What are the consequences of static electricity on LEDs?
We often encounter situations where LEDs do not light up. Packaging companies, application companies, and units and individuals may encounter them. This is the dead light phenomenon that people in the industry say. The reason is 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 operation of other LED lamps;
Second, the internal connection lead of the LED lamp is disconnected, causing no current to pass through the LED 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 must 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. 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, causing the PN junction of the LED chip to fail, increasing the leakage current and turning it into a resistor
There are countless electronic components damaged by static electricity all over the world, causing tens of millions of dollars in economic losses. 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. 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, the grounding resistance is generally required to be 4 ohms. The grounding resistance must 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. Anti-static measures are not in place. This is the test record of grounding resistance that most companies cannot find. Even if the grounding resistance test is done, it is once a year or once every few years. If there is a problem, check the grounding resistance. Resistance test This is a very important task, at least 4 times a year (one test every quarter), in some places with high requirements, a ground resistance test must be done every month. The soil resistance varies with the seasons. In spring and summer, there is more rain and the soil wet ground resistance is easier to achieve. In autumn and winter, the 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 management system. You can design a form for testing grounding resistance. Both grounding resistance testing and packaging companies and LED application companies need to fill in the form for various equipment names, record the grounding resistance of each equipment, and file the tester’s signature.
Human body static electricity can cause great damage to the LED. Wear anti-static clothing and wear an electrostatic ring. The electrostatic ring should be well grounded. If the staff violates the operating rules, they should receive corresponding warning education and also serve as a notice. The role of 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 with our hands, the result can be imagined. The chip or LED will be damaged to varying degrees. Sometimes a good device will be inexplicably broken by our hands. This is The fault of static electricity.
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 causes of the dead light phenomenon caused by the open circuit of the solder joints in the internal connection 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 packaged in bracket rows 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 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 bracket. The iron bracket row must be silver plated. The silver plating has two functions. One is to prevent oxidation and rust, and the other is to facilitate welding. The plating quality of the bracket row 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 meticulous. The current should be controlled during electroplating. The thickness of the silver coating should be controlled. 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. I have seen that some brackets rust after being discharged in a 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 will be 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 process in the packaging process must be carefully operated, and the negligence of any link is the cause of the dead light
In the dot and die bonding process, there are too many or less silver glue dots. If there is too much glue, it will return to the gold pad of the chip, causing a short circuit, and the chip will not stick firmly if it is missing. The same is true for double-soldered chips with insulating glue. If the insulating glue is applied too much, it will return to the gold pad of the chip, resulting in a false solder during soldering and thus 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 adjustment should be moderate and the pressure should be high. It is easy to crush the chip, and it is easy to solder if it is 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. It should be moderate. In short, the adjustment of the parameters of the gold wire ball welding machine should be good for welding. The material is qualified if tested with a spring torque tester ≥ 6g. 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 single-soldered chip is 1.5-2 chip thickness, and the arc of double-soldered chip is 2-3 chip thickness. The arc height 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.
In short, there are many reasons for the occurrence of dead lights, which can not be listed one by one. From packaging, application, to use, dead lights may appear in all links. How to improve the quality of LED products is for packaging companies and application companies to attach great importance to and study carefully. The problem, from the selection of chips and stents to LED packaging, the entire process flow must be operated in accordance with the ISO2000 quality system. Only in this way can the quality of LED products be improved comprehensively, and long life and high reliability can be achieved. In the circuit design of the application, choosing varistors and PPTC components to perfect the protection circuit, increasing the number of parallel circuits, using constant current switching power supplies, and adding temperature protection are all effective measures to improve the reliability of LED products.
