Flexible and cost-effective packaging method for Chip-on-Board Leds with adjustable beam angle (Fleko)
In chip-on-board (CoB) technology, the LED chips are placed directly on the PCB and contacted by bonding wires. The process offers a high degree of flexibility, best positioning accuracy and enormous potential for miniaturization, thermal management and integration. In addition to the LED chips, additional drivers and / or evaluation electronics can be integrated on the module. The chips are also usually protected by an optical encapsulation or a glass cover. For many applications, however, a limitation of the emission angle of the LED is necessary, which is realized with an optical lens. To date, three main methods have been established for the manufacture or attachment of such a lens:
1. Enclosing the lenses in a metal housing (TO housing) and subsequent sticking on the board. This has the disadvantage that the adhesive bond dissolves at high temperatures (> 120 ° C). In addition, the technology is expensive and inflexible.
2. Placement of the lenses directly on the chip. For this purpose, epoxy resins are usually used, which are molded on the chip. These resins have the disadvantage that they are not temperature resistant (<85 ° C) and can tear off the bonding wire due to their thermal expansion coefficient during heating or cooling from the chip. Another disadvantage is the moldability of the resins. Currently it is not possible to produce lenses with small radiation angles (<45 °). For this, a TO housing would have to be used.
3. The Total Internal Reflection (TIR) Lens: This is an effective way to reduce the beam angle. However, it is very expensive and requires complex components, which in turn makes them very expensive and useless for small batch production. In addition, TIR lenses are comparatively large and unsuitable for the desired market of sensor technology and medical technology, where the installation space is usually very limited.
Because of this situation, a new low-cost and easy-to-use packaging method for lens shaping and assembly for high density LEDs is desirable. In the project a technologically new approach is to be pursued, the application by means of a dropping method, whereby the viscosity of the materials but also the surface energy of the substrate play an important role. The drop method has the advantage that the production is considerably simplified and cheaper because expensive stamps are superfluous. With the PTFE coating of the housing surface, very good results for reducing the emission angle have been achieved so far. Due to the very low surface tension of the PTFE, the polymer lenses form very large contact angles and thus small radiation angles. The coating of the housing with graphene oxide is also investigated.
left: COB-LED with untreated housing surface, right: housing surface treated with PTFE solution, middle: the corresponding radiation characteristics