LED Packaging and Efficacy Advances Boost Lumen Density

By Steven Keeping

Contributed By Electronic Products

Despite impressive luminosity from individual LEDs, lighting engineers often have to group several together to achieve the required light output from their fixture. The downsides of this method are that the array of LEDs takes up valuable space, matching the correlated color temperature (CCT) across the array is expensive, it increases assembly costs, and more complex optics are required to form the desired beam pattern.

LED makers are addressing these drawbacks in two ways: repackaging existing devices into smaller form-factors reduces the space needed for the LEDs on the lighting fixture’s PCB, and boosting the luminosity of individual LEDs so that fewer are needed for a given output. This article provides an overview of the manufacturers’ progress in both areas.

The disadvantages of LED arrays

Although the efficacy and luminosity of LEDs has improved dramatically, single devices still do not produce enough output for a mainstream lighting application. For example, a single 100 W, 120 V incandescent bulb generates 1,700 lumens (at an efficacy of around 17 lm/W). In comparison, a lower-priced LED such as Cree's CLN6A generates 87 lm (300 mA, 3.8 V, 76 lm/W). A lighting designer would need 20 of these Cree devices to provide approximately the same output as the bulb.

This presents problems because of the physical space that the LED array takes up. Moreover, grouping LEDs also introduces challenges from illumination, optical, and manufacturing viewpoints.

Mature products such as the CLN6A are packaged on leaded substrates (Figure 1). Such devices typically measure around 7 by 5 mm (including leads). Some space is also required between devices for ease-of-assembly, electrical isolation, and thermal considerations, resulting in a “lighting array” that measures perhaps 10 cm². That might not sound too bad, but by the time heat sinks and optics are added, the resulting luminaire could become quite bulky.

Cree CLN6A

Figure 1: The conventionally-packaged Cree CLN6A measures 7 by 5 mm (including leads).

Worse yet, using up to 20 devices makes it difficult to optimize the quality of the light. Designers try to match the CCT and luminosity of the individual devices to ensure the consumer does not notice any variation. While LED manufacturers offer a sorting service that groups LEDs into bins of approximately the same CCT and luminosity, it does increase the costs, especially if the designer wants the very close match demanded of a quality product. (See the TechZone article “Defining the Color Characteristics of White LEDs.”)

A second problem with a large array of LEDs is that the devices age at different rates. The life of LEDs is typically determined by the time it takes for the device’s output to fade to 70 percent of its output when new (70 percent being the point that the consumer notices the difference between an old and new device). It is not difficult to imagine many of the LEDs in a fixture could have plenty of life left while the others have “failed”, resulting in the consumer junking the unit.

Third, one of the key advantages of single LEDs is that they approximate a point light source, simplifying the optics required to direct the output into a consumer-friendly pattern. An array of many devices spreads the light source, requiring the designer to commit more time and resources to the beam-forming optics.

Finally, assembling an array of many LEDs is difficult and time consuming, increasing cost. LED makers can eliminate the assembly task by offering pre-assembled modules, but the designer is still faced with a bill-of-materials (BOM) price premium for the privilege, compared with buying and assembling the devices in-house.

The amazing shrinking LED

One way to overcome some of these problems is to shrink the package size for a given light output (manufacturers refer to this a “improving the lumen density”). Moving from the leaded component form-factor described above to a leadless surface-mount device reduces the footprint of contemporary LEDs by around 75 percent. The OSLON SSL white LED from OSRAM (258 lm at 800 mA, 3.3 V, 125 lm/W) measures 3 by 3 mm (Figure 2).


Figure 2: Footprint of OSRAM’s SSL white LED.

Philips Lumileds has taken LED packaging a step further with their recently released LUXEON® Q (135 lm/W at 5700K). The company says that as the device is designed as a direct drop-in replacement for products that use the standard 3535 surface-mount package, it is currently no smaller than contemporary devices (measuring in at 3.5 by 3.5 mm). However, the LUXEON Q is the first high-power LED based on the LUXEON Flip Chip die, Philips Lumileds’ Chip Scale Package (CSP) device architecture, and a form-factor that can be shrunk considerably once lighting manufacturers redesign older products to accommodate it.

Hot on the heels of the LUXEON Q comes the LUXEON Z. The device produces 105 lm (500 mA, 2.85 V, 74 lm/W) within a footprint measuring 2 by 1.6 mm (about 75 percent smaller than the 3535 surface-mount package, Figure 3).

Philips Lumileds’ LUXEON Z

Figure 3: Philips Lumileds’ LUXEON Z has a footprint of 2 by 1.6 mm.

Figure 4 compares the new Philips Lumileds LUXEON Z with leadless- and leaded-LED packaging.

Philips Lumileds LUXEON Z footprint comparison

Figure 4: LED footprint comparison between Philips Lumileds LUXEON Z, leadless, and leaded-LED packaging.

An alternative approach to packaging has been taken by Cree with their CXA integrated array. To form the CXA package, the company has integrated multiple die under one uniform phosphor disc. The footprint of the device measures 13.3 by 13.3 mm, so this is not a tiny device. However, with an output of 425 lm (400 mA, 9 V), only four are needed to match the luminosity of a 100 W incandescent bulb, simplifying assembly, thermal management, and CCT matching.

The benefits of improved efficiency

As well as decreasing the package size to improve lumen density, manufacturers are also continually improving the efficacy of their products such that fewer devices are needed to attain a given light output (see the TechZone article “LED Efficacy Improvement Shows No Signs of Slowing”). At the high end of Cree’s product range is the XLamp® XM-L, which the company claims is the highest-performance single-die white LED for lighting applications. The device can produce 250 lm (700 mA, 2.9 V, 123 lm/W) in typical use or over 800 lm if the current is pushed to a maximum of 3 A. Depending on the drive current, between two and seven of the devices are all that is needed to match to output of a 100 W incandescent bulb.

Seoul Semiconductor has also recently introduced a technology that claims to boost the output of a single LED by up to five times so two devices could do the work of 10 older LEDs. Called “nPola,” the approach relies on manufacturing the LED’s active region in a “nonpolar” direction. Such a technique dramatically reduces microcracks or threading dislocations in the LED. These dislocations are known to reduce efficacy because they form centers where electron and hole recombination does not result in an emitted photon (see the TechZone article “What’s Next for High-Power LEDs?”).

A combination of package shrinkage and improved device efficacy is resulting in escalating lumen density. This means that designers can use fewer LEDs to meet their luminosity requirements, simplifying the product’s design, easing manufacture, and reducing cost. Moreover, by using fewer LEDs in a product, fewer design resources are required for the lighting fixture’s power supply (see the TechZone article “High Current, High Brightness LEDs Simplify Power Supply Solutions”).

Another major benefit of using fewer LEDs is that it allows designers to create novel compact lighting form-factors that were previously impractical.

For more information about the devices described in this article, click on the links provided to access product information pages on the Digi-Key website.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Steven Keeping

Steven Keeping is a contributing author at Digi-Key Electronics. He obtained an HNC in Applied Physics from Bournemouth University, U.K., and a BEng (Hons.) from Brighton University, U.K., before embarking on a seven-year career as an electronics manufacturing engineer with Eurotherm and BOC. For the last two decades, Steven has worked as a technology journalist, editor and publisher. He moved to Sydney in 2001 so he could road- and mountain-bike all year round, and work as editor of Australian Electronics Engineering. Steven became a freelance journalist in 2006 and his specialities include RF, LEDs and power management.

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Electronic Products

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