Factors affecting LED light decay and their solutions

Cause of LED light decay

At present, the white light on the market LED its light decay may be one of the first problems to enter the civil lighting. For LED, there are two main factors:

I. LED of the quality of the product itself: The LED chip used in the

1. Has poor physique and fast brightness attenuation.

2. Production process has defects, LED the chip heat dissipation can not be well derived from the PIN foot, resulting in the chip temperature is too high to increase the chip attenuation.

II. Conditions of use:

1.LED is constant current drive, some LED use voltage drive reason to make LED attenuate.

2. Drive current is greater than the rated drive condition.

There are many reasons for the light decay of LED products, the most important is the problem of heat, although many manufacturers do not pay special attention to the problem of heat dissipation in secondary LED products, but these secondary products will be used for a long time, the degree of light decay than the LED products with a focus on heat dissipation. LED the thermal resistance of the chip itself, the influence of silver glue, the heat dissipation effect of the substrate, and the colloidal and gold wire aspects are also related to light decay.

Factors affecting LED quality light decay of lamps and lanterns.

A single LED white lamp on the aging board detected data, and the LED white lamp assembly into a lamp aging detected data, must be a bit different.

The magnitude of this difference depends on the electrical parameters when the LED works and the design of the luminaire, and the environment in which the luminaire is used.

First, choose what kind of LED white light.

And that's important, LED the quality of the white light is an important factor. To give some examples, the same white light lamp, which is encapsulated with mil white light section chip of crystal element 14, is made of common epoxy resin and LED white light glue and package glue. The attenuation data is 70% after a thousand hours in a 30 degree environment ,45% in the same aging environment if encapsulated with D kinds of low aging glue, and 12% in the same aging environment if encapsulated with C kinds of low aging glue. if B class of low-aging glue is encapsulated, in the same aging environment, the thousand-hour light decay is -3%; if A class of low-aging glue, in the same aging environment, the thousand-hour light decay is -6%.

Why do different packaging processes lead to how big differences?

A major reason is that LED chips are afraid of heat. A short period of time to heat more than 100 degrees, that is not important, afraid of long-term under the high temperature, is a great damage to the LED chip. generally speaking, the thermal conductivity of ordinary epoxy resin is very small, so when LED chip lights up to work, LED chip to emit heat, and ordinary epoxy has limited thermal conductivity, so, when you measure the temperature of the LED bracket from the outside of the white lamp has 45 degrees, LED the chip center temperature inside the white lamp is likely to exceed 80 degrees. LED temperature node is actually 80 degrees, then, when the LED chip in the temperature of the temperature, is very tormented, which speeds up the aging of the LED white lamp.

As the LED chip is working, the center temperature produces a high temperature of 100 degrees, which can immediately lead the heat out through the bracket pin 98%, thus reducing the heat damage to it. As a result, LED the white light holder temperature is 60 degrees, its chip center temperature may be just 61 degrees.

From the above data can be seen, choose what kind of packaging process of LED white lamp, directly determine the light decay of LED lamps.

Secondly, LED lamp bead working environment temperature.

According to the data of single LED white lamp aging, if only one LED white lamp lights up to work, and at the same time, the ambient temperature in which it is located is 30 degrees, then the support temperature of single LED white lamp working will not exceed 45 degrees. At this time, the life of this LED will be ideal.

When 100 LED white lights work at the same time and the interval between them is only 11.4 mm, then the bracket temperature of the LED white lamp around the lamp stack may not exceed 45 degrees, but those LED white lights in the middle of the lamp stack may reach a high temperature of 65 degrees. At this time, LED lamp bead is a test. If the LED white lights are gathered in the middle, the light decay will be faster in theory, and the LED white lights around the lamp stack will slow down a little.

but if LED lamp beads are separated by more than 25 mm, then the heat that they diverge from each other will not accumulate so much. at this time, the temperature of each LED white lamp holder should be less than 50 degrees below, which is more conducive to the normal operation of the LED.

If LED work in a colder environment, the average annual temperature may be only about 15 degrees, or less, that would be longer for LED.Alternatively, when LED work, there is a small fan blowing beside it to help it disperse the heat, which is also quite helpful for LED life. Anyway, we should know that LED is afraid of heat, the higher the temperature, the shorter the LED life, the lower the temperature, the longer the LED life. LED the ideal operating temperature is of course between minus 5 and zero degrees. But this is basically impossible.

As a result, we understand the ideal working parameters of LED lamp beads, as far as possible in the design of lamps, strengthen the heat conduction, heat dissipation function. Anyway, the lower the temperature, the longer the LED life.

Third, the working electrical parameters design of LED lamp beads.

According to the experimental results, LED the lower the driving current, the smaller the heat emitted, of course, the smaller the brightness. LED the design of the solar lighting circuit, the driving current of the LED is generally only 5-10 mA; the number of lamp beads used in lamps has a large number of products, such as more than 500, or more, the driving current is generally only 10-15 mA, and the general popular LED application lighting driving current, only 15-18 mA, few people design the current to more than 20 mA.

The experimental results also show that under the driving current of 14 mA, and the air temperature of the lid is 71 degrees, the low-aging product ,1000 hours light decay to zero ,2000 hours light decay to 3%, which indicates that the use LED this low-aging white lamp in such an environment has reached its maximum, and then is a damage to it.

Because the aging plate does not have heat dissipation function, so the heat generated when LED work is basically unable to transmit to the outside. experimentally proved this. The air temperature inside the aging plate has reached a high temperature of 101 degrees, and the surface temperature of the lid on the aging plate is only 53 degrees, a difference of dozens of degrees. This shows that the designed plastic cover basically does not have the function of heat conduction and heat dissipation. But in the general design of lamps and lanterns, the function of heat conduction and heat dissipation is considered. Therefore, to sum up, the design of the working electrical parameters of the LED lamp bead should be based on the actual situation, if the heat conduction and heat dissipation function of the lamp is very good, it is OK LED increase the driving current of the white lamp a little, because the heat generated by the work of the LED lamp bead can be exported to the outside instantly, without damage to the LED, is the best care for the LED. On the contrary, if the heat conduction and heat dissipation function of the lamp is sloppy, it is best to design the circuit a little smaller, let it put less heat out. As mentioned earlier, researchers believe that because the Auger effect is an internal formation mechanism, LED light decay can not be eliminated, but can be minimized.

Relationship between white light LED light decay and materials

With the advent of blue light LED, the combination of fluorescent body and blue light LED can easily obtain white light LED, which is the most mature white light packaging method in the industry. With the advent of blue light led, the combination of fluorescent body and blue light LED can easily obtain white light LED, which is the most mature white light packaging method in the industry. At present, white light LED has become a lighting source, and general household lighting has become a reality. However, in the process of using more white light products attenuation is large, can not be suitable for the lighting market, Taiwan Hongcai Optoelectronics for the high-end lighting market demand, increase the research and development of white light, through changing packaging technology and material collocation to develop low attenuation white light products, for the LED lighting industry slightly modest. Below is our company in the packaging process summed up five points of experience, and you industry colleagues to exchange, with a view to white light LED packaging technology promotion.

1、Wafer on white light LED light decay

From the results of current experiments, the effect of wafer on light decay can be divided into two categories: first, the different material of wafer leads to different attenuation. At present, the commonly used blue-ray wafer substrate material is silicon carbide and sapphire, the general structure of silicon carbide is designed as single electrode, its heat conduction effect is better, sapphire is generally designed as double electrode, heat is difficult to derive, and the heat conduction effect is poor; second, the size of wafer, when the wafer material is the same, the size difference of different attenuation difference is also different.

2. effect of solid crystal base glue on white light LED light decay

Epoxy resin insulating adhesive, silicone insulating adhesive and silver adhesive are commonly used in white LED packaging industry. Each of the three has advantages and disadvantages, in the selection of a comprehensive consideration. Epoxy resin insulation adhesive thermal conductivity is poor, but high brightness; silicone insulation adhesive thermal conductivity is slightly better than epoxy resin, high brightness, but because of the silicon composition of a certain proportion, the residual silicone resin next to the solid wafer and epoxy resin in the fluorescent glue will produce the phenomenon of interlayer, after cold and impact will produce peeling lead to dead lamp; silver adhesive thermal conductivity than the first two, can prolong the life of the LED chip, but silver glue absorption of light is relatively large, resulting in low brightness. For the double-electrode blue-ray wafer, the control of glue quantity is also very strict when using silver glue to fix crystal, otherwise it is easy to produce short circuit, which directly affects the good product rate of the product.

3. phosphors on white LED light decay

A variety of ways to achieve white light LED, the most commonly used and most mature is by applying a layer of yellow phosphor on the blue chip, so that the blue light and yellow light mixed into white light, so the material of the phosphor has a great impact on the attenuation of white LED. the most mainstream phosphors in the market are YAG yttrium aluminum garnet phosphors, silicate phosphors, nitride phosphors. compared with blue LED chips, phosphors have the effect of accelerating aging white light LED, and the influence of phosphors from different manufacturers on light decay is also different, which is closely related to the raw material composition of phosphors. Lehman Optoelectronics selected the best material of white phosphors, making the white LED compared with the peer in attenuation control has a great improvement.

4. Fluorescent Glue on the LED of White Light

The white LED, fluorophores used in traditional packaging generally use epoxy resin or silica gel, and the results of light decay experiments show that the white LED life of powder with silica gel is obviously longer than that of epoxy resin. One of the reasons is that the silica gel LED, encapsulated into finished product by the above two methods has stronger UV resistance than epoxy resin and better heat dissipation effect than epoxy resin; but under the same conditions, the initial brightness of silica gel powder is lower than that of epoxy resin powder, the most important reason is that the refractive index of silica gel (1.3-1.4) is lower than that of epoxy resin (more than 1.5), so the initial light efficiency is not as high as that of epoxy resin.

5. Scaffold on white light LED light decay

LED bracket mainly have copper bracket and iron bracket. The copper support has good thermal conductivity and high price. while the thermal conductivity and conductivity of the iron support are relatively poor, which is easier to rust, but the price is cheap.

LED on the market mostly use iron supports. Different materials of scaffolds have different effects on LED performance, especially on light decay. this is mainly due to the fact that the thermal conductivity of copper is much better than that of iron. the thermal conductivity of copper is 398 W (m.k), while the thermal conductivity of iron is only about 50 W (m.k), which is only 1/8 of the former, and the thickness of the plating layer of the scaffold is also closely related. At the same time, we should pay attention to whether the bowl cup size of the bracket matches the luminous chip and the mold particle, and its matching quality directly affects the optical effect of the white light LED, otherwise it is easy to cause the spot shape asymmetry, yellow ring, and black spot, which directly affects the quality of the product.

As white light LED is applied in lighting, customer demand is also constantly improving, we must constantly innovate, improve our product performance to improve our process, as far as possible to meet customer needs. to make high-performance white light LED products, whether the material selection and collocation is the best directly affects the white light LED attenuation and quality. therefore, good materials plus the best collocation, plus the matching of the unique manufacturing technology of taiwan hongcai optoelectronic shares is the key to do a good job of white light technology. For Taiwan's Hongcai Optoelectronics, we must give full play to our own advantages, make continuous efforts, increase research and development, develop products more suitable for customers' requirements, and contribute to the national semiconductor lighting energy-saving industry.