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​Thoughts on Reliability Testing of LED Lighting(II)
- May 24, 2017 -

Third, the failure of LED light source module reasons

LED light source module generally consists of substrate, chip, packaging materials (including phosphor), lens composition, some modules also include heat sink and thermal silica, popular packaging methods are DOB and COB. Because the LED light source module design diversity and composition of the complexity, so the cause of failure is also very much, generally include the following:

1. Packaging material degradation

LED in the daily life of the process, a long time work will make LED blue and GaN systems in the band between the radiation generated by the combination of ultraviolet radiation and temperature rise caused by LED external packaging materials (such as epoxy resin) There is a large decrease in the optical transparency of many polymers, resulting in a decrease in the luminous efficiency of the LED.

For this degradation of the packaging material will cause the LED light efficiency to reduce this problem, D. L..Barton et al had done research and testing. Experiments show that when the ambient temperature of the LED is 95 , the driving current is greater than 40mA, the LED pn junction temperature exceeds 145 ℃, this temperature is to make the packaging material to achieve the critical state of discoloration. If the encapsulation material is carbonized even under high current conditions, an opaque substance is formed on the surface of the device or a conductive path is formed, causing the device to fail.

2. Pollutant welding

LED contaminant welding refers to the LED in the packaging process, LED chip electrode is droplets, oil, fiber, dust and other substances covered by pollution, resulting in some or all of the LED solder joint defect caused by defects, which is harmful Maximum LED welding defects.

According to experiments, when the contaminants cover the entire solder joint, the metal-dielectric-metal structure, also known as the tunnel junction, is formed at the weld. In the process of the device glow, due to the presence of tunnel junction, LED chip peak wavelength luminous intensity will be reduced to normal when 60%. Therefore, the LED package welding defects for reliability testing is very necessary.

3. Solid crystal primer caused by the failure

In the white LED industry commonly used in the solid crystal epoxy resin insulation, silicone resin, silver plastic, and the three have their own advantages and disadvantages, in the selection should be taken into account. Epoxy resin: Insulation plastic thermal conductivity is poor, but the high brightness; silicone insulation: glue thermal effect slightly better than the epoxy resin, high brightness, but because of a certain proportion of silicon, solid silicon chip next to the remaining silicone resin and fluorescent Of the epoxy resin will produce a combination of the phenomenon, after the hot and cold shock will produce peeling leading to dead lights; silver plastic thermal conductivity than the first two are good, you can extend the life of LED chips, but the silver Absorption is relatively large, resulting in low brightness. For the two-electrode blue chip in the use of silver plastic solid crystal when the control of the amount of glue is also very strict, or prone to short circuit, a direct impact on the product yield. Therefore, for different types of device products, to properly use different solid crystal primer, so as to better reduce the device failure caused by it.

4. Phosphor failure

There are many ways to achieve a white LED, the most commonly used, the most mature one is generated by the LED chip to stimulate the yellow phosphor yellow phosphor, so the phosphor material on the impact of white LED attenuation greatly. The most mainstream white phosphor in the market is YAG aluminum garnet phosphor, silicate phosphor, nitride phosphor. Compared with the blue LED chip, the failure of the phosphor will lead to LED light attenuation acceleration, thereby reducing the life of LED. Experiments show that the phosphor at a temperature of 80 , the excitation efficiency will be reduced by 2%, after cooling and recovery, and this very short time a test shows that the LED temperature will cause the performance of the phosphor down, and LED long time Work at high temperatures, will cause irreversible decline in the phosphor, LED will generally appear blue wavelength shift problem.

Therefore, the white light LED light attenuation or even a large part of the reason is that the heat under the effect of rapid degradation of phosphor performance. Therefore, the quality of the phosphor itself has a very important effect on the normal luminous life of the LED.

5. Failure caused by heat dissipation problems

LED is a solid-state semiconductor devices, and LED chip surface area is small, working current density, and for lighting often require multiple LED combination. LED density, resulting in high thermal density of the chip, and junction temperature rise will lead to reduced light output, chip to speed up degeneration, shorten the device life. Table 1 gives the thermal conductivity of several different materials. It can be seen that the current technology in the preparation of power LED, the most mature technology, the most used sapphire substrate thermal conductivity of only 35 ~ 46W / (m × K), less than 1 /

If you want to take into account the practical application of the color drift of the adverse effects of thermal design should also limit the maximum junction temperature. As the LED chip input power continues to improve, these power LED packaging technology to put forward higher requirements, and now the heat problem has become a key factor restricting the development of high power LED.

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  6.LEDGaN-based epitaxial material defects caused by failure

Due to the absence of a substrate material that is compatible with GaN, there are a number of defects in GaN films in most of the current LED devices. GaN material and the current mainstream substrate sapphire lattice constant mismatch rate of 14%, while the growth of GaN material on the sapphire substrate dislocation density of 108 / cm3 ~ 1010 / cm3.

In the preparation of LED, the defects of the material will adsorb the carrier, so as to form a non-radiating composite center in the active layer, which increases the absorption of light, resulting in the decrease of the luminous efficiency of the LED. When the current is large enough , The carrier will occur radiation recombination, but it will cause lattice vibration, lattice thermal motion will accelerate the formation of defects, resulting in degradation of LED heterojunction. The metal electrode in contact with the device under the action of electrical stress and thermal stress will migrate along the dislocation, resulting in low-ohmic ohmic circuit, which will lead to the device optical power drop and leakage current increase. Therefore, to improve the quality of epitaxial materials, reducing the density of materials in the defect can effectively improve the reliability of LED devices.

7. Failure caused by electrostatic damage

GaN material has a wide band gap of 3.39 eV, high resistivity. Therefore, the GaN-based LED chip in its production, transportation process generated by the electrostatic charge is easy to accumulate and produce a high electrostatic voltage. The structure of the GaN-based LED device of the sapphire substrate is very small for the electrostatic bearing capacity and is susceptible to electrostatic breakdown by its generation. In the case of no static protection, the body of the static electricity easily lead to LED breakdown, LED devices are static breakdown will cause permanent failure.

8. P-type GaN ohmic contact aging

Meneghesso et al., In the analysis of the failure process of GaN, through the LED device before and after degradation of the IV characteristics, Meneghesso et al believe that these changes are due to P-GaN transparent conductive film and metal wire electrode ohmic contact under the influence of large current and heat Degradation, resulting in an increase in series resistance, resulting in a current-intensive effect, which makes the luminous efficiency of the decline; in the case of high current injection, the defect will increase, resulting in an increase in leakage current. Therefore, the ohmic contact of the metal electrode of P-GaN plays an important role in the optical performance of the LED.

In addition to the above reasons, the remaining reasons for failure include chip and substrate welding holes, spallation, lens yellowing, cracking, chip open circuit, short circuit and so on.


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