The inclusion of phosphor into a high brightness light-emitting diode (LED) package is a complicated task since LEDs are encapsulated with a phosphor and epoxy mixture to convert blue photons to white light. Moreover, this common practice may cause high temperatures and fractures in the gold wire bonds of the chip or solder balls due to local heating and thermal stresses leading to device failures. Furthermore, at elevated junction temperatures, the light conversion efficiency of the phosphor reduces and decreases the overall optical efficiency of an LED. Although, remote phosphor technique has been already applied to LED systems, the high power requirements have needed better performing methods. Thus, an immersion liquid cooled remote phosphor-coated system has been proposed and experimentally and computationally investigated. First, a set of experiments was performed, which includes the combined effects coming from both optical and thermal improvements with the proposed liquid cooled remote phosphor-coated technique, where the total light extraction enhancement was obtained in excess of 25%. Then, the same problem has been computationally studied for investigation of solely optical enhancements, which has shown that remote phosphor-coated LED package with a liquid coolant of suitable refractive index at the optical path has enhanced the overall lumen performance about 13%, whereas the rest of the improvements of 12% were due to thermal enhancements.
Light-emitting diode (LED)-based automotive lighting systems pose unique challenges, such as dual-side packaging (front side for LEDs and back side for driver electronics circuit), size, harsh ambient, and cooling. Packaging for automotive lighting applications combining the advanced printed circuit board (PCB) technology with a multifunctional LED-based board is investigated with a focus on the effect of thermal conduction-based cooling for hot spot abatement. A baseline study with a flame retardant 4 technology, commonly known as FR4 PCB, is first compared with a metal-core PCB technology, both experimentally and computationally. The double-sided advanced PCB that houses both electronics and LEDs is then investigated computationally and experimentally compared with the baseline FR4 PCB. Computational models are first developed with a commercial computational fluid dynamics software and are followed by an advanced PCB technology based on embedded heat pipes, which is computationally and experimentally studied. Then, attention is turned to studying different heat pipe orientations and heat pipe placements on the board. Results show that conventional FR4-based light engines experience local hot spots (ΔT>50°C) while advanced PCB technology based on heat pipes and thermal spreaders eliminates these local hot spots (ΔT<10°C), leading to a higher lumen extraction with improved reliability. Finally, possible design options are presented with embedded heat pipe structures that further improve the PCB performance.
Thermoelectrics have been investigated for their cooling and energy harvesting uses over the last
six decades. Those devices can be bought from a number of commercial suppliers.
Thermotunneling (TT) devices, on the other hand, have been known only for the last two decades,
and nobody has been able to practically manufacture or demonstrate the performance of those
devices. In this study, we will discuss the high thermodynamic efficiency of these systems and
design bottlenecks to reach the high efficiencies such as thermal back path and electrical losses.
Concepts for possible device designs will be discussed in detail. Efficiency of those devices will
be compared with the conventional power generation as well as solid-state power generation
systems. Thermodynamic limits of TT systems will be compared, and first order economic
analysis will be performed.
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 mm2, 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.
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 $/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.
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.