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Analysis of high power white LED applications and LED chip heat dissipation
- May 07, 2017 -

Although it seems quite good in terms of the characteristics, but in fact there are some shortcomings, like in the service life, only 3,000 hours or so, plus the price is too expensive is not easy to solve things, perhaps the price is too expensive The problem can take some time to drop some, but now the level of 300,000 yen to see, to fall to 3,000 or even 300 yen, it takes more than 10 years time.

For today, white LED is still there is poor luminous uniformity, closed material life is not long, and can not play the white LED is expected to use the advantages. But the demand level, not only the general use of lighting, with the mobile phone, LCDTV, automotive, medical and other widely used positive, making the most appropriate development of white LED technology research results are quite concerned about.

Increase the amount of light by increasing the wafer area

Expect to improve the luminous efficiency of white LED, there are two major directions, is to improve the area of LED chips, that is, the current area of 1m ㎡ small chip, the luminous area increased to 10m ㎡ or more, to increase the amount of light , Or put several small chips together in the same module.

Although the LED chip area to be large, to get a lot of brightness, but because of the large area, in the application process and the results will appear counterproductive phenomenon. Therefore, in view of such problems, some of the LED industry to improve the electrode structure, and the structure of the flip chip, the chip surface to improve, to achieve 50lm / W luminous efficiency.

For example, in the case where the light-emitting layer is close to the vicinity of the package, the light is emitted from the outside, so that the electrode is not shielded, but the disadvantage is that the heat generated is not easily dissipated.

Instead of increasing the wafer surface, it is absolutely possible to increase the brightness in one breath by adding the wafer area, since the improved portions of the wafer can not be reflected when the light is scattered from the inside of the wafer, A little limit, according to the calculation, the best play light efficiency LED chip size is about 7m square meters.

The use of several small area LED chip to quickly increase the luminous efficiency

And large area LED chip compared to the use of low-power LED chip package into the same module, which is able to quickly achieve high brightness requirements, for example, Citizen will be eight small LED package together, so that the module luminous efficiency Reached 60lm / W, called the industry's first case.

But this approach also caused some doubts, because it is more than one LED package in the same module, so the module must be placed in some insulating materials, so as to avoid short-circuit between the LED chip, but this one It will increase the cost of a lot.

The explanation for Citizen is that, in fact, the magnitude of the impact on costs is quite small, since less than one percent of these insulating materials are compared to the overall cost ratio and can be made with existing materials Application, these insulating materials do not need to re-develop, do not need to add new equipment to respond.

Although Citizen's interpretation is theoretically reasonable, it is a challenge for less experienced industry, because both in terms of good, R & D, and production engineering need to be overcome.

Of course, there are other ways to achieve the goal of improving the luminous efficiency, many industry found in the LED sapphire substrate to produce uneven structure, which may be able to improve the light output, so there is gradually towards the wafer surface to establish Texture or PhotONics crystal structure.

For example, Germany's OSRAM is such a framework to develop a "ThinGaN" high-brightness LED, OSRAM is formed in the InGaN layer of metal film, and then stripped of sapphire. In this way, the metal film will produce the effect of mapping and get more light out, and according to the OSRAM data show that this structure can get 75% of the light extraction efficiency.

It is expected that the luminous efficiency of 50 lm / W is expected to be achieved by the use of a flip chip structure. Since the light emitting layer is close to the vicinity of the package, the light of the light emitting layer is dissipated to the outside, so that the electrode is not shielded.

Of course, in addition to the need for light to take out the chip out of the effort, because expect to be able to get higher light efficiency, in the package part of the need to do some improvement. In fact, each additional project will have some impact on the efficiency of light extraction, but this does not mean that because the packaging process will certainly increase the higher light loss, as Japan OMROM developed by the plane light source Technology, it can greatly improve the efficiency of light extraction, such a structure OMROM is the LED light emitted by the use of LENS optical system and reflective optical system to do the control, so OMROM called "Doublereflection optical system."

With such a structure, the light loss caused by the LED of the conventional shell-type package or the like can be improved for the wide-angle reflection of the package to obtain higher light efficiency, and further, the processing is performed on the Mesh formed on the surface, and The formation of double-layer reflection effect, this way, in fact, can get a good light out of the efficiency control. Because of this special design, these use of reflective effect to achieve high light out of the efficiency of the LED, the main purpose is for LCDTV backlight applications.

The importance of encapsulating materials and fluorescent materials is increasing

But if you want to use as LCD TV backlight applications, then the need to overcome the problem will be more, because the LCD TV continuous use of time is up to several hours, or even 10 hours, so, because of such a long time Of the use of white light LED used as a backlight must have a continuous use will not have the brightness of the situation.

Has been published high-power white LED, its luminous power is a low power white LED brightness of several times, so the hope that the use of high-power white LED instead of fluorescent lamps as a lighting device, there must be a difficult to overcome is The brightness of the situation.

For example, white light LED for a long time continuous use of 1W of electricity, will cause continuous use of the second half of the time gradually reduce the brightness of the phenomenon, of course, not only high-power white LED will appear such a situation, low-power white LED There is such a problem, but because, low-power white because the application of different products, so it will not particularly highlight such a problem.

The greater the current used, the higher the brightness obtained, which is generally the idea of achieving high brightness for the LED, but because the current used increases, the drawback is that the encapsulation material is able to withstand this For a long time because of the heat generated by the current, but also because of such continuous use, often the thermal resistance of packaging materials will drop to 10k / w below.

High-power LED heat is a few times lower than the low-power LED, therefore, there will be with the temperature rise, and the luminous power to reduce the problem, so in the high heat resistance of high-quality packaging materials development, it is relatively significant important.

Perhaps in the 20 ~ 30lm / W following the LED, these problems do not exist, but once faced more than 60lm / w high-power LED, you have to find a way to solve, because the impact of thermal effects , Definitely not only the LED itself, but will be troubled by the overall application of the product, so if the LED can be resolved in this regard, then, you can also reduce the application of the product itself, the cooling burden.

Therefore, in the face of continuous improvement of the current situation at the same time, how to increase the heat resistance, but also the urgent need to be overcome at this stage, in all respects, in addition to the material itself, but also from the chip to the packaging materials Of the heat resistance, thermal structure, packaging materials to the PCB board between the heat resistance, thermal structure, and the PCB board cooling structure, which need to be a holistic consideration.

For example, even if the heat resistance between the wafer and the package material can be solved, the heat dissipation efficiency of the LED wafer is also increased due to the poor heat dissipation effect from the package to the PCB. So, like Panasonic in order to solve this problem, from 2005 onwards, put the circular, linear, surface type of white LED, and PCB substrate design into one, to overcome may appear in the package from the PCB The problem of heat disruption.

However, not all of the industry are like Panasonic, the packaging materials to the PCB board between the heat resistance are considered, because the industry's strategic relationship, some industry to the substrate design as the goal, only for the PCB board Of the cooling structure to improve.

There are a lot of industry, because they do not produce the relationship between the LED, so only in the PCB to do some research and development, but only in the end is still not enough, so the need to choose a good heat white LED. Metal material, with the white LED package in the cooling slot tightly connected to complete with a heat sink design of high power white LED and PCB board connection, to heat capacity.

However, it seems that just because it is expected to achieve heat, and a simple thing to be complicated, in the end this is not consistent with the concept of cost and progress to today's application level, it is difficult to make a judgment, In fact, some industry is looking at this aspect, such as Citizen published in 2004 the product is able to package from the thickness of 2 ~ 3mm heat dissipation out of the heat, to provide the application can be used because of a heat sink Of the high-power white LED, PCB board to allow the thermal design to play.

Changes in packaging materials to improve the white LED life of the original 4 times

Of course, the problem of heat is not only the impact of the performance of the brightness, but also on the life of the LED itself challenges, so in this part of the LED continue to develop packaging materials to respond, continue to improve the LED brightness generated Impact.

In the past, epoxy resin used as a packaging material had poor heat resistance, and it was possible that the epoxy resin had been discolored before the life of the LED wafer itself was reached. Therefore, in order to improve the heat dissipation, And must allow more of the current to get released, which is an important part of this architecture.

In addition, not only because the heat phenomenon will produce epoxy resin, and even short wavelengths will also cause some problems on the epoxy resin, which is because the white LED light spectrum, also contains short wavelength light, and ring Oxygen resin is quite easy to be white light LED in the short wavelength of light damage, even low-power white LED has been to cause damage to the epoxy resin, not to mention the high-power white LED contains short wavelength light more, then Deterioration of the natural also accelerated, and even some products in the continuous lit after the service life of less than 5,000 hours.

So, with its constant overcoming because of the old packaging materials - epoxy resin caused by discoloration, it is better to develop a new generation of packaging materials, perhaps a good choice. At present in the solution to the problem of life in this area, many LED packaging industry are facing to abandon the epoxy resin, and change the use of silicone and ceramic as a packaging material, according to statistics, because the change in packaging materials, in fact, can improve the LED life.

On the data point of view, instead of epoxy resin packaging materials - silicone resin, it has a high heat resistance, according to the test, even in the 150 ~ 180 degrees Celsius high temperature, it will not change the color of the phenomenon seems to be A nice wrapping material.

Because the silicone resin can disperse blue and near ultraviolet light, so compared with epoxy resin, silicone resin can inhibit the material because of short-wavelength light and the degradation caused by the phenomenon, while reducing the rate of light penetration rate of decline.

So, with the current application point of view, almost all of the high-power white LED products have been modified silicone resin as a packaging material, for example, because the short wavelength of light caused by the impact of the part of the wavelength of 400 ~ 450nm light , Epoxy resin about a few bits.

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