Multichannel LED arrays
Ready-made and custom-assembled
Brief description of modern multichannel LED arrays with medium power, both off-the-self and custom-assembled.
LED arrays, multichannel
LEDs together with modern driver modules make it possible design custom light sources for measuring spectral responses. Compared to multi-monochromator based approaches, the availabilty of LEDs has made it a lot simpler and cheaper to simultaneously irradiate a target with arbitrary sets of multiple wavelengths. The limitation is that the wavelength of the peak of emission of LEDs is a property of each LED, and these peaks are significantly wider than those from a high-resolution monochromator. An alternative, used in the past, are sets of band-pass interference filters. These are usually single band pass filters and although two- and three-band pass filters exist, when using them there is no easy way of adjusting the irradiance in one band separately of others.
Arrays are assembled from bare LED chips, while modules are normally assemble from individually packages LED chips. Currently, many LED modules are assembled from LED chips packaged individually or in small arrays as SMDs (surface mounted devices). For the measurement of response and action spectra it is frequently impossible to find off-the-shelf parts that are well suited to the task. One needs to either use a sub-optimal set of wavelengths or have arrays or modules custom assembled, which is surprisingly easy and relatively cheap.
Two channel arrays, including chip-on-board (COB) modules, have become relatively common as two channels of white light of warmth (two colour temperatures, such as 2700 K and 6500 K), which are used as light sources of tunnable colour temperature varying smoothly between the two extreme values. They are mainly used as light sources for photography and video, but are nowadays also advertised as a more comfortable household and workplace illumination.
1 Chips
LED chips are available in many different peak-emission wavelengths and power ratings. One constraint for research reproducibility is that the design-life of LEDs is rather short given rapid development of new types, which replace similar older types but with improved performance. On the other hand use-life is several times longer than design-life. Given the relatively low cost of LEDs, in most cases it is best to buy more pieces than needed at a given time, and stock them as spare parts.
“Design lifetime” means the length of time that a specific type, with no change in specifications, remains available for purchase in quantity from the manufacturer. The “use lifetime” is normally given in hours of use at rated power (current and voltage) until a given decrease in light output due to ageing. The mean-time-to-failure (MFT) is expected length of time 50% of the sold units will remain functional. The actual life time is lengthened considerably when the LEDs are run at a lower power than they are rated at, and it is shortened is they are not cooled down to the maximum rated temperature.
2 Single-chip packages
The traditional LEDs encased in plastic with leads for connection are becoming a rarity and being replaced by other packages that are small. Surface mounted device (SMD) is the most commonly type of packaging for electronic components, including single-chip LEDs. SMD packages are sometimes used also for clusters of 2, 3 or even 4 chips. These can be very small, a common size for 1 W LEDs is a square \(3 \times 3\,\mathrm{mm}\) and \(1\,\mathrm{mm}\) thick. The photograph below shows a slightly larger LED, rated foe 2.3 W of power dissipation. This is the same LED shown in the last section describing soldering of SMD LEDs.
3 Arrays
LED arrays with between 1 and 12 channels are available of-the-shelf in some configurations, while suppliers of custom-assembled LED arrays with 1 to 12 channels abound. Some suppliers have minimum quantities of 1 to 10 arrays per custom-assembled type or set of chip types. As arrays are assembled from individual chips, and for some types manual assembly is relatively simple, these arrays can combine chips of different wavelengths and from different chip manufacturers. Each array channel is usually wired independently, and its output can be dimmed and/or switched on and off separately from other channels. Each channel can contain a set of chips emitting all at the same wavelength or a mix of chips, such as alternating chips of two different wavelengths. In custom assembled arrays, chips tend to be less densely packed, while some off-the-shelf types such as COBs pack multiple high power chips into a very small area. When few arrays of a given form factor and power are needed, they are assembled onto of-the-shelf carriers or boards.
4 RGB and RGBW modules
Red-green-blue and red-green-blue-white arrays are common off-the-shelf parts used in large-sized screens and other uses that take advantage of the trichromatic vision of humans. Small sized arrays have one LED chip of each colour and larger ones, frequently used for illumination for video recording have multiple clusters of chips, with each cluster having RGB or RGBW chips. In this case the chips are individually mounted in SMD packages.
5 An off-the-shelf seven-channel array
Assembly of individual LED chips into an array is a specialized job that can be done manualy under a (stereo) microscope using special tools and potting compounds, or using robotic machines. The choice tends to depend on the number of arrays to be assembled. The array shown in the photographs below used to be available of-the-shelf as a standard type produce in large quantities. In this case the seven chips are packaged together into a special SMD package, and then this single SMD is soldered into a copper metal core board.
6 A custom-assembled 12-channel array
A custom assembled 12-channel LED array containing 120 LED chips, 10 of each of 12 different peak emission wavelengths. In this case bare chips were mounted and wired onto a standard 120 W array package and “potted” using clear silicone resin. Given the very high power disipation, the array is mounted on an aluminum heat sink, using a heat transfer compound.
Assembled by ShenZhen Weilli using CREE, Luminus and possibly other suppliers’ 1W-LED chips. Light emitting area 24 mm x 24 mm, array 40 mm x 56 mm.
7 Modules assembled using SMD LEDs
Modules are assembled either on glass-fibre-epoxi circuit boards or metal core (solid aluminium or solid copper) circuit boards, of which several types are readily available of-the-shelf both unpopulated and populated with LEDs. They are relatively easy to assemble as in SMDs LEDs chips are mounted on a carrier, protected and ready to be soldered in the same way as most other SMDs used in modern electronics. For assembly with manual equipment the trickiest aspect is the dosing of the solder paste evenly and sparingly to ensure that the light emitting surface remains level with the board and other SMD LEDs on the same board. When using metal core boards, there are some restrictions about what methods for heating the solder work well.
Epoxi-fiber-glass boards are cheaper than metal-core ones but they do not conduct heat as efficiently, thus can be used with lower power chips and only if these are not densely packed. Aluminium-core boards are commonly used as they ensure good conduction of heat from the SMD packages through the board to an underlying heat-sink or other cooling system. Copper-core board conduct heat even better that aluminium but are quite a lot more expensive.
When larger quantities of modules with special shapes or arrangements and wiring of LEDs are needed, custom module boards can be designed, made and assembled by special order. Because of set-up costs, special designs when robotically assembled make economic sense usually only for series of not fewer than 100’s of modules, but this is changing rapidly.
Unpopulated boards for the assembly of modules are more easily available for single-channel than multichannel designs. The foot-print (the soldering pads at the back of SMD LEDs) vary in design with the size and power of the packaged chips and among manufacturers. Boards need to match the SMD LED type for successful assembly.
8 Soldering of SMD LEDs
I wrote this section as a separate post on 2021-10-16. I moved it here as works better here.
High power SMD (surface mounted device) LEDs are normally soldered to a printed circuit board that has a metal core to help conduct the heat generated away. SMD devices are usually small, sometimes very small. Soldering them with a soldering iron is to say the least, challenging as unlike components installed with pins going through holes in the board SMDs just sit on its surface.
The use of electronic devices in “trough-hole” packages is decreasing rapidly, so availability is limited. I recently bought a mini-hot plate and the photographs below shows how it works. Alternative approaches are re-flow ovens and hot air guns. The hot-plate I show here (MHP30 from Miniware, Guangzhou City, China) is specially handy when working with small individual boards and not too many of them. The size of the ceramic plate is 30 mm \(\times\) 30 mm, and can reach 300 C.
When using a soldering iron one adds the solder using a lead/tin “wire” that has a core of flux, when using a hot plate for re-flow soldering one uses a paste with the flux as matrix carrying small beads of lead/tin or other alloy. As temperature increases gradually the flux melts first, cleaning the metal surface and acting as a wetting agent for the metal that melts at a higher temperature. The solder paste can be applied with a stencil, but in this example with a “large” LED \(4 \times 4\) mm device I applied it directly with a syringe and a very small tip. I could have used slightly less solder paste, but not much less as the metal pads on the board are larger. Through this series of images the temperature of the board raises from ambient to nearly 250 C.
[2024-07-11]
A new lager version of the hot plate (MHP50 from Miniware, Guangzhou City, China) is now also available. The size of the MHP30 hot plate is \(30 \times 30\,\mathrm{mm}\) while the size of the MHP50 is \(50 \times 50\,\mathrm{mm}\)
Independently developed firmware for the MHP30 is available at https://github.com/Ralim/IronOS. It adds a few useful features but can be a bit more complex to set-up for use than the original firmware.