The core issue Of led package and application is how to improve the capacity of heat elimination of LED Since LED invented, full colors and high brightness have been achieved, Meanwhile, white light has also been produced based on blue and purple LED. Compared with incandescent lamp and fluorescence lamp, LED has a series of advantages like small volume, full solid state, long lifetime, environmentally-friendly and low power, so it has been applied to automotive lighting, decorational lighting, cell flashlight in middle and big size, Light source module of display of NB, LCD and TV.
Due to these advantages, LED has become one of the most promising high-tech field in 21th century. LED is a kind of electroluminescent device, which is made of compounds of Ⅲ ~ Ⅳ group such as gallium phosphide (GaP), gallium arsenide phosphorus (GaAsP), and semiconductor. In an electric field enviroment, the electrons and cavity-radiation will compound then generate electrochromic effect, at the same time, some of the power is translated into light capacity, namely quantum effect while the crystal lattice is generated by non-radiation compound surges then the rest power will be translated into heat.
Presently, white light of high brightness has been up to 130 lm/w, and high power white color of 100 lm/w has been commercialized. The single LED device has achieved 1.5kw from a few milliwatts initially. For the high power LED over 1w, the electro-optical conversion efficiency is about 30 percent at present, the balance translated into heat while the chip’s size is 1mm*1mm~2.5mm*2.5mm. That is to say, the power-density of chips is very large. Different from traditional lighting devices, there is not infrared spectrum included in white light-emitting spectrum of LED. so the heat cannot be released relied on radiation. Then, how to improve ability in heat-elimination will be one of the complex problems which decide if high power LED can be put into industrialization or not.
Thermal effect on the impact of high-power LED For a single LED.
If the heat concentrated in a very small chip can not be discharged effectively will result in the chip’s temperature raises, and cause the thermal stress non-uniform distributed, decline in efficiency of chip’s light-emitting and excitation fluorescence powder .Study shows that, when the temperature is above a certain value, the failure rate of the device will rise by exponential increase. For example, when the temperature of devices raises 2 ℃, the reliability will decrease by l0%. In order to ensure the life of the device, the temperature of the general requirements of pn junction should be below 110 ℃. As the temperature of pn junction rises, the wavelength from LED white light-emitting devices will be red-shifted. According to the statistics.
At 100 ℃ temperature, wavelength can be red-shifted 4 ~ 9 nm and result in decreased absorption of YAG fluorescence powder, Overall luminous intensity will be reduced, the white color may vary.
At room temperature, When it rise l ℃. The luminous intensity of LED will be in a corresponding reduction about l%. When the device from the ambient temperature rose to l20 ℃. Decline in brightness will be as much as 35%. When numbers of components arrange with dense and make up of white LED lighting systems. Heat dissipation problem will became worse and worse. So it is a prerequisite of application in power type LED to solve heat dissipation problem.
Facing the conundrum of heat-elimination of high power led, the device designers and manufacturers have optimized the design of heat-elimination system on structure, material and craft.
For example, for the packing structure, adopt big area chips out-inside, Metal circuit board structure, heat-conductor slot structure and low current array structure. and choose appropriate base and pasted material, use silicone resin instead of epoxy resin.
After the packing frame is decided by packing material, we can choose different material to reduce the heat-resistance of the system to improve the capacity of electric conducted. Currently, manufacturer from home and abroad usually choose the better material for the base panel, pasted material and packing material.
(1) epoxy resin.
As the LED device packaging materials, Epoxy resin insulates from electricity excellently, adhere well and dielectric, However, epoxy resin has its disadvantages, it is hygroscopic, ageing easily, heat-resistant poly, easy to turn its color when in high temperature and irradiated by shortwave for a long time, all of these factors will affect the lifespan of led devices. At present, many of the packing trade worker change to use silicone and ceramic instead of epoxy resin to extend the lifetime of LED products.
(2) pasted material.
It is very important to choose proper chips as the bottom base material and make sure that the thickness is very low during production. The 3 common materials are heat conducted glue, conductive silver paste and tin-based glue. Though the hardening temperature of heat conducted glue is low(<150°C), it conducts heat bad. The hardening temperature of conductive silver paste-type are generally lower than 200 ~ C with a good thermal conductivity properties and pasted capacity. However, the silver-paste will heat during enhancing its brightness, at the same time toxic metals such as lead will be caused, so it is not the best choice for paste materials. Compared with the first two, the thermal conductivity of conductive tin-based paste is the best of the three pastes with a wonderful electronic conducted capacity.
(3) Base panel material.
For high power LEDs, we may choose ceramic, Cu/Mo panel or Cu/W panel alloy as the heat conducted material to solve the rupture in the pole down-lead caused by the heat expansion between the chips material and the heat-elimination material.However, the cost of these alloys is too high to be produced in a large scale for a low cost. So choosing aluminums which can conduct heat well and the copper-plate as the base panel material for heat-eliminating is one of the important studies at the present.
Overall, the key of the technique of packing objects will be a new packing frame of low thermal barrier, excellently heat-elimination and low mechanical stress. No matter what the materials and frames are,we must solve a problem, that is, the heat-elimination from the chip’s junction temperature to extension part, extension part to packing base, packing base to cooling device. And heat conduction channel of solid-state light source is made up of by these three taches, any link of the three is weak will result in the light resource destroyed. There are three ways to conduct heat from the junction temperature to its surrounding enviroment. They are conducting, convecting and radiating. That is to say, we must use high thermal conductivity coefficient materials in the three parts to improve heat-elimination and reliability to the best.
At present, many of power LED driving electrical current can reach to 70ma, 100ma or 1a. With the power increasing, the most important problem to achieve bulk production will be solving the heat-elimination. Based on the aspects mentioned above, we have studied heat-elimination from the following aspects:
(1) We usually choose the materials which is good at heat conducting like aluminum or copper to make the radiator to quicken the heat–elimination from the underlay to the surrounding environment.
Then fix a fan and a loop heat pipe to force cooling. No matter considered the cost or the appearance, It is inadvisable to use external cooling device. Therefore,According to the energy conservation, using piezoelectricity ceramic as the heat-elimination device, and transforming the heat to vibration to consume the heat directly will be one of the most important studies in the future.
(2) Inner quantum effect decides the quatity of heat produced.
During the growing of the gallium, ameliorating the frame of materials, optimizing growing parameters, getting a high quality extension part, improving inner quantum efficiency of the devices to reduce heat-produced fundamentally will quicken the speed of heat exchange from the chip’s junction temperature to the extension part.
(3) As for the high power LED devices, its total resistance is all of the heat sinks’s thermal resistances from PN junction to its surroundings.
Including the heat sink and thermal resistance of LED itself inside, the thermal resistance of the heat conducting colloidal between the inner heat sink and PCB, PCB and external heat sink, thermal resistance of the external heat sink etc. Every heat sink in the loop of heat conducting will cause barrier to some extent. After a longterm study, reducing the quatity of inner heat sink, and using thin film technology to make the indispensable pole heat sink and insulation layer directly to the mental heat-elimination devices will reduce the total thermal resistance substantially. This technology is potential to become the leading direction on high-power LED heat-elimination packing.
(4) As for the under layers, Good thermal conductivity materials should be considered first.
Such as giving priority to aluminum-based when choosing MC-PCB, ceramic, DBC, composite metal substrate. So that the heat can be conducted from the extension part to the outmost panel easily. By optimizing the thermal design of MCPCB plate, or fixing the ceramic to the the mental substrate directly to form mental based LTCC-M, the under layers will be of a good heat conducting, low coefficient of thermal expansion.
