Bonding bare die LEDs on PET foils for lighting applications: Thermal design modeling and bonding experiments

Integration of LEDs on flexible foil substrates is of interest for flexible lighting applications and flexible photonic devices. A matrix of LEDs on a foil combined with a diffuser can be a potential alternative for flexible OLED lighting devices. Preferably, these LEDs are integrated in an unpackaged, bare die form as it reduces cost, footprint and thickness. As a substrate, preferably low cost materials like polyesters (PET) are being used, especially for large area devices. However, the use of these materials imposes some limitations. Most notably, the low temperature stability (<100ºC continuous use temperature) of these materials limits the maximum temperatures during the manufacturing process and poses constraints on the thermal design of the device. The current paper describes the results of research on possibilities for integrating bare die LEDs with low cost flexible PET foils. Finite element (FE) thermal modeling has been performed of possible designs of adhesively bonded LED-on-foil and laminated LED-in-foil configurations. The role of the different materials and the effect of their geometries on the temperature distribution in the simulated devices are discussed. The results give insight in measures that can be taken to keep the temperature of all the components of the device within operational limits. For LEDs bonded on Cu-PET foil the modeled temperature distributions are compared to experimentally observed temperature distributions in LED on PET foil reference devices using infrared thermal imaging. Besides this, initial studies on directly bonding LEDs on etched Cu on PET substrates using anisotropic conducting adhesives and isotropic conducting adhesives were performed. An experimental comparison is made between the different techniques based on temperature/humidity reliability and flexural stability of the bonded LEDs, based on these preliminary results.