The mixture of gallium nitride (GaN) and indium nitride (InN) to form indium gallium nitride (InGaN) has become one of the more popular technologies for making LEDs, particularly blue and green. Though the technology’s history can be traced back to the 1990s, high-power InGaN technology is much more recent.
Available products in this field can be applied to modern-day applications, where there are advantages and disadvantages of standard versus high-power products. Standard LEDs have advantages in being easier to design, as well as being smaller and cheaper than their high-brightness counterparts. However, some applications, such as street lighting and automotive headlights, require the extra power. There are still many applications that fall between these two uses.
The fastest growing application area is in illumination due to the LED’s color saturation, energy saving, and long life. White and colored LEDs are finding uses in architectural lighting, entertainment, decorative lighting, and video walls. Red, green, and blue LEDs are becoming popular due to their abilities to combine to make other colors, and these can now be put together in a single package.
Products and applications
This article will look at two LEDs that can be used for outdoor lighting applications: the HLMP-CB1A-XY0DD from Avago Technologies and the C503B-GAN-CB0F0791 from Cree. These two products can both be used for outdoor signage but care must be taken in preparing and mounting them, and these precautions are looked at in detail. To put this into practice, the discussion will now look at how the Cree XBDROY-00-0000-000000L01 XLamp product can be used in optical design packages. There will also be a discussion on the benefits and disadvantages of standard versus high-power products using the Lumex SML-LX1206UWW-TR as an example.
Blue and green LEDs for outdoor lighting
Avago Technologies’ 5 mm blue and green LEDs in the HLMP-Cx1A range (such as the HLMP-CB1A-XY0DD) are untinted and non-diffused. The efficient InGaN material lets them produce well-defined spatial radiation patterns at specific viewing-cone angles. Their advanced optical-grade epoxy construction provides high-temperature and moisture-resistant performance in outdoor signal and sign applications, which can be used for commercial advertising, traffic signs, and variable message signs. The epoxy contains an ultra-violet inhibitor to reduce the effects of long-term exposure to direct sunlight. They are available in 470 nm blue and 525 nm green.
Figure 1: Schematic of an HLMP-Cx1A LED with dimensions in millimeters (top) and inches (bottom).
The leads of the LED lamp may be preformed or cut to length before insertion and soldering on a PCB. For better control, it is recommended to use proper tools to form and cut the leads precisely to the correct length rather than doing it manually. If manual lead cutting is necessary, the leads should be cut after the soldering process. The solder connection forms a mechanical ground that prevents traveling into the LED package after lead cutting; this can cause mechanical stress. This is recommended for hand solder operation, as the excess lead length also acts as a small heatsink.
Care should be taken during PCB assembly and the soldering process to prevent damage to the LED components. These may be effectively hand soldered to the PCB, but this is only recommended under unavoidable circumstances such as rework. The closest manual soldering distance of the soldering heat source (the soldering iron’s tip) to the body is 1.59 mm. Soldering the LED using the soldering iron tip closer than this might damage the LED.
Electro-static discharge (ESD) precautions must be properly applied on the soldering station and to those working there to prevent ESD damage to the LED components as they are ESD sensitive. The soldering iron used should have a grounded tip to ensure that the electrostatic charge is properly grounded.
The manufacturer recommends that only bottom pre-heaters are used as this will reduce thermal stress experienced by the LED. Wave soldering parameters should be set and maintained according to the recommended temperature and dwell time. The manufacturer also recommends checking the soldering profile daily to ensure that it always conforms to the recommended soldering conditions.
Remember that different sizes and designs of PCBs will have different heat masses that might cause a change in temperature experienced by the board with the same wave-soldering settings. Therefore, it is recommended that the soldering profile be recalibrated before starting work on a different type of PCB.
Any alignment fixture that is being applied during wave soldering should be loosely fitted and should not apply weight or force on the LED. A non-metal material is recommended by the manufacturer as it will absorb less heat during the wave-soldering process.
At elevated temperatures, an LED is more susceptible to mechanical stress. The PCB must therefore be allowed to cool down to room temperature before handling, including removing the alignment fixture or pallet.
If the PCB board contains both through-hole LEDs and other surface-mount components, it is recommended that surface-mount components be soldered on the top side of the PCB. If surface-mount components need to be on the bottom side, these components should be soldered using reflow soldering before inserting the through-hole LEDs.
The recommended solder is either Sn63 leaded or SAC305 lead-free. The solder bath temperature should be 255˚C ±5˚C with a maximum peak temperature of 260˚C. Dwell time should be between three and five seconds. The board should be cooled to room temperature before applying any mechanical force.
Green LEDs for outdoor signage
The Cree GAN range of round 5 mm blue and green LEDS can be used for electronic signs and signals, motorway, variable message, advertising, or fuel station signs, and other outdoor applications as their light output is geared for sunlight readability. They are made with an advanced optical-grade epoxy that gives high temperature and high moisture resistance. The C503B-GAN-CB0F0791 is a good example in this range with a 15˚ viewing angle and a luminous intensity between 16,800 and 90,500 mcd. It is a green LED with a wavelength between 520 and 535 nm.
When using these for any of the above applications, care and precautions need to be taken to get the best out of the products. For example, chemical liquids should not be used to clean the products; instead, wipe the LED with alcohol at normal room temperature and allow to dry for about 15 minutes before use. Ultrasonic cleaning can be used, but this needs to be pre-qualified to ensure that it does not damage the LED. If in doubt, check with the manufacturer first.
The leads should be formed before soldering, not during or after, and should be bent at a point at least 3 mm from the base of the package. Care should be taken not to stress the LED package during the forming. For manual soldering, do not use a soldering iron above 35 W or let it reach a temperature above 300˚C. For solder dipping, the solder bath temperature should not be above 260˚C. The maximum soldering time for both is three seconds and should be carried out at least 3 mm from the base of the package.
Figure 2: Recommended wave soldering for the Cree GaN LEDs.
For wave soldering, the peak preheat temperature should be 110˚C with the total preheat time no more than a minute. The peak profile temperature should be 260˚C. The dwell time above 200˚C should not exceed three seconds. Again, avoid stressing the LED package, especially when heated. In fact, the package should not be subjected to shock or vibration until it has cooled down to below 40˚C. Such stress can even be caused by the PCB warping or by clinching or cutting the LED leads.
ESD and electrical overstress (EOS) can damage these LEDs, and therefore, ESD wrist and shoe straps and antistatic gloves should be used when handling the products. All the devices and equipment used must also be properly grounded. The circuit design should avoid the possibility of EOS.
Heat management is also important in the final application. The heat that surrounds the LED when used can cause it to fail if it is too high. To check for individual LEDs, it is best to refer to the de-rating curve in the manufacturer’s documentation. The documentation will also show the maximum rating for the reverse voltage; exceeding this can cause the LED to fail.
Cree also makes LED lamps, such as the XBDROY-00-0000-000000L01 XLamp product. The solid-state LED emitters combine InGaN materials with the company’s proprietary device technology and silicon carbide (SiC) substrates for the general illumination market. The blue light from these types of lamps is emitted from the InGaN epi layer and yellow light from the top of the bevel cuts on the top surface of the chip. This makes it difficult to design very-narrow beam optics that produce a uniform beam pattern in intensity and color. Bevel cuts on the top surface of the LED improve light extraction.
Radiant imaging source imaging goniometers (SIG) have been used to create LED radiant-source model data files. The SIG system captures a precise model of a light source’s near-field output. The image data and model file generated from it can then be used to provide a complete characterization of the light source output. This can be used for design evaluation and imported into major optical design packages to allow accurate design of LED optical systems.
In addition, ray set files containing an arbitrary number of rays can be generated by ProSource software for export to other optical and illumination system design software packages such as ASAP, Fred, LightTools, LucidShape, TracePro, and Zemax.
If an LED light pattern does not fit the application, secondary optics can be used to change the light pattern from the LED. Secondary optics perform three basic functions: collimating (increasing the light applied to the target), diverging or diffusing (spreading the light in a wide pattern), and illuminating (controlling the distribution of light so it falls where needed and not where it is not wanted).
What this means in practice is that the secondary optics, including lenses and reflectors, modify the output beam of the LED such that the output beam of the finished lamp efficiently meets the desired photometric specifications. To direct light onto a target, secondary optics collimate the light into a controlled beam, illuminating the targeted area. Collimated light rays propagate in parallel, although perfect collimation is not possible due to diffraction and the finite size of the emitter. However, smaller light sources can make collimating optics more effective. Besides collimating light, secondary optics can also be designed to improve color uniformity and light distribution within the target area.
High versus standard power for LEDs
One of the biggest factors that has affected LEDs in recent years is the question of power with high power (more than 1 W) and brightness LEDs hitting the market. The question is whether it is beneficial to use a bank of standard LEDs, such as the Lumex SML-LX1206UWW-TR, or a single high-power LED. Lumex actually put this to the test in an application that needed 90 lumen of power and compared one high-power LED to an array of six standard products.
The design process required for a standard LED array can be easier than that required for a high-power technology because of simplified thermal-management considerations. In this example, the 1 W LED was driven by 350 mA of current compared with just 120 mA for the array. The high-power unit needed a heatsink and metal-core PCB to ensure the junction temperature was not high enough to cause a loss in efficacy, a decrease in life hours, or color degradation. Standard LEDs do not require heatsinks, metal-core PCBs, capacitors, or resistors, so they are easier to design, test, and manufacture. This simplified process not only saves time and money during the manufacturing process, it also speeds up time-to-market.
The most expensive design addition for the high-power LED is the heatsink, which can be made from relatively inexpensive aluminum, but also more expensive materials such as copper and silver, which have the advantage of being more conductive. Similarly, high-power LEDs need the metal-core PCB to serve as another passive cooling technique for controlling junction temperature. These boards dissipate heat more efficiently than the more affordable FR4 PCBs used with standard LEDs, but can be up to five times more expensive. Cost savings by using the standard array can be up to 60%.
Despite there only being one unit in a high-power implementation, the addition of the bulky cooling apparatus means the whole installation takes up more room than using an array of six LEDs. In fact, the high-power unit can in some cases take up to twice the space of the standard products.
However, there are still applications that need the high-power technology including general outdoor lighting, large area indoor lighting, and automotive forward lighting. Smaller, battery-operated or portable devices such as consumer electronics, accent lighting, and standard indicator lights have long used standard LEDs and will likely continue to do so. Other applications fall into a gray area, such as small-space lighting including glove compartments and cabinets, interior and exterior signage, dental and medical devices, and industrial control status indicators.
A recent example was the inside cabin in a refrigerator, where the design called for three high-power LEDs. Lumex provided an alternative using eighteen standard-brightness 5 mm LEDs with specific housing. The result was the lighting cost about half as much and generated less heat. However, sometimes the opposite is true, as was shown by the case of a supplier of calibration-monitoring systems. It used a 7.6 by 7.6 cm board containing thirty-six 0.25 W, standard surface-mount LEDs. These were replaced with nine 1 W components without changing the power requirements. Despite the need to add a metal-core PCB, the high-power technology was cheaper and took up less space.
The Lumex SML-LX1206UWW-TR mentioned is a 3.2 by 1.6 mm surface-mount ultra-white LED with an operating and storage range between -25 and +85˚C. Its peak wavelength is 550 nm.
Figure 3: The Lumex SML-LX1206UWW-TR surface-mount ultra-white LED.
Conclusion
The revolution in LED technology has seen these devices used for ever more powerful lighting applications, whether it be street lights, automotive lighting, or outdoor signage. The appearance of high-power LEDs has boosted their use for these types of applications, but, as we have shown here, it is not always necessary to jump to the high-power version for every application. In some cases, a bank of standard LEDs can be just as effective, sometimes in a smaller space, and can save money. However, each application must be considered on its merits, and, whatever the choice, great care needs to be taken when using these LEDs.
