Why does the phenomenon of dead light appear in the welding process of LED lamp beads?

Now the quality of LED beads is uneven. As long as the raw materials used are a little different, the quality and cost will be very different. Moreover, the packaging technology of LED lamp bead manufacturers also plays a great role. If you give the best chips and other materials to ordinary manufacturers, the quality will not go up. Because of poor technology, the quality of LED beads is very unstable, and the phenomenon of dead lights often occurs.

At present, the quality of LED beads is good: Corey, Riya, Osram, Tong Jia, Toyota Synthetic, etc. The manufacturers of high-quality LED lamp beads are basically in the United States, Japan and Taiwan Province Province, and the packaging technology in Chinese mainland is not very mature.

There are two main situations in which LED lamp beads die:

First, the leakage current of LED is too large, which leads to the failure of PN junction, which makes the LED light point not bright, and generally does not affect the work of other LED lights;

Second, the internal connection lead of the LED lamp is disconnected, which leads to the LED lamp being dead due to no current flowing. This situation will affect the normal operation of other LED lamps, because the working voltage of LED lamps is relatively low (the working voltage of red, yellow and orange LEDs is 1.8V-2.2V, and the working voltage of blue, green and white LEDs is 2.8-3.2V), and generally series and parallel connection are adopted to adapt to different working voltages. The more LED lights are connected in series, the greater the impact. As long as one of the LED lights is open, the whole string of LED lights in the series circuit will not light up, which shows that this situation is much more serious than the first situation.

The dead light of LED 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 a key problem that packaging application enterprises need to solve.

The following are some analysis and discussion on the causes of LED lamp bead death.

1. Static electricity damages the LED chip, making the PN junction of the LED chip invalid, increasing the leakage current and turning it into resistance.

Static electricity is a very harmful demon. There are countless electronic components damaged by static electricity in the world, resulting in tens of millions of dollars in economic losses. Therefore, preventing electrostatic damage to electronic components is a very important task in the electronic industry, and led packaging application enterprises must not take it lightly. Problems in any link will cause damage to LED, which will make LED performance worse or even fail. We know that the static electricity (ESD) of human body can reach about 3 thousand volts, which is enough to break down and damage LED chips. In the LED packaging production line, it is also very important whether the grounding resistance of all kinds of equipment meets the requirements. Generally, the grounding resistance is required to be 4 ohms, and in some demanding occasions, the grounding resistance even reaches ≤2 ohms.

These requirements are familiar to people in the electronics industry, and the key is whether they are in place and recorded when they are actually implemented. According to the author's understanding, the anti-static measures of general private enterprises are not in place, which means that most enterprises can't find the test records of grounding resistance. Even if they do the grounding resistance test once a year, or once every few years, or check the grounding resistance when there is a problem, they don't know that the grounding resistance test is a very important job, and it should be done at least four times a year (once every quarter), and some places with high requirements have to do the grounding resistance test once a month.

Soil resistance will change with the seasons. Wet grounding resistance is easy to reach when there is more rain in spring and summer, and may exceed the specified value when there is less dry soil moisture in autumn and winter. The purpose of recording is to save the original data for future reference. Conform to ISO2000 quality management system. The meter can be designed to test the grounding resistance, which should be completed by packaging enterprises and LED application enterprises. As long as the names of various devices are filled in the form, the grounding resistance of each device is recorded and signed by the tester, it can be archived.

Human static electricity is also very harmful to LED. Wear anti-static clothing at work, and the wrist strap and wrist strap should be well grounded. There is a wrist strap that does not need to be grounded, so the anti-static effect is not good. It is not recommended to use this product. If the staff violates the operating rules, they should receive corresponding warning education, which also plays a role in informing others. The amount of static electricity in the human body is related to the different fabrics and clothes worn by people and everyone's physique. When we undress in autumn and winter and at night, it is easy to see the discharge phenomenon between clothes, and the voltage of this electrostatic discharge is 3 kV.

The ESD value of silicon carbide substrate chip is only 1 100 volts, and the ESD value of sapphire substrate chip is even lower, only 500-600 volts. A good chip or LED, if we hold it with our hands (the body has not taken any protective measures), the result can be imagined. Chips or LEDs will be damaged to varying degrees, and sometimes a good device will be broken from our hands inexplicably, which is the disaster caused by static electricity.

If packaging enterprises do not strictly follow the rules of grounding gas, it will be the enterprises themselves that suffer, reduce the qualified rate of products and reduce the economic benefits of enterprises. If the enterprise equipment and personnel using LED are poorly grounded, it will also cause LED damage, and rework is inevitable. According to the requirements of LED standard manual, the lead of LED is not less than 3-5 mm away from the colloid, so it should be bent or welded. However, most application companies do not do this. Just welding directly with the thickness of a PCB (≤2 mm) will also cause damage or damage to the LED, because too high welding temperature will affect the chip, make the chip characteristics worse, reduce the luminous efficiency, and even damage the LED. This phenomenon is very common. Some small enterprises use manual welding, using a 40-watt ordinary soldering iron, and the welding temperature cannot be controlled. The temperature of soldering iron is above 300-400℃, and too high welding temperature will also cause dead lamp. The expansion coefficient of LED leads at high temperature is several times higher than that at 150℃, and the internal gold wire solder joint will be pulled apart due to excessive thermal expansion and cold contraction, resulting in dead lamp phenomenon.

2. Cause analysis of dead lamp phenomenon caused by open circuit of internal connection solder joint of 2.LED lamp.

2. 1 The imperfect production technology and backward inspection methods of incoming materials in packaging enterprises are the direct causes of LED dead light.

LED brackets are generally encapsulated by bracket rows, which are made of copper or iron metal materials and stamped by precision molds. Because copper is more expensive, the cost is naturally high. Due to the fierce market competition, in order to reduce the manufacturing cost, cold-rolled low-carbon steel stamping led brackets are used in most markets. The iron bracket row needs silver plating, which has two functions, one is to prevent oxidation and rust, and the other is to facilitate welding. Electroplating quality of bracket row is very important. It is related to the life of LED. The pretreatment before electroplating should be carried out in strict accordance with the operating procedures, and the processes such as rust removal, oil removal and phosphating should be meticulous. When electroplating, it is necessary to control the current and the thickness of silver coating. Too thick and too thin coating will affect the quality.

Because the general LED packaging enterprises don't have the ability to detect the electroplating quality of the bracket row, this gives some electroplating enterprises an opportunity to make the silver plating layer of the electroplated bracket row thinner and reduce the cost. IQC, a general packaging enterprise, lacks the means to detect the bracket row and the instrument to detect the thickness and fastness of the bracket row, so it is easy to get away with it. I have seen some stents rust after being put in the warehouse for a few months, let alone used, which shows how poor the electroplating quality is. The products made of this rack row will certainly not take long, let alone 30,000-50,000 hours, 10000 hours.

There is a simple reason. There is a southerly wind for some time every year. In this weather, the humidity in the air is high, which easily leads to embroidery of badly plated metal parts and failure of LED components. Even the packaged LED will die because the silver coating is too thin, the adhesion is not strong, and the solder joint is separated from the bracket. This is the common lamp we encounter. In fact, the internal solder joint is separated from the bracket.

2.2 Every working procedure in the packaging process must be carefully operated, and the negligence of any link is the cause of death.

In the process of point-to-die bonding, you can't point more or less silver glue (for single solder joint chips). Too much glue will flow back to the gold pad of the chip, causing a short circuit. Without the chip, it will not stick firmly. The same is true for the insulation glue of double solder joint chip. If there is too much insulating glue, it will flow back to the gold pad of the chip, resulting in virtual soldering and dead light. Without the chip, it won't stick well, so the glue must be just right, not too much and not too little.

Welding process is also very important. The pressure, time, temperature and power of the gold wire ball welder should be properly matched. Except for the fixed time, the other three parameters are adjustable. Pressure adjustment should be moderate. If the pressure is high, it is easy to crush the chip. If it is too small, it is easy to solder. Generally, it is better to adjust the welding temperature to 280℃, and the adjustment of power refers to the adjustment of ultrasonic power. It is not good to be too big or too small, but to be moderate. In short, the spring torsion tester detects that the material to be welded is more than 6 grams, and the parameters of the gold wire ball welding machine are qualified.

The parameters of the gold wire ball welder should be tested and revised every year to ensure that the welding parameters are in the best state. In addition, there are requirements for the radian of welding wire. 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. Arc height can also lead to LED quality problems. When the arc height is too low, the phenomenon of dead lamp is easy to occur during welding, and when the arc height is too high, the current impact resistance is poor.

3, the method of identifying virtual welding dead lamp

Use a lighter to heat the lead wire of unlit LED lamp to 200-300℃, take off the lighter, and connect the LED with 3 V button cell according to the positive and negative poles. If the LED lamp can be lit at this time, it will change from bright to not bright with the decrease of lead temperature, which proves that the LED lamp is virtual welding. The reason why heating can light up is to use the principle that metals expand when heated and contract when cooled. When the LED leads are heated, the expansion and elongation are connected with the internal solder joints. At this time, turn on the power supply, and the LED can emit light normally. When the temperature drops, the LED leads shrink to the normal temperature and are disconnected from the internal solder joints, so the LED lamp will not light up. This method is effective after repeated experiments.

Weld the two leads of the virtual welding dead lamp on the metal strip, soak in concentrated sulfuric acid to dissolve the colloid outside the LED, and take it out after the colloid is completely dissolved. Observing the welding situation of each solder joint under a magnifying glass or microscope, we can find out whether the problem is primary welding or secondary welding, whether the parameters of the gold wire ball welding machine are set incorrectly or other reasons, so as to improve the method and technology and prevent the phenomenon of virtual welding from happening again.

Users who use LED products will also encounter the phenomenon of dead light, which is the phenomenon of dead light after LED products have been used for a period of time. There are two reasons for death ray. If the dead lamp is open, the welding quality is poor, or there is a problem with the electroplating quality of the bracket, the leakage current of the LED chip will increase, and the LED lamp will not light up. At present, many LED products do not provide anti-static protection in order to reduce costs, and are easily damaged by induced static electricity. Thundering in rainy days is easy to produce high-voltage static electricity induced by power supply lines and peak pulses superimposed on power supply lines, which will damage LED products to varying degrees.

In a word, there are many reasons for death ray, which cannot be listed one by one. Dead light may occur in all aspects of packaging, application and use. How to improve the quality of LED products is a problem that packaging enterprises and application enterprises should attach great importance to and seriously study. The whole process from chip and bracket selection to LED packaging should be operated according to ISO2000 quality system. Only in this way can the quality of LED products be comprehensively improved, and long life and high reliability can be achieved. In the applied circuit design, selecting varistor and PPTC components to improve the protection circuit, increasing the number of parallel circuits, adopting constant current switching power supply and increasing temperature protection are all effective measures to improve the reliability of LED products. As long as the packaging application enterprises operate in strict accordance with ISO2000 quality system, the quality of LED products will definitely reach a new level.