By BizLED Bureau
Apr 10, 2017: Most often the basis for unique selling points of several products in the market is the greatest power densities in the smallest space. These criteria can be realized by the chip on board (COB) LEDs technology. Direct contacting of the semiconductors (LEDs) on ?olychlorinated biphenyls (PCBs) allows for optimal thermal management, high packaging density, and therefore, long-lasting and high-performance COB LED modules.
COB LED technology
Under this technology, the LED chips are in the form of a semiconductor chip, which is neither connected nor encased. The semiconductor chip is called a ‘Die’. This LED chip is processed by a special procedure known as ‘Die Bonding’. Here every chip is placed on the PCB and using the ?Wire Bonding? method, connected to the contact surface of the PCB, which are called ‘Pads’. For contacting, gold wires in the micrometer range are used. The COB LED light technology allows almost limitless freedom of scope for the PCBs and thus are the basis for totally unique LED solutions.
COB LED module
LED technology allows better color mixing and simplified thermal management for backlighting large LCD displays. Usually, LED chips are being mounted on substrates to create discrete LED components, which are 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 considerable amount of heat that can easily damage the chip or degrade its performance. So, tight (and expensive) process control is needed for this type of assembly. In fact, the substrate cost is almost always the second-highest in an LED component, exceeded only by the cost of the chip itself.
Another approach, known as COB packaging, seems capable of fulfilling all the requirements of backlighting. This method mounts the LED chip directly onto the printed circuit board, using a conductive adhesive that reduces costs by removing the substrate and complicated solder reflow assembly process. Besides, direct attachment can reduce the pitch between LED chips from the traditional 5 mm to approximately 2 mm and lower the overall height of the light source.
Lessening the LED pitch reduces the color-mixing area needed, which means that the area of light loss is smaller. To attain high coupling efficiency from the light source to the lightguide plate, a reflector is incorporated into the COB package to produce an oval radiation pattern. A slender radiation pattern on the X-axis allows more light to enter the lightguide plate, while a wider radiation angle on the Y-axis increases color mixing.
Simple thermal management
A metal core printed circuit board is used in the package for a low thermal resistance. This allows heat generated by the LED chip to be transferred to the heat sink through the shortest possible thermal path, which inhances the life span. Apart from that, the heat transfers more easily through three layers than through five. The COB packages are mounted directly onto the back metal (with thermal compound at the interface), so that the heat generated by the LED chips spreads easily and efficiently on the large metal frame for efficient dissipation without additional heat sinking. In the demonstration configuration, the whole backlight unit can keep temperature below 60°.
In comparison to assemblies using conventional discrete LED packages, a COB approach can be compared with RGB LED-based LCD backlighting.
The COB LEDs packaging has a slim outline, produces better color mixing, and requires simple thermal management and potentially lowers costs. All these better match the needs of customers. Besides, assembling a complete backlight by using the COB LED package is similar to that using fluorescent lamps, which makes a changeover quite simple.
The first step of COB package is to cover wafer placement point on the substrate surface by thermal conductive epoxy (usually is epoxy resin mixed with silver particles). Then the wafer is put on the substrate surface directly, and the fix wafer to the substrate firmly by heat treatment. Next, is to establish electrical connection between wafer and the substrate by wire bonding method.
Advantages of COB LEDs
COB LED lights can save about 30% cost in different applications, mainly in LED package cost, light engine manufacturing costs and in secondary light distribution costs.
Performance wise, through the rational design and micro-lens molding, COB LED light module has less defects of point and glare light, as well as other faults that exist in the light source device. If we add some red chips appropriately, it can improve the CRI effectively.
Application wise, COB module makes production of lighting manufacturer more convenient and simple and, as a result, reduces cost effectively. In production, the existing technology and equipment can support high yield and large-scale COB module manufacturing.
COB LED disadvantages
COB packaging technology, however, has some limitations as well. How to improve the reliability of the light source and its operating temperature are the major limitations. However, most of theCOB packaging companies use aluminum substrate as the material. Aluminum COB reliability is low because of its greater thermal resistance, and good for high light attenuation. The ceramic substrate is an ideal COB material, but its cost is relatively high, especially for the power less than 2W.