Edison invented the light bulb and brought light to the entire human race. In fact, human beings have never stopped pursuing and exploring new lighting technologies and technologies. In recent years, LED lighting technology has developed rapidly, and the splendid LED lighting decorations decorate high-rise buildings. Widely used in outdoor lighting, traffic lights, car lights, LCD backlight and other fields, such as the 2008 Beijing Olympic venues, some lighting facilities will use LED products. The 21st century will be the era of LED lighting, because LED products have the following advantages over traditional lighting products: 1. Energy saving, using 3 V~5 V power supply; 2.
Long life, in theory, LED products can make
100,000 hours; 3. Environmentally friendly, green products, without the harmful mercury element in traditional lighting products.
2. Introduction to LED Light Emitting Diodes
L E D is the three-letter abbreviation of L i g h t E m i t t i n g D i o d e, abbreviated as light-emitting diode in Chinese. L E D chips are generally composed of compounds such as G a N , G a As , G a P, etc. Its core is a PN junction, so it has a general diode Body forward conduction, reverse cut-off, breakdown characteristics, and also have luminescence
Characteristic, under the forward voltage, the electrons inside the chip crystal transition from a high-energy state to a low-energy state, converting electrical energy into light energy. LED single tube is an important component of colorful LED lighting engineering components. The main materials of LED are LED chip, bracket, silver glue, gold wire, epoxy resin, as shown in Figure 1:
The failure of a single LED tube will cause great trouble to the entire LED lighting product, because most of the LED lighting products are installed on outdoor buildings and cars, and maintenance and replacement are inconvenient, so the reliability of the LED single tube is particularly important.
3. Analysis of the failure cause of LED single tube
First of all, we define the five important points in the bonding wire as points A ~ E, the first welding point A of the gold wire is spherical, the diameter of the sphere is about 3-5 times the diameter of the wire, the deformation is good, the wire and the ball are concentric; One solder joint generally adopts thermosonic ball key
LED single tube structure diagram
Thermosonic ball bonding is a technology that combines ultrasonic vibration, heat and pressure to form soldered gold balls, and is currently the most popular bonding method in the semiconductor packaging industry. The second solder joint E is shaped like a wedge, which is also very similar to a fish tail. The width of the wedge is about 3-5 times the wire diameter, as shown in Figure 2:
In the following, based on my experience in my work, I will discuss several reasons and solutions for the failure of single LED lights:
(1) The contact of point A is not good, that is, the bonding wire gold ball and the chip electrode are soldered together. In this case, the LED lamp may also appear unstable and flicker. The solder ball is detached from the chip electrode as a whole, there is no real fusion between the gold ball and the electrode, and there is generally no residual gold on the electrode surface, as shown in Figure 3:
The root causes of virtual welding are:
① The chip electrode is dirty, oily, oxidized or the chip electrode is scratched. The storage environment of the chip is particularly important.
Schematic diagram of LED single tube bonding alloy wire
Generally stored in an electronic drying box, temperature: 20 - 30 ℃; humidity: 0 % - 40 % RH, this
In this way, it can not only prevent the oxidation of the chip electrodes, but also prevent the chip from being rubbed by the outside world, causing chip cracks or electrode scratches.
② At present, the chip electrodes can be divided into A u electrodes and Al electrodes. The solderability of the chip electrode materials is not good, which will also cause virtual welding or difficult electrode bonding. Generally, the A u electrode is used for gold wire, and the Al electrode is used for aluminum wire, because the fusion between the same materials is better.
③ The welding parameters of the gold ball are not well set, such as the pressure, power, and time are not large enough.
Modify the bonding parameters appropriately, and continue
14 Trials in small batches for best welding results.
(2) In the process of gold wire bonding, the setting of the wire arch wire is too high or too low, or the rivet is worn too much, the life of the rivet has reached the limit, the wire feeding clip is abnormal, the ball burning current is too large, etc., It will cause damage to point B, so that subsequent LED lamps are prone to failure when applied under harsh conditions. The bad phenomenon can be reflected through the test of cyclic damp heat, rapid temperature change, high temperature and high humidity. Due to the different thermal expansion coefficients of epoxy resin, LED chip, gold wire and other materials, under these harsh conditions, the stress inside the LED lamp becomes large, and the point B is pulled off. As shown in Figure 4.
(3) It is also a common problem that the gold wire is disconnected from point D. After point D is disconnected, there will be some residual gold on the bracket. The reasons for the disconnection of point D are: ① The life of the rivet has reached the limit and the wear is too much; it can be replaced by a new rivet. The reference life of the bonding rivet is generally 2 million points; ② The welding parameters of the automatic welding table are not well set during welding, such as insufficient welding time and welding pressure. The defects can be improved by modifying the welding parameters. In addition, after the welding of the second solder joint is completed, silver glue is added to the solder joint to protect the solder joint by bonding the silver glue to the bracket, which can prevent the gold wire from disconnecting from points D and E.
(4) The adhesion between the silver glue and the bottom of the bowl is not good. When the temperature rises, the epoxy/silver glue expands so that the chip is connected to the bottom of the bowl. After cooling and shrinking, it is separated from the bottom of the bowl and disconnected. Therefore, the LED light The performance is unstable and flickering, and the heating pin can return to normal. The root causes of the failure are as follows:
① The material of the silver glue is not good. Pay attention to the quality of the incoming silver glue, such as particle size and bonding performance.
② Pay attention to the use and storage conditions of silver glue. Silver glue is generally stored in a refrigerator at 0 ∽ 5 ℃. Before use, it should be warmed at room temperature (about 25 ℃) for 0.5 ∽ 1 hour. The shelf life is 7 days at room temperature, otherwise it may deteriorate and can no longer be put into use. In addition, before use, pay attention to whether the production date of the whole bottle of silver glue has passed the shelf life.
③ Strictly control the sintering time and temperature of the silver paste, the sintering temperature is generally 170 ± 10 ℃, and the sintering time is 60-70 minutes.
(5) The reverse leakage current IR value is seriously out of specification. When the IR value is tens of microamps or even hundreds of microamps, the LED can still emit light, but if the chip PN junction has been damaged, as the LED usage time becomes longer, the IR leakage current
Test Technology Volume
Test Technology will gradually increase, when IR reaches mA level, LED will be dim or not bright, at this time PN junction has been broken down. The characteristic curve of LED chip can be measured by X J 4 8 1 0 semiconductor characteristic grapher , as shown in Figure 5:
Causes of leakage: ① E S D Static electricity
The large current will break down the PN junction of the chip; ② The large current from the outside will burn the LED chip during misoperation, such as the forward current/voltage regulation is too large during the test, the positive and negative electrodes are reversed, etc.; in severe cases, the chip surface There may be burn marks or the gold wire is burnt black; ③ For the two-electrode chip, the gold ball is too large or offset, short-circuit the P pole and N pole of the chip, and push the gold ball beyond the dividing line, and the chip leakage phenomenon disappears , the photoelectric parameters return to normal;
④ For single-electrode chips, if too much silver paste is dispensed during die bonding, the N and P electrodes of the chip will be short-circuited, so the height of the silver paste is required not to exceed 2/3 of the chip height.
The above has analyzed several common causes of LED single tube failure. Try to avoid the occurrence of defects during the production and use of LEDs. After tracing the cause of failure, corresponding countermeasures should be taken in time, so as to improve the product process yield and reduce LED lighting components. Repair rate.
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