low thermal resistance packaging technology
In particular, high-power LED package, the key technologies include:
(A) low thermal resistance packaging technology
For the current level of LED light effect, since about 80% of input energy into heat, and the LED chip area is small, therefore heat is the LED chip packaging must be resolved key issues. Include chip layout, packaging material selection (substrate material, thermal interface materials) and processes, heat sink design.
Studies have shown that encapsulation interface is also a great influence on the thermal resistance, if not correctly handle the interface, it is hard for a good cooling effect. For example, a good interface contact at room temperature under high temperature there may be interface gap, the substrate warpage may also affect the bonding and local cooling. The key to improve the LED package to reduce the interface and the interface contact resistance and enhance heat dissipation. Therefore, the chip and heat thermal interface material between the substrate (TIM) selection is very important. TIM commonly used in LED packaging and thermal conductivity for the conductive adhesive gel, due to low thermal conductivity, generally 0.5-2.5W/mK, resulted in a high interface thermal resistance. The use of low temperature or eutectic solder, solder paste, or nano-particles inside the doped conductive adhesive as a thermal interface material, can greatly reduce the interface thermal resistance.
The thermal resistance LED package includes materials (substrate and heat sink thermal structure) within the thermal resistance and the interface thermal resistance. Role is to heat the substrate to absorb heat generated by chips, and conduction to a heat sink to achieve heat exchange with the outside world. Commonly used thermal substrate materials including silicon, metal (such as aluminum, copper), ceramic (eg Al2O3, AlN, SiC) and composite materials. Such as the Nichia company's third-generation LED substrate using CuW to do would be 1mm in CuW flip-chip substrates, reducing the package thermal resistance, enhanced light-emitting power and efficiency; Lamina Ceramics companies have developed low-temperature co-fired ceramic metal substrate, in Figure 2 (a), and the development of the corresponding LED packaging technologies. The technology is suitable for eutectic solder was prepared high-power LED chips and the corresponding ceramic substrate, the LED chip and substrate and then directly welded together. As the substrate integrated layer of eutectic solder, electrostatic protection circuit, drive circuit and control compensation circuit, not only simple in structure, but also because of the high thermal conductivity materials, thermal interface, a small, greatly improving the thermal performance for high-power LED array package propose a solution. Germany Curmilk company develops a high thermal conductivity copper ceramic plate from the ceramic substrate (AlN or Al2O3) and conductive layer (Cu) sintered at high temperature under high pressure without the use of bonding agents, so a good thermal conductivity, high strength, insulation strong, shown in Figure 2 (b) below. In which aluminum nitride (AlN) for the thermal conductivity of 160W/mk, thermal expansion coefficient of 4.0 × 10-6 / ℃ (of thermal expansion coefficient of silicon 3.2 × 10-6 / ℃ equivalent), thereby reducing the package thermal stress.
(B) high rates of take-ray structure and process packaging
In the LED using the process, the radiative recombination photons produced by the loss generated when firing outwards mainly include three aspects: the internal structure of chip defects and material absorption; photons in the outgoing interface, the refractive index difference due to reflections caused by the loss; as a result of angle of incidence greater than the total reflection critical angle of total reflection caused by the loss. Therefore, a lot of light can not reach all parts of the external from the chip out. Through the chip surface coated with a layer of relatively high refractive index transparent adhesive layer (Potting), since the adhesive layer is between the chip and the air, thus effectively reducing the loss of photons at the interface and improve the efficiency of taking light. In addition, the role of potting chip also includes a mechanical protection, stress release, and as a light guide structure. Thus, requiring a high transmittance, high refractive index, thermal stability, good fluidity, easy to spray. To improve the reliability of LED package, also called potting with a low moisture absorption, low stress, anti-aging properties. Now commonly used, including epoxy resins and silicone sealant. Silica gel has a light transmission rate due to the high refractive index of large, good thermal stability, stress is small, low moisture absorption characteristics, much better than epoxy resin, high-power LED packages are widely used, but the higher cost. Studies have shown that improving the refractive index silica gel can be effective in reducing the photon refractive index of the physical barrier caused by the loss, improve external quantum efficiency, but the performance silica gel influenced by environmental temperature. As the temperature increases, the silica gel inside the thermal stress increased, leading to lower refractive index of silica gel, thus affecting the LED luminous efficiency and light intensity distribution.
The traditional way is to phosphor phosphor coating with Potting mix, then coated on the chip. As the phosphor coating can not be precisely controlled thickness and shape, leading to inconsistent outgoing light color, blue, or yellowish light appeared biased. The Lumileds, the development of conformal coating (Conformal coating) technology to achieve uniform phosphor coating to protect the light color uniformity, shown in Figure 3 (b). But studies show that when the phosphor coating on the chip surface directly, due to the presence of light scattering, a light efficiency is low. For this reason, the United States Institute Rensselaer proposed a photon scattering extraction process (Scattered Photon Extraction method, SPE), through the surface of the chip layout of a focusing lens, and phosphor-containing glass placed in a certain position from the chip, not only increasing the device reliability, but also improves the optical efficiency (60%), shown in Figure 3 (c).
Phosphor's role is to light and colors combined to form white light. Its characteristics include particle size, shape, light-emitting efficiency, conversion efficiency, stability (thermal and chemical), etc., in which light-emitting efficiency and conversion efficiency is the key. Studies showed that with increase in temperature, reduce the quantum efficiency of phosphor, a light decrease in the wavelength of radiation can also change, causing white LED color temperature, color changes, higher temperatures will accelerate the aging of phosphors. The reason is that phosphor coating by epoxy or silicone and phosphor deployment made of poor heat dissipation, as are purple, or ultraviolet radiation, the temperature of quenching and aging-prone, so light and reducing efficiency. In addition, the high temperature sealant and the thermal stability of the phosphor is also problematic. As the phosphor used for more than 1um in size, refractive index greater than or equal to 1.85, while the silica refractive index of about 1.5 in general. As the refractive index mismatch between the two, and the phosphor particle size is much larger than light-scattering limit (30nm), thus the surface of the phosphor particle light scattering, reducing the efficiency of a light. By silica gel mixed with nano-phosphors, the refractive index increased to 1.8 and above can reduce the light scattering and improve the efficiency of LED Idemitsu (10% -20%), and can effectively improve the quality of light and colors.
Overall, the out to improve the LED light efficiency and reliability, packaging layer are gradually being transparent high refractive index glass or ceramic glass to replace the trend, by doped or coated with phosphor on the inside glass surface, not only improve the uniformity of phosphor, but also improve the efficiency of encapsulation. In addition, the reduction in the direction of LED light out of the number of optical interfaces, but also to improve the efficiency of a light and effective measures.
low thermal resistance packaging technology