Stanton Earl Weaver

Thermal Management of LEDs: Package to System

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 general illumination. The increased electrical currents used to drive the LEDs have focused more attention on the thermal paths in the developments of LED power packaging. The luminous efficiency of LEDs is soon expected to reach over 80 lumens/W, this is approximately 6 times the efficiency of a conventional incandescent tungsten bulb. Thermal management for the solid-state lighting applications is a key design parameter for both package and system level. Package and system level thermal management is discussed in separate sections. Effect of chip packages on junction to board thermal resistance was compared for both SiC and Sapphire chips. The higher thermal conductivity of the SiC chip provided about 2 times better thermal performance than the latter, while the under-filled Sapphire chip package can only catch the SiC chip performance. Later, system level thermal management was studied based on established numerical models for a conceptual solid-state lighting system. A conceptual LED illumination system was chosen and CFD models were created to determine the availability and limitations of passive air-cooling.

Thermal challenges in the future generation solid state lighting applications: light emitting diodes

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.

Chip-scale thermal management of high-brightness LED packages

The efficiency and reliability of the solid-state lighting devices strongly depend on successful thermal management. Light emitting diodes, LEDs, are a strong candidate for the next generation, general illumination applications. LEDs are making great strides in terms of lumen performance and reliability, however the barrier to widespread use in general illumination still remains the cost or

Effects of Localized Heat Generations Due to the Color Conversion in Phosphor Particles and Layers of High Brightness Light Emitting Diodes

/Lumen. LED packaging designers are pushing the LED performance to its limits. This is resulting in increased drive currents, and thus the need for lower thermal resistance packaging designs. As the power density continues to rise, the integrity of the package electrical and thermal interconnect becomes extremely important. Experimental results with high brightness LED packages show that chip attachment defects can cause significant thermal gradients across the LED chips leading to premature failures. A numerical study was also carried out with parametric models to understand the chip active layer temperature profile variation due to the bump defects. Finite element techniques were utilized to evaluate the effects of localized hot spots at the chip active layer. The importance of “zero defects” in one of the more popular interconnect schemes; the “epi down” soldered flip chip configuration is investigated and demonstrated.

Chip to System Levels Thermal Needs and Alternative Thermal Technologies for High Brightness LEDS

The efficiency and reliability of the solid-state lighting devices strongly depend on successful thermal management. Light emitting diodes, LEDs, a strong candidate for the next generation general illumination applications are of interest. Typical white LEDs start with either blue or near UV light generated by the active quantum layers. The light is guided through a transparent encapsulant filled with micron sized phosphor particles. The phosphor particles up-convert the short wavelength light to desired colors, producing white light. Due to low quantum efficiency, during the conversion, localized heating of small particles occurs. Experimental results with high brightness LED packages showed that there is significant light output reduction. Idealized numerical models through Finite element technique were created to evaluate the effects of localized heat generations at particles and layers. Results showed that as small as a 3 mW heat generation on a 20 μm diameter spherical phosphor particle might lead to excessive temperatures which can be a major source of light output degradation and reliability concern for high brightness LEDs.Copyright

Effect of chip and bonding defects on the junction temperatures of high-brightness light-emitting diodes

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 has allowed them to penetrate specialty and general illumination applications. The increased electrical currents used to drive the LEDs have resulted in higher heat fluxes than those for average silicon integrated circuits (i.e., ICs). This has created a need to focus more attention on the thermal management engineering of LED power packages. The output of a typical commercial high brightness, 1 mm 2 , LED has exceeded 100 1m at drive levels approaching 3 W. This corresponds to a heat fiux of up to 300 W/cm 2 . Novel thermal solutions need to address system architectures, packaging, phosphors for light color conversion, and encapsuianfs and fillers for optical extraction. In this paper, the effect of thermal management on packaging architectures, phosphors, encapsulants, and system design tire discussed. Additionally, discussions of microscopic defects due to packaging problems as well as chip active layer defects are presented through experimental and computational findings.

Micro Fluidic Jets for Thermal Management of Electronics

Light-emitting diodes (LEDs) are a strong candidate for the next-generation general illumination applications. LEDs are making great strides in brightness performance and reliability; however, the barrier to widespread use in general illumination still remains the cost (dollars per lumen). LED packaging designers are pushing the LED performance to its limits. This is resulting in increased drive currents and thus the need for lower-thermal-resistance packaging. The efficiency and reliability of solid-state lighting devices strongly depends on successful thermal management, because the junction temperature of the chip is the prime driver for effective operation. As the power density continues to increase, the integrity of the package electrical and thermal interconnects becomes extremely important. Experimental results with high-brightness LED packages show that chip attachment defects can cause significant thermal gradients across the LED chips, leading to premature failures. Perfect chip and interconnect structures for highly conductive substrates showed only a 2 K temperature variation over a chip area of approximately 1 mm 2 , while defective chips experienced greater than 40 K temperature variations over an identical area. A further numerical study was also carried out with parametric finite-element models to understand the temperature profile variation of the chip active layer due to the bump defects. Finite-element models were utilized to evaluate the effects of hot spots in the chip active layer. The importance of zero defects in one of the more popular interconnect schemes--the epi-down soldered flip-chip configuration--is investigated and demonstrated.

Ce3+-based phosphors for blue LED excitation

Micro fluidics devices are conventionally used for boundary layer control in many aerospace applications. Synthetic Jets are intense small scale turbulent jets formed from entrainment and expulsion of the fluid in which they are embedded. The idea of using synthetic jets in confined electronic cooling applications started in late 1990s. These micro fluidic devices offer very efficient, high magnitude direct air-cooling on the heated surface. A proprietary synthetic jet designed in General Electric Company was able to provide a maximum air velocity of 90 m/s from a 1.2 mm hydraulic diameter rectangular orifice. An experimental study for determining the thermal performance of a meso scale synthetic jet was carried out. The synthetic jets are driven by a time harmonic signal. During the experiments, the operating frequency for jets was set between 3 and 4.5 kHz. The resonance frequency for a particular jet was determined through the effect on the exit velocity magnitude. An infrared thermal imaging technique was used to acquire fine scale temperature measurements. A square heater with a surface area of 156 mm2 was used to mimic the hot component and extensive temperature maps were obtained. The parameters varied during the experiments were jet location, driving jet voltage, driving jet frequency and heater power. The output parameters were point wise temperatures (pixel size = 30 μm), and heat transfer enhancement over natural convection. A maximum of approximately 8 times enhancement over natural convection heat transfer was measured. The maximum coefficient of cooling performance obtained was approximately 6.6 due to the low power consumption of the synthetic jets.Copyright

EXPERIMENTAL INVESTIGATION OF MICRO/NANO HEAT PIPE WICK STRUCTURES

The luminescence of Ce3+ in host lattices based on Y3Al5O12 garnets that can be used in blue LED based solid state lighting sources is discussed. Specifically, the effect of Ga3+ and Tb3+ substitution for Al3+ and Y3+, respectively, on the emission color and thermal quenching of these phosphors is described with the initial implications for white light devices.

Development of a Compliant Nanothermal Interface Material

The performance of electronic devices is limited by the capability to remove heat from these devices. A heat pipe is a device to facilitate heat transport that has seen increased usage to address this challenge. A heat pipe is a two-phase heat transfer device capable of transporting heat with minimal temperature gradient. An important component of a heat pipe is the wick structure, which transports the condensate from the condenser to the evaporator. The requirements for high heat transport capability and high resilience to external accelerations leads to the necessity of a design trade off in the wick geometry. This makes the wick performance a critical parameter in the design of heat pipes. The present study investigates experimental methods of testing capillary performance of wick structures ranging from micro- to nano-scales. These techniques will facilitate a pathway to the development of nano-engineered wick structures for high performance heat pipes.Copyright