Cause analysis of LED dead light

- Feb 11, 2021-

        LED not bright is the dead light phenomenon mentioned by people in the industry, and the reasons are nothing more than two situations:

   One, the excessive leakage current of the LED causes the failure of the PN junction and the LED light does not light up. This situation generally does not affect the work of other LED lights;

Second, the internal connecting 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, orange LED working voltage 1.8 V-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 light The internal wiring is open, which will cause the entire string of LED lights in the series circuit to be off. It can be seen that this situation is much more serious than the first situation.

  LED dead light is the key that affects 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, causes the PN junction of the LED chip to fail, and the leakage current increases, turning into a resistor

   Static electricity is a very harmful devil. There are countless electronic components damaged by static electricity all over the world, causing economic losses of tens of millions of dollars. 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 make the LED performance worse or even invalid. We know that the human body (ESD) static electricity can reach about 3,000 volts, 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 even reach ≤ 2 ohms.

   These requirements are familiar to people in the electronics industry. The key is whether they are in place and whether they are recorded in actual implementation. According to the author's understanding, the anti-static measures of general private enterprises are not in place. This is the test record of ground resistance that most companies cannot find. Even if the ground resistance test is done, it is once a year or every few years, or there may be problems. Check the grounding resistance from time to time. Everyone knows that the grounding resistance test is a very important job, 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. 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 of various equipment names, and record the ground resistance of each equipment, and the tester’s signature can be archived.

Human body static electricity can cause great damage to the LED. 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 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 worn by people 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 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 (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 cause 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, causing dead lights.

  2. The cause analysis of the dead light phenomenon caused by the open circuit of the solder joints in the internal connection of the LED light

  2.1 Incomplete production processes of packaging companies and backward inspection methods for incoming materials are the direct cause of LED dead lights

Generally, LEDs are packaged in bracket rows. The 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 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 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. Thickness is costly, and too 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 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 such weather, the humidity in the air is high, which can easily cause poorly plated metal parts to embroider and make LED components fail. Even the packaged LED will have weak adhesion due to the thin silver plating layer, and the solder joints will be separated from the bracket, resulting in dead lights. This is what we encountered when the light does not turn on when it is used properly. In fact, the internal solder joints are separated from the bracket.

  2.2 Every process in the packaging process must be carefully operated, and the negligence of any link is the cause of dead lights

  In the dot and die bonding process, the number of silver glue (for single solder joint chips) is not enough. If there is 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 double-soldered chips with insulating glue. If the insulating glue is applied too much, it will return to the gold pad of the chip, which will cause the virtual solder during welding and cause dead lights. If the chip is missing, the glue will not stick, 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. Welding temperature is generally adjusted at 280℃. Power adjustment refers to the ultrasonic power adjustment. It is not good to be too large or too small. Moderate is the degree. 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 are progressively 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 a single solder joint chip is 1.5-2 chip thickness, and the arc height of a double solder joint chip is 2-3 chip thickness. The arc height will also cause LED quality problems. Too low will easily cause dead lights during welding, and too high an arc will have poor resistance to current impact.

  3, the method of identifying dead lights

Use a lighter to heat the LED leads to 200-300℃ for the non-lit LED lights, remove the lighter, and connect the positive and negative electrodes with a 3 volt button battery to the LED. If the LED lights can light up at this time, but the temperature of the lead decreases The LED light changes 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 with 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 the normal temperature state and disconnecting from the internal solder joints, the LED light will not light up. This method is effective after repeated attempts.

Solder the two leads of the dead lamp with this kind of virtual welding to a metal strip, soak it in a thicker sulfuric acid to dissolve the external colloid of the LED. After the colloid is completely dissolved, take it out. Observe the welding condition of each solder joint under a magnifying glass or a microscope. You can find out whether the problem is the first or second welding, the parameter setting of the gold wire ball welding machine, or other reasons, so as to improve the method and process to prevent the phenomenon of false welding from happening again.

Users of LED products will also 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 dead lights are due to poor welding quality or the quality of bracket plating. If there is a problem, the increase of the leakage current of the LED chip will also cause the LED light to not light up. Nowadays, many LED products do not add anti-static protection to reduce the cost, so it is easy to damage the chip by induced static electricity. Lightning on rainy days 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 damage the LED products to varying degrees.

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, there may be dead lights in all aspects. 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 applied circuit design, choosing varistor 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. As long as the packaging and application companies operate strictly in accordance with the ISO2000 quality system, they will definitely be able to bring the quality of LED products to a new level.

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