LED failure analysis case with poor antistatic index exposed

　　LED electrostatic failure principle:

　　Due to the existence of different levels of static electricity in the environment, a certain amount of static electricity charges with opposite polarities will accumulate at both ends of the PN junction of the LED chip through static induction or direct transfer , forming different levels of static electricity voltage. When the static electricity voltage exceeds the maximum tolerance of the LED, the static electricity charge will be discharged between the two electrodes of the LED chip in a very short time (nanoseconds), thereby generating heat. The conductive layer and PN junction light-emitting layer inside the LED chip form a high temperature of more than 1400℃, which causes local melting into small holes, resulting in LED leakage, dimming, dead light, short circuit and other phenomena.

　　LEDs damaged by static electricity often have dead lights and leakage. LEDs with minor static damage generally have no abnormalities, but at this point, the LED already has certain hidden dangers. When it is damaged by secondary static electricity, the probability of dimming, dead lights, and leakage increases. Based on the data and experience summarized by Jinjian Detection's case analysis over the years, when the LED chip is slightly and unnoticed by static electricity, it is time to use a scanning electron microscope to magnify it more than 10,000 times for further diagnosis to prevent a higher probability of failure.

　　LED static breakdown point

　　Blue LED electrostatic breakdown point under a scanning electron microscope (magnification: 13,000 times)

　　The anti-static index depends on the LED chip, but LED lights are more susceptible to static damage

　　The anti-static index of LED lamp beads depends on the LED light-emitting chip itself, and has little to do with the packaging material and packaging process, or the influencing factors are very small and subtle; LED lamps are more susceptible to electrostatic damage, which is related to the distance between the two pins. The distance between the two electrodes of the LED chip bare crystal is very small, generally within 100 microns, while the LED pin is about 2 millimeters. When the electrostatic charge is to be transferred, the larger the distance, the easier it is to form a large potential difference, that is, a high voltage. Therefore, it is often more likely to cause electrostatic damage accidents after the LED lamp is sealed.

　　Good antistatic index is a comprehensive reflection of the comprehensive performance of LED

　　The antistatic index of LED is not just a simple reflection of its antistatic strength. People who understand the design and manufacturing of LED chip epitaxy know that the antistatic ability of LED chip is closely related to its leakage value and overall reliability. It is also a comprehensive reflection of comprehensive quality and reliability, because LED with high antistatic ability often has good optical and electrical properties.

　　Good antistatic index of LED not only means that it can be applied to various products and various environments, but also reflects the reliability of LED's comprehensive performance. According to the actual measurement of antistatic index of LED of different brands by Jinjian, the antistatic index of LED of international LED manufacturers is generally good, while the antistatic index of some B products, miscellaneous brands, and Korean chips is still very low. The level of antistatic ability of LED is the core embodiment of LED reliability. Even if the brightness and electrical index of LED are very good, once its antistatic index is low, it is easy to die due to electrostatic damage. Testing the antistatic index of LED is a very effective quality control method, and it is urgent to effectively evaluate the antistatic ability of LED. Companies familiar with LED manufacturing are well aware that the quality of products in China's LED industry is uneven, and the stability of LEDs of different qualities varies greatly, which makes many LED users confused and even suffer from it. Among them, the most serious losses are caused by poor anti-static LEDs, such as dimming, dead lights, and leakage. In particular, some low-quality Taiwanese and Korean chips are flooding into China. Even for products from large manufacturers, middlemen often pass off inferior products as good ones, and many companies are facing huge risks. Jinjian believes that as long as LED packaging companies use LED chips with higher anti-static capabilities and do a good job of the packaging process, the products will definitely be reliable and stable. LED lighting manufacturers and LED users should frequently test the anti-static capabilities of lamp beads. Selecting LEDs with high anti-static capabilities is the core of controlling LED quality.

　　Detection method:

　　Many companies evaluate the anti-static performance of LEDs by "trying out a batch to see the results". In fact, this is a long-term, error-prone, costly and risky evaluation method. These companies often learn from their mistakes in LED anti-static testing, and because they are not familiar with LED anti-static testing, in most cases, this is a last resort.

　　Static electricity breakdown of LED is a very complicated process. Therefore, the simulation design when testing LED anti-static is also a very complicated and rigorous test. Jinjian believes that the use of anti-static test related instruments is the most standardized, scientific, objective and direct method. When testing LED anti-static, static electricity must be applied directly to the two pins of the LED, and the discharge waveform of the instrument has strict standard regulations. There are two modes, human body mode and mechanical mode, which are used to measure the anti-static ability of the object being tested.

　　Human body model: When static electricity is applied to the object being measured, a 330 ohm resistor is connected in series and applied. This simulates the charge transfer when a person comes into contact with a device. The contact between a person and an object usually also acts at 330 ohms, so it is called the human body model.

　　Mechanical mode: Static electricity is applied directly to the device under test, and the simulation tool mechanically transfers the electrostatic charge directly to the device, so it is called mechanical mode.

　　The two test instruments also have some differences in the internal electrostatic charge storage energy and discharge waveform. The results of the human body model test are generally 8-10 times that of the mechanical model. The LED industry and many companies now use the human body model indicators.

　　Testing standards:

　　IEC61000-4-2 of the International Electrotechnical Commission

　　ANSI-ESDSTM5.1.2-1999 of the International Electrostatics Association

　　JESD22-A114/115c of the Joint Electron Devices Committee

　　Test sample types:

　　Chip bare crystal, pin-type lamp beads, conventional SMD lamp beads, piranhas, high-power lamp beads, modules and digital tubes, LED lamps.

　　LED antistatic index:

　　LED can refer to the voltage level classification in the current authoritative International Electrostatic Association (ANSI) standard:

　　LED antistatic index 　　case analysis 1:

　　The customer sent 16 packaged blue LED lamp beads, which were tested by spectroscopy but not by aging and anti-static environment tests. The customer asked to find the leakage cause of the LED chip . After the leakage point was found by laser scanning microscope and chip quality was identified, Jinjian found that too many chip defects made the chip vulnerable to static shock and poor reliability. We recommend that enterprises use LED chips with higher anti-static index.

　　LED Chips

　　Most LED chips such as green, blue, white, and pink LEDs are dual-electrode structures. The thickness between the two electrode layers is much thinner than that of single-electrode LEDs, and the materials are also different. Therefore, their anti-static properties are often weaker. Therefore, blue, green, and white LEDs are more likely to fail and leak electricity.

　　Laser scanning microscope, chip leakage point detection

　　The reverse voltage is 1V, the leakage current is measured to be 0.159mA, and the laser scanner observes the leakage on the chip

　　Jinjian selected a lamp bead, used a fine needle to pick off the surface silicone, and performed a scanning electron microscope microscopic inspection on the exposed chip. Through the scanning electron microscope microscopic inspection, it was found that the non-electrode material layer of the chip was melted and formed into a small hole, and the morphological characteristics were electrostatic breakdown points. The laser scanning microscope was used again to confirm that the defect point displayed was an electrostatic breakdown point.

　　In addition to the electrostatic breakdown points, Jinjian found a large number of black voids on the surface of the chip epitaxial layer. These defects indicate that the epitaxial layer crystal quality is poor and there are defects inside the PN junction. These defects make the chip susceptible to electrostatic damage and have poor anti-static ability. We recommend that epitaxial manufacturers perform TEM and SIMS analysis on epitaxial wafers to further analyze the causes of voids and improve the production process.

　　During the chip quality identification process, Jinjian found that there were many defective holes on the chip surface. These holes are the external manifestation of the poor quality of the chip crystal.

　　Case Study 2:

　　The customer sent an LED digital tube for testing, and the sample had reverse leakage. The customer asked Jinjian Testing to analyze the cause of the failure. Jinjian Testing conducted an LED anti-static test and found that the chip's anti-static ability was 500V, which was extremely poor. It had almost no anti-static ability under normal conditions. The lamp beads were easily damaged by static electricity during production and use, resulting in leakage. The customer was advised to strengthen the inspection of incoming chips.

　　We selected a bad sample, connected the positive probe to pin 1 and the negative probe to pin 12, that is, K1 was reversely connected, and found that it had a reverse current of 4mA. Then the positive probe was connected to pins 8, 9, 10, and 11, and the negative probe was connected to pin 12, and 1A, 2A, 3A, and 4A could all emit light. That is to say, when K1 was reversely connected to 1A, 2A, 3A, and 4A, 1A, 2A, 3A, and 4A, 1A, 2A, 3A, and 4A could all emit light, further indicating that the sample K1 was abnormal and could conduct in the reverse direction.

　　Digital tube electrical performance test

　　Digital tube antistatic test data

　　The red LED chip is a single-electrode structure. The material, thickness, and substrate material between its two electrodes are different from those of the blue-green LED with two electrodes. The static energy it bears is much higher than that of the double-electrode LED. In terms of the principle of dam collapse, the "dam" of the red LED is much thicker, and the material used is better than that of the double-electrode LED. Its anti-static capacity is naturally much higher. However, the anti-static capacity of this chip is too poor, only 500V. The following figure is a scanning electron microscope observation of the static breakdown point of the red chip.

　　Scanning electron microscope observation picture of electrostatic breakdown point of red light chip