The popular high-power LEDs in use today are all formed of three basic elements:
- A ceramic substrate, generally made of aluminium oxide (also known as alumina) or of aluminium nitride.
- The LED chip, consisting of a die and a phosphor coating. This is affixed on top of the substrate.
- An overmoulded silicone lens which encapsulates the LED chip
Figure 1. The 219C from Nichia, a high-power lensed LED (Image credit: Nichia)
An example, the 219C high-power LED from Nichia, is shown in Figure 1. Small variations in the design of high-power LEDs can be found between one manufacturer and another, and even between different product families made by a single manufacturer. Nevertheless, the basic recipe is almost universally followed. This is because it offers numerous advantages:
- The ceramic substrate is robust and large enough to be easily handled on the production line, and may be assembled on a luminaire’s PCB using a standard surface-mount assembly process.
- The substrate can accommodate a Zener diode to protect the LED against Electro-Static Discharge (ESD), which may be generated when the device is handled in the factory.
- The lens protects the chip and any bonding wires.
- The lens adapts the refractive index so as to maximize light extraction from the die.
- Larger ceramic substrates can accommodate multiple LED chips, providing a higher output from a single package.
OEMs which use high-power LEDs will be aware of certain drawbacks to this type of device. The substrate gives the device a relatively large board footprint – noticeably larger than the footprint of the chip alone. In addition, the lens has a magnification factor which has the effect of making the light source appear bigger than it really is.
Last but not least, the manufacturing cost of the complex package assembly tends to make the unit cost of a high-power LED higher than that of other LED types.
The CSP: A Smaller and Cheaper Light Source
The introduction of the CSP (Chip-Scale Package) LED was intended to counter the limitations that inhibited the adoption of high-power LEDs in certain applications.
The main aim of the design of CSP LEDs was to remove as many of the packaging elements found in high-power LEDs as possible, in order to reduce the size of the device, and to reduce the cost of manufacturing it. In doing so, the LED industry was drawing on packaging know-how acquired by the semiconductor component industry more than 20 years previously, when it achieved a similar reduction in the size and cost of discrete diodes and transistors, as well as of some Integrated Circuits (ICs).
The CSP LED consists of the LED die (an epi layer on a sapphire substrate) and a silicone/phosphor encapsulant: this eliminates the high-power LED’s ceramic substrate and silicone lens. As a result, the CSP LED gains a more compact footprint, hardly larger than that of the LED die itself, and a light source which appears to be smaller, because there is no lens to magnify it. CSP LEDs thus provide higher luminance, and are cheaper to produce.
Once again, however, the benefits are offset by some drawbacks. Assembling a small CSP LED on a PCB calls for an accurate and tightly controlled mounting process. The precisely designed PCB also needs to be accurately printed, and extra care might therefore need to be taken in commissioning the PCB from a high-quality supplier.
In addition, a CSP LED is generally more sensitive to ESD than high-power LEDs are. This is not a problem if proper ESD mitigation practices are followed in the factory. But the CSP package style is less tolerant of incorrect materials handling processes.
The decision to use a CSP LED also has ramifications for the optical performance of the luminaire. Of course, optical designers regard small size and high luminance as desirable attributes.
Unfortunately, the optical radiation pattern of conventional CSP LEDs is not so desirable. Conventional CSP LEDs emit light from all four sides as well as from the top surface. A fraction of the light is emitted below the horizon, potentially resulting in optical losses.
When CSP LEDs are assembled in dense clusters, these side emissions lead to further optical loss through shadowing. Worse, the side emissions can cause optical crosstalk: this is where neighbouring LEDs absorb each other’s light output, producing colour shifts.
A final disadvantage is that early CSP LEDs are not compatible with most existing secondary optics.
New DMC Type of LED Offers Different Optical Attributes
Now LED manufacturer Nichia has introduced a new generation of ‘DMC’ LEDs which eliminates the drawbacks of the early CSP products.
Nichia’s DMC LEDs (DMC stands for Direct Mountable Chip) feature a special reflective ‘wall’ which blocks the light emitted at the sides of the die, and redirects it forwards (see Figure 2).
Figure 2. The innovative design of the Nichia DMC eliminates side emissions. (Image credit: Nichia)
This offers two important benefits. The first is much improved light extraction from the LED assembly. This reduction in optical losses helps DMC LEDs to achieve high board-level efficacy (lm/W) when compared to conventional CSP LEDs. Nichia’s DMC LEDs offer efficacy typically just 7% lower than that of its domed high-power LEDs.
The second benefit of the absence of side emissions is that the DMC LEDs can be assembled in dense clusters, with no risk of optical shadowing or crosstalk. Losses from optical crosstalk reduce luminous flux and efficacy (see Figure 3).
Figure 3. Even when packed closely together, DMC LEDs maintain flux with almost no optical losses. (image credit: Nichia)
The colour shift from which conventional CSP LEDs suffer is shown in Figure 4. This demonstrates clearly that the more tightly packed the LEDs are, the further the complete light module’s colour shifts from the individual LEDs’ nominal value. In contrast, Nichia’s DMC LEDs maintain consistent chromaticity, regardless of spacing.
Figure 4. Conventional CSP LEDs suffer from colour shift when assembled in tightly packed arrays. (Image credit: Nichia)
The DMC LEDs, then, have a chromaticity and efficacy advantage over conventional CSP LEDs. They also, like conventional CSP devices, have a size advantage over packaged high-power LEDs. Figure 5 shows the great reduction in the size of the secondary lens when using a DMC LED, thanks to both its smaller physical size and its higher luminance.
Figure 5. DMC LEDs require smaller secondary optics than conventional domed LEDs. (Image credit: Nichia)
How to Build DMC LEDs into Luminaire Designs
There are various ways for lighting OEMs to take advantage of the attractive characteristics of this new type of LED package from Nichia.
Users of traditional light engines will begin to see new products emerging which offer greatly increased light density, helping them to achieve reduced system cost. As an example, Rena now uses Nichia DMC LEDs to make double-density streetlight modules in a Zhaga-compatible form factor (see Figure 6).
Figure 6. The Rena Standard (top) and Double Density (bottom) streetlight modules. (Image credit: Rena)
The Rena modules benefit from excellent thermal design, an important feature of any DMC-based light engine that clusters multiple high-power LEDs in a small board area, as well as optional active thermal feedback to the LED driver.
Another way to use DMC LEDs is to assemble them in dense clusters – the gap between LEDs can be as small as 200μm. This is ideal for applications which require a small light source with high luminance, or where the luminaire’s shape requires a light source with a shape that is difficult to achieve with conventional LED packages (see Figure 7).
One other application for Nichia DMC LEDs is in tunable white light sources: these use a combination of LEDs with different CCT values.
Figure 7. Nichia DMC LEDs can be tightly packed in clusters that provide very high luminance. (Image credit: Rena)
For any given target flux, the space occupied by a tunable white light source will be bigger than that of a fixed-CCT light source, since the fixed-CCT light source can run all its LEDs at a 100% duty cycle.
Here, the small size of the DMC LEDs and the ability to assemble them in densely packed clusters can enable OEMs to achieve a light density rivalling that of systems using Chip-on-Board (CoB) LED packages (see Figure 8).
Figure 8. With less than 0.3mm spacing between LEDs, 68 DMC E21A LEDs fit well within a 20mm diameter LES and outputs over 10klm of high CRI tunable white light with an excellent uniformity and color over angle. (Image credit: Rena)
New Opportunities for Lighting OEMs
CSP LEDs opened new horizons for solid state lighting system designers. Now the introduction of Nichia’s DMC technology helps these designers to overcome some of the biggest limitations of the CSP LED.
Future Lighting Solutions and its industrial partners have invested in the technology to enable lighting OEMs to take advantage today of the DMC LED products from Nichia, while benefiting from a faster time-to-market and reduced design risk.