Cheaper and Brighter: Innovations in Thermal Radiation

zhuyuan1117 · 发表于 2010-1-6 13:35:21

Too expensive and too dim: the two major problems that light-emitting diode (LED) lighting has faced. Manufacturers are resolving them through continuing innovation, slashing cost while boosting emission efficiency.

"When you mention that it will take three years to depreciate initial installation costs, even firms seriously considering LED lighting start to hesitate," says a source at one LED lighting manufacturer. At present, LED lighting prices are several to tens of times higher than conventional lighting, raising the bar pretty high for corporations and general consumers alike. The cost of these products is, of course, largely due to the price of the key component: the LED chip/package. Cost-control measures, though, are also essential in peripherals such as power supply and case.

When it comes to brightness, as discussed in Part 1, total efficiency is still not as high as that achieved by high-frequency (HF) fluorescent tubes. LED chip manufacturers are already boosting emission efficiency through innovations in chip and package architecture, materials and more, because high emission efficiency will mean lower packaging costs, reduced heating and other advantages. In addition to package emission efficiency, heat radiation performance is another crucial factor. If LED package heat can be efficiently released to minimize heating, service life would be increased even for the same chip/package, and emission efficiency would rise (Fig 1).

Simple, Inexpensive
The biggest obstacle to widespread adoption of LED lighting is price. The need to slash cost is especially strong in products that are also marketed to general consumers, such as the bulb-type LED lights sold at mass merchandisers. We took a look at the bulb-type LED lights from Toshiba Lighting & Technology Corp of Japan to see how they dropped costs.

The company's products use a diecast Al case, with the LED package mounted on a metal-base substrate on the upper side of the case. An acryl dome is placed on the very top to diffuse the light. The aluminum external case contains a resin case holding the power supply circuit board (Fig 2). On the outside of the case there are 16 heat radiation fins, allowing heat generated by the LED package, power supply, etc, to escape.

There are no major differences in shape between the old and new models, but the new design omits the paint and uses the bare aluminum surface, and also eliminates the decorative ring. Both were cost-control measures.

A look inside the case, though, shows major differences in the power supply circuit. In the old design, the rear of the circuit board inside the resin case was packed with filler, but in the new design the board is large, and no filler is used.

"We had problems shrinking the power supply in the old design, so we reviewed it and simplified," explains Koji Sato, senior manager, LED Business Div, LED Business Planning Dept of Toshiba Lighting & Technology. Engineers thinned down the surface contacting the metal base substrate to expand the space for the power supply, and the more volume they gained the cheaper the design became. Another cost-cutting measure was switching from glass epoxy resin to paper phenol for the power supply circuit board. The thinner case also contributed, slightly, to lower material cost.

Both old and new LED packages are manufactured by Nichia Corp of Japan, but while the old package mounts six chips connected in parallel, the new one has only three chips in series. This design change reduces power supply current, thereby reducing heating in the power supply, and eliminating the need for the filler in the case to act as a conductive channel for power supply heat.

Lining up 400 LEDs
Rohm Co Ltd of Japan is working to reduce costs through innovations in LED packaging. The firm's downlight, for example, uses an array of 400 LED packages, each with a 1.6 x 0.8mm footprint and an output of 0.1W (Fig 3).

Most recent LED lighting uses "high-intensity" LED packages of 1W or above, and the trend is to minimize the number of LEDs mounted. Rohm, however, instead found it was cheaper to mount a lot of low-intensity LEDs, according to

Hideaki Shikata, group general manager, Lighting Div, Discrete Module Production Headquarters at the firm.
The packages are being volume produced for general-purpose application, making them quite inexpensive, and because so many of them are mounted in each package, average variation in package characteristics is minimal. "Unless high-intensity LED packages are available in volume production at low cost, we have no intention of using them," reveals Rohm's Shikata.

The company is also developing other types of LED lighting, including bulb-type, baselight and linear. All use 0.5W-output LEDs with 2 x 4mm footprints; the bulb-type light uses over 70 such LEDs, while the baselight, linear and other designs use over 400.

As it turns out, mounting lots of little LEDs together offers advantages not only in terms of cost, efficiency and the like, but also functionally, as illumination. LEDs output highly directional light, which can alienate some users. If the chips are aligned as shown in Fig 3, though, areal light sources can be achieved even without using diffusers or light guides, for example. Areal lighting provides excellent illumination distribution, with no overlapping shadows.

Heat Radiation Critical
Rohm was able to mount so many low-intensity chips at low cost because they use their own packages, and handle the mounting on their own existing equipment. Many other manufacturers tend to use high-intensity chips to reduce package procurement, mounting and other costs.

The big problem here is with heat radiation. When high-intensity chips are used, heat sources are concentrated, making heating common. "I think we'll reach the limits of natural cooling in another two or three years," warns Hisashi Hattori, senior partner/PhD of Multi-task Company Ltd of Japan. Demand for even brighter lights, though, continues to drive an increase in absolute heat output.

When LEDs get hot, forward voltage and emission efficiency drop, and service life shortens. High-intensity packages tend to get quite hot, and also require expensive heat-resistant materials, which further pushes up the cost. In other words, heat radiation is a critical factor in efficiency, cost and life.

As the number of high-intensity packages increases, more and more LED lighting designs use metal base substrates, but it is simultaneously becoming increasingly difficult to secure adequate thermal radiation with these substrates. As a result, a variety of new, radiation-efficient structures have been proposed for substrates.

Denki Kagaku Kogyo KK of Japan has developed the Advanced Grade Solid-bump Process (AGSP) substrate with enhanced heat radiation performance, based on technology developed by Daiwa Kougyo Co Ltd of Japan. According to Naomi Yonemura, technical general manager, Electronic Products Department, Electronic Materials Div, Electronic Materials Business Unit of Denki Kagaku Kogyo, "While the radiation performance of the metal base substrate has been enhanced by improving thermal conductivity, a physical property, we concentrated on the mechanical structure."

The structure embeds copper bumps in an insulating resin with high thermal conductivity, channeling the heat from the LED through the bumps to the outside of the package (Fig 4). Heat radiation is efficient as long as the heat sink, case, etc, are in physical contact with each other. "A metal base is ample for a 40W-equivalent LED, but when the LED output gets up to 100W-equivalent, you need AGSP," says Denki Kagaku Kogyo's Yonemura. The copper bumps can be formed with diameters of about 4mm, large enough for LED chip mounting.

Ceramic substrates with silver paste printed on an aluminum nitride (AlN) base, for high thermal conductivity, are commonly used as substrates for high-intensity LEDs, but AlN is expensive to manufacture. While the thermal radiation performance of AGSP is not as high as that of a ceramic substrate, it offers good performance for a relatively low cost.

Denki Kagaku Kogyo is evaluating volume production now, aiming for full-scale commercial operation in about 2010.

Further cost reduction is essential, though, because even though it is relatively cheap, it is still about double the cost of metal base. The firm hopes to refine its bump-forming technology, and leverage volume-production effects to drop the price to about the level of metal base solutions.

Nippon Tungsten Co Ltd of Japan has taken the same approach, tweaking package substrate to improve thermal radiation, in its LED package. Their solution integrates a copper leadframe with a special ceramic which can be sintered at low temperature. Heat is channeled through the leadframe, while the high heat resistance of the ceramic package assures durability.

A source at the firm explains that the ceramic material is inexpensive, yielding thermal characteristics equivalent to AlN at about half the cost. Substrate packaging freedom is high because a copper leadframe is used, making it possible to mount flip-chips as well. Performance evaluation and volume-production investigation are under way now, with release planned for 2010.

Better Package Structure
In addition to enhancing thermal radiation performance, other approaches being explored as means of boosting emission efficiency include changes to the structures of chips, packages, etc, and the use of new materials.

High-intensity LED packages are becoming more widely used in lighting applications, whether they mount multiple tiny chips in a single package, or only a single large chip. The light path from generation to emission is longer in the large chips, and attenuation therefore reduces efficiency. This problem has been resolved recently by modifying the chip architecture to form a gallium nitride (GaN) layer on the substrate.

OSRAM Opto Semiconductors GmbH of Germany, for example, uses a laser to strip the sapphire substrate off a GaN-based chip, then sandwiches it to a germanium wafer using its "ThinGaN" technology. ThinGaN LEDs emit 97% of generated light from the chip surface, achieving high optical utilization.

The firm has also added a phosphor distribution method called "chip level conversion" (CLC), as shown in Fig 5. Phosphors are coated directly onto the chip emission surface, taking advantage of the fact that the ThinGaN design emits directly from there. Blue light and yellow light are emitted from the same surface. Light distribution is especially clean when combined with a lens.

Conventional GaN-based blue LEDs emit light from the sides of the emission layer. This requires the use of a reflector when combined with a lens, and the reflection reduces optical utilization efficiency. In addition, while blue light is only emitted from the chip, yellow light is emitted from the encapsulation resin, which has phosphors embedded in it. This means the two colors have light sources of different sizes, often leading to color variation.

The new Golden Dragon Plus package released in 2008 from the same firm features enhanced encapsulation resin shape. Resin is transparent, with the surface formed into a convex lens (Fig 5). The lens-shaped surface minimizes total reflection at the resin-air interface, boosting the light output from the resin by 10% to 15%, according to a source at the firm. With a flat surface, light emitted from the chip with a shallow angle of incidence is reflected, attenuating inside the resin mass.

Improvements in chip crystalline structure, layer structure, etc, are also being made to boost quantum efficiency.

Hidenaga Warashina, senior manager, Business Development, Opto Semiconductors of OSRAM Ltd Japan claims, "We will be able to supply chips achieving emission efficiencies of 130lm/W in the first half of 2010." Details have not been announced, but continuing development of ThinGaN can be expected to provide significant improvement in emission efficiency.

Nano-Etching, Cheap
SCIVAX Corp of Japan proposes using nanoprinting technology to etch details into chip sides, layer interfaces, etc, and thereby boost optical output efficiency. Three-dimensional (3D) patterns only several hundred nm in size refract the light, preventing the light from the emission layer from reflecting at the interface (Fig 6).

The idea of nano-etching the chip surface was proposed quite some time ago, and some LED chip manufacturers have already adopted it. Norimichi Okuda, vice president of SCIVAX Corp, however, explains they can deliver higher emission efficiency at low cost: "Traditionally it was difficult to process large areas. With nanoprinting, however, the cost of processing a single LED wafer is only a few hundred yen."

Resin is spin-coated onto the p-type GaN layer surface of the LED wafer, and then a 3D-patterned silicon transfer die pressed against the resin to transfer the pattern. Reactive ion etching strips the GaN from the p-type GaN depressions to reveal the 3D pattern. Island height and diameter are about 200nm, but must be optimized to match specific characteristics such as light wavelength and chip composition.

Numerical simulation results indicate the technique could boost energy emitted from the chip by 20% to 30%. The same process could also be used on the sapphire substrate before the emission layer is formed, suppressing reflection at the interface between the sapphire and the buffer film. In addition, says a source at the firm, the process also reduces lattice defects during GaN crystal growth.

Triple Emission Efficiency
Mitsubishi Chemical Corp of Japan, which recently announced its new entry into the LED lighting sector, hopes to achieve a major increase in emission efficiency using chips made with a new substrate material called m-plane GaN.

The firm acquired the LED operation of Mitsubishi Cable Industries Ltd of Japan in 2008, and entered into a licensing agreement with Cree Inc of the US in Jan 2009 for m-plane GaN technology. The company plans to use the technology to supply white LEDs combining outstanding color characteristics with high emission efficiency, cooperating with lighting system manufacturers to make and market complete fixtures.

LED chips using m-plane GaN substrates use GaN crystal cut on the non-polar m-plane, with GaN layers grown on top. Engineers believe that it can provide performance superior to the sapphire substrate in common use now, according to Shin Kawana, deputy general manager, Business Planning Group, SSLD Department, Information and Electronics Div at the firm: "With m-plane GaN it should be possible to roughly triple emission efficiency, which means 200lm/W to 300lm/W."

The development of m-plane GaN substrate LEDs is under way at other companies, universities, etc, as well, but problems with poor productivity and high cost have been cited. Mitsubishi Chemical uses a relatively low-cost liquid-phase growth technique. The firm's Kawana revealed they hope to cut manufacturing cost to the level of blue LED chips by about 2015.

The company currently uses a conventional LED package, with a combination of blue LED and yellow phosphors (Fig 7). The next step will be to apply the firm's expertise in phosphors to combine red and green phosphors with the blue LED, creating an LED package with outstanding color characteristics. This type of package generally suffers from low emission efficiency, but the company hopes to boost that to 100lm/W by 2010 by enhancing the phosphors.

The m-plane GaN substrate is a good choice for future generations, too. Engineers are working on a package for high emission efficiency and good color characteristics by combining the substrate with a near-ultraviolet LED and red, green and blue phosphors. Sample-shipment is scheduled to begin before the end of fiscal 2009, with volume production slated for 2011.

Cheaper and Brighter: Innovations in Thermal Radiation

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