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How to prevent failed LED pixels on large LED Mesh Screens?

LED Lamp is the key to production volume and reliability. How to reduce and eliminate failed led and increase production volume and reliability is a key issue that manufacturers need to deal with. Here are some analysis and discussions on some causes of failed LED.

  1. Static electricity damages the LED large screen chip, causing the PN junction of the LED large screen chip to fail, and the leakage current becomes large and becomes a resistor.

There are countless electronic components damaged by static electricity all over the world, causing economic losses of tens of millions of dollars. Therefore, preventing static electricity from damaging electronic components is a very important task in the electronics industry. Companies that package and use LEDs must not take it lightly. Any problem in any link will cause damage to the LED, which will cause the performance of the LED to deteriorate and lead to failure. We know that the human body (ESD) static electricity can reach three thousand volts, and the feet can break down and damage the LED chip. In the LED packaging and disassembly production line, whether the grounding resistance of various devices can meet the requirements is also very important. Generally, grounding is required. The resistance is 4 ohms, and the grounding resistance should be less than or equal to 2 ohms in some cases with high requirements.

Human body static electricity can also cause great damage to the large LED screen. When working, wear anti-static clothing and wear an electrostatic ring. The electrostatic ring is poorly grounded. There is no static ring that does not need to be grounded. The anti-static effect is not good. It is recommended not to use this kind of product. If workers violate the operating rules, they should receive considerable warning education and also serve as a notice to others. The amount of static electricity in the human body depends on the clothes of different fabrics worn by people and the size of each person. It is very difficult to see the discharge between clothes when we take off at night in autumn and winter. The voltage of this type of electrostatic discharge is less than 3000 Volt. While the ESD value of silicon carbide substrate chips is only 1100 volts, the ESD value of sapphire substrate chips is even lower, as long as 500-600 volts. A good chip LED, if we take it by hand (without any protective measures on the body), the result can be imagined. The chip or LED will be damaged to varying degrees. Every time a good device passes our The hand is broken for no apparent reason, and that is caused by static electricity.

If the sealing and dismantling companies do not strictly follow the grounding regulations, it will result in a decline in the product qualification rate and reduce the economic efficiency of the company. Also, if the equipment and personnel of the companies using LEDs are poorly grounded, they will cause LED damage, and rework is inevitable. According to the requirements of the LED standard user manual, the lead of the LED should be no less than 3-5 mm away from the colloid, and be bent or soldered. However, most of the companies that use it do not do that, but just separate the thickness of a PCB board ( ≤2mm) is indirect soldering, it will also cause damage or damage to the LED because too high soldering temperature will affect the chip, it will deteriorate the chip characteristics, reduce the luminous efficiency, and damage the LED. Such phenomena are not uncommon. . Some small companies use manual soldering and use a 40-watt ordinary soldering iron. The soldering temperature cannot be controlled. The soldering iron temperature is above 300-400℃. Excessive soldering temperature will cause LEDs. The expansion-contraction ratio of LED leads is 150℃ at high temperatures. The expansion coefficient on the left and the left is several times higher, and the internal gold wire solder joints will be pulled apart due to excessive heat shrinkage and cold shrinkage, resulting in a failed LED phenomenon.

  1. Analysis of the reasons for the phenomenon of failed LED caused by open solder joints in the internal connection of LEDs

2.1The incomplete production process of the packaging and demolition enterprises and the backward inspection methods of incoming materials are the indirect reasons for the formation of LEDs

Generally, LEDs for receiving racks, sealing, and disassembling are used. The racks are made of copper or iron metal materials by precision die stamping. Because copper is more expensive, the cost is naturally high. Affected by the fierce market, the factors are affected. To reduce the formation In this market, most of the market uses cold-rolled low-carbon steel to stamp the LED racks. The iron racks must be silver-plated, and silver-plated does not have two functions. One is to prevent oxidation and rust, and the other is to facilitate welding. The amount of electroplating in the rack is very critical, it is related to the life of the LED, the treatment before electroplating should be carried out in strict accordance with the operating procedures, the processes such as rust removal, degreasing, and phosphating should be perfunctory, and the electroplating should be controlled For good current, the thickness of the silver-plated layer should be well controlled. Too thick the coating is expensive and too thin affects the amount.

Since the general LED packaging and disassembly companies do not have the ability to inspect the amount of electroplating in the rack, it gives some electroplating companies no gaps, so that the silver plating layer of the electroplating rack is thinned, and the cost and income are reduced. The general packaging and dismantling companies IQC There is a lack of inspection methods for the receiving rack, and there is no instrument to detect the thickness and fastness of the coating of the receiving rack, so it is more difficult to get confused.

I have seen that few racks rust after being stored in the warehouse for a few months. Not to mention using them, it can be seen that the amount of electroplating is not bad. Products made with such a rack will not last long, not to mention 30,000 to 50,000 hours, 10,000 hours is a problem. The reason is very simple. In that weather, the humidity outside the air is high, and it is very difficult to form poorly plated metal embroidery, which makes the LED components fail. Even if the LED is packaged and disassembled, the silver-plated layer is too thin and the adhesion is not strong, and the solder joints will be separated from the frame, resulting in an led failure.

2.2 Every process outside the sealing and unpacking process must be done in practice, and any negligence in any link is the cause of the led lamp.

During the dot and failed bonding process, the silver glue (for single solder joint chips) cannot be used as much as possible. If there is too much glue, it will return to the gold pad of the chip, forming a short circuit, and the chip will not stick firmly. The same is true for double-welded chips with insulating glue. If the insulating glue is applied too much, it will return to the gold pad of the chip, forming a virtual solder during welding and causing LEDs. If the chip is missing, the glue will not stick, so the glue must be just right, neither more nor less.

The welding process is also very important. The four parameters of the gold wire ball welding machine, namely pressure, time, temperature, and power, must be properly matched. In addition to the fixed time, the other three parameters are adjustable. The pressure is properly adjusted. It’s hard to crush the chip if it’s too large, and it’s hard to solder it if it’s too small. The welding temperature is generally adjusted to 280℃. The power adjustment refers to the ultrasonic power adjustment. It is not good if it is too large or too small. The adjustment of the parameters of the gold wire ball welding machine is based on welding.

A good material is qualified if tested with a spring torque tester ≥ of 6 grams. Every year, the various parameters of the gold wire ball welding machine must be progressively tested and corrected to ensure that the welding parameters are in the best form. Also, there is no requirement for the arc of the bonding wire. The arc height of a single solder joint chip is 1.5-2 chip thickness, and the arc height of a double solder joint chip is 2-3 chip thickness. The arc height will also cause the amount of LED large screens. The problem is that if the arc height is too low, it is difficult to cause a dead light phenomenon during welding. If the arc height is too high, the resistance to the current impact is poor.

  1. Method to identify LEDs

Use a lighter to heat the LED leads to 200-300°C for the non-lighting LED lights, remove the lighter, and use a 3-volt button battery to reverse and connect the negative electrode to the LED. If the LED lights can be lit at this time, but the temperature of the lead will decrease as it goes out If the LED light turns from bright to not bright, it proves that the LED light is soldered. The reason why the heating can light up is to use the principle of metal heat shrinkage and cold shrinkage. When the LED lead is heated, the internal solder joints are connected. At this time, the current is turned on, the LED can generally emit light, and the temperature decreases with the temperature. The lead shrinks and returns to the normal temperature state, and the internal solder joints are disconnected, and the LED light will not light up. This type of method is effective after repeated attempts. Weld the two lead wires of the dead lamp of that kind of virtual welding on a metal strip, soak it with concentrated sulfuric acid to dissolve the external colloid of the LED. After the colloid is completely dissolved, take it out. Observe the welding condition of each solder joint under a magnifying glass or a microscope. It is possible to find out whether the problem is the first or second welding, whether the parameter setting of the gold wire ball welding machine is incorrect, or other reasons, to improve the method and process to prevent the phenomenon of false welding from occurring again.

There is no problem with the amount of plating on the shelf, and the leakage current of the LED chip will be too large to cause the LEDs to fail to light.

At present, many LED products do not have anti-static protection to reduce the cost, so it is difficult for them to be damaged by static electricity. On rainy days, it is difficult to experience power supply lines, high-voltage static electricity, and spikes superimposed on power supply lines, which will cause LED products to suffer varying degrees of damage.

There are not many reasons for the occurrence of LEDs, and they cannot be listed one by one. There is no possibility of LEDs in all aspects of packaging, use, and use. How to increase the quantity of LED products is a high priority for packaging and dismantling companies and users. Only in that way can the output of LEDs be fully improved, and long life and high reliability can be achieved. In terms of the circuit design being used, choosing varistors and PPTC components to complete the protection circuit, adding parallel circuits, using constant current switching currents, and deleting temperature protection are all ineffective measures to improve the reliability of LED large-screen products.

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