Jonathan Wafer, Osram Opto Semiconductors Inc.
LEDs have entered the mainstream in many applications as a result of the increases in their luminous flux, the availability of an extensive range of colors, and their inherent technological benefits compared with conventional filament-based light sources. Moreover, improvements in lifetime and the development of electronic and optical solutions have led to the rise of an extensive value chain of suppliers, solution providers and enablers that has aided in the proliferation of LED-based products in the absence of solid-state standards.
Over the past decade, automotive lighting and the illumination of personal communication devices such as cell phones and personal data assistants have driven volume and revenue growth for the LED market. LED usage within the automotive sector falls into two application areas: interior and exterior lighting. The benefits include reduced warranty replacement costs, improved lifetimes, high luminance, reduced power and heat, saturated colors and design flexibility.
Interior lighting applications include switches, audio/climate control systems, entertainment systems, instrumentation and driver/passenger cockpit illumination, inclusive of reading and map lighting. The majority of these applications requires the backlighting of buttons, isolated areas and icons. Such illumination typically is harmonized or monochromatic in color throughout the cockpit and passenger compartment. Over time, each OEM has developed a brand color identity that can be observed in these modules upon ignition.
Figure 1. The CIE chromaticity diagram shows the colors available in LEDs.
Exterior automotive lighting applications include brake/taillights or rear combo panels, center high-mount stop lamps, side markers, and forward lighting or headlights. With the exception of the last two, these applications employ red-orange or yellow LEDs based on AlInGaP (Figure 1). The rear-panel exterior lighting applications also benefit from the fast switching time of LEDs, typically 30 to 100 ns, a safety advantage over the conventional incandescent-based lighting.
Figure 2. A time line shows the growth in LED efficiency over time.
These benefits, combined with the improvements in LED efficiency, the cost reductions over time, and an increasingly educated supply chain and design culture, have led to LED installment rates of more than 75 percent on new automotive designs. The power efficiency increases of LEDs over the past 40 years are based on the enhancement of InGaN and GaN technologies, the engine driving LED technology growth in exterior lighting, illumination and general lighting applications.
The efficiency enhancements are the result of improvements in light generation within the chip and on the ability to extract light from the chip and package. Thin-film technology represents the latest breakthrough in extraction efficiency and luminance (Figure 3). Thin-film LEDs use a unique liftoff process to create a highly efficient, scalable, top-emitting structure.
Figure 3. The evolution and associated extraction efficiencies of common InGaN structures display progression toward higher efficiencies.
Another advantage of LEDs over traditional lighting technologies is the fact that they are clean, mercury-free sources. Many are available in environmentally friendly, or “green,” packaged devices. This is becoming increasingly important as governments worldwide express a greater interest in limiting the use of potentially harmful substances.
For example, a European Parliament and Council directive on hazardous substances in electrical and electronic equipment will restrict by weight the usage of some elements and flame-retardants, effective July 1, 2006 (see table). This directive has a far smaller impact on LEDs than on traditional lighting technologies. Historically, LED packaging materials and construction schemes have not used such materials, the exception being the lead in electroplated eutectic SnPb-based terminations for soldering purposes.
Notably, the migration of standard LED through-hole and surface-mount technology packages to lead-free terminations has mirrored that of other semiconductor components. Such devices benefit from many years of process development, mechanization and high-speed volume manufacturing. Based on the knowledge and expertise within the semiconductor industry, a 100 percent tin plating finish with a flash nickel barrier layer has emerged as the best process for the fabrication of lead-free terminations that are compliant with the new environmental directive.
Efficiency increases have enabled LED designs in applications that were once thought to be beyond the technology. LEDs have migrated from being small-chip, low-flux indicators to being high-power, low-energy light sources. Large chip-based white LEDs are revolutionizing lighting design, surpassing many conventional light sources in power efficiency and other key lighting parameters.
Future applications, such as projection light sources, forward automotive lighting and backlighting of large-area liquid crystal displays, are under development. The commercialization of LEDs for lights on emergency vehicles, beacons and strobes, for architectural and theme lighting, and for large-area video display billboards and signage is on the rise. In all of these cases, just as with the pioneering efforts in LED-based traffic signal designs, suppliers and end users have benefited from the advantages of LED technology.
LEDs may be expected to continue their inroads into traditional lighting applications as their power efficiencies and volumes increase, helping to drive down costs. The convergence of solid-state lighting, electronics, optics and assembly techniques will generate faster, smarter lighting systems, providing illumination, entertainment, visual effects and longevity beyond that from conventional sources.
Furthermore, the power savings associated with environmentally safe LED lighting solutions has sparked the interest of governmental and regulatory agencies, which will create additional technology funding and legislation. Such support and action will drive the technology further toward standardization.
Meet the author
Jonathan Wafer is product marketing manager of LED and intelligent display products at Osram Opto Semiconductors Inc. in San Jose, Calif.; e-mail: email@example.com.