The phenomenon of LED dead lights often occurs in the LED lighting industry, which seriously affects the quality and reliability of the products, and is also a concern of many manufacturers. What causes the LED dead light? How to avoid the phenomenon of LED dead light is the focus of this article.
Causes of LED dead lights
The causes of LED dead lights are nothing more than two situations:
First, the excessive leakage current of the LED causes the failure of the PN junction and the LED lamp does not light up. This situation generally does not affect the work of other LED lamps;
Second, the internal connecting lead of the LED lamp is disconnected, causing the LED to have no current passing through and causing a dead lamp. This situation will affect the normal operation of other LED lamps. The reason is that the working voltage of the LED lamp is low (red, yellow and orange LED working voltage). 1.8V-2.2V, blue, green and white LED working voltage 2.8-3.2V), generally need to be connected in series and parallel to adapt to different working voltages, the more LED lights in series, the greater the impact, as long as there is one LED If the internal wiring of the lamp is open, the entire string of LED lights in the series circuit will not light up. It can be seen that this situation is much more serious than the first situation.
Analyze the situation of LED dead lights
LED dead lights are the key to 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 an analysis and discussion on some of the causes of dead lights.
1. Static electricity damages the LED chip
Static electricity damages the LED chip, causes the PN junction of the LED chip to fail, increases the leakage current, and becomes a resistance. Static electricity is a very harmful devil. There are countless electronic components damaged by static electricity worldwide, causing thousands of Ten thousand dollars in economic losses. Therefore, preventing static electricity from damaging electronic components is a very important task in the electronics industry, and LED packaging and application companies must not take it lightly. Any problem in any link will cause damage to the LED, and cause the performance of the LED to deteriorate or even become invalid. We know that the human body (ESD) static electricity can reach about three kilovolts, which is enough to break down and damage the LED chip. In the LED packaging production line, whether the grounding resistance of various equipment meets the requirements is also very important. Generally, the grounding resistance is required to be 4 Ohm, the grounding resistance of some high-demand occasions should even reach ≤ 2 ohms. These requirements are familiar to the people in the electronics industry. The key is whether they are in place and whether there is a record in actual implementation.
It is understood that ordinary private enterprises have not done enough anti-static measures. This is why most companies cannot find the test records of grounding resistance. Even if the grounding resistance test is done, it is once a year, or once every few years, or when there is a problem. Check the grounding resistance. Everyone knows that the grounding resistance test is a very important task, at least 4 times a year (once every quarter). In some places with high requirements, a grounding resistance test must be done every month. The soil resistance varies with the seasons. There is more rain in spring and summer, and the soil wet ground resistance is easier to achieve. In autumn and winter, dry soil has less moisture, and the ground resistance may exceed the specified value. Recording is to preserve the original data. It will be well documented in the future. Comply with ISO2000 quality control system. You can design a form for testing ground resistance. Both ground resistance testing and packaging companies and LED application companies need to fill in the form of various equipment names, record the ground resistance of each equipment, and save it with the signature of the tester.
Human body static electricity can also cause great damage to LEDs. Wear anti-static clothing and wear an electrostatic ring. The static ring should be well grounded. There is a kind of static ring that does not need to be grounded. The anti-static effect is not good. It is recommended not to use the strap. For this kind of product, if the staff violates the operating rules, they should receive corresponding warning education, and at the same time play the role of notifying others. The amount of static electricity in the human body is related to the clothes of different fabrics that people wear and each person's physique. It is easy to see the discharge between clothes when we take off clothes at night in autumn and winter. The voltage of this kind of electrostatic discharge is three thousand volts.
While the ESD value of silicon carbide substrate chips is only 1100 volts, the ESD value of sapphire substrate chips is even lower, only 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. Sometimes a good device passes through our hands. It is inexplicably broken, this is the fault of 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 benefits of the company. If the equipment and personnel are also poorly grounded, the company that uses LEDs will also cause damage to the LED. 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. However, most application companies have not done this, but only separated by the thickness of a PCB board (≤ 2mm) is directly soldered, which will also cause damage or damage to the LED, because too high soldering temperature will affect the chip, which will deteriorate the chip characteristics, reduce the luminous efficiency, and even damage the LED. This phenomenon is 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 can also cause dead lights. The expansion coefficient ratio of LED leads at high temperatures is about 150℃. The expansion coefficient is several times higher, and the internal gold wire solder joints will be pulled apart due to excessive thermal expansion and contraction, resulting in a dead light phenomenon.
2. Analysis of the cause of the dead light phenomenon caused by the open circuit of the internal connection solder joint of the LED light
The incomplete production process of packaging companies and the backward inspection methods of incoming materials are the direct causes of LED dead lights. Generally, LEDs encapsulated in bracket rows are made of copper or iron metal materials and stamped by precision molds. Because copper is more expensive, the cost is naturally high. Affected by the fierce competition in the market, in order to reduce manufacturing costs, most of the market Cold-rolled low-carbon steel is used to stamp the LED brackets. The iron brackets must be silver-plated. Silver plating has two functions. One is to prevent oxidation and rust, and the other is to facilitate welding. The plating quality of the brackets 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 procedures such as rust removal, degreasing and phosphating should be meticulous. The current should be controlled during electroplating. The thickness of the silver coating should be controlled well. The coating should be too thick. Thickness is high in cost, and thinness affects quality. Because the general LED packaging companies do not have the ability to inspect the plating quality of the bracket row, this gives some electroplating companies the opportunity to thin the silver plating layer of the electroplated bracket row and reduce the cost. Insufficient inspection means, no instrument to detect the thickness and fastness of the plating layer of the bracket row, so it is easier to get confused.
Some brackets rusted after being discharged in the warehouse for a few months. Not to mention using them, it can be seen how poor the quality of electroplating is. 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 very simple. There is a period of south wind every year. In this kind of weather, the humidity in the air is high, which can easily cause poorly plated metal parts to embroider and make the LED components ineffective. Even the packaged LED will have weak adhesion due to the thin silver-plated layer, and the solder joints will be separated from the bracket, resulting in dead lights. This is what we encountered when the light did not turn on when it was used properly. In fact, the internal solder joints were detached from the bracket.
Every year, the various parameters of the gold wire ball welding machine must be tested and corrected to ensure that the welding parameters are in the best condition. In addition, the arc of the bonding wire is also required. The arc height of the single-soldered chip is 1.5-2 chip thickness, and the arc of the double-soldered chip is 2-3 chip thickness. The degree of arc will also cause LED quality problems, and the arc is high. Too low will easily cause dead lights during welding, and too high an arc will result in poor resistance to current impact.
3. The method to identify the dead lamp
Use a lighter to heat the LED lead to 200-300℃ for the unlit LED light, remove the lighter, and connect the positive and negative electrodes with a 3 volt button battery to the LED. If the LED light can light up at this time, but the temperature of the lead decreases The LED light turns from bright to not bright, which proves that the LED light is soldered. The reason why the heating can light up is to use the principle of metal thermal expansion and contraction. When the LED lead is heated, the expansion and extension is connected to the internal solder joint. At this time, the power is turned on, the LED can normally emit light, and the LED lead shrinks as the temperature drops. After returning to normal temperature and disconnecting from the internal solder joints, the LED light will no longer light up. This method is effective after repeated attempts. Weld the two lead wires of the dead lamp of this 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 the problem 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 occurring again.
However, even for the exhibits at the China Electronics Show, 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 The welding quality is not good, or there is a problem with the plating quality of the bracket, and the increase of the leakage current of the LED chip will also cause the LED light to not light. Nowadays, many LED products do not have anti-static protection in order to reduce the cost, so it is easy to damage the chip by induced static electricity. Lightning on a rainy day is prone to high-voltage static electricity induced by the power supply line, as well as spikes superimposed on the power supply line, which will cause different degrees of damage to the LED products.
