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 lamp does not light up. This situation generally does not affect the work of other LED lamps;

Second, the internal connection lead of the LED light is disconnected, causing the LED to have no current passing through and producing 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 LED lights 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 of the causes of dead lights:

1. Static electricity causes damage to the LED chip, which makes the PN junction of the LED chip fail, and the leakage current increases, becoming 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 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 LED performance worse or even invalid. We know that 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, it is also very important whether the grounding resistance of various equipment meets the requirements. Generally, the grounding resistance is required to be 4 Ohm, and in some occasions with high requirements, the grounding 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. According to the author's understanding of the general private enterprises, the anti-static measures are not in place, which is why most enterprises cannot find the test records of grounding resistance. Check the grounding resistance from time to time, but the grounding resistance test is a very important job, at least 4 times a year (test once a quarter), and some places with high requirements, a grounding resistance test is required every month.

Soil resistance will vary with the change of seasons. There is more rain in spring and summer, and the soil wet grounding resistance is easier to achieve. In autumn and winter, the 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 should 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 the LED is also very large. You should wear anti-static clothing and wear an electrostatic ring when working. The electrostatic ring should be well grounded. There is an electrostatic ring that does not need to be grounded. For this kind of product, if the staff violates the operating procedures, they should receive corresponding warning education, and also play the role of notifying 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 body does not take any protective measures), the result can be imagined, the chip or LED will be damaged to varying degrees, and sometimes a good device passes through our hands. It is inexplicably broken, and this is the fault of static electricity.

If the packaging company does not strictly follow the grounding regulations, the company itself will suffer, which will reduce the product qualification rate and reduce the economic benefits of the company. If the equipment and personnel are poorly grounded in the same LED company, the LED will be damaged. Inevitably. 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 glue, and the legs 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 excessive soldering temperature will affect the chip, make the chip characteristics worse, 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. Excessive soldering temperature will also cause dead lights. The expansion coefficient ratio of LED leads at high temperature is about 150 °C 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

2.1 The production process of packaging enterprises is not complete, and the inspection methods for incoming materials are backward, which is the direct cause 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 plating 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 southerly wind. In such weather, the humidity in the air is high, and it is easy to cause embroidery of metal parts with poor electroplating, which makes LED components fail. Even if the packaged LED is too thin, the silver-plated layer will be too thin and the adhesion will not be strong, and the solder joint will be separated from the bracket, resulting in a dead lamp phenomenon. 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.

2.2 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, the silver glue (for a single solder joint chip) can not be applied more or less. Too much glue 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 the double-soldered chip insulation glue. Too much insulation glue will return to the gold pad of the chip, causing 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 critical. 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 ℃. The adjustment of power refers to the adjustment of ultrasonic power. Too large or too small is not good. The degree is moderate. In short, the adjustment of various parameters of the gold wire ball welding machine is based on the welding If the material is tested with a spring torque tester or more 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 lights during welding, and if the arc height is too large, the resistance to current impact is poor.

3. The method of identifying the dead lamp of virtual welding

Use a lighter to heat the LED leads to 200-300°C for the unlit LED lights, remove the lighter, and use a 3-volt button battery to connect the LEDs according to the positive and negative poles. If the LED lights can light up at this time, but as the lead temperature decreases The LED light changes from bright to non-bright, which proves that the LED light is a virtual welding. The reason why the heating can be turned on is that the principle of metal thermal expansion and cold contraction is used. When the LED lead is heated, it expands and expands and connects with the internal solder joint. At this time, when the power is turned on, the LED can emit light normally, 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 of this virtual welding to a metal strip, soak it in concentrated sulfuric acid to dissolve the external colloid of the LED, take it out after all the colloid is dissolved, and observe the welding condition of each solder joint under a magnifying glass or microscope. You can find out whether it is the first welding or the second welding problem, whether the parameter setting of the gold wire ball welding machine is incorrect, or other reasons, so as to improve the method and process and prevent the phenomenon of false welding from happening again.

Users who use 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-circuit dead lights are caused by poor welding quality or the quality of bracket plating. There is a problem, the increase of the leakage current of the LED chip will also cause the LED light not to light up. Nowadays, many LED products do not have anti-static protection in order to reduce costs, so it is easy to damage the chip by induced static electricity. Thunderstorms on rainy days are prone to high-voltage static electricity induced by power supply lines, as well as spikes superimposed on power supply lines, which will cause LED products to suffer varying degrees of damage.

In short, there are many reasons for the occurrence of dead lights, which cannot be listed one by one. The phenomenon of dead lights may occur in all aspects from 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 comprehensively improved, and long life and high reliability can be achieved. 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. As long as the packaging and application companies operate strictly in accordance with the ISO2000 quality system, the quality of LED products will be brought to a new level.

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