UV LED Encapsulated, global leaders, NICHIA chemical industry, for more than a decade research of LED encapsulation technology, create other not to do, do what others had already innovation, known as perfectly.

In response to market demand, NICHIA chemistry developed Flip-Chip LED new technology in March, 2015 --- “Direct Mountable Chip”, which its size is 1010 standard, that is 1mm*1mm. The mass production has been started in October 2015. In the future, lighting and liquid crystal application will be imported. The estimated production scale in 2016 will be three times that of 2015. There is a special mention of chemistry --- DMC (Flip Chip), which at present the cost is relatively high, but the expected cost can be further reduced in the future under the new equipment investment.

The following is the research on the technology of the power GaN LED photoelectric device overlaying welding industry, including the UV photoelectric about development course, product application, research method, technical route and key problem solving.

Domestic and Foreign Technology

GaN UV LED, as a new generation environment-friendly solid-state light source, has become the focus of the industry. In 1992, Nakamura, known as the father of blue light, has been successfully prepared Mg with Type P GaN. Subsequently adopted InGaN / GaN high UV LED luminance was prepared by hetero-structure in 1993 and 1995. He won the Nobel Prize for physics in 2014.

At present, high power, high brightness white LED has become a hot spot in the field of lighting. Although white LED luminous efficiency has reached 170lm/W, but from its theoretical value 250lm/W has a certain gap. Therefore, it is a key technical problem to further improve its luminous efficiency to power white LED.

Generally speaking, there are two ways to improve the luminous efficiency of LED, which is to improve the internal quantum efficiency and the efficiency of light extraction. On the other hand, how to improve the heat dissipation is another key to the development of power LED devices.

With the increase of LED power, especially the development of solid-state lighting technology, new and high requirements are proposed for the optical, thermal, electrical and mechanical structure of LED packaging. It can be seen that the encapsulation technology of high efficiency, low thermal resistance and high reliability is the only way for high-power LED to become practical and industrialized. Flip-Chip Technology, named anti-crystal encapsulation, is a mature chip encapsulation technology in IC encapsulation technology. Because of the requirements of high performance packaging, the power LED encapsulation based on the coating technology is considered to be the key technology and development trend of encapsulated power type with high brightness LED.

In the traditional horizontal and vertical wafer structures, the absorption of the positive electrode and the total reflection critical Angle of the GaN-Air interface will greatly influence the efficiency of the optical extraction. On the other hand, in the traditional packaging structure, the heat of the LED chip needs to be transmitted to the conductive substrate via the substrate sapphire (its thermal conductivity is only 38W/m.K), and the thermal resistance of the chip is larger. The flip chip technology and the inversion structures, the sapphire substrate inverted chip, chip welding directly on the thermal conductive substrate and electrode and the substrate are connected at the bottom, also avoids the traditional packaging chip height difference lineup of difficult problems. At this point, the light comes out from the transparent sapphire substrate at the top of the chip. On the one hand, it avoids the shielding of the metal electrode, and also increases the total reflection critical Angle of the optical interface, so it can effectively improve the efficiency of optical extraction. Metal electrode, on the other hand, micro convex point and high thermal conductivity of silicon, metal or ceramic substrate, such as direct contact, the current flow is shortened, decreased resistance, heat quantity is reduced, and the combination of this makes low thermal resistance, is a good way to increase the cooling capacity. In addition, since there is no positive out light gold, white LED products of phosphor coating process is relatively easy to implement, especially the phosphor powder coating process, the product of the light color consistency will be greatly improved. Compared with the traditional packaging, the inversion structure has the advantages of simpler packaging process, lower encapsulation cost and higher packaging yield. The overlaying structure consists of substrate, UBM, welding ball and chip. The method of connecting the chip to the substrate is often used Eutectic-Welding technology.

Eutectic-Welding, called low melting point alloy welding, has many advantages such as high thermal conductivity, small connection resistance, uniform heat dissipation, high welding strength and good process consistency. Therefore, it is especially suitable for welding of power devices with high power and high heat dissipation requirements. The basic feature is that two different metals can form an alloy at a fraction of the temperature of each melting point. The common crystalline metal layer of common inversion LED is usually as Au/Sn alloy (Au80Sn20), which the temperature is 282℃. Eutectic welding is divided into direct welding and flux soldering. Direct welding is a chip that has a eutectic alloy at the bottom directly under the eutectic and the eutectic pressure is no more than 50g. This kind of method is helpless flux, clean technology, high yield, but one-time investment big. Another way of eutectic flux eutectic, electrode size according to flip the UV LED chips, on the base board Au/Sn alloy plating layer in advance, and then point flux on base board, the LED chip is fixed on the alloy layer of base slabs should, in the process of industrial production can use ordinary solid crystal machine in a dispensing head, add the reflow oven, make form the eutectic alloy melt welding welding joint. It is difficult to control the amount of eutectic flux in this process, and the backflow curve should be explored according to different reflux furnace, and it is difficult to control its stability. The advantage is that the process is less invested.

Two methods of eutectic all needs to support sustainable Au/Sn melt temperature (greater than 320 ℃), the gilt of the substrate surface roughness is less than 2 microns, otherwise it will cause melting eutectic material can't completely fill the interface uneven places. It will not only increase the thermal resistance of the device, but also make the combination of the chip and the substrate unstable, affecting the packaging quality. In addition, new solid crystal materials appeared. On January, 2014, Dexerials shows the conductive adhesive, conductive particles only 5 microns in size, the use of conductive adhesive, strong after the substrate, then make the P/N pole insulation completely, the conductive particles burst, to complete the current conduction. The Au/Sn alloy eutectic need operating temperature above 300 ℃, and use the LEP conductive adhesive, near the operating temperature control at 180 ℃, so the heat conducting substrate selectivity more, can use glass and PET substrate.Therefore, the cost can be saved from each stage of the chip, substrate and equipment, and the LED manufacturer only needs to buy the hot press to match the LEP conductive adhesive, and the estimated overall cost will be reduced by about 30% compared with the Au/Sn eutectic.

In 2001, it was first proposed by Wierer et al., that the light extraction efficiency increased to 1.6 times of the structure. Shchekin et al in 2006 in flip chip AlGaInNLED chips on the basis of making the film inversion structure of UV LED chips, the structure with laser stripping technology to remove the sapphire substrate and thinning the n - GaN at the bottom of the eigen GaN materials, the light output power of the LED chip, compared with ordinary inversion structure promoted two times, under the current 350 ma, the structure of the external quantum efficiency reached 36%.

In terms of improving the efficiency of light extraction, improving heat dissipation performance, and inverting welding technology, GaN-based inversion LED has done a lot of academic research work. Meanwhile, industry is following closely.Some manufacturers based on reverse technology, introduced chip level CSP packaging products. For example, Taiwan semiconductor has introduced the latest unpackaged chip technology called ELC, no encapsulation PoD module in Taiwan semiconductor solid state lighting, PhilipsLumileds’LUXEONFlipChip, LUXEONQ, CREE of XQ-B, XQ-ELED and other products. Samsung recently introduced the latest products, including medium power LM131A, high-power LH141A and tube lamp module.

(Please continute to read The Questions of Technology about LED Chip, Encapsulation, Illumination(2))