak and accumulate charges. The staff, equipment, tools, Equipment, etc., have anti-static measures, and there are equipment that neutralizes electrostatic charges in these work areas. These are the conditions for electrostatic safety zones.
① Equipotential working system
The equipotential working system or zero-potential working system refers to the establishment of a grounding system for the equipment on the SMT production line, so as to leak the electrostatic charge accumulated by the human body, tools, equipment, materials, etc. into the ground in time , As shown in Figure 5.22. It also includes:
Metal curtain. The door curtain is placed on the door of the workshop. It is constructed of several pieces of metal. It can also be composed of cloth rope containing metal wire or carbon fiber, which is slightly smaller than the width of the door. A grounded stainless steel plate is placed on the corresponding floor. After this arrangement, all personnel entering the workshop will step on the stainless steel plate and touch the metal jacket, thereby leaking the electrostatic charge of the human body to the ground to ensure the safety of static electricity when people enter the workshop.
Ground bus. Connect the incoming end of the grounding bus and the outgoing end of the installed equipment to the ground. The resistance between the bus bar and the grounding copper plate should be less than lOQo
Metal floor. The floor where the operator stands is made of 0.8mmX600mmX 1000mm stainless steel plate, and the stainless steel plate is connected with the ground bus with a wire. The method is: one end of the wire is welded to the stainless steel plate first, and the other end is connected in series to prevent electric shock accidents. It is very important to connect a resistor with a resistance value of 1MI) to the bus bar.
Ground connection. The grounding bus is connected to the body of the assembly equipment, which also means the connection to the workbench in the workshop (such as inspection workbench, repair workbench); the grounding bus is a copper wire with a diameter of 3.2mm, and it must be connected to the equipment or the floor to ensure good contact. The value of measuring its resistance should be lower than lOflo
① Anti-static equipment
In the static safe working area, measures should be taken as far as possible to reduce the generation or accumulation of electrostatic charge, that is, the workbench, workbench, turnover box, etc. in the static safe working area should be equipped with anti-static equipment,
There are many types of anti-static equipment subdivided, and the technical parameters are different due to different manufacturers. At the same time, in addition to anti-static indicators, there should also be many physical performance requirements, which should be contacted by the manufacturer. In particular, it is necessary to point out: In the electrostatic safe working area, strict requirements must be strictly required to ensure that the electrostatic charge in the working area cannot damage the electrostatic sensitive devices.
① Anti-static equipment
Non-conductive objects or insulators cannot move freely in them. Therefore, grounding cannot be used to leak the charges. Only static elimination equipment can be used to eliminate static charges on charged objects. The general method is to use a device that blows out the ion wind and applies the principle of corona discharge to neutralize the electrostatic charge on the electrostatically charged objects with positive and negative ions blown to the designated space to achieve the purpose of eliminating static electricity.
Static eliminator is a liquid substance, sprayed on the surface of an object, will change its conductivity, reduce the generation of frictional static electricity, and will greatly reduce the volume resistance or surface resistance, accelerate the leakage of electrostatic charge, to eliminate or prevent electrostatic charge The purpose of accumulation.
In addition to the static elimination equipment mentioned above, you should also pay attention to the environmental conditions of the static safe working area, such as dust, smoke, and the temperature and humidity of the environment to suppress the generation of static electricity. In particular, humidity management is very important. In the north, especially in the dry winter, the damage caused by static electricity is quite serious. But in summer when the air is humid, the static value will be much lower. The static electricity value generated under these two different humidity conditions will be tens of times different, so we must pay great attention to the appropriate management of the environment. When the relative humidity value is lower than 50%, it is already in a dangerous limit state. At this time, there are various methods for artificial humidification. The most thorough method is to install a humidification device in the air conditioning system in the entire SMT production line plant to make the gas from the centralized air conditioner contain a certain amount of water, and there is no centralized humidification. If the conditions of the factory building, artificial humidification can be carried out according to the specific situation, and the number of daily artificial humidification and the amount of humidification can be determined with reference to the humidity value. For key anti-static areas, local key humidification can be set up, and industrial or household humidifiers can be used.