THERMAL CHALLANGES IN THE FUTURE GENERATION SOLID STATE LIGHTING APPLICATIONS: LIGHT EMITTING DIODES

Mehmet Arik, James PetroskF, Stanton Weavery

General Electric Company Corporate Research and Development

Energy and Propulsion Technology Laboratories Thermal Systems Laboratory One Research Circle ES-102

Niskayuna, NY 12309 arik@ crd.ge.com

General Electric Company GELcore

6180 Halle Dr Valley View, OH 44125

Jim.Petroski @ gelcore.com

yGeneral Electric Company Corporate Research and Development

Micro and Nan0 Structures Technology Lab Electronic Structures and Materials Program

One Research Circle Bldg. KW, Room 81 432 Niskayuna, NY 12309

ABSTRACT

Light emitting diodes, LEDs, historically have been used for indicators and produced low amounts of heat. The introduction of high brightness LEDs with white light and monochromatic colors have led to a movement towards specialty and general illumination applications. The increased electrical currents used to drive the LEDs have focused more attention on the thermal paths in the level-1 packages and developments in LED power packaging. The luminous efficiency of LEDs is expected to reach over 80 Lumens/Watt that is approximately 6 times more than one tungsten bulb. The thermal challenges of these products in many applications will open new research areas for engineers from chip level to system level thermal management.

INTRODUCTION

Seventeen percent of the primary energy consumption in homes is lighting applications. The move towards more conservative, energy saving, plans have attracted very intense attention to LEDs over the last decade. General Electric sold the first LEDs in 1962. Performance of LEDs has been greatly improved since then. Over the years, LEDs have been used for indicator lights and produced a low amount of heat. Fig. 1 presents the application opportunities for LEDs based on light

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output. Although in the early days of development, researchers dealt with low levels of lumens, today it is a lot easier to get 10-20 based LED devices bringing new opportunities. The introduction of high brightness LEDs with white light and monochromatic colors has led to a movement towards specialty and general illumination applications. This has brought the thermal challenges to the designers attentions.

High brightness solid state lighting products, LEDs, have about 5000 times higher surface heat fluxes than the ultimate light and heat source, the sun. Technological advances in the microelectronics industry have led to a rapid increase in the transistor density and speed of conventional electronic chips, and hence an increase in the dissipated heat fluxes. Based on the 1997 SIA Packaging Technology Roadmap, it may be anticipated that a single microprocessor chip may reach heat fluxes in excess of 31 W/cm2 by 2006 [SIA, 19971. A typical LED power- package has a lmm2 surface area with a total heat generation of 1 W. This corresponds to a heat flux of 100 W/cmz. This is approximately three times higher than SIA’s 2006 packaging roadmap. Therefore, thermal engineers should focus on designing smart, efficient, and low thermal enveloped structures. A personnel computer has a chassis that in reality provides enough space to cool a microprocessor, while an LED application does not have that luxury. Therefore, while the package level design will present enough challenges,. system level design parameters should also be selected carefully.

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In this paper, the history, present status and future potential of solid-state lighting with high brightness LEDs are discussed. Potential applications of LEDs are presented and discussed along with the thermal challenges. The need for the innovative cooling techniques will be brought to the attention of the scientific community.

LED PACKAGING DEVELOPMENT

Historical DeveloPment

Light emitting diodes are found today in more applications than ever before due to new developments. However, the LED itself is not a new device, as it was first noticed at the beginning of the 20th century. In 1907, H. J. Round, an engineer working with Marconi, first noted the illumination of a crystal while working on a point contact crystal detector. It was later observed by the Russian scientist O.V. Losov in 1922 and published in a series of four patents from 1927 to 1942. None of this created enough interest to develop the device [Craford,

It was not until 195 1 that the LED resurfaced again as science became interested in solid-state devices. Research during the 1950s led to the refinement of the device and the first commercialized LEDs were marketed in the late 1960s.

The first practical visible spectrum emitting LED was invented in 1962 by Nick Holonyak Jr. of General Electric [Craford, 20001. He discovered that the wavelength of a Gallium Arsenide diode could be shifted from the infrared to the visible specr” merely by changing the chemical composition of the crystal to Gallium Arsenide Phosphate. This allowed the development of the first solid-state lamps. These devices had distinct advantages in terms of vibration, shock resistance, lifetime and power consumption over filament bulbs. These early solid-state lamps found uses in indicator type applications. Advances continued with the development of improved 111-V and 11-VI compounds. The 111-V and III-VI materials are GaN, AlGaN, InGaN, and InAlGaN, known as “wide band gap semiconductors”. New processes for expitaxial growth, such as liquid phase and more recently MOCVD, were developed. Devices of varying colors such as red, orange, yellow, and green soon appeared. Additionally continual improvement in quantum efficiency was demonstrated.

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Various types and colors LEDs have manufactured as illustrated in Fig. 2.

Today, the LED market consists of two sectors, low power indicators and specialty/general illiimination systems. The commodity-priced, inexpensive indicator LEDs are used to replace small indicator bulbs in various devices. A typical 5mm indicator construction is shown in Fig. 3.The LED’s inherent advantages of low power consumption, shock resistance and temperature tolerant design led to-wide acceptance in this field. Developments in the 1980s and 1990s led to the introduction of high brightness LEDs, where the devices produced enough light to provide illumination, although initially at very low levels. This occurred through the use of AlInGaP semiconductors for red and yellow wavelengths, and InGaN .for blue and green applications, along with the development of the MOCVD growth processes. Since 1995, the brightness levels of LBDs has been growing at an exponential rate due to waferldie, packaging, and optical improvements. This has resulted in the opening (of general and specialty illumination markets, with products such as outdoor displays, signage, portable/ fmed lighting, automotive, and backlighting.

Manufacturers and Market Expectations

The majority of all the LED lamps and displiays in the world are supplied from or manufactured in Japim and the surrounding Pacific Rim. This is due to the fact that European and US based manufacturers source the assembly of their LED complonents in Asia. The major manufacturers can be broken down into four world geographic regions such as Japan, United States, Europe and the Asia-Pacific. The bulk of the Asia-Pacific suppliers are located in Taiwan with a few others .in Korea. Mainland China is fast becoming a site for manufacturing operations for Taiwanese suppliers.

The major manufactures in Japan consist of Stanley, Matsushita, Nichia, Sanyo, arid Kodenshi etc accounting for approximately 50 percent of the world miirket. The second largest supplier is the Asia-Pacific regions including Lite-On, Everlight, Ledtech etc. The Asia-Pacific region accounts for approximately 23 percent of all world sales. North America is the third largest suppliix of LED components. The major manufacturers include CREE, Hewlett-Packard, Fairchild Optoelectronics and Uniroyal Optoelectronics. The North American world market share is estimated at 15 percent, .while Europe is the fourth largest supplier.

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The use of the LED components can be broken down into five major markets, automotive, indicator, communications, signs and illumination. The automotive market includes automobiles, truck, buses, and all terrain vehicles etc. The automobile use includes center high mounted stop lamps, instrument cluster lighting, side markers stop/tail/tum signals, switch illumination, and interior lighting. An LED’s long lifetime and reliability offers an advantage over incandescent bulbs in places that are difficult or time consuming to replace. LEDs have seen an increase in use in truck and bus exterior lighting. The major factor for their popularity in this use is low maintenance, high vibration resistance, and low power consumption. The majority of the LEDs are used for marker and clearance lighting. The indicator market includes the consumer, computer, office equipment, industrial and instruments. The communication sector entails residential and business phones, cellular telephones and switching equipment. Residential and business phones use LEDs for indicator and backlighting purposes. The area that has seen dramatic growth is cellular phone LCD and keypad backlighting. An average cell phone uses up to 3 LEDs, and an estimated 450 million cellular phones will be sold in 2001.

The sign market has become a popular one for LEDs. This market consists of moving message panels, dot matrix tiles, highway, traffic signal, channel letter, and full motion video LED signs. Moving message, dot matrix and exit type signs are the most prevalent LED signs. The major uses include visual communication and advertising. Highway signs or variable message signs consist of two types, fmed and portable generator operated signs. Maintenance and excellent visibility are the main drivers for using LEDs in this application. Traffic signals are an emerging use for potential high volumes of LEDs. The main reason for their use is energy and maintenance savings. Typical payback in energy savings alone is 3 years. The number of 5mm lamps used a typical 12” assembly exceeds 100, however recent power LED light engines may use as little as 18 large LEDs. North America is the major market for traffic signals at this time, however slow growth is expected in Europe and Asia. Channel letters, those large exterior letters depicting store names, are a huge potential market. Current channel letters are constructed using custom-built neon tubes. LEDs in “on wire form” are now shipping for channel letters. LEDs in this application offer a large savings in initial installation, energy and maintenance costs.

General illumination is the ultimate vision for LEDs. Since the introduction of the white LED manufactures have been investing in products that will someday replace the traditional light bulb. White LEDs can be built in three different ways. The newest way is to utilize a GaN device emitting in the 380-470 nm range combined with a phosphor. Another method uses the combined light of a red, green, and blue LED in a single lamp. A cheaper and less popular method uses a two chip design where an amber and blue LED are combined. LEDs in lighting have the advantages of precise wavelengtbkolor output, directed light output, long lifetime, vibration resistance, energy savings and a small flat package. Current applications for white LED lighting include architectural lighting, decorative lighting, flashlights, backlighting of large displays etc. The limiting factor for LED use in general illumination is the high cost per lumen. In order to be viable the LED lumen cost today of about $0.35 must be reduced to $ 0.01. For example; a typical incandescent bulb produces approximately 600 lumens and retails for less than $1. A similar amount of light output from LEDs would cost about $60 based on current LED lamp pricing. Large increases in efficiency and reduction in cost will be necessary to make the LED successful in the general illumination market. In 2000 the worldwide market for LED lamps reached $ 1 billion. LED systems sold more than $3.5 billion, and the market is expected to climb to $ 10 billion and beyond during the next decade as numerous applications are found for these robust devices. LED based systems market expectations are shown in Fig. 4 presenting an expectation of approximately 1 iX1o9 USD.

LED Packaaina Roadmap

LEDs used for indicator lamps during early technology adoption were packaged in two-lead devices similar in shape to small bulbs. These “lamp” packages came in 3mm and 5mm diameter sizes, also called T1 and Tl-%I respectively, although some lamps were made in sizes up to 10 mm in diameter.

As high brightness LEDs were developed, these lamp package styles were used but the thermal limitations of the packages became readily apparent. As the LED currents exceeded 50 mA, the generated heat resulted in unacceptable high die junction temperatures, which degraded device performance and life. As a result, new LED packages were developed to cope with higher power devices. One of the first of these was the Piranha first developed by HP and this package type is commonly used

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in automotive taillights and is also the basis of the GE TetraTM Channel Letter product. Recently another higher power device, known as the Barracuda, is the foundation of the Lumileds Luxeonm LED package that supports currents as high as 350 mA and a power of approximately 1W. The historical development of the packaging of LEDs is shown in Fig. 5.

Other illumination LEDs are made for small applications and are made as surface mount devices. These are frequently used for backlighting applications in cell phones, displays, etc., and use a modest amount of power.

Fig. 2 presents various Led packages. Conventional through hole 3, 5 and 10 mm round and various oval styles are shown in several colors. The square and rectangular shapes are most widely used and are known as the present Chip LED. The SMD Chip LED comes in domed and flattop configurations, typically with no reflector. The Piranha package is of the power through- hole variety and is used in the automotive and signage markets. . The TopLED (Osram) is also given in the figure. They are produced in flattop and domed P K C 2 and PLCC4 designs. Typical applications are found in automotive interiors. There are thousands of packagdchip combinations that account for the mostly low illuminatiodbrightness applications.

THERMAL MANAGEMENT OF LEDs

The efficiency of the solid-state lighting products strictly depend on the junction temperature as presented in Fig. 6. In addition, the available life of LEDs are also dependent on the device junction temperature. Currently, the most efficient LED package has a thermal resistance of 20 WW. If one considers running the system at 3 V with 350 mA, it will cause a 25-degree temperature gradient between the junction and the substrate. If a single LED is placed on a 0.5 cm2 Aluminum substrate, this will lead to a heat flux of 2 W/cm2. System level design requires a heat sink in the passive cooling regime.

Although it's a fairly new market, LEDs as an alternative to conventional lighting products, brings some demanding challenges. A typical LED lighting system is faced with the issues of decreasing the thermal resistance from junction to the substrate, and the availability of the orientation independent cost-effective thermal solutions. In the close future researchers will be faced with the additional issues of local hot spots resulting in high temperature gradients on the epitaxial layer.

LEDs have design issues in thermal management due to their small size and general lack of a sound thermal path. Since LED usage began with indicator lights anti display segments, the primary packaging of die has been to create indicators rather than illuminators, and this requires small currents and generates little heat. Because the LEDs are more sensitive to temperature than standard silicon chips (LED junction temperatures need to be below 125"C), more attention must be paid to thermal design issues even for low power illumination applications. With the invention of high brightness LE:Ds, interest in higher drive currents and greater power dissipation beg" First generation brighter dies wen: packaged in the 5mm systems and rated for apprommately 4VDC anld 20mA (80mW power). These 5mm packages can be over-driven to higher currents but the thermal path out the caihde leg of the lead frame quickly becomes too Ki t ing to remove heat. With a thermal resistance of 240-275 W W , better thermal paths were needed to remove the heat from the LEDs to the system application.

Current high brightness LED designs have begun to migrate away from the 5mm laimp style of packaging and into custom packages better designed for heat removal. With the release of the Piranha and Barracuda-style packages, heat dissipation of 0.5-1 Wlpackage is now possible. Larger die would result in this number increasing more. This trend combined with applications that use multiple LEDs now cause system designers to look at system level dissipations of 5-110 W for small applications and 20W or more for larger systems. This combined with the relatively low maximum junction temperature of the LED means the overall system must be very thermally efficient. For example, a 10 W system in a worst-case 50°C ambient environment and a design goal of 100°C LED junction requires a 5 WW system resistance. This resistance normally has to be achieved in 21 manner consistent with lighting product design - a low cost system, lack of any active cooling method, and re liable in a variety of environmental conditions. Incandescen thalogen lighting technology uses both radiation and natural convection to achieve this, primarily via radiation from the hot filament. But the low ]LED junction temperature requirement forces a passively cooled design to rely mostly on natural convection, and this is a paradigm shift in lighting.

Therefore a typical thermal design problem for a LED system revolves around efficient LED packaging for low junction to case thermal resistance, attention to a conductive thermal path to minimize resistance, and finally an efficient natural convection system to remove the heat to the ambient. Some systems may be able to rely on active cooling or a large conductive sink, but this is not the usual case for lighting applications.

Illustrative example: A typical Led application consists of a GaN on sapphire LED attached GaN side

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down (flip chip) to a silicon sub-mount. This assembly is then attached to a skeletonlmetal cup. The cup has a conical housing in which to place the chip and the sub- mount. The conical angle at the walls is vital to obtain the most effective light extraction from the chip package. Later this open space is filled with a phosphor/epoxy or phosphor/silicone mixture and then an epoxy encapsulate is molded to form a lens which protects and guides the light from the device. This structure is usually attached to a metallic board that will later be connected to the heat sinking system, usually a conventional heat sink.

A simple LED system is modeled by Finite Element Method by utilizing ANSYS5.7. The geometry of the system is close to a typical system. A 1 W heat generation is applied at the epitaxial layer that is connected to the Si Sub-mount. The length of the chip is approximately 1 mm, while the thickness is around 200 pm. A thin layer is connecting the chip to sub-mount. This might be a solder layer or a thin silver-filled epoxy or any alternative layer with acceptable electrical and mechanical requirements. The sub mount is connected to the cup; by another layer of silver filled epoxy. A natural convection heat transfer is applied to the backside of the lcm radius substratdcup. The fictitious temperature is 30 OC, while the heat transfer coefficient is 10 W/m*-K [Incropera and DeWitt, 19951.

Figs. 7 through 9 present the results of the FEM analysis. Even though very high end of natural convection cooling is applied the junction temperature is found to be approximately 351 OC. This is almost three times higher than what a typical LED package can handle. To solve this simple problem a designer will need either more aggressive cooling, forced convection, or increased surface area through a heat sink. This example with the best thermal interface performances created about 8 K N thermal resistances from junction to the board. However, in most of the practical applications, manufacturing problems, material imperfections doubles or triples this number. Cost is also a driving factor in interface material selection. The main focus should be on decreasing the thermal resistance path in the package. Secondly the system level design should maximize utilization of the enclosure structure to aid in dissipating heat.

Thermal measurements are also a challenge for the experimentalists since very small surface areas must be dealt with [Acharya and Vyavahare, 19991. With a system length of 1-1.5 mm, a macro-conventional device such as a thermocouple would lend itself to measurement errors. Typical infrared cameras lack in resolution, and even the best infrared cameras don't have the resolution to identify

hot spots on the chip surface. Current low cost approaches rely on generating a forward voltage (V,) vs. temperature correlation. This is done by heating the device and making pulsed Vf measurements at low current over a temperature range. Although effective for average junction temperature, this method does not allow for identification of hot spots on the chip surface. It is also time consuming and device dependent. Perhaps special measurements techniques should be developed to measure the temperatures at the desired locations. This may lend itself to "test LEDs" with on chip sensors. Some of the techniques utilized in MEMS technology such as thin film thermocouples, diodes; or perhaps sutured resistor layers will be handy to solve the thermal measurement issues.

SUMMARY AND CONCLUSIONS

A brief overview of the solid-state lighting history and markets is presented. The energy saving advantages and robustness of LEDs are discussed briefly. These solid-state lighting devices have become of interest over the last decade leading to the development of efficient, high brightness and cost-effective packaging. Thermal problems due to higher and higher heat dissipation for every generation of LEDs will be a challenge for thermaI engineers. A typical LED system is demonstrated through E M . It showed a heat flux of 100s of times higher than the sun.

REFERENCES

A.D. Kraus and A. Bar-Cohen, "Thermal Analysis and Control of Electronic Equipment". University of Minnesota, 1996.

Acharya, Y.B., and Vyavahare, " Temperature characteristics of the device constant (n) of a light emitting diode", Solid State Electronics, Vol: 43, 1999.

Craford, M.G., "Visible Light-Emitting Diodes: Past, Present, and Very Bright Future", MRS Bulletin, October 2000.

Incropera, F. and DeWitt, D., Fundamentals of Heat Transfer, 1995.

SIA, 1997, "The national technology roadmap for semiconductors: Technology needs - 1997, Semiconductor Industry Association, Washington DC.

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Figure 1. Application opportunities for LEDs based on light output

Figure 2. Various LED Packages.

Figure 3. Schematic view of a 5 mm LED and cathode lead.

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$12,000 - $10,000 -

$8,000 - $6.000 - $4,000 - $2,000 -

$11,134

$0 I 2000 2001 2002 2003 2004

Figure 4. General market trends for the LED based lighting

systems

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- 120 &

3 U

-9 100 9 A .Fp a 80

1992 1996 1998 1999 & Beyond

Figure 5. LED Power Packaging History.

I I

5 0 0 50 100 150

Junckn Tem perabre E 1

Figure 6. Variation of the light output with the junction temperature.

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Figure 7. Temperature distribution of a typical LED system.

Figure 8. Temperature distribution of a LED layers front and back views.

Figure 9. Heat Flux distribution of the LED system.

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