1 Introduction
The light-emitting diode (LED) birth date, full color and high brightness, and developed on the basis of the blue LED and violet LED white light LED. The history of it is the general term for man lighting brings a Pentium. From incandescent lamps and fluorescent lamps, LED with its small size, all solid, long-lived life, environmental protection, energy saving, and a series of advantages, has been widely used in automotive lighting, decorative lighting, phone flash, medium and large size, that is NB and LCD. The light source module such as a TV display.
The 2l centuries of the most promising high-tech areas has become one of the LED is a injection electroluminescent light emitting devices. Ⅲ ~ Ⅳ aromatic compound, such as phosphating gallium (GaP), gallium arsenide phosphide (GaAsP), a semiconductor made of the electric field under the action of the ~ I-DN. The electrostrictive effect which occurs in the radiative recombination of electrons and holes will be part of the energy is converted to light. Quantum effects, without radiative recombination gave birth to the occurrence of lattice vibrations to the rest of the energy is converted into heat. Now, high-brightness white LED in the laboratory has reached a level of 1001m / W 501m / w power white LED has entered commercialization, a single LED devices jumped from the first few milliwatts 1,5 kW. Greater than 1W-level high-power LED
, the eras of electro-optical conversion efficiency of about 15%, and the remaining 85% is converted into heat. The chip size is only 1mm × 1mm ~ 2. 5mm ~ 2. 5mm. Means that the power density of the chip different from the conventional lighting device, the emission spectrum of the white LED is not included in the infrared part. So its heat can not rely on the release of radiation. Therefore, how to improve the heat dissipation capability is one of the high-power LED to achieve industrialization key technical problems to be solved.
2. Thermal effects of high-power LED
In terms of a single LED. If the thermal energy is concentrated within a small chip size can not be effectively dissipated. Will lead to the elevated temperature of the chip. The thermal stress caused by the non-symmetry distribution chip light emitting efficiency and phosphor lasing efficiency decreased. The study shows that when the temperature exceeds a certain value. The failure rate of the device will show a climb exponentially. Component temperature rise 2 ℃, reliability will drop l0% l. Order to ensure the lifetime of the device, generally require pn junction temperature below 110 ℃. Followed by the temperature rise of the pn junction. Redshift data count data show that the emission wavelength white LED devices. In the 100 ° C the temperature. The wavelength can redshifts 4 ~ 9 nm. Thus lead to the YAG phosphor absorption rate, total luminous intensity cut, white chromaticity worse. Similar at room temperature, the temperature is increased by 1 ° C. LED luminous intensity will be a corresponding reduction of about l%. When the device is increased from ambient temperature to l20 ℃. The brightness decline by as much as 35%. When the multiple LED set them in intensive constitute white light illumination system. More serious heat dissipation need someone to answer questions. Prerequisite to solve the heat has become a power LED applications need someone to answer questions.
3. Domestic and foreign research progress
For high-power LED package heat dissipation problems. By default, the device at home and abroad and manufacturers in terms of layout, materials and workmanship the device thermal system optimized default. For example. Flip-chip package layout, accepted a large area layout, metal circuit board layout, the thermal trough layout, microfluidic array layout; material selection, choose a suitable substrate material and adhesive materials instead of epoxy, silicone resin resin.
3.1 Package layout
Acquaintance decision of high-power LED package heat dissipation problems, the international development of a variety of layout are:
(1) Silicon flip-chip layout (FCLED)
The traditional LED acceptance suits layout, the above typically coated with a layer of epoxy. Below accepted sapphire as a substrate. Since the poor thermal conductivity of the epoxy resin. Sapphire is a poor conductor of heat, and heat can only rely on the chip below the pin shed, before and after the two are caused by cooling difficulties. Affect the performance and reliability of the device.
In 2001. LumiLeds company developed A1GaInN power flip-chip layout. Figure 1 shows a chip being installed layout and flip layout collation LED chip
connected by a bump flip to the silicon. So. High Power LED breeds heat does not have to occur via the chip sapphire substrate. But directly to the higher thermal conductivity of silicon or of ceramic substrate, and then spread to the metal base, due to its active heating zone closer to the cooling body. Therefore, to reduce the internal thermal resistance of the heat sink 21. The thermal resistance of this type of layout views calculating the minimum can be achieved 1. 34K / W. Actually done 6 ~ 8K / W, the light exit rate also increased about 60%. However, the thermal resistance of the heat sink thickness is directly proportional. Therefore limited by the the silicon mechanical strength and thermal conductivity. Difficult by thinning wafers to further reduce the thermal resistance of the internal heat sink, which restricts further improve its heat transfer performance.
(2) The metal circuit board layout
The metal PCB layout using aluminum and other metals with excellent thermal conductivity temper. PCB board of the PCB board of the chip package to a copper electrode that is covered with a few millimeters thick, or the chip is encapsulated in the metal sandwich. Then encapsulated into the heat sink, in order to solve the heat dissipation of LED power increases need someone to answer questions. Acceptance of the layout was suffering from a good heat dissipation characteristics, and greatly improve the input power of LED.
The U.S. the UOE company Norlux series LED. Packaged products assembled in the metal core PCB with aluminum sandwich panel. Herein PCB board is used as the electrode connection wiring of LED devices. The aluminum sandwich As a heat sink cooler. Figure 2 shows the metal circuit board layout. Its defect is that the dissection of the PCB is a poor conductor of heat. It will hinder the heat conduction. According to the research, the OSRAM company Golden Dragon series white LED chips
LW W5SG flip in of a 3ram to 3mm. And horizontal placement of the metal circuit board, in the LED devices and between the metal circuit board, the thermal interface material of the coating 1898In-Sil 8, the system thermal resistance approximately 66. 12K/Wt ".
(3) The layout of the micro-pump
Sheng Liu et al in 2006 through a micro-pump system installed on the radiator solve LED heat need someone to answer questions, and invented its thermal performance is superior to the heat pipe and heat sink. In the closed system, water, under the action of the micro pumped into the LED's bottom groove small endothermic and a small water container, and then returned, and then by a fan endothermic. Figure 3 shows a layout of the type of micro-pump. It can be external thermal resistance reduced to O. 192K / W. And encapsulation [41. Preferably of such layout of the micro-pump refrigerant. However, if the first two layout, the internal interface thermal resistance, the thermal conductivity is compromised. And the layout is too complex.
3.2 Packaging materials
After determining the package layout. You can select different materials to further reduce the system thermal resistance, improve system thermal performance. Now, at home and abroad often substrate materials, adhesive materials and packaging materials merit.
(1) Substrate material
For high-power LED acquaintance decision between the chip and cooling materials due to heating and expansion mismatch caused by the electrode lead off the need to answer the questions. Optional of ceramic, Cu / an Mo plate and Cu / W board alloy is used as the heat sink material, but the production cost of these alloys is too high, and not conducive to large-scale, low cost production. The choice of good thermal conductivity of aluminum plate, copper plate as a heat sink substrate material is the focus of current research circle.
(2) Adhesive materials
Appropriate choice of the chip substrate adhesion material. And in the batch production process to ensure adhesion to a thickness of as small as possible. This to guarantee the thermal conductivity characteristics of the device is very important. Is usually used three kinds of thermal plastic, conductive silver paste and tin pulp material adhesion. LED devices although lower hardening temperature thermal plastic packaging material. Having excellent electrical insulating properties, adhesion and dielectric properties, but the epoxy resin having a hygroscopicity, aging, poor heat resistance, high temperature and shortwave light, easy to change color, and before curing has some toxicity, so the LED devices the life of the impact. Eras LED packaging industry to switch to silicone and porcelain pottery instead of epoxy resin as a packaging material, in order to improve the life of the LED.
3.3 Summary
Overall. The layout of the new package with low thermal resistance, good heat dissipation ability and low mechanical stress is a key technology for the package. The chip junction layout and materials need to be addressed to the epitaxial layer epitaxial layer to the package substrate, package substrate cooling need someone to answer questions to the three aspects of the cooling device. By these three links constitute the heat conduction path of the solid-state lighting source. Which emerge any weak links will make the LED light source destroyed. Junction to ambient thermal conductivity style pattern of conduction, convection, radiation three kinds. This means, in order to enhance the thermal performance and reliability of the power LED to the highest. The three link must accept high coefficient of thermal conductivity of the material.
4. Trends
Eras. Power type LED drive current can be up to 70mA, lOOmA even lA level. Followed the work of the current increase cooling needs someone to answer questions has become a prerequisite for high-power LED to achieve industrialization. In accordance with the aspects of the heat dissipation of the LED device. From the following aspects to improve the thermal performance of high power LED
.
(1) LED produce heat, depending on the internal quantum effect. Among the GaN growth process, improved material layout. Optimizing the growth parameters, was suffering from high-quality epitaxial wafers, device internal quantum efficiency, to cut heat gestation occurred from the foundation, to speed up the chip junction to the thermal conductivity of the epitaxial layer.
(2) select the aluminum-based metal core printed circuit board (MC-PCB), of ceramic, DBC, a composite metal substrate such as a good thermal conductivity material used as the substrate, from the epitaxial layer in order to expedite thermal energy circulated to the heat radiation substrate. MCPCB board by optimizing the thermal default. Or porcelain pottery directly bind to the metal substrate to form a metal-based low-temperature sintering of ceramic (LTCC-M) substrate to be suffering from the thermal conductivity performance. Heating the small expansion coefficient of the substrate.
(3) In order to make the heat energy on the substrate more rapidly profile sheets to the surroundings. Usually chosen good thermal conductivity metal material of aluminum, copper, etc. as a heat sink. Plus compulsory cooling mounted fan and loop heat pipe. Both from the point of view of cost or appearance. LED lighting should not accept external cooling device. Therefore, in accordance with the law of conservation of energy, the use of piezoelectric ceramic pottery as a heat sink, heat vibration style pattern directly into thermal energy consumption will become one of the focus of future research.
(4) For the high-power LED devices, the total thermal resistance is the pn junction to ambient thermal several heat sink thermal resistance and on the road, including the Conceiving LED itself internal heat sink thermal resistance, the internal heat sink to the PCB thermal plastic thermal resistance between the thermal resistance of the thermal glue between the plate, PCB board with an external heat sink, an external heat sink thermal resistance, etc., the heat transfer loop in each of a heat sink on the heat transfer will cause some obstacles Thus, too long-term studies that. Reduce the number of internal heat sink, and acceptance of thin film technology will be essential interface electric hot Shen, the insulation layer is built directly on the metal radiator. Able to significantly reduce the total thermal resistance. Such technology has the potential to become a mainstream After high-power LED cooling package direction.