The phenomenon of LED dead lights often appears in the LED lighting industry, which seriously affects the quality and reliability of products, and is also a concern of many manufacturers. What causes LED dead lights? How to avoid the phenomenon of LED dead lights is the focus of this article.
Causes of LED dead lights
There are two reasons for LED dead lights: First, the excessive leakage current of the LED causes the PN junction to fail, making the LED light off. This situation generally does not affect the work of other LED lights;
Second, the internal connection lead of the LED lamp is disconnected, causing the LED to pass through without current and causing a dead lamp. This situation will affect the normal operation of other LED lamps, because the operating voltage of the LED lamp is low (red yellow orange LED operating voltage 1.8V-2.2V, blue-green-white LED working voltage 2.8-3.2V), generally connected in series and parallel to adapt to different operating voltages, the more series LED lights, the greater the impact, as long as there is one LED Opening the internal wiring of the lamp will cause the entire series of LED lights of the series circuit to not light, which can be seen that this situation is much more serious than the first situation.
Analyze and discuss the causes of LED dead lights
LED dead lights are the key to affecting product quality and reliability. How to reduce and eliminate dead lights and improve product quality and reliability is a key issue that packaging and application companies need to solve. The following is some analysis and discussion on some causes of dead lights.
1. Static electricity causes damage to the LED chip
Static electricity damages the LED chip, invalidates the PN junction of the LED chip, increases the leakage current, and turns into a resistance. Static electricity is a very harmful devil. There are countless electronic components damaged by static electricity in the world, causing thousands Ten thousand dollars in economic losses. Therefore, preventing static electricity from damaging electronic components is a very important task in the electronics industry. LED packaging and application companies must not take it lightly. Any problem in any link will cause damage to the LED, causing the LED performance to deteriorate or even fail. We know that the static electricity of the human body (ESD) can reach about three thousand volts, which is enough to damage the LED chip. In the LED packaging production line, it is also very important whether the grounding resistance of various devices meets the requirements. Generally, the grounding resistance is 4 Ohm, the grounding resistance of some occasions with high requirements should even reach ≤2 ohms. These requirements are familiar to people in the electronics industry. Guan Jian is in place and has a record in actual implementation.
It is understood that in general private enterprises, anti-static measures are not in place. This is the test record that most companies cannot find the ground resistance. Even if the ground resistance test is done, it is once a year, or once every few years, or when there is a problem. Check the grounding resistance, but I do n’t know that the grounding resistance test is a very important job. At least 4 times a year (tested once every quarter). Some places with high requirements will be tested once a month. The soil resistance will vary with the seasons. There is more rain in spring and summer, and the wet ground resistance of the soil is easier to reach. In autumn and winter, the dry soil has less moisture, and the ground resistance may exceed the specified value. The record is to save the original data. There will be evidence in the future. Comply with ISO2000 quality control system. You can design a table for testing the grounding resistance yourself. Both the grounding resistance testing and packaging companies and the LED application companies need to do it. Just fill in the names of various devices in the table, measure the grounding resistance of each device, and record the tester's signature.
The static electricity of the human body is also very damaging to the LED. You should wear anti-static clothing and wear an electrostatic ring when working. The electrostatic ring should be well grounded. There is an electrostatic ring that does not need to be grounded. If this kind of product violates the operation regulations, it should receive corresponding warning education and also serve as a notice to others. The amount of static electricity in the human body is related to the different fabrics worn by people and their physical fitness. When we take off our clothes in autumn and winter and night, we can easily see the discharge between the clothes. The voltage of this electrostatic discharge is 3,000 volts.
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, only 500-600 volts. A good chip or LED, if we take it by hand (the body has not made any protective measures), the result can be imagined, the chip or LED will be damaged to varying degrees, and sometimes a good device passes through our hands It was inexplicably broken, and this 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 losses, which will result in a decline in product qualification rates and reduce the economic benefits of the companies. LED companies that also use LEDs will also cause damage to the LEDs if the equipment and personnel are poorly grounded. Inevitable. According to the requirements of the LED standard manual, the lead wire of the LED should be no less than 3-5 mm away from the colloid, bent or soldered, but most application companies have not done this, but only separated by the thickness of a PCB board (≤ 2 mm) directly soldered, which will also cause damage or damage to the LED, because excessively high soldering temperature will affect the chip, will deteriorate the chip characteristics, reduce luminous efficiency, and even damage the LED, this phenomenon is 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 ℃. Excessively high soldering temperature will also cause dead lights. The expansion coefficient of LED leads at high temperature is about 150 ℃ The coefficient of expansion is several times higher, and the internal gold wire solder joints will pull the solder joints apart due to excessive thermal expansion and contraction, causing a dead light phenomenon.
2. Analysis of the cause of the dead light phenomenon caused by the open connection of the internal connection of the LED lamp
2.1 The production process of packaging companies is not complete, and the incoming inspection methods are backward. It is the direct cause of LED dead lights. LEDs are usually packaged by bracket rows. The bracket rows are made of copper or iron metal materials through precision mold stamping. It is relatively expensive, and the cost is naturally high. In order to reduce manufacturing costs, most of the market uses cold-rolled low-carbon steel to bring stamped LED brackets. The iron bracket row has to be silver-plated, and there are two silver-plated The first is to prevent oxidation and rust, and the second is to facilitate welding. The plating quality of the bracket row 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. Derusting, degreasing, and phosphorus Processes such as chemical treatment should be meticulous, the current should be well controlled during electroplating, the thickness of the silver plating layer should be well controlled, if the plating layer is too thick, the cost is too high, and the thickness is too thin to affect the quality. Because the general LED packaging companies do not have the ability to check the plating quality of the bracket row, this gives some electroplating companies an opportunity to thin the silver plating layer of the plated bracket row and reduce cost. The inspection method is lacking, and there is no instrument for detecting the thickness and fastness of the coating of the bracket row, so it is easier to get confused.
Some of the brackets rusted in the warehouse after a few months, let alone used, it can be seen how bad the plating quality is. The products made with such a bracket row will definitely not last long, not to mention 30,000 to 50,000 hours, 10,000 hours are a problem. The reason is simple. There are a period of south wind days every year. In such weather, the humidity in the air is high, which is easy to cause poorly electroplated metal parts to embroider, and invalidate the LED components. Even if the encapsulated LED is too thin, the adhesion of the silver plating layer is not strong, and the solder joint is separated from the bracket, causing a dead light phenomenon. This is that the light that we have encountered and used well does not turn on. In fact, the internal welding point is separated from the bracket.
2.2 Every process in the packaging process must be carefully operated. Any negligence in any link is the cause of the dead light. In the spot and solid crystal process, the silver glue (for the single solder joint chip) has more or less points, and more glue It will return to the gold pad of the chip, causing 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, resulting in a virtual soldering during welding and thus 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 critical. The four parameters of pressure, time, temperature and power of the gold wire welding machine must be properly matched. In addition to the fixed time, the other three parameters are adjustable. The pressure adjustment should be moderate and the pressure is large. It is easy to crush the chip, and too small is easy to solder. The welding temperature is generally adjusted at 280 ° C. The power adjustment refers to the ultrasonic power adjustment. If it is too large or too small, it is not good. It is moderate. In short, the adjustment of various parameters of the gold ball welding machine is good for welding. The material shall be 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 state. In addition, the arc of the welding wire is also required. The arc height of the single solder joint chip is 1.5-2 chip thickness, and the arc height of the double solder joint chip is 2-3 chip thickness. If it is too low, it will easily cause the dead light phenomenon during welding. If the arc is too high, the current impact resistance will be poor.
3. The method of identifying the virtual welding dead light
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 positive and negative electrodes, if the LED light can be lit at this time, but as the temperature of the lead decreases The LED light changes from bright to non-lighting, which proves that the LED light is a virtual solder. The reason why the heating can light is to use the principle of metal expansion and contraction. When the LED lead is heated, the expansion and elongation are connected to the internal solder joints. At this time, when the power is turned on, the LED can emit light normally. The LED lead shrinks as the temperature drops After returning to the normal temperature state, disconnecting the internal solder joint, the LED light will not light up. This method is tried and tested. Weld the two leads of this virtual welding dead lamp on a metal bar, soak it with concentrated sulfuric acid to dissolve the external colloid of the LED. After the colloid is completely dissolved, take it out and observe the welding situation of each welding spot under a magnifying glass or microscope. You can 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.
However, even the exhibits at the China Electronics Fair, users who use LED products will encounter the phenomenon of dead lights. This is the phenomenon of dead lights after LED products have been used for a period of time. There are two reasons for dead lights. Open circuit dead lights It is because the welding quality is not good, or the quality of the bracket plating is problematic, and the increase in the leakage current of the LED chip will also cause the LED light to be off. Many LED products do not have anti-static protection in order to reduce costs, so it is easy to damage the chip by induced static electricity. Thunderstorms on rainy days are prone to high-voltage static electricity induced by the power supply lines, and spikes superimposed on the power supply lines, which will cause LED products to be damaged to varying degrees.
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