Electrostatic hazards and protective measures in the process of LED display production

- May 15, 2019-

In recent years, LED display production technology in China is becoming more and more mature, and the application field is becoming more and more popular. However, at present, most LED display manufacturers are not fully capable of producing such products, which brings potential risks to LED display products and even affects the whole market. How to standardize the production, how to produce a real sense of low attenuation, long life LED display products? In this paper, the harm of static electricity in LED display production and its protection method are discussed.


Causes of static electricity:


From the microscopic point of view, according to the theory of atomic physics, when the electricity is neutral, the material is in the state of electric equilibrium.


Macroscopically, the reasons are as follows: friction between objects generates heat and stimulates electron transfer; Contact and separation between objects produce electron transfer; The uneven distribution of surface charge caused by electromagnetic induction; The combined effect of friction and electromagnetic induction.


Electrostatic voltage is produced by the contact and separation of different kinds of materials. This effect is known as triboelectrification, and the voltage produced depends on the nature of the material being rubbed against each other. Because the LED display in the actual production process is mainly the human body and the related components of direct contact and indirect contact electrostatic. So according to the characteristics of the industry we can do some targeted electrostatic preventive measures.


The harm of static electricity in the process of LED display production


If anti-static is neglected in any link of production, it will cause electronic equipment failure or even damage.


When semiconductor devices are placed alone or loaded into a circuit, static electricity can cause permanent damage to these devices even if they are not energized. As we all know, LED is a semiconductor product. If the voltage between two or more pins of LED exceeds the breakdown strength of the component medium, it will cause damage to the component. The thinner the oxide layer, the LED and drive IC will be more sensitive to electrostatic, such as the solder is not full, solder quality problems, etc., will produce a serious leakage path, resulting in devastating damage.


Another failure is due to the node temperature exceeds semiconductor silicon melting point (1415 ℃). Electrostatic pulse energy can produce local heating, so there is a direct breakdown of the lamp and IC failure. This failure can occur even if the voltage is lower than the breakdown voltage of the medium. A typical example is that the LED is a diode composed of PN junction, and the breakdown between the emitter and the base electrode will sharply reduce the current gain. The LED itself or the IC in the drive circuit may not be affected by static electricity immediately after functional damage, these potentially damaged components usually show in the use of the process, so the life of the display is fatal.


Protective measures against static electricity in LED production


Grounding is to directly discharge the static electricity through the wire connection to the earth, which is the most direct and effective anti-static measures, for the conductor usually use the method of grounding, we require the manual use of tools grounding, with grounding anti-static bracelet, and work table grounding.


(1) in the production process, the workers must wear earthing electrostatic bracelet. Especially in the process of pin cutting, plug - in, debugging and post - welding, and make good supervision, quality personnel must do at least every two hours to do the bracelet static test, make a test record.

(2) in the welding, the soldering iron should be anti-static low-voltage thermostatic soldering iron, and maintain good grounding.

(3) in the assembly process, try to use low-voltage dc electric screwdriver with grounding wire (commonly known as telex batch).

(4) ensure the effective grounding of production bench, glue filling table, aging rack, etc.

(5) we require the production environment to be grounded by copper wires. For example, floors, walls and ceilings used in some occasions should be made of anti-static materials. Normally, even if common plasterboard and lime coating metope are ok also, but prohibit to use ceiling of plastic goods and common wall paper or plastic wall paper.


Anti-static ground wire embedding:


(1) the lightning rod of the factory building is generally welded together with the reinforced concrete of the building to properly ground. When lightning strikes, the ground and even the whole building will become the discharge point of high voltage and strong current. It is generally believed that there will be a "step voltage" within 20M of the discharge junction, that is, it is no longer an ideal zero potential within this range. In addition, the zero line of three-phase power supply is impossible to be absolutely balanced, and there will be unbalanced current generated and flowing into the zero line, so the embedded point of the anti-static ground line should be 20 meters away from the ground of the building and equipment.


(2) embedding method: in order to ensure the reliability of grounding, there should be at least three points of grounding, that is, dig a pit at a depth of more than 1.5m every 5m, and drive the iron pipe or Angle iron above 2m into the pit (that is, Angle iron into the pit at a depth of more than 2m), then weld the three places together with a 3mm thick copper strip, and weld 16m2 insulated copper core wire into the room as the main line.


(3) pits with a moderate amount of charcoal powder and industrial salt, in order to increase the soil electrical conductivity, the landfill after measured with grounding resistance tester, grounding resistance should be less than 4 Ω, and tested at least once a year.


Ii. Storage and handling of LED display components


Electrostatic sensitive components will be exposed to areas with static electricity during storage and transportation. Electrostatic shielding can reduce the influence of external static electricity on electronic components. The most common method is to use electrostatic shielding bags and anti-static circulation boxes as protection. In addition anti - static clothing on the body has a certain electrostatic shielding effect.


Therefore, we require workers to wear cordless electrostatic ring and gloves, wear anti-static clothing and anti-static shoes, etc., and use anti-static material box, PCB anti-static material frame, stainless steel turnover car and other professional equipment in the turnover process, to avoid direct contact with the human body turnover.


To: as the anti-static clothing, is to use special synthetic fiber woven cloth, usually rub friction does not produce static electricity. But it is not a electrostatic shielding clothing, it can not eliminate other material produced by static electricity. So the correct wear should be only a shirt, or underwear, inside outside the anti-static clothing. In the winter wear more chemical fiber, woollen clothes etc, again without wearing anti-static clothing in order to use.


Therefore, in the process of storage and handling, it is necessary to control the environmental temperature and humidity, wear a good electrostatic bracelet, correctly wear electrostatic clothing. Do hand and product "insulation", prevent sweat stain pollution products, is very necessary.


Temperature and relative humidity control


Especially the SMT electronic homework, have high requirements for temperature and humidity, general temperature control in the 18 ~ 28 ℃, too high or too low will affect the normal operation of equipment and precision; Relative humidity should be in 50%~85%, too low is easy to produce static electricity, too high equipment easy dew, solder paste moisture content increase, so should strengthen monitoring and regulation, for anti-static, autumn and winter when relative humidity is low, can be solved by humidifier or wet cloth dragging method.


Monitoring and recording


Anti-static measures to be responsible for the implementation of a special person, and the formation of a system, to do a good job in the implementation of the work, or all the investment in hardware facilities, may not play a practical role.


(1) personnel: it shall be jointly managed, tested and recorded by two people. In most cases, two people shall cooperate with each other to prevent staff turnover.

(2) test and record: do a good job in electrostatic protection, and complete the following test and record every day:


A. Electrostatic test point -- -- electrostatic ground.

B. Measure the temperature of the electric soldering tip and ground.

C. Grounding of small tin furnace and temperature measurement of tin furnace.

D. Ground measurement of test instruments.

E. Electrostatic ground test of bracelet.

F. measurement and control of indoor temperature and relative humidity.


(3) check the implementation of staff's clothing and anti-static regulations in the work area.


(4) under certain conditions, the static voltage should be measured on the work site and assembly line with the electrostatic tester under various conditions, the electrostatic voltage should be generally less than 100V, special circumstances should be less than 25V.


Training and literacy


Anti-static knowledge and measures should be taken as an important part of staff training, so that every employee can understand and understand, improve the anti-static awareness of all staff, from top to bottom to implement anti-static requirements, forming good professional habits. Such as: entering the workshop to replace anti-static clothing, wear anti-static shoes, must wear electrostatic ring and gloves before contact with components, PCB board or sensitive devices to hold the edge as far as possible, avoid contact with its lead and wiring, and consciously abide by and implement the anti-static system and regulations.


In fact, anti-static should prevent and inhibit the generation of static charge, accumulation, and quickly, safely and effectively eliminate the static charge has been generated as the basic principle. But many anti-static measures are actually a set of system engineering, a link of omission may appear "a thousand miles of dam collapse in the nest", not immodesty.


LED display manufacturing process is a very rigorous, subtle process, each link should not be neglected. The electrostatic protection of LED display is an important link in LED display production. At present, the industry's understanding of electrostatic protection is not deep enough to meet the needs of professional LED display production.