What are the consequences of static electricity on LEDs?
We often encounter the situation that the LED does not light up. Packaging companies, application companies, and units and individuals may encounter them. This is what people in the industry call the dead light phenomenon. There are only two reasons for this:
First, the leakage current of the LED is too large, which causes the pn junction to fail, so that the LED light does not light up. This situation generally does not affect the work of other LED lights;
Second, the internal connection lead of the led light is disconnected, resulting in no current passing through the led and a dead light. This situation will affect the normal operation of other led lights. 1.8v-2.2v, blue-green-white LED working voltage 2.8-3.2v), generally connected in series and parallel to adapt to different working voltages, the more LEDs in series, the greater the impact, as long as there is one LED The open circuit of the internal connection of the lamp will cause the entire string of LED lights in the series circuit to 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 affecting product quality and reliability. How to reduce and eliminate dead lights and improve product quality and reliability are key issues that packaging and application companies need to solve. The following is some analysis and discussion on some causes of dead lights,
1. Static electricity damages the led chip, which makes the pn junction of the led chip fail, the leakage current increases, and becomes a resistor
There are countless electronic components damaged by static electricity all over the world, causing economic losses of tens of millions of dollars. Preventing static electricity from damaging electronic components is a very important job in the electronics industry. Enterprises in LED packaging and application must not take it lightly. Any problem in any link will cause damage to the LED, making the performance of the LED worse or even invalid. The static electricity of the human body (esd) can reach about 3,000 volts, which is enough to 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, and some occasions with high requirements The ground resistance should even reach ≤ 2 ohms. These requirements are familiar to people in the electronics industry, and the key is whether they are in place when actually implemented and whether there is a record. The anti-static measures are not in place, which is why most companies cannot find the test records of the grounding resistance. Even if the grounding resistance test is done, it is once a year, or once a few years, or when there is a problem, check the grounding resistance, but do not know the grounding Resistance test This is a very important job, at least 4 times a year (test once a quarter), some places with high requirements, a ground resistance test is required every month. Soil resistance will vary with the change of seasons. There is a lot of rain in spring and summer, and the grounding resistance of wet soil is easier to achieve. In autumn and winter, dry soil moisture is less, and the grounding resistance may exceed the specified value. The purpose of recording is to preserve the original data. There will be evidence at a later date. In line with iso2000 quality management system. You can design a form to test the grounding resistance, and the grounding resistance testing packaging companies and LED application companies must do it. Just fill in the names of various devices in the form, and record the grounding resistance of each device, and the tester's signature can be archived.
The damage of human body static electricity to LEDs is also very large. When working, you should 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 different fabrics people wear and their physique. When we take off our clothes in autumn and winter, we can easily see the discharge between clothes. The voltage of this electrostatic discharge is 3,000 volts. The esd value of silicon carbide substrate chips is only 1100 volts, and 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, and sometimes a good device will be inexplicably broken after passing through our hands, this is the Static to blame.
According to the requirements of the LED standard manual, the lead of the LED should be no less than 3-5 mm away from the gel, and the feet should be bent or welded, 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 the excessive 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 °C. Too high soldering temperature will also cause dead lights. The expansion coefficient is several times higher, and the internal gold wire solder joints will pull the solder joints apart due to excessive thermal expansion and contraction, resulting in a dead lamp phenomenon.
2. Analysis of the cause of the dead lamp phenomenon caused by the open circuit of the solder joint of the LED lamp
The incomplete production process of packaging enterprises and the backward inspection methods for incoming materials are the direct causes of LED dead lights.
Generally, LEDs encapsulated by brackets are used. The brackets are made of copper or iron metal materials and stamped by precision molds. Because copper materials are 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 They are all made of cold-rolled low carbon steel to bring stamping led brackets. The iron brackets are plated with silver. 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 processes such as rust removal, oil removal, and phosphating should be meticulous. The current should be controlled during electroplating, and the thickness of the silver-plated layer should be well controlled. Thick cost is high, too thin 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 an opportunity to thin the silver-plated layer of the plated bracket row and reduce the cost. There is a lack of inspection methods, and there is no instrument to detect the thickness and fastness of the plating layer of the bracket row, so it is easier to get away with it. The author has seen that some brackets rust after being placed in the warehouse for a few months, not to mention the use, which shows how poor the quality of electroplating is. Products made with such a bracket row are definitely not used for a long time, not to mention 30,000-50,000 hours, 10,000 hours are a problem. The reason is very simple. Every year there is a period of south windy weather. In such weather, the humidity in the air is high, which can easily cause embroidery of metal parts with poor electroplating and make LED components fail. Even if the packaged LEDs are too thin, the silver-plated layer will not have strong adhesion, and the solder joints will be separated from the brackets, resulting in dead lights. This is what we have encountered when a well-used light is not lit, in fact, the internal solder joint is detached from the bracket.
Every process in the packaging process must be carefully operated, and any negligence in any link is the cause of the dead light.
In the spotting and die-bonding process, it is impossible to spot too much or too little silver glue. Too much glue will return to the gold pad of the chip, causing a short circuit. The same is true for the double-soldered chip insulating glue. Too much insulating glue will return to the gold pad of the chip, resulting in virtual welding during soldering and resulting in dead lights. A few chips will not stick well, so the glue must be just right, neither too much nor too little. 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 coordinated. Except for the fixed time, the other three parameters are adjustable. The adjustment of the pressure should be moderate and the pressure should be large. It is easy to crush the chip, and if it is too small, it is easy to be soldered. The welding temperature is generally adjusted at 280 °C. The power adjustment refers to the adjustment of the ultrasonic power. Too large or too small is not good, and the degree is moderate. In short, the adjustment of the parameters of the gold wire ball welding machine is based on the welding If the material is tested with a spring torque tester or greater than 6 grams, it is qualified. Every year, the parameters of the gold wire ball bonding machine are inspected and calibrated to ensure that the welding parameters are in the best state. In addition, there are also requirements for the arc of the welding wire. The arc height of a single-solder chip is 1.5-2 chip thicknesses, and the arc height of a double-solder chip is 2-3 chip thicknesses. The arc height will also cause LED quality problems. If it is too low, it is easy to cause dead lamp phenomenon during welding, and if the arc height is too large, the resistance to current impact is poor.
In short, there are many reasons for the occurrence of dead lights, which cannot be listed one by one. Dead lights may occur in all aspects of packaging, application, and use. How to improve the quality of LED products is a matter for packaging companies and application companies to attach great importance to and study carefully. The whole process from the selection of chips and brackets to LED packaging must be operated in accordance with the iso2000 quality system. Only in this way can the quality of LED products be improved in an all-round way, in order to achieve long life and high reliability. In the circuit design of the application, choosing varistor and pptc components to improve the protection circuit, increasing the number of parallel circuits, using constant current switching power supply, and adding temperature protection are all effective measures to improve the reliability of LED products.
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