Visit：2167   Date：2023-11-13

With the rapid development of modern technology, the application fields of LED beads have become increasingly widespread, and at the same time, the number of LED brands and models on the market has gradually increased, which is mixed with a large number of low-quality LED beads. Next, we will analyze the defects of these low-quality LED lamp beads and the reasons for the defects.

1： There are many and complex reasons for dead lights (when the positive and negative poles of an LED are connected to a standard voltage and the light is not on or slightly on), mainly analyzed from the perspective of static electricity and packaging.

2： Color deviation (refers to the deviation between the white light emitted by an LED and the standard color temperature, with an error value greater than 10%), which is caused by poor heat dissipation, causing the junction temperature of the LED beads to be too high; The application of fluorescent powder is uneven, and areas with thick coatings are prone to yellowing due to low color temperature; Poor quality of fluorescent powder; Improper mixing of adhesive powder ratio, etc.

3： Flashing (LED lights intermittently turn on and off without human control), caused by unstable driving power supply and intermittent current; Another issue is that packaging materials such as lenses deform under force, resulting in poor contact with the gold wire.

4： Large light attenuation (after using LED for a certain period of time, the luminous flux tested is significantly lower than the luminous flux before use, with a ratio of less than 0.9), which is caused by poor heat dissipation and prolonged overheating, leading to the aging of LED beads; Improper mixing of adhesive powder.

5： Chip failure, chip quality issues (cracks or damage); Poor adhesion between the chip and the substrate can cause severe light decay or dead light.

6： Improper packaging process, resulting in poor quality of encapsulated lamp beads, such as yellowing, bubbles, black spots, corrosion, etc; Thermal stress failure, poor heat dissipation, leading to an increase in junction temperature; Electrical overstress failure, overcurrent or electrostatic breakdown of the chip; The unstable driving power supply will burn off the gold wire. Assembly failure, poor installation and assembly leading to device failure.

The above are some of the situations and causes of LED defects that we usually encounter. Below, we will talk about how to solve these common problems.

Check the bracket: blackening of the bracket indicates corrosion, the silver plating layer on the bracket is too thin, and the bracket is detached from the welding point.

Check dispensing: Check if the solid crystal adhesive itself has expired or failed, and then the amount of solid crystal adhesive should be appropriate. If the amount is too small, the thrust is not enough, and the chip cannot be firmly adhered; Excessive dosage causes the colloid to return to the chip gold pad, causing a short circuit. The selection of curing conditions should be carried out according to the standard curing conditions as much as possible.

Check welding: The parameter settings of the welding machine should be reasonable, with a time of no more than 5 seconds, moderate pressure, and excessive pressure that can easily crush chips; If it is too small, it can lead to false soldering, and the temperature should be controlled around 280 degrees to effectively prevent static electricity. The arc height of the gold wire should be reasonable, as if the arc height is too high, the gold wire will be burnt black when subjected to high current impact; If the arc height is too low, the chip will be burned due to excessive temperature during welding.

Research data shows that if the LED chip emits 100% light at a junction temperature of 25 degrees Celsius, then when the junction temperature rises to 60 degrees Celsius, its emission is only 90%; When the junction temperature is 100 degrees, it drops to 80%; 140 degrees is only 70%. It can be seen that improving heat dissipation and controlling junction temperature are very important things. In addition, the heating of LEDs can also cause spectral shift, increase in color temperature, increase in forward current (under constant voltage power supply), increase in reverse current, increase in thermal stress, and accelerate the aging of fluorescent powder epoxy resin. So, the heat dissipation of LEDs is the most important issue in the design of LED lighting fixtures.

How is the junction temperature of LED chips generated? It is mainly caused by two factors. Firstly, the internal quantum efficiency is not high, which means that when electrons and holes recombine, photons cannot be generated 100%, commonly referred to as "current leakage", resulting in a decrease in the recombination rate of PN region carriers. The leakage current multiplied by the voltage is the power of this part, which is converted into thermal energy, but this part does not constitute the main component because the internal photon efficiency is now close to 90%. Secondly, the photons generated internally cannot be fully emitted to the outside of the chip and ultimately converted into heat, which is the main reason because currently the so-called external quantum efficiency is only about 30%, and most of them are converted into heat.

The heat dissipation of LED light emitting diodes is increasingly valued by people because the light decay or lifespan of LEDs is directly related to their junction temperature. Poor heat dissipation leads to high junction temperature and short lifespan. What are the cooling solutions for LED chips?

In recent years, with the efforts of industry experts, the following improvement plans have been proposed for the heat dissipation problem of LED chips: 1. Increasing the amount of light emitted through the area of LED chips. 2. Encapsulate several small area LED chips. 3. Change LED packaging materials and fluorescent materials.

So can the heat dissipation problem of LED products be completely improved through the above three methods? In fact, it is remarkable. Firstly, although we increase the area of the LED chip to obtain more luminous flux (in units of light, the number of beams passing through a unit area in time is called luminous flux, in ml), we hope to achieve the desired effect. However, due to its actual large area, some opposite phenomena have occurred in the application process and structure.

In fact, there is another effective way to improve the heat dissipation problem of LED chips, which is to replace plastic or organic glass with silicone resin as the white light packaging material. Replacing packaging materials not only solves the heat dissipation problem of LED chips, but also improves LED lifespan. Why is it necessary to use silicone as the packaging material in LEDs now? Because silicone has an absorption rate of less than 1% for light of the same wavelength. However, epoxy resin has an absorption rate of up to 45% for 400-459nm light, which can easily lead to severe light decay due to aging caused by long-term absorption of this short wavelength light.

The characteristic of LED chips is that they generate extremely high heat in a very small volume. And the heat capacity of LED itself is very small, so it must be conducted out at the fastest speed, otherwise it will generate a high junction temperature. In order to extract heat as much as possible from the chip, many improvements have been made to the chip structure of LEDs. In order to improve the heat dissipation of LED chips themselves, the main improvement is the use of substrate materials with better thermal conductivity.

Even if the heat resistance between the chip and the packaging material can be solved, the poor heat dissipation from the packaging to the PCB board can also cause an increase in LED chip temperature and a decrease in luminous efficiency. So we can also integrate circular, linear, and surface shaped LEDs with PCB substrates to overcome the problem of heat dissipation interruption that may occur between the packaging and the PCB board.

Therefore, in the face of continuously increasing current, how to increase heat resistance is also an urgent issue to be overcome at this stage. From various aspects, in addition to the material itself, it also includes the heat resistance between the chip and packaging materials, thermal conductivity structure, thermal resistance between packaging materials and PCB boards, thermal conductivity structure, and PCB board heat dissipation structure, all of which require overall consideration.