LED technology is improving by leaps and bounds, or, more appropriately, watts and lumens. Riding a wave of better fabrication technologies, today’s LEDs are cheaper, brighter and offer more color choices. And, they still feature inherent longevity, durability and the ability to operate at a wide — and growing wider — temperature range.

The LEDs of the past were often simple devices, able to let the user know that the product was on or off, but not much else. No longer are they just an afterthought, LEDs are playing key roles in backlighting LCDs and as panel indicators — not to mention their use in illumination, signage and decoration applications.

Nokia’s new 7900 Prism phone uses RGB LEDs to allow the user to select from up to 49 colors to illuminate the phone’s keypad. “It is purely aesthetic, but it makes for a nice, upscale feature,” says Keith Nowak, a senior communications manager at Nokia. “This helps differentiate the phone, which is aimed at a style-conscious consumer, from other phones on the market.”

LEDs are being used in cellphones for backlighting and aesthetic purposes such as this phone that uses Avago Technologies’ LEDs.

LEDs are also becoming popular choices for backlighting LCDs in larger applications. Earlier this year, Philips Electronics announced it was switching from fluorescent backlighting for larger LCD panels to LED backlighting.

William Chu, regional sales manager for Kingbright Corp., City of Industry, Calif., a manufacturer of more than 28,000 types of LEDs, says that his company has started to see increased interest from electronics manufacturers, especially those that make handheld devices such as cell phones, PDAs, and digital cameras.

Brightness is one of the reasons why more manufacturers are turning to LEDs. The level of brightness offered by LEDs have been rising for at least the last two decades, says Richard Saffa, vice president of Carrollton, Texas-based Optek Technology’s Visible LED Business Unit. Twenty years ago, LEDs were only about 0.2 millicandelas (mcd) in brightness. “You could barely tell it was on, but that was sufficient for applications of that time period,” he says.

Manufacturers of cellphones and other portable devices are turning toward LED technology. Photo: Avago Technologies.

The low luminous levels were the result of the limitations of the materials used to build the LED. Today, the semiconductor dies that are the heart of what gets made into an LED have improved to the point where luminous intensity is in the thousands of millicandelas. A candela is the measure of how much light is produced at the light source. Today’s red LEDs can go higher than 12,000 mcd, green can go up to 22,000 mcd, and blue to more than 5,000 mcd.

One of the things that designers should be aware of is that there can be variations in brightness levels from LED to LED, says Jeff Oliveros, director of engineering for Lumex, Palatine, Ill. Variations can also occur in color as well, which is important as color LEDs continue to enter the marketplace, he says. A human’s eye has different sensitivities to color. For instance, a human eye is not as sensitive to red as it is to other colors. If one red LED is 25 percent brighter than its neighbor, most consumers cannot tell the difference. But, with colors such as green and yellow, a 10 percent difference may be noticed.

Suppliers are aware of this and many have introduced into their production processes a binning process in which like items are sorted into bins. Oliveros says that his company bins for both wavelength, which is the color, and also intensity. “Binning is critical if you have an array of LEDs because you don’t want one side looking duller than another, or one side brighter than the other side,” he says.

LEDtronics’ RGB-1007-001 is a low profile LED that features luminous intensities that range from 230 mcd to 750 mcd with current between 20 mA and 25 mA.

Beyond brightness, a broader color palette has been one of the biggest developments in LED technology. Just about every color in the rainbow can be achieved through one of two methods — by using a phosphor, or by combining red, green and blue LED materials onto a single semiconductor chip.

The less expensive method is adding the phosphor, a tried-and-true technique that allows for a range of colors to be attained at a relatively inexpensive cost. “We have new colors coming out all the time because of the industry’s capability of phosphoring the chip in different ways to make any shade of color the designer wants,” says Saffa.

By phosphor coating the lens, different colors and shades are developed. For instance, a white LED might be available in cool white, pale white and incandescent white. The cool white would have the least amount of phosphorous and would be brighter. The incandescent white would have the most and would be the dimmest, while the pale white would be somewhere between the two extremes.

The OVSPRGBCR4 LED from Optek offers a 130 Deg viewing angle and an ultra-low profile of 1.5 mm.

More expensive is placing the different color RGB chips into a single discrete package. Its strength is in the millions of possible colors it can generate by varying the individual color’s intensity levels, says Saffa. For some applications a fourth die or chip of an amber or yellow color wavelength is added to give even more options and enhance color rendering.

While this technique is more expensive, costs are expected to drop across the board as it has for blue LEDs. For years, blue had been expensive to make, and purchase, and its intensity was not as good as other colors. Today, newer fabrication technologies and materials including the use of indium gallium nitride (InGaN) materials have lowered the cost of blue LEDs.

According to Man Yu, engineering manager for Kingbright, the brightness of a blue LED has doubled in the past 12 months, at the same time its price has fallen. “As the trend continues, we anticipate getting brighter blue LEDs at the same price of what we have seen right now, or lower.”

The Nokia 7900 Prism phone offers 49 LED color choices to illuminate the keypad.

White is another popular color, and is a direct result of the development of blue. There are different methods to create white. One is through combining the RGB to create white. Another technique is to place a yellow phosphor over a blue LED, says Alvin Yeoh, marketing program manager for San Jose, Calif-based Avago Technologies’ chipLED products. Yeoh, who is based in Malaysia, says that this is still the main method used. “When the wavelengths of the light from the blue LED goes through the phosphor, it converts it so what the eyes sees is actually white,” he says.

No matter the way the color is achieved, designers now have a myriad of choices. Just like paint, all white is not white, with many companies offering gradations of white from which to choose. “Depending on the look the designers want to achieve they can choose a cool white, which is more of a fluorescent type light, or something that is a warm white that gives you more of an incandescent type color,” says Janie Haynie, product marketing director for Optek Technology. “Of course, they can also choose a cool blue or a hot red.”

New fabrication technology has also helped to reduce operating voltages. LEDs in most colors operate in the range of 2 VDC to 2.5 VDC, but other colors such as blue and bright green require higher operating voltages. “The fabrication technology has lowered the operating voltage for blue LEDs to the level that is much more application friendly,” says Yu. “It is around 3 to 3.3 volts, which is much more user friendly than before, when it was about 4 to 4.5 volts.”

Kingbright’s new blue SMD LEDs features an ultra-thin design (2.0 mm x 1.25 mm x 0.4 mm) with wide viewing angle of 110 degrees, making handheld devices and consumer electronics lighter, smaller, and sleeker.

This is important when power efficiency is pivotal. “In terms of cell phones and all portable electronics, battery life is a major issue,” says Yeoh. “What our customers always want to do is get the maximum light output with minimum power or minimum current.”

Despite their size, LEDs are a robust product, physically tougher than incandescent bulbs, but they still must reckon with some electrical and heat issues. Often, the LED will require a current-limiting resistor to protect it from over-current. A diode is also used to protect the LED from over voltage, reverse voltage and electrostatic discharge. Oliveros says that LED drivers allow for a constant current so the current does not fluctuate or adjust as the LED heats up.

In terms of ESD protection, some companies have added specialized diodes. Kingbright, for example, added a Zener diode to protect its new blue product. The diode permits current to flow in the forward direction like a traditional diode, but also in the reverse direction if the voltage is too large for the LED to handle. According to Yu, the Zener diode will help protect against the LED from ESD of up to 5,000 volts.

The HSMR-CL25 LED from Avago Technologies is indium gallium nitride blue and is ultra thin.

Heat can be another important issue because as the temperature rises, brightness falls. If the LEDs are driven harder, used in a multi-array scenario, or placed too close to a hot operating environment, failure — meaning a 50 percent or more drop in luminous intensity — could occur. But, many suppliers say that while heat still must be considered, it is less of a challenge. In the past, as temperature neared 55 DegC, the light output would decrease. Today, says Kingbright’s Chu, many LEDs operate all the way to 85 DegC. Some of Kingbright’s specialty products can top 100 DegC.

LEDs on cell phones and appliances generally do not have to worry about heat issues, says Saffa, because the LEDs themselves generate little heat. It is often other areas of the design that can be troublesome. Saffa suggests that using proper circuit board design is a way to minimize most heat-related problems.

Other ways to protect the LED from heat include using heat sinks or changing the substrate. Lumex, for instance, offers both types of products. Some LEDs feature aluminum heat sinks that can be attached to the LED to dissipate the heat. If the footprint is too small that may not be possible, says Oliveros. In this case, Lumex offers the Koolbright LED product that uses a ceramic substrate to help protect the LED from the heat.

Kingbright 0.4 mm Ultra-Thin 0805 Blue SMD LED’s features a built-in Zener diode that can withstand ESD voltage up to 5,000 V.

Desired viewing angles are another important design consideration when specifying LEDs. LEDs emit light in a single direction, which means that it needs a diffuser lens to achieve a desired beam pattern. According to Jordon Papanier, marketing manager for LEDtronics Inc., Torrance, Calif., LEDs are available in many different viewing angles from 8 degrees to 180 degrees, and there are ways to get a wider viewing angle for LEDs like an inverted LED chip with a reflector cup.

However, he cautions that the more the LED light is spread out, the lower the light output. A 5 mm, 3,000-Kelvin white LED with a viewing angle of 16 degrees puts out about 14,000 mcd. The same LED with a viewing angle of 55 degrees will put out 3,000 mcd, which is a loss of 11,000 mcd at a focal point of 16 degrees.

In terms of overall size, LEDs have gotten smaller and smaller. Today, they come in several standard sizes including 0805, which measures 2.0 mm x 1.25 mm; 0603, which measures 1.6 mm x 0.8 mm, and 0402, which measures 1 mm x 0.5 mm.

Lumex Koolbright LED product uses a ceramic substrate that helps protects the LED from the heat.

While getting smaller is usually better, in some cases the 0402 maybe too small for application scenarios that use pick-and-place machines. These tiny LEDs maybe too small for the machines to pick up, some suppliers say.

So, while for some the overall footprint may be small enough, its profile may not be svelte enough. Because of that, some LEDs are being offered in reduced profiles. Companies such as Avago and Kingbright have developed slim products. Avago Technologies product, the HSMR-CL25 (blue) and HSMW-CL25 (white), are available in the 0603 format and feature a 0.25 mm profile.

The Kingbright product is in the 0805 format, and its profile is 0.4mm “The slim design offers flexibility for different designs, especially in membrane-switch applications,” says Yu. “Then you can put the LED behind the membrane switch to create a button effect.”

Yu, as well as all of the suppliers interviewed, believes that the trend to brighter, more efficient LEDs will continue. With this, they say, the opportunities to improve and differentiate products will also continue to rise.

For more information, email:
Avago Technologies: support@avagotech.com
Kingbright Corp.: sales1@us.kingbright.com
LEDtronics Inc.: jpapanier@ledtronics.com
Lumex Inc.: Lmxsales@lumex.com
Optek Technology.: visibleled@optekinc.com