High Power LED Application and Chips Heat Dissipation
Posted: 2015-12-01 02:53:47  Hits: 878
Today, white light LED still has the disadvantages of poor uniformity of luminance and short service life of sealing materials, which makes it fail to perform its received advantages. As for requirements level, with its application in mobile phone, car, medical and other fields, white light LED research results have gained more and more attention.
To improve the luminous efficiency of white LED, there are two general direction, one of them is to enlarge the LED chip area, which is to say, increase 1m㎡ small chip to more than 10m㎡, thereby increase the amount of light. Or put a few small chips encapsulation together in the same module. Though, large-scale LED chip area will obtain high brightness, but because of too large area, there will appear the opposite result in its application. Therefore, in view of this problem, part of the LED industry enterprisers improves the electrode structure and the structure of the flip chip, so as to improve LED chip surface and increase luminous efficiency up to 50 lm/W. In part of the white LED flip chip packaging, for example, because the light-emitting layer is close to encapsulation, when the light of the light-emitting layer is out, the electrode will not be covered, but the heat will be not easy to dissipate.
Another one is making use of several small LED chips to quickly increase luminous efficiency. Compared with large area of LED chips, using small power LED chips encapsulation into the same module has the ability to quickly reach the requirements of high brightness. But it also cause some doubts, because it is more than LED encapsulated on the same module, so in the module there must be put into some insulation materials, so as not to cause short circuit between the LED chip, however, in this way the cost will increase a lot. It is explained that in fact, the influence of the amplitude to cost is relatively small, because compared with the overall proportion costs, these insulation materials only cover less than one percent, and we can make use of the existing application material for insulation, which does not need to develop and also doesn't need to add new equipment. It may be reasonable but for relatively inexperienced operators, this is still a challenge, because no matter on the yield, research and development, production engineering, there are a lot of problems need to be overcome.
The increasing importance of packaging materials and fluorescent light materials
But if it is expected to LCD TV backlight application, there will be more problems to overcome, because the using time of LCD TV is several hours, therefore, white LED backlight must have the ability to continually shine without declining. Currently published high power white LED, its luminous power is dozens of times than low power white LED brightness. So using high power white LED to replace fluorescent lamp as lighting, one must overcome difficulties is diminishing brightness. White LED, for example, the long time continuous use 1w power, which causes the brightness of the continuous use of the second half of the time gradually reduce.
Larger the current will lead to higher brightness, and this is general idea for LED, however, because the use of current is added, so the disadvantage is that whether encapsulation materials have the ability to withstand such heat for a long period of time. And also because such a continuous use, usually encapsulation materials heat resistance will fall below 10k/w. Calorific value of high power LED is larger than low power LED, therefore, it will appear that as 
temperature rises, the luminous power reduces. Therefore the development of resistance to high encapsulation materials is very important. Perhaps LEDs under 20~30 lm/W do not have all these problems, however, once appears more than 60 lm/W high luminous power LED, we have to find a way to solve it, because the thermal effect influences not only LED itself, but on the whole application products bring distress.
Changing of encapsulating material increases the service life of white light LED
Hot issues, of course, is not only the brightness performance impact, at the same time, it will appear to the life of the LED itself, so in this part, the LED constantly develop packaging material to meet the requirements. Epoxy resin which is used as packaging materials in the past has poor heat resistance, which may appear off-color situation before the arrival of the life span of the LED chip itself, therefore, in order to improve the heat dissipation, and it must allow more current release. In addition, not only because hot phenomenon will have consequences for epoxy resin, even short wavelength may cause some problems to it, this is because the epoxy resin is quite easy to be damaged by short wavelength of white LED light. Just the low power white LED can cause the destruction of the epoxy resin, let what's more, the high power white LED contains more short wavelengths, then the deteriorating process also accelerates, and even some products service life is less than 5000 hours after continuous light.
So, rather than constantly overcome discoloration problem because the old packaging materials, we should go towards development of a new generation of packaging materials, which perhaps is a good choice. At present to solve problems of its service life, many LED packaging industry abandons epoxy resin and adapts silicon resin and ceramic as a packaging material, which according to statistics can improve the life of the LED. As data point of view, instead of the epoxy resin encapsulation material - silicon resin, has higher heat resistance, and according to the test, even in the high temperature of 150 ~ 180 degrees Celsius, it will not change color, which appears to be a good packaging material. Because silicone can disperse blue and near ultraviolet light, so compared with epoxy resin, it can inhibit the materials degradation phenomenon caused by current and short wavelength light. Therefore, almost all of the high power white LED products have substitution of silicone as the material of packaging.

High Power LED Application and Chips Heat Dissipation

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