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led chip package lighting technology development status and key issues to be broken analysis

REVIEW: When someone said LED packaging junior high school students will do, Nichia Chemical smiled. However, to their raw materials and equipment to do an independent LED packaging products, they certainly can not do it. Professionals Kanmen Road, layman watch. In this paper, the development process, product application, research method, technology route and key problems of LED optoelectronic are introduced.

When someone said LED packaging junior high school students will do, Nichia Chemical smiled. Those who say this and those media journalists generally university degree, but to give them raw materials and equipment to do an independent LED packaging products, they certainly can not do it. Professionals Kanmen Road, layman watch. There is always a person in this world, see “one two three” will say that writing is so simple.

LED packaging global leader Nichia Chemical more than ten years tirelessly on the LED packaging technology research, to others do not create, to others have been innovative, well known to do the ultimate.

In response to market demand, Nichia Chemical in March 2015 developed Flip Chip (Flip Chip) LED new technology “direct installation chip” (DirectMountableChip), size is 1010 specifications, is 1mmx1mm, has started in October 2015 Mass production, the future will be gradually into the lighting and LCD applications, estimated 2016 production scale will reach 3 times in 2015. Nichia Chemical specifically mentioned, DMC (Flipchip) products, the current cost is still relatively high, but the future investment in new equipment, the expected cost can be further reduced.

In this paper, the development process, product application, research method, technology route and key problems of LED optoelectronic are introduced.

Technical Development and Status Quo at Home and Abroad

GaN based light emitting diode (LED) as a new generation of environmentally friendly solid state light source, has become the industry’s focus. 1992, the “father of blue,” said Nakamura repair II successfully prepared the Mg doped p type GaN, then in 1993 and 1995 using InGaN GaN heterostructure successfully prepared a high brightness blue LED, And thus gains the 2014 Nobel Prize in Physics.

At present, high power, high brightness white LED has become a hot spot in the development of lighting. Although the luminous efficiency of white LED has reached 170lm W, but its theoretical value of 250lm W there is still a certain gap, so to further improve its luminous efficiency as a power based white LED a key technical problem. In general, there are two ways to improve the luminous efficiency of LED, respectively, to improve its internal quantum efficiency and light extraction efficiency. On the other hand, how to improve the cooling capacity of power LED devices has become another key.

With the LED power increases, especially the development of solid state lighting technology, LED packaging of the optical, thermal, electrical and mechanical structure put forward new and higher requirements. This shows that the study of high light extraction efficiency, low thermal resistance, high reliability, high power LED packaging technology is the practical and the industrialization of the only way.

Flip Chip (Flip Chip), also known as the flip chip packaging technology, IC packaging technology in a mature chip packaging process. Because to meet the requirements of the high performance package, based on flip chip technology, power LED package is considered to be the key technology and development trend of high brightness LED package power type.

In the traditional horizontal and vertical wafer structure, the absorption of the front electrode and the critical angle of total reflection of the GaN Air interface will greatly affect the light extraction efficiency. On the other hand, in the traditional package structure, the heat of the LED chip Through the substrate sapphire (the thermal conductivity is only 38W mK) conduction to the thermal conductivity of the substrate, thermal path is longer, so the chip thermal resistance. The use of flip chip flip chip structure of the structure, the sapphire substrate chip inversion, the chip directly welded to the thermal conductivity of the substrate, the electrode at the bottom of the mutual connection with the substrate, it avoids the traditional packaging chip to bring the difference between the level difference Difficult problem. The outgoing light is emitted from the transparent sapphire substrate at the top of the chip.

On the one hand, this kind of flip chip structure can increase the light extraction efficiency while avoiding the obscuration of the outgoing light from the metal electrode, and also increases the total reflection critical angle at the light exit interface. On the other hand, the micro Metal, or ceramic substrate directly, resulting in shorter distance of current flow, lower resistance, lower heat generation, and lower thermal resistance, which can improve the heat dissipation capacity. In addition, because there is no positive light at the impact of gold, white LED products, the phosphor coating process is relatively easy to implement, especially the phosphor coating process, the color consistency of their products will be greatly improved. Compared with traditional packaging, flip chip structure also has a simpler packaging process, lower packaging costs, higher yields and other advantages of the package.

The flip chip structure consists of substrate, UBM, solder balls and chips. The way in which the chip is connected to the substrate is often Eutectic welding. Eutectic welding, also known as low melting point alloy welding, with high thermal conductivity, connection resistance, uniform heat dissipation, high welding strength, process consistency and many other advantages, it is particularly suitable for high power, high thermal requirements of power devices Of the welding. Its basic feature is: two different metals can be far below the temperature of their melting point in accordance with a certain proportion of the formation of alloys. Common flip chip LED eutectic metal layer is generally Au Sn alloy (Au80Sn20), Au80Sn20 eutectic temperature of 282 ℃.

Eutectic welding is divided into direct welding and flux welding. Direct welding is the eutectic alloy at the bottom of the chip in the eutectic machine under direct thermal pressure eutectic, eutectic pressure does not exceed 50g. The way no flux, no cleaning process, high yield, but a one time investment. Another eutectic flux eutectic, according to the flip chip LED chip electrode size, in the substrate pre plated with Au Sn alloy layer, and then point the substrate flux, the LED chip will be fixed to the substrate corresponding to the alloy Layer, the industrial production process can be used for an ordinary solid crystal dispensing head can be, and then into the reflow furnace, the alloy melt to form eutectic welded joints. The amount of eutectic flux is more difficult to control, and the reflow curve should be explored according to different reflow furnace, and it is difficult to control its stability. The advantage is that the process investment is small.

Two eutectic methods are required to support gold tin melting temperature (greater than 320 ℃), requires the substrate surface roughness of the gold plated layer is less than 2 microns, or will cause melting eutectic material can not completely fill the interface uneven place, not only will Increase the thermal resistance of the device, and even make the combination of chip and substrate is not stable, affecting the quality of packaging.

In addition, the new solid crystal material also appeared. In January 2014, Dexerials demonstrated conductive adhesives with a conductive particle size of only 5 microns. After the conductive paste was applied to the substrate, the P N poles were completely insulated, and the conductive particles were ruptured to complete current conduction. The current eutectic of Au Sn alloy requires an operating temperature of 300 ° C or more. The use of LEP conductive paste and the operating temperature is controlled at about 180 ° C, so that the thermal conductivity of the substrate is increased, and the glass substrate and the PET substrate can be used. As a result, cost savings can be achieved from the wafer, substrate and equipment stages, while LED manufacturers only need to purchase a hot press with LEP conductive adhesive, which is estimated to reduce overall costs by about 30% compared to the Au Sn eutectic approach.

In 2001, the flip chip LED by Wierer, who first proposed, the light extraction efficiency is increased to 1.6 times the structure of the installed. In 2006, Shchekin et al. Fabricated a flip chip LED chip based on the AlGaInN LED chip, which uses laser lift off technology to remove the sapphire substrate and thin the intrinsic GaN material under the n GaN. The external quantum efficiency of the structure is 36%, driven by 350mA current. Focused on improving the light extraction efficiency, improve thermal performance, flip chip technology, etc., GaN based flip chip LED do a lot of academic research.

At the same time, the industry is also closely follow up. Some manufacturers to flip chip technology, based on the introduction of a chip level CSP packaging products. Such as: Taiwan’s Crystal Power introduced the latest non encapsulated chip technology ELC, Taiwan’s Taiwan Semiconductor product packaging solid state lighting package PoD, Philips Lumileds introduced LUXEO NFlip Chip, LUXEONQ, CREE’s XQ B, XQ ELED products. Samsung recently introduced the latest flip chip products, including the power LM131A, high power LH141A and downlight modules.

Research methods and technical routes

The basic research ideas are: LED model design cooling capacity of the software simulation, analysis and optimization an optical design of the LED flip chip welding coated phosphor LED product molding.

(1) the selection of materials standards. The choice of materials should be based on the need to meet certain physical properties: such as welding materials, welding stability, thermal conductivity and conductivity, packaging materials, light transmission, thermal stability, resistance to external forces and hardness, density, refractive index uniformity And stability, water absorption, turbidity, the maximum long term working temperature, anti static and so on.

(2) package design and preparation of support

① thermal conductivity of high thermal conductivity substrate material, the choice of materials are aluminum (thermal conductivity of 231W mK), copper (385W mK), ceramic materials, aluminum nitride (320W mK), silicon (191W mK) ;

② In order to avoid the fusion of the two electrodes in the eutectic, according to the size of the chip and the electrode design of a suitable height and size of the middle of the insulation barrier.

(3) LED product model structure of the cooling of the software simulation, analysis and optimization of structural parameters. According to the design of the sample model, the simulation software is used to analyze the simulation results, and the LED model is optimized by changing the structure parameters to obtain the optimized LED product model with excellent heat dissipation capacity.

(4) to conduct an optical design of LED. The optical design simulation software (such as TracePro, etc.) for flip chip, package stand, mold, the lens design. The purpose is to achieve the best light efficiency.

(5) flip chip eutectic welding technology

① In the bottom of the package bracket, solder paste to the external circuit of the positive and negative, the location of the barrier layer has a certain degree of insulation bonding layer to avoid the fusion of the two, the height is higher than the solder paste height.

led chip package lighting technology development status and key issues to be broken analysis

② The flip chip positive and negative poles are precisely aligned with the circuit of the package holder and bonded to the bottom of the bracket. The flip chip is firmly welded to the holder by the eutectic bonding process and the temperature is controlled.

(6) phosphor coating technology. Taking into account the flat phosphor coating process can achieve the phosphorous soil concentration, thickness and shape controllability. Uniformity of the spatial distribution of the light spot and the uniformity of the chromaticity and luminance of the tube.

(7) Molding or lens molding, into the product. According to the primary optical design structure, a transparent epoxy resin or silicon resin is used to mold an arc traveling lens above the light output of the chip to increase the light extraction efficiency.

The key issues to be addressed

(1) It is one of the key technologies to solder the chip on the substrate firmly through eutectic welding technology. Specifically relates to the electrode alignment, eutectic process temperature control, solid crystal strength, chip and substrate surface roughness.

(2) to ensure that the positive and negative electrodes in the process of fusion will not become a key issue. In particular to how to design and fabricate an intermediate barrier layer to achieve an effective barrier between positive and negative electrodes.

(3) phosphor coating technology, mainly phosphor coating uniformity, thickness and shape.

(4) the light efficiency, is a technically important one, to avoid the manufacturing process, especially the welding temperature on the flip chip chip quantum well damage, resulting in lower light efficiency.

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