We often encounter that the LED does not shine, packaging enterprises, applications enterprise and the use units and individuals, are likely to encounter, this is the people within the industry said death light phenomenon. But the reason is nothing more than two kinds of circumstances:
One, The leakage flow is overlarge through the LED, which causes to the failure of the PN junction, make the LED lamp does not light, in this case , it usually does not affect the other LED lamp work.
Second, the internal connection lead wire of LED lights is off, causing LED no current through, the lamp die, it will affect the normal work of other LED lamps, because of LED lamps low working voltage (red yellow orange LED working voltage 1.8V-2.2V, blue green white working voltage LED 2.8-3.2V), generally in series and in parallel connection to adapt to different working voltage, the more LED lamp in series, the more influence exists, as long as there is one LED lamp internal connection lead wire off, will cause the whole series of LED lamp with series circuit not bright, obviously this situation is more serious than the first case.
LED dead light is the key problem to influence the quality and the reliability of the product, how to reduce and eliminate dead lamp, improve product quality and reliability, is the key problem for encapsulation, application enterprises to solve. Below is some analysis and Discussion on some causes of death lamp,
Static electricity is a kind of great harm devil, electronic components are too many to count because of electrostatic damage in the world, resulting in tens of millions of dollars in economic losses. So to prevent electrostatic from damaging electronic components is a very important work for electronic industry, LED encapsulation, application enterprises must not let it down. Any link problem will cause damage to LED, will make the LED performance deteriorated even be failure. We know that the human body (ESD) electrostatic can reach three thousand volts, which is enough to cause LED chips breakdown, in LED packaging production line, all kinds of equipment grounding resistance is to meet the requirements or not, which is also very important, general requirements for grounding resistance is 4 ohms, and for some demanding occasions the grounding resistance must even achieve ≤ 2 ohm
People are familiar with all these requirements in the electronics industry, The key point is or is not in the actual implementation and if there is a record or not. According to the author's understanding about the general private enterprises, anti-static measures do not place, the majority of enterprises can not check test records of grounding resistance, even if doing the grounding resistance test , it is once a year, or every a few years, or when meeting questions, check the grounding resistance, they do not know that the ground resistance test is a very important work, at least 4 times a year (once every quarter ), In some applications requiring high, we need to do a grounding resistance test per month.
The soil resistance changes with the seasons, there is a lot of rain in spring and summer, the grounding resistance is easier to achieved due to soil moisture, but in autumn and winter the soil is dry soil m, the grounding resistance is likely to exceed the specified value, doing the record in order to save the original data, do it in a data. In accordance with the ISO2000 quality management system. Ground resistance measurement can be done by designing their own form, LED packaging companies and LED application companies both have to do grounding resistance testing , as long as the device name is filled in the form, the measurements of the grounding resistance for all the equipment are recorded, then add test signatures, then it can be archived.
The electricity from human body make the LED great damage, when working, the workers should wear anti-static clothing with electrostatic ring, anyway, static ring should be well grounded, the antistatic effect of one kind of electrostatic ring is not good ,it is recommended not to use with this product. if the staff is in violation of operating procedures, he should be take warning education accordingly, at the same time also played the role of notice to others. How much electrostatic human body carries is relevant to different fabrics clothes he wears and different constitution . In autumn and winter, when we take off our clothes, it is very easy to see the discharge phenomenon between the clothes, the electrostatic discharge voltage is three thousand volts.
The silicon carbide substrate chip ESD value is only 1100 volts, the sapphire substrate chip ESD value is lower, only 500-600 volts. A good chip or LED, if we use the hand to take (the body without any protective measures), one can imagine the result , the chip or LED will be subject to varying degrees of damage, and sometimes rather baffling ,a good device through our hands would be bad ,the electrostatic bring disaster upon ruin.
If Packaging companies do not in accordance with the ground rules strictly, the enterprises themselves will buy it, because it will cause the product qualified rate drops and reduce the economic benefits , also if the application enterprises personnel and equipment grounding bad can cause damage to LED, remade work can hardly be avoided. In accordance with the LED standards Manuals, the LED lead wire from the gel should be not less than 3-5 mm , but most companies do not have to do this when doing LED bending or welding. but only at a thickness of PCB (≤ 2 mm) is directly welded, it will also cause harm or damage to LED, because of high welding temperature will have an impact on the chip, will make the chip performance deterioration, low luminous efficiency, even damage the LED, this kind of phenomenon it is often seen. Some small businesses to use manual welding, using 40 Watt ordinary iron, welding temperature can not control, solder iron temperature at 300-400 ℃, the high temperature welding can also cause dead lights, LED lead coefficient ratio at 150 ℃ expansion coefficient is several times higher expansion at high temperature, the internal wire joint because of too large thermal expansion and contraction welding point open, causing death light phenomenon.
Generally the manufacturers use the support platoon to package LED, support platoon uses copper or iron metal materials by precision die stamping, because copper is more expensive, cost is high, due to fierce market competition factors, in order to reduce the production cost, the market mostly adopts cold rolled low carbon steel to press LED support platoon, iron bracket should be silver-coated, silver has two functions, one is to prevent rust, two is a convenient to weld, plating quality for the support platoon is critical, it is related to the LED life, the treatment before plating should be strict according to the operating rules, rust removal, degreasing, phosphating process should be strict according to the operating rules ,when doing electroplating , we should take good control of current, silver plating layer thickness should be controlled, if coating is too thick,the cost is too high ,if too thin, it will influence the quality.
Generally LED packaging enterprises do not have the capacity to test the electroplating quality of the support platoon, which gives a number of electroplating enterprises to have a loophole to exploit to make the silver plating layer of the support platoon be thin in order to reduce cost, general packaging enterprise IQC lack the inspection methods for the support platoon, no detection instrument to judge the thickness and fastness of the support platoon coating, so the platoons are very easy to slip by IQC. I have seen some support platoons which start to rust in the warehouse after several months, don't say the usage quality of platoons, visibly how bad the support platoons are. The product made from such support platoon certainly , don't say 3-5 million hours, 10000 hours are a problem.
The reason is simple, every year there are a period of Southerly days when the humidity in the air is very high in such weather, very easy to cause LED component rusted because of the difference of metal plating . Even if the LED which have been packaged will be dead because the silver plating layer is so thin that adhesion is not strong, the solder joint and the support will be separated. This is why we encountered the situation where it doesn’t shine suddenly when using normally, actually the internal solder is divorced from the support.
In the process of the solid crystal, silver glue (for single joint chip) if putting more or less will not work, much gum will return to the chip pads, causing a short circuit, if less gum, chip will sticky in prison. For Double joint chip ,the insulating rubber is the same, more insulating glue will return on the chip’s gold pad to result in cold solder join, and then lead the LED light dead; but if less the adhesive on the LED chip is not strong, so the dispensing must be just perfect, neither more nor less.
The welding process is the key, the pressure, temperature, time, power ,the four parameters of the gold wire ball-bonder have to be just perfect, except the time is fixed, the other three parameters are adjustable, pressure regulation should be moderate, if too much pressure can easily crush chips, too small to make the cold solder joint. The general regulation at 280 ℃ for good welding temperature, power regulation refers to the ultrasonic power regulation, too big, too little is not good, moderate degree is good. in short, the parameters regulation of the gold wire ball welding machine, as to the welding material, using spring torque meter to detect , If it is ≥ 6 grams, which is qualified.
The enterprises should do the detection and correction work to all the parameters of gold wire bonder each year, to ensure the welding parameters in the best condition. Besides, the wire radian has its requires, too. a single solder joint chip’s arc height for 1.5-2 chip thickness, double joint chip’s camber is 2-3 chip thickness, arc height will cause the quality problems of LED, arc height is too low ,which easily cause death light phenomenon when welding, arc height is too large anti- current impact is bad.
Firstly use the lighter to heat The LED lamp’s wire to 200-300 deg (at the LED is not working condition), remove the lighter, then sue 3 volt button cell is connected to the LED according to the positive, negative, and at this time if the LED lamp can light up, but white the lead wire temperature decreased,LED lamp light is from the light to not bright, which proved that LED lamp. Heating can light the reason is the use of the principle of thermal expansion and contraction of the metal, elongation and internal solder on expansion of LED wire heating, then switch on the power, LED can be normal lighting, with the temperature dropping LED lead shrink back to normal state, and internal solder disconnect, LED lamp is not bright, this test is efficacious.
Let the dead lights’ two lead wire solder on a metal strip, immerse with thick sulfuric acid to make the LED external colloidal dissolved, then get it out when colloidal dissolved, observe the welding condition at each point in a magnifying glass or microscope, it can find out if the problem took place at the first welding or the second welding, or the parameters are set incorrectly on the gold wire bonder, or other reasons in order to improve the method and process in cast of the recurrence of the false solder phenomenon.
LED product Users will encounter death light phenomenon, LED products die after a period of use , there are two reasons, If there is a broken circuit in the dead lamp, it means the welding quality is not good or there is a problem with the quality of rack plating, If LED chip leakage current increases, it will cause the LED lamp not bright. Now a lot of LED products in order to reduce costs without adding antistatic protection, so prone to damage chip because of electrostatic induction phenomenon. On the rainy day it is prone to induct high voltage electrostatic on the supply line, and the superimposed spike on a power supply line will make the LED products be subject to vary degrees of damage.
There are a lot of reasons for dead lamps, we can’t write one one lists, from the package, application, to the various links of usage, it likely happens dead lamp phenomenon, how to improve the quality of LED products, it should attach great importance and conscientiously study and for the application and the packaging industry enterprise. From the chip,rack selection to LED packaging’s entire process will be in accordance with ISO2000 quality system to operate. Only in this way the quality of the LED products can fully improve in order to achieve long-life, high reliability. In the application circuit design, they should select varistors and PPTC element to perfect protection circuit and increase parallel way, using constant current switching power supply, adding a temperature protection are all the effective measures to improve the reliability of LED products. As long as the encapsulation and application enterprise operates strictly according to ISO2000 quality system, we can make a new step on the quality of LED products.