For the existing LED light efficiency level, about 80% of the input power is converted into heat, and the LED chip area is small, so the chip heat dissipation is the key problem that the LED package must solve.Mainly including chip layout, packaging material selection (substrate material, thermal interface material) and technology, heat sink design.

The thermal resistance of LED package mainly includes internal thermal resistance and interface thermal resistance of materials (heat dissipation substrate and heat sink structure).The role of the radiating substrate is to absorb the heat generated by the chip and conduct it to the heat sink to achieve heat exchange with the outside world.Commonly used thermal substrate materials include silicon, metals (such as aluminum, copper), ceramics (such as AlN, SiC) and composites.For example, the third generation LED of Nichia company USES CuW as the substrate, and the 1mm chip is inverted on the CuW substrate, which reduces the packaging thermal resistance and improves the luminous power and efficiency.Lamina Ceramics has developed low temperature co-fired ceramic metal substrates and LED packaging technology.In this technology, a high-power LED chip and a corresponding ceramic substrate are firstly prepared for eutectic welding, and then the LED chip is directly welded to the substrate.Due to the integration of eutectic welding layer, electrostatic protection circuit, drive circuit and control compensation circuit on the substrate, not only the structure is simple, but also due to the high thermal conductivity of the material, the thermal interface is less, greatly improve the heat dissipation performance, for the high-power LED array package solution.The copper clad ceramic plate with high thermal conductivity developed by Curmilk company in Germany is made of ceramic substrate (AlN or) and conductive layer (Cu) sintered under high temperature and high pressure. No binder is used, so it has good thermal conductivity, high strength and strong insulation.The thermal conductivity of aluminum nitride (AlN) is 160W/mk and the coefficient of thermal expansion is (which is similar to that of silicon), thus reducing the thermal stress of the package.

The results show that the packaging interface has a great influence on the thermal resistance.For example, a well-contacted interface at room temperature may have interfacial gaps at high temperatures, and the buckling of the substrate may affect bonding and local heat dissipation.The key to improve the LED package is to reduce the interface and interface contact thermal resistance, enhance the heat dissipation.Therefore, the selection of thermal interface material (TIM) between chip and thermal substrate is very important.The commonly used TIM for LED packaging is conductive adhesive and thermal adhesive. Due to the low thermal conductivity, it is generally 0, 5-2 and 5W/mK, resulting in high interface thermal resistance.However, using low temperature or eutectic solder, solder paste or conductive adhesive with nanoparticles as the thermal interface material can greatly reduce the thermal resistance of the interface.

During the use of LED, the losses caused by the emission of photons generated by the radiation combination mainly include three aspects: the internal structure defects of the chip and the absorption of materials;The reflection loss caused by the refractive index difference of the photon at the exit plane;And the total reflection loss caused by the incident Angle greater than the critical Angle of total reflection.As a result, much light cannot escape from the chip to the outside.By coating the chip surface with a transparent rubber layer with a relatively high refractive index (potting rubber), the loss of photons in the interface is effectively reduced and the efficiency of taking light is improved because the rubber layer is between the chip and the air.In addition, the role of potting adhesive also includes the mechanical protection of the chip, stress release, and as a photoconductive structure.Therefore, the requirements of high light transmittance, high refractive index, good thermal stability, good liquidity, easy to spray.In order to improve the reliability of LED packaging, the potting adhesive is also required to have the characteristics of low moisture absorption, low stress and aging resistance.The commonly used sealant includes epoxy resin and silica gel.Silica gel is better than epoxy resin because of its high light transmittance, high refractive index, good thermal stability, low stress and low moisture absorption.The results show that increasing the refractive index of silica gel can effectively reduce the photon loss caused by the refractive index physical barrier and improve the external quantum efficiency.As the temperature increases, the thermal stress inside the silica gel increases, resulting in a decrease in the refractive index of the silica gel, thus affecting the LED light efficiency and light intensity distribution.

The action of phosphor powder is in light color compound, form white light.Its characteristics mainly include particle size, shape, luminous efficiency, conversion efficiency, stability (heat and chemistry), among which luminous efficiency and conversion efficiency are the key.Studies have shown that as the temperature rises, the quantum efficiency of phosphors decreases, the light output decreases, and the radiation wavelength will change, which will cause the change of color temperature and chroma of white LED, and the higher temperature will accelerate the aging of phosphors.The reason is that the phosphor coating is made of epoxy or silica gel and phosphor powder, the heat dissipation performance is poor, when subjected to violet or ultraviolet radiation, easy to occur temperature quenching and aging, so as to reduce the luminous efficiency.In addition, the thermal stability of sealant and phosphor at high temperature is also a problem.Because the size of common phosphors is more than 1um, the refractive index is greater than or equal to 1, 85, and the silica gel refractive index is generally about 1, 5.Due to the mismatch of refractive index between them and the fact that the size of phosphor particles is much larger than the light scattering limit (30nm), there is light scattering on the surface of phosphor particles, which reduces the light output efficiency.By adding nano phosphors into silica gel, the refractive index can be increased to more than 1 and 8, the light scattering can be reduced, the LED light output efficiency (10%-20%) can be improved, and the light color quality can be effectively improved.

The traditional coating method of phosphor powder is to mix the phosphor powder with the sealant and then apply it on the chip.Due to the inability to accurately control the coating thickness and shape of the phosphor powder, the color of the outgoing light is inconsistent, resulting in blue or yellow light.However, the Conformal coating technology developed by Lumileds can achieve the uniform coating of phosphors and ensure the uniformity of light color.However, when the phosphor is directly coated on the surface of the chip, the luminous efficiency is low due to the light scattering.In view of this, Rensselaer institute has proposed a Photon scattering Extraction method (SPE), which not only improves the device reliability, but also greatly improves the light efficiency (60%) by placing a focusing lens on the chip surface and placing the glass sheet containing phosphor powder in a certain position away from the chip.

In general, in order to improve the luminous efficiency and reliability of LED, the packaging adhesive layer has a tendency to be gradually replaced by high-index transparent glass or microcrystalline glass, etc. By adding or coating phosphors on the glass surface, not only the uniformity of phosphors is improved, but also the packaging efficiency is improved.In addition, reducing the number of optical interfaces in LED direction is also an effective measure to improve the luminous efficiency.

After more than 40 years of development, LED packaging technology and structure has gone through four stages.

Pin packaging is commonly used 3-5mm packaging structure.Generally used for small current (20-30ma), low power (less than 0, 1W) LED packaging.Mainly used for instrument display or indication, large scale integration can also be used as a display screen.Its disadvantage is that the package thermal resistance is large (generally higher than 100K/W), short life.

Surface assembly technology (SMT) is a packaging technology that can be directly pasted and welded to a specified location on the PCB surface.Specifically, it is to use specific tools or equipment to point the chip pin at the pattern of the solder pad precoated with adhesive and solder paste, and then directly attach to the PCB surface without drilling the mounting hole, after wave soldering or reflow soldering, so that the device and the circuit to establish a reliable mechanical and electrical connection.SMT technology has the advantages of high reliability, high frequency characteristics, easy to realize automation, and so on.

COB is the abbreviation of Chip On Board. It is a packaging technology that directly pastes the LED Chip to the PCB Board by glue or solder, and then realizes the electric interconnection between the Chip and the PCB Board through lead bonding.PCB board can be a low cost fr-4 material (glass fiber reinforced epoxy), or a high thermal conductivity metal or ceramic matrix composite (such as aluminum or copper coated ceramic substrate).Lead bonding can be used under high temperature thermosonic bonding (gold wire ball welding) and under normal temperature ultrasonic bonding (aluminum wedge welding).COB technology is mainly used for LED package of high-power multi-chip array. Compared with SMT, it not only greatly improves package power density, but also reduces package thermal resistance (generally 6-12w /m, K).

SiP(System in Package) is a new Package integration method developed on the basis of System on Chip(SOC) in recent years to adapt to the portable development of the whole machine and the requirements of System miniaturization.For sip-led, not only can multiple light-emitting chips be assembled in one package, but also various types of devices (such as power supply, control circuit, optical microstructure, sensors, etc.) can be integrated together to build a more complex and complete system.Compared with other packaging structures, SiP has the advantages of good process compatibility (using the existing electronic packaging materials and processes), high integration, low cost, more new functions, easy block testing and short development cycle.According to the technology type, SiP can be divided into four types: chip cascade, module, MCM and 3D (3D) package.

Currently, to replace incandescent and high-voltage mercury lamps, high-brightness LED devices must increase the total flux, or available flux.The increase of luminous flux can be realized by improving the integration degree, increasing the current density, using large size chip and so on.All these will increase the power density of LED, such as poor heat dissipation, will lead to high junction temperature of LED chip, thus directly affecting the performance of LED devices (such as reduced luminous efficiency, red shift of outgoing light, reduced service life, etc.).Multi-chip array packaging is currently one of the most feasible solutions for obtaining high-light flux, but the density of LED array packaging is limited by price, available space, electrical connections, and especially heat dissipation.Because of the high density integration of luminescent chip, the temperature on the radiating substrate is very high.Commonly used heat sink structure is divided into passive and active heat dissipation.In general, fins with high finned coefficient are used for passive heat dissipation, which dissipates heat to the environment through natural convection between fins and air.The scheme has the advantages of simple structure and high reliability, but because of the low natural convection heat transfer coefficient, it is only suitable for low power density and low integration.For high-power LED package, active heat dissipation must be adopted, such as fin + fan, heat pipe, liquid forced convection, microchannel refrigeration, phase change refrigeration, etc.

In terms of system integration, Taiwan new strong light electric company has developed 72W, 80W high-brightness white LED light source by using system encapsulation technology (SiP) and efficient heat dissipation module via fin + heat pipe.Due to the low thermal resistance of the package (4, 38℃/W), when the ambient temperature is 25℃, the LED junction temperature is controlled below 60℃, thus ensuring the service life and good luminous performance of the LED.Huazhong university of science and technology adopted COB packaging and micro-spray active cooling technology to package the 220W and 1500W super power LED white light source.

Four, packaging production technology

Wafer bonding technology refers to the fabrication and encapsulation of Chip structure and circuit are all carried out on the Wafer. After the encapsulation is completed, cutting is carried out to form a single Chip.The corresponding Die bonding refers to that after the chip structure and circuit are completed on the chip, the chip is cut to form a Die, and then the individual chip is packaged (similar to the current LED packaging process).Obviously, the chip bonding package is more efficient and of higher quality.Since the packaging cost accounts for a large proportion of the manufacturing cost of LED devices, changing the existing packaging form of LED (from chip bonding to chip bonding) will greatly reduce the packaging manufacturing cost.In addition, chip bonding and packaging can also improve the cleanliness of LED device production, prevent the damage of chip scribing and splicing before bonding to device structure, improve the package yield and reliability, so it is an effective means to reduce the packaging cost.

In addition, for high-power LED Packaging, packaged-less Packaging should be adopted as far as possible in the chip design and Packaging design process, and the Packaging structure should be simplified to minimize the number of thermal and optical interfaces, so as to reduce the thermal resistance of Packaging and improve the light output efficiency.

V. package reliability testing and evaluation

The failure modes of LED devices mainly include electrical failure (such as short circuit or circuit failure), optical failure (such as yellowing of potting adhesive caused by high temperature, deterioration of optical properties, etc.) and mechanical failure (such as lead fracture, disconnection, etc.), which are related to the packaging structure and process.The service life of LED is defined by mean failure time (MTTF). For lighting purposes, it generally refers to the service time when the output flux attenuation of LED is 70% of the initial value (for display purposes, it is generally defined as 50% of the initial value).Because of the long life of LED, the method of accelerating environmental test is usually adopted for reliability test and evaluation.The test contents mainly include high temperature storage (100℃, 1000h), low temperature storage (-55℃, 1000h), high temperature and high humidity (85℃/85%, 1000h), high and low temperature circulation (85℃ ~ -55℃), thermal shock, corrosion resistance, solubility resistance, mechanical shock, etc.However, accelerated environmental testing is only one aspect of the problem, and the prediction mechanism and method of LED life is still a difficult problem to be studied.