The LED dead light phenomenon may be encountered by consumers ranging from led light package companies, downstream finished products companies to using units and individuals. There are two reasons for this: First, the LED leakage current is too large to cause the failure of the PN junction, which makes the LED light not light up. This type of situation generally does not affect the work of other LED lights; second, the LED The internal connection lead of the lamp is disconnected, and the LED has no current passing through and produces a dead lamp. This kind of situation will affect the general operation of other LED lamps. The reason is that the voltage of the LED lamp is low (the red, yellow, orange, and orange LED work voltage is 1.8 V—2.2V, the blue, green and white LED working voltage is 2.8—3.2V). Generally, it must be connected in series and parallel to smoothly cope with different working voltages. The more LED lights in series, the greater the impact, and only the external Without an open circuit in the internal wiring of the LED lamp, the zero string of LED lamps forming the series circuit will not light up. It can be seen that this kind of situation is much more severe than the first kind of situation. LED dead lights are the key to the production volume and reliability. How to reduce and eliminate dead lights and improve the production volume and reliability is a key issue that the packaging and use companies need to deal with, For example, MOKOLight PCB LED assembly process is carried out in strict accordance with relevant standards, and the dead light rate of LED products produced is very low. Let’s do some analysis and discussion on some reasons for the formation of dead lights.
Static electricity damages the LED chip, causing the PN junction of the LED chip to fail, and the LED light package current becomes large and becomes a resistor
Static electricity is a kind of devastating devil. 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. LED ligth package and LED display companies 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 light package production line, whether the grounding resistance of various equipment can meet the requirements is also very important. Generally, grounding resistance is required. It is 4 ohms, and the grounding resistance is less than or equal to 2 ohms in some cases with high requirements.
Human body static electricity can also cause great damage to LEDs. When working, wear anti-static clothing and wear static rings. Static rings are poorly grounded. There is no static ring that does not need to be grounded. The anti-static effect is not good. It is recommended If you do not use this kind of products, 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 phenomenon between clothes when we take off clothes at night in autumn and winter. The voltage of this type of electrostatic discharge is less than three thousand. Volt. The ESD value of the silicon carbide substrate chip is only 1100 volts, and the ESD value of the sapphire substrate chip is even lower, as long as 500-600 volts. A good chip or 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 by us. His hands are unexplainably broken, which is caused by static electricity.
If the packaging company does not strictly follow the grounding regulations, it will be the company itself that will suffer, which will result in a decline in the product qualification rate and reduce the economic efficiency of the company. If the equipment and personnel of the same company use LEDs, they will also cause LED damage and rework if the equipment and personnel are poorly grounded. It 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 gel, and be bent or soldered, but 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 also cause dead lights. The expansion-contraction ratio of LED leads is 150℃ at high temperatures. The expansion and contraction coefficients on the left and the left are 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 dead light phenomenon.
The cause analysis of the phenomenon of dead light caused by the open circuit of the solder joints in the internal connection of the LED light
- The incomplete production process of led light package enterprises and the backward inspection methods of incoming materials are the indirect reasons for the formation of LED dead lights
Generally, LEDs are used for receiving racks, sealing and disassembling. 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 and the factors, in order 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 important, 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 light package 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 of the rack in the electroplating rack can be thinned, and the cost and income can be reduced. There is a lack of inspection methods for the receiving rack, and there is no instrument for detecting the thickness and fastness of the coating of the receiving rack, so it is more difficult to get confused.
The author has seen that few racks have rusted after being in the warehouse for a few months. Not to mention the use, it can be seen that the amount of electroplating is not much different. Products made with that sort of shelf will definitely not last long, not to mention 30,000 to 50,000 hours, 10,000 hours will be a problem. The reason is very simple. There is no southerly wind for a period of time every year. In such weather, the humidity outside the air is high, and it is very difficult to form poorly plated metal parts and make the LED components fail. Even if the packaged LED is too thin, the silver-plated layer will be too thin, and the solder joint will be detached from the rack, resulting in a dead light phenomenon. That is, the light that we have used well does not turn on, in fact, the internal solder joints are taken away from the rack.
- Every process outside the LED light package process must be done according to the actual operation, and any negligence in any link is the cause of the dead light
In the dot and die bonding process, the silver glue (for single solder joint chips) cannot be taken 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 solder joint 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 soldering and causing a dead light. If the chip is missing, the stickiness is not strong, so the glue must be just right, neither more nor less. The welding process is also very important. The four parameters of pressure, time, temperature, and power of the gold wire ball welding machine 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 at 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 various parameters of the gold wire ball welding machine is based on welding. A good material is qualified if tested with a spring torque tester ≥ 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. In addition, 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 height of the arc will also cause the quantity of LEDs. 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 current impact will be poor.
Method to identify LED dead light
Use a lighter to heat the LED lead to 200-300℃ for the non-lighting LED light, remove the lighter, and use a 3 volt button battery to connect the negative electrode to the LED. If the LED light can light up at this time, but the lead temperature 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 wire 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 of the LED. The lead shrinks back to the normal temperature state, and the internal solder joints are disconnected, and the LED lights will not light up. Such methods are efficacious 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 all the colloid is 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 it is the first or second welding, whether the parameter setting of the gold wire ball welding machine is incorrect, or other reasons, so as to improve the method and process to prevent the phenomenon of false welding from happening again.
Users who use LED products will also encounter the phenomenon of dead lights, that is, after LED products have been used for a period of time, there will be dead lights. There are no two reasons for dead lights. Open dead lights are due to poor welding volume or racking. There is no problem with the amount of electroplating. If the leakage current of the LED chip is increased, the LED light will not light up. At present, many LED products do not have anti-static protection in order to reduce the cost, so it is difficult to be exposed to static electricity to damage the chip. In rainy days, thunder is difficult to appear on the power supply line. High-voltage static electricity, as well as the spikes superimposed on the power supply line, will cause the LED products to suffer varying degrees of damage.
There are not many reasons for the occurrence of dead lights, and they cannot be enumerated one by one. There is no possibility of dead lights in every link from packaging, use, to use. How to increase the quantity of LED products is for packaging and dismantling companies and using companies to pay a lot of attention And the actual research issues, from the chip, the selection of racks, to the LED light package, all processes must be carried out in accordance with the ISO2000 quantity system. Only in that way can the output of LEDs be able to increase comprehensively and achieve long life and high reliability. 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 products.