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Popular Science: Possible Causes of Damage to LED Beads

2019/04/20

The phenomenon of LED dead light 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 the LED dead light? How to avoid the phenomenon of LED dead light is the focus of this paper.

Reasons for LED Dead Lights

There are two reasons for the LED dead lamp: one is that the leakage current of the LED is too large to cause the PN junction to fail, so that the LED lights do not turn on, which generally does not affect the work of other LED lights;

Secondly, the disconnection of the internal connection lead of the LED lamp causes the dead light due to no current passing through the LED. This situation will affect the normal operation of other LED lamp. The reason is that the low working voltage of the LED lamp (red, yellow and orange LED working voltage 1.8V-2.2V, blue, green and white LED working voltage 2.8-3.2V) generally uses series and parallel connection to adapt to different working voltage. The more series LED lamp is connected. The greater the impact, as long as one of the internal connection of the LED lights is open, the whole series of LED lights in the series circuit will not turn on. It can be seen that this situation is much more serious than the first one.

Analysis of LED Dead Lamp

LED dead light is the key to affect the quality and reliability of products. How to reduce and eliminate dead light and improve the quality and reliability of products is the key problem to be solved by packaging and application enterprises. The following is some analysis and discussion of the causes of the dead light.

1. Electrostatic damage to LED chips

Electrostatic damage to the LED chip, make the PN junction of the LED chip failure, leakage current increase, become a resistance static electricity is a great harm to the devil, the world because of electrostatic damage of electronic components countless, causing tens of millions of dollars in economic losses. Therefore, preventing electrostatic damage to electronic components is an important work in the electronic industry. Enterprises encapsulating and applying LED must not take it lightly. Any part of the problem will cause damage to the LED, so that the performance of the LED will deteriorate or even fail. We know that the static electricity of human body (ESD) 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 all kinds of equipment meets the requirements is also very important. Generally, the grounding resistance is required to be 4 ohms. In some high-demand situations, the grounding resistance should even be less than 2 ohms. These requirements are familiar to people in the electronic industry. The key is whether they are in place and whether they are recorded when they are actually implemented.

It is known that in general private enterprises, anti-static measures are not in place. This is that most enterprises can not find the test records of grounding resistance. Even if grounding resistance is tested once a year, or several years, or when there are problems, check grounding resistance. It is very important to do grounding resistance test at least four times a year (once a quarter). In some places where the requirements are high, the grounding resistance is tested once a month. Soil resistance varies with the season. Rainfall in spring and summer is more frequent, soil wet grounding resistance is easier to achieve. Dry soil moisture in autumn and winter is less, and grounding resistance may exceed the specified value. Recording is to preserve the original data, so that it can be checked in the future. It conforms to ISO2000 quality control system. Testing grounding resistance can be designed by oneself. Grounding resistance testing packaging enterprises and LED application enterprises all need to do this. As long as the names of various devices are filled in the form, the grounding resistance of each device is measured and recorded on the record, the tester can sign and save it.

The damage of human static electricity to LED is also great. When working, anti-static clothing should be worn with electrostatic rings. The electrostatic rings should be grounded well. There is a kind of electrostatic rings without grounding. It is not good to use this kind of products. If the staff violates the operating rules, they should receive corresponding warning education and also serve as a warning to others. The amount of static electricity in human body is related to the different fabrics and clothes that people wear and their physique. When we take off clothes in autumn and winter night, we can easily see the phenomenon of discharge between clothes. The voltage of this kind of static electricity discharge is 3,000 volts.

The ESD value of SiC substrate chip is only 1100 volts, while that of sapphire substrate chip is lower, only 500-600 volts. A good chip or LED, if we take it by hand (without any protective measures), the result can be imagined that the chip or LED will be damaged to varying degrees. Sometimes a good device will inexplicably break down through our hands, which is caused by static electricity. If packaging enterprises do not strictly follow the grounding rules, they will suffer from the loss of the enterprise itself, which will result in the decline of product qualification rate and reduce the economic benefits of the enterprise. If the equipment and personnel of enterprises using LED are not grounded properly, it will also cause the damage of the LED, and rework is inevitable. According to the requirements of the standard manual for the use of LED, the lead spacing of the LED should be no less than 3-5 mm to bend or weld, but most of the application enterprises have not done this. Instead, the thickness of a PCB plate (<2 mm) separated by it will be directly welded. This will also cause damage or damage to the LED, because too high welding temperature will affect the chip and will deteriorate the chip characteristics. It is common to reduce the luminous efficiency and even damage the LED. Some small enterprises use manual welding, using 40-watt ordinary soldering iron, the welding temperature can not be controlled, soldering iron temperature above 300-400 degrees C, too high welding temperature will cause dead light, LED lead expansion coefficient at high temperature is several times higher than the expansion coefficient around 150 degrees C, the internal gold wire solder joint will be pulled open because of excessive thermal expansion and cold contraction, resulting in dead light phenomenon.

2. Cause analysis of dead lamp caused by solder joint opening in internal connection of LED lamp

2.1 The direct cause of LED dead light is the incomplete production process of packaging enterprises and the backward inspection methods of incoming materials. In order to reduce the manufacturing cost, cold rolled low carbon steel is mostly used in the market to bring stamping LED brackets. Iron brackets have to be silver plated and silver plated for two purposes. One is to prevent it from happening. The second is easy welding. The plating quality of the support plating is very important. It relates to the life of the LED. The pretreatment before plating should be carried out strictly according to the operation rules. The processes of rust removal, oil removal and phosphating should be meticulous. The current should be well controlled during plating. The thickness of the silver plating should be well controlled. The thickness of the silver plating layer is too thick and the cost of the plating is too high, which affects the quality. Because ordinary LED packaging enterprises do not have the ability to test the plating quality of the support plating, which gives some plating enterprises an organic opportunity to take advantage of, so that the silver plating layer of the plating support plating can be thinned and the cost expenditure reduced. Generally, IQC packaging enterprises lack the means to test the support plating, and there is no instrument to detect the thickness and fastness of the support plating, so it is easy to blur through.

Some of the brackets rusted several months after they were discharged into the warehouse, not to mention the use, so we can see how poor the quality of the electroplating is. The products made with such brackets will certainly not last long, let alone 30-50,000 hours, 10,000 hours will be a problem. The reason is very simple. Every year, there is a period of south wind days. In such weather, the humidity in the air is high. It is easy to cause poor plating of metal parts to embroider and make the LED components invalid. Even the encapsulated LED will die because the silver coating is too thin and the adhesion is not strong, and the solder joint and the bracket are separated. This is what we have come across when the well-used lamp does not turn on. In fact, the internal solder joint is separated from the bracket.

2.2 In the packaging process, every process must be carefully operated, any link negligence is the cause of dead light in the point, crystallization process, silver glue (for single solder chip) points more or less can not be done, more glue will return to the chip gold pad, resulting in short circuit, less chip and sticky. The same is true for spot insulating glue of double solder joint chips. If more insulating glue is added, it will return to the gold pad of the chip, resulting in virtual soldering during soldering, which will result in dead light. A little bit less chips and sticky, so dispensing must be appropriate, neither more nor less. Welding process is also critical. The four parameters of pressure, time, temperature and power of the golden wire ball welding machine should be matched properly. In addition to the fixed time, the other three parameters are adjustable. The adjustment of pressure should be moderate. The chip is easy to crush when the pressure is large, and the virtual welding is easy when the pressure is too small. Welding temperature is generally adjusted at 280 C. Power regulation refers to the ultrasonic power regulation, too large or too small is not good, moderate degree. In short, the adjustment of the parameters of the golden wire ball welding machine, to welded materials, using a spring moment tester to detect more than 6 grams, is qualified.

Every year, all parameters of the gold wire ball welding machine are checked and corrected to ensure that the welding parameters are in the best state. The arc height of single solder joint chip is 1.5-2 chip thickness, and the arc height of double solder joint chip is 2-3 chip thickness. The arc height will also cause the quality problems of LED. The arc height is too low to cause the dead lamp phenomenon in welding, and the arc height is too high to resist the current shock.

3. Method of Identifying Virtual Welding Dead Lamp

If the LED lamp can be lit at this time, but with the lead temperature decreasing, the LED lamp will not be lit, which proves that the LED lamp is virtual welding. The reason for heating energy is that the principle of metal thermal expansion and contraction is used. When the LED lead is heated, the expansion and extension are connected with the internal solder joint. When the power supply is connected, the LED can normally emit light. With the temperature decreasing, the LED lead shrinks back to the normal temperature state. When the LED lead is disconnected from the internal solder joint, the LED lamp will not be lit. This method has been tried and tested repeatedly. By welding the dead lamp two leads of this virtual welding on a metal strip and immersing them in concentrated sulfuric acid, the colloid outside the LED is dissolved, and then the colloid is removed. Observing the welding situation of each solder joint under a magnifying glass or a microscope, we can find out whether the parameters of the gold wire ball welding machine are incorrectly set or other reasons, so as to improve the method and process. Prevent the phenomenon of virtual welding from happening again.

However, even the exhibits in China's electronic exhibition, users of LED products will encounter the phenomenon of dead light. This is the phenomenon of dead light after a period of time when LED products are used. There are two reasons for dead light. Open-circuit dead light is bad welding quality, or the quality of bracket plating is problematic. Increased leakage current of LED chips will also cause the LED lamp not to turn on. Nowadays, many LED products do not add antistatic protection in order to reduce costs, so it is easy to be damaged by induction static electricity. In rainy days, lightning prone to high voltage electrostatic induction of power supply lines, as well as the spike pulse superimposed on power supply lines, will cause different degrees of damage to LED products.

Yiwu Fuwei e-commerce firm (IACS Co., Limited) is a very professional Led manufacturer who produces high quality outdoor led light. Our popular products are solar led lights, UFO lights, underwater lights and so on. If you have any question about the led product, please feel free to contact us.