The LED dead light phenomenon may be encountered by consumers from packaging companies and downstream finished product companies to the units and individuals used. The reason is that there are two types of conditions: First, the leakage current of the LED is too large to form a PN junction failure, which makes the LED lamp not bright. That kind of situation generally does not affect the work of other LED lights; Second, the LED The internal connection leads of the lamp are disconnected, resulting in a dead light generated by the LED without current. Such a situation will affect the general work of other LED lights. The reason is that the low voltage of the LED light (red yellow orange LED work voltage 1.8 V-2.2V, blue-green-white LED working voltage 2.8-3.2V), generally need to be connected in series or parallel to meet different working voltages, the more series LED lights, the greater the impact, only the external If there is no open circuit in one LED lamp, the zero-string LED lamp that will form the series circuit will not light. It can be seen that this kind of situation is more severe than the first kind of situation. LED dead lights are the key to affecting the output volume and reliability. How to reduce and eliminate dead lights and improve the output volume and reliability are the key issues that companies need to deal with. The following is some analysis and discussion on some reasons for the formation of dead lights.
1. Static electricity damages the LED chip, invalidates the PN junction of the LED chip, and the leakage current becomes large, becoming a resistor
Static electricity is a kind of extremely harmful devil. There are countless electronic components damaged by static electricity in the world, resulting in economic losses of tens of millions of dollars. Therefore, the prevention of static electricity damage to electronic components is a very important task in the electronics industry. LED packaging and LED display companies must not take it lightly. If any problem occurs, it will cause damage to the LED and deteriorate the performance of the LED to failure. We know that the human body (ESD) static electricity can reach about 3,000 volts, the foot can break the LED chip and damage it. In the LED packaging production line, whether the grounding resistance of various devices can meet the requirements is also very important. Generally, the grounding resistance is required. It is 4 ohms, and the grounding resistance is not required in some occasions with high requirements, so as to reach ≤2 ohms.
The static electricity of the human body is also very damaging to the LED. When working, wear anti-static clothing and wear an electrostatic ring. The electrostatic ring is poorly grounded. There is no type of electrostatic ring that does not require grounding. The anti-static effect is not good. It is recommended Do not use such products. If workers violate the operation rules, they should receive considerable warning education and also serve to inform others. The amount of static electricity in the human body is related to the different fabrics worn by people and the body size of each person. When we take off our clothes in autumn and winter and night, it is very easy to see the discharge phenomenon between the clothes. The voltage of such electrostatic discharge is not three thousand. Volt. While the ESD value of the silicon carbide substrate chip is only 1100 volts, 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 (the body does not do any protective measures), the result can be imagined, the chip or LED will encounter different degrees of damage, and a good device will pass us The hand is inexplicably broken, that is the disaster caused by static electricity.
If packaging companies do not strictly follow the grounding regulations, it is the companies themselves that will suffer a loss of product qualification rate and reduce the economic impact of the company. If the companies that use LEDs also have bad grounding of equipment and personnel, the LEDs will be damaged and reworked 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 from the colloid, and it is bent or soldered, but most users do not do that, but only the thickness of a PCB board ( ≤2mm), it will be indirectly soldered, which will also cause damage or damage to the LED. Due to the excessively high soldering temperature, the chip will be affected, the chip characteristics will be deteriorated, the luminous efficiency will be reduced, and the LED will be damaged. Such phenomena are not uncommon. . Some small enterprises 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 form a dead light. The expansion coefficient of the LED lead is higher than 150 ℃ at high temperature. The expansion coefficient of Zuo Zuo is several times higher. The internal gold wire solder joints will pull the solder joints apart due to excessive heat shrinkage and cold shrinkage, forming a dead light phenomenon.
2. Analysis of the cause of the phenomenon of dead light phenomenon caused by the open connection of the internal connection of the LED lamp
2.1 The production process of packaging enterprises is not complete, and the methods of incoming inspection are backward, which is an indirect reason for the formation of LED dead lights.
Generally, LEDs are packed and unpacked. The racks are made of copper or iron metal materials through precision mold stamping. Due to the higher cost of copper, the cost is naturally high. Due to the fierce market, the factors are affected. In this market, most of the market uses cold-rolled low-carbon steel to bring stamping LED storage racks. The iron storage racks have to be silver-plated. Silver plating has no two uses. One is to prevent oxidation and rust, and the other is to facilitate welding. The amount of plating in the storage rack is very critical. It is related to the life of the LED. The treatment before plating should be carried out in strict accordance with the operating procedures. The processes of derusting, degreasing, and phosphating should be perfunctory and should be controlled during electroplating. For good current, the thickness of the silver plating layer should be well controlled. If the plating layer is too thick, the cost is high, and if it is too thin, the amount of influence will be affected. Since the general LED packaging companies do not have the ability to check the amount of rack plating, it gives some electroplating companies no gaps, making the silver plating layer of the plating rack thinner, reducing cost and income. IQC The inspection method of the storage rack is lacking, and there are no instruments for detecting the thickness and fastness of the coating layer of the storage rack, so it is more difficult to pass through.
I have seen that some of the rack discharges are rusting after a few months in the warehouse. Not to mention the use, it can be seen that the amount of electroplating is not bad. Products made with that sort of storage rack are definitely not going to last long. Not to mention 30,000 to 50,000 hours, 10,000 hours are a problem. The reason is that it is very simple in the south wind days where there is no period of time every year. In such weather, the outside humidity of the air is large, and it is difficult to form poorly electroplated metal parts to embroider, which makes the LED components ineffective. Even if the packaged LED is too thin, the silver plating layer is too thin and the destructive force is not strong, the solder joint take-off frame is detached, forming a dead light phenomenon. That is, the light that we have encountered is not working well, in fact, the internal welding point collection rack is detached.
2.2 Every procedure outside the packaging process must be carried out pragmatically, any negligence in any link is the reason for the formation of dead lights
In the process of spot bonding and crystal bonding, the silver glue (for single-soldered chips) can be selected more or less, and more glue 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 the double solder joints of the chip and the insulating glue. If the insulating glue is added, it will be returned to the gold pad of the chip, forming a virtual soldering during welding, which will cause a dead light. The chip is too little and not sticky, so the glue must be just right, neither more nor less. The welding process is also very critical. The four parameters of pressure, time, temperature and power of the wire ball welding machine must be properly matched. In addition to the fixed time, the other three parameters are adjustable, and the pressure adjustment should be appropriate. Large capacity is difficult to crush the chip, and too small is difficult to solder. Welding temperature is generally adjusted at 280 ℃ is better, power conditioning refers to ultrasonic power conditioning, too large and too small are not good, in order to adapt to the degree, divided, the adjustment of various parameters of the wire ball welding machine, to welding Good material is qualified if it is detected with a spring torque tester ≥ 6 grams. Every year, the various parameters of the gold wire ball welding machine are progressively tested and corrected to ensure that the welding parameters are in the best form. In addition, the arc of the welding wire is also not required. The arc height of the single solder joint chip is 1.5-2 chip thicknesses, and the arc height of the double solder joint chip is 2-3 chip thicknesses. If the arc height is too low, it is difficult to form a dead light phenomenon during welding. If the arc height is too large, the current impact resistance is poor.
3. The method of identifying dead lights
Use a lighter to heat the LED lead to 200-300 ℃, remove the lighter, use a 3 volt button battery to connect the LED to the reverse and negative poles, if the LED light can be lit at this time, but the temperature decreases as the lead is extinguished The LED light changes from bright to unlit, which proves that the LED light is a virtual solder. 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 expansion and contraction take the internal solder joint to turn on. At this time, when the current is turned on, the LED can generally emit light. The lead shrinks back to the normal temperature form, and the internal solder joint is disconnected, and the LED light is not lit. Such methods are tried and tested. Weld the two leads of this kind of virtual soldering dead light on a metal strip, soak it with thick sulfuric acid to dissolve the external colloid of the LED, remove it after all the colloid is dissolved, and observe the welding situation of each welding spot under a magnifying glass or microscope It is possible to find out whether it is the first welding or the second welding, is the parameter setting of the gold ball welding machine incorrect, or other reasons, so as to improve the method and process to prevent the phenomenon of virtual welding from happening again.
Users who use LED products will also encounter the phenomenon of dead lights. That is, after using the LED products for a period of time, the phenomenon of dead lights occurs. There are no two types of reasons for dead lights. Open-circuit dead lights are due to poor welding volume or storage. There is no problem with the amount of electroplating. If the leakage current of the LED chip is increased, the LED light will not be turned on. In order to reduce costs, many LED products are not equipped with anti-static protection, so the phenomenon of being damaged by static electricity will appear. Thunder tolerant on rainy days is difficult to appear when the power supply line feels high-voltage static electricity, and the spikes superimposed on the power supply line will cause LED products to be damaged to varying degrees.
There are many reasons for dead lights, and they cannot be listed one by one. There is no possibility of dead lights from packaging, use, and use. How to increase the amount of LED products is a major concern for the packaging and use enterprises. In order to confirm the research problem, from the chip and shelf selection to the zero packaging process of LED packaging, it must be carried out according to the ISO2000 measurement system. As long as the output volume of LEDs can be comprehensively improved, long life and high reliability can be achieved. In terms of the circuit in use, selecting a varistor and a PPTC element to improve the protection circuit, adding parallel circuits, using a constant current switching current, and deleting temperature protection are all invalid measures to improve the reliability of LED products.
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