LED technology offers better color mixing and simplified thermal management for backlighting large LCD displays.
Conventionally, LED chips have been mounted on substrates to create discrete LED components, which were attached to a printed circuit board. The solder reflow process typically used to attach the component to the circuit board subjects the LED chip to a substantial amount of heat that easily can damage the chip or degrade its performance. Hence, tight (and, therefore, expensive) process control is required for this type of assembly. In fact, the substrate cost is almost invariably the second-highest in an LED component, exceeded only by the cost of the chip itself.
A different approach, called chip-on-board packaging, seems capable of meeting all the requirements of backlighting. This method mounts the LED chip directly onto the printed circuit board using a conductive adhesive, which helps reduce costs by eliminating the substrate and complicated solder reflow assembly process. In addition, direct attachment can reduce the pitch between LED chips from the conventional 5 mm to approximately 2 mm and can lower the overall height of the light source (Figure 1).
Decreasing the LED pitch reduces the color-mixing area required (Figure 2), which means that the area of light loss is smaller. To achieve high coupling efficiency from the light source to the lightguide plate, a reflector is incorporated into the chip-on-board package to produce an oval radiation pattern. A narrow radiation pattern on the X-axis allows more light to enter the lightguide plate, whereas a wider radiation angle on the Y-axis enhances color mixing.
Figure 2. Reducing the pitch decreases the size of the color-mixing area.
Simple thermal management
A metal core printed circuit board is used in the package to provide a low thermal resistance, allowing heat generated by the LED chip to be transferred to the heat sink via the shortest possible thermal path (Figure 3), which increases the life span; moveover, the heat transfers more efficiently through three layers than through five. The chip-on-board packages are mounted directly onto the back metal (with thermal compound at the interface), so that the heat generated by the LED chips spreads efficiently on the large metal frame for efficient dissipation without additional heat sinking. In the demonstration configuration, the entire backlight unit can maintain a temperature below 60°.
Figure 3. In comparison with the conventional approach, the thermal path is reduced. (DA = die attach, MCPCB = metal core printed circuit board.)
When compared with assemblies using conventional discrete LED packages, a chip-on-board approach can be compared with RGB LED-based LCD backlighting. The chip-on-board leds packaging has a thin outline, it produces better color mixing, and it requires simple thermal management and potentially lowers costs, all of which better match the requirements of customers. In addition, assembling a complete backlight using the chip-on-board led package is similar to that using today fluorescent lamps, which makes a changeover relatively simple.
Source: http://www.ledaladdin.com/led_technology/chip_on_board_leds.html
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