A comparison of LED technologies

SMD, AOB or COB: There are many light emitting diodes and a correspondingly large number of abbreviations. We have compiled a comparison of all technologies including their advantages and disadvantages. We also explain where they are best applied.

An LED pixel basically consists of two parts: the LED element or light-emitting diode and the control element in form of a circuit board. So-called SMD LEDs are the most common and most established. SMD stands for „surface-mounted-device“, i.e. a component that is fixed onto a surface. In this case, this refers to the LED that is soldered with its connection surface directly onto a circuit board. However, this is only one of many technologies, all of which have advantages and disadvantages. An overview:

■ SMD-Chip-LED: The standard SMD type. The individual LEDs are mounted directly on the circuit board and are surrounded by a transparent epoxy resin housing. The emitted light of the LED overlaps and is widely scattered. Due to the different wavelengths of the light, screens using this technology appear slightly reddish from the side. The pixels are placed individually, which makes the surface rough – you can clearly feel it when you slide a finger over the panel. One advantage is that the pixels can easily be replaced if they are damaged.

■ SMD-Top-LED: DWith SMD top LED, the emitted light is directed to the front by a funnel-shaped plastic housing that surrounds the LED. The housing is filled with epoxy resin. Due to the directed light emission, the image is brighter from the front. When viewed from the side, however, the image appears much darker and is more difficult to see than with a chip LED. Repairing or replacing individual pixels again is quite easy.

Because SMD technologies are mounted directly onto the circuit board, the construction is delicate. For a very high resolution the pixel spacing, and the respective components must be very small. SMD-LEDs are therefore not suitable for crystal-clear images in the high-resolution area. Other, more stable technologies are required:

■ GOB (Glue on Board): The technology is very similar to SMD top LED. With GOB, however, a layer of epoxy resin is applied over the entire LED surface, which encloses and seals the pixels. The LEDs are thus optimally protected; they are very stable – ideal for outdoor use and a large pixel pitch. However, in the case of a defect, the entire module must be replaced. The direct repair of individual pixels is not possible. The coating also causes the display to lose a significant amount of color quality. The light is refracted by the resin, which is particularly noticeable when displaying black images. The picture then looks rather greyish.

■ AOB (Admixture On Board): The AOB technology is also similar in its construction to SMD top LED. This time there is an extra silicon layer between the individual pixel housings for stabilization. When touching with a bare finger, the individual pixels can be felt like SMD. In contrast to GOB they can be repaired from the top if necessary. But this is not always possible. In this case the whole module has to be replaced as well.

■ COB (Chip-on-Board): The light emitting diodes are placed unencapsulated directly onto the PCB – the board for the LED control. This means the LEDs are not wired from above as usual, but directly from below. This is known as “ Flip Chip“. Without wiring on the surface, the individual pixels can move even closer together. This also increases the light yield: because no wires, no shadows. Especially when mini or micro LEDs are used, very small pixel pitches can be achieved with COB LEDs. A rather thin epoxy resin coating compared to GOB makes the LED a bit more robust without affecting the colors too much.

■ IMD (Integrated Mounted/Matrix Devices): The bridging technology between SMD and COB. The term Mounted or Matrix varies by manufacturer. An IMD pixel can best be described as a 4in1 LED. In this case, four SMD LED pixels are combined into one mechanical unit. This allows for an optically very fine pixel pitch, while the actual mechanical spacing is in dimensions known from other applications. However, the image of the IMD LED can appear grainy and irregular with a larger pixel pitch between the individual IMD units. The reason: With larger pixel pitches, the distance of the LEDs within the mechanical 4-block units is smaller than between the respective blocks. At pixel pitches of 1.5 millimeters and more, a black cross is therefore lasered into the middle of the unit to make the pixel pitch appear more even.

■ MiniLED und MicroLED: In contrast to common belief, the term does not refer to a manufacturing process, but only to the chip size, which each manufacturer defines for themselves. For example, MiniLEDs and MicroLEDs are combined in the previously described production processes such as COB or IMD to achieve finer pixel pitches of less than 1 millimeter. In the case of SMD LEDs or similar technologies, the housing limits the minimum pixel pitch. These technologies are therefore not suitable in this case.