Large area flexible lighting foils using distributed bare LED dies on polyester substrates

Integration of LEDs on flexible foil substrates is of interest for flexible lighting applications and for backlights for flexible displays. Such a large area lighting device can be made by integrating a matrix of closely spaced LEDs on a flexible foil substrate. Preferably, these LEDs are integrated unpackaged, i.e. as bare dies, as this reduces footprint, thickness and cost. As substrates, low cost materials like polyethylene terephthalate (PET) should preferably be used. However, the use of these materials also imposes limitations. Especially, their low thermal stability limits the maximum temperatures during the processing and the thermal dissipation of the LED during operation will pose constraints on the thermal design. This paper describes the results of research on possibilities for integrating bare die LEDs with such low cost flexible PET foils. Bonding of LED dies on PET substrates with copper circuitry using conductive adhesives was performed. Both anisotropic conducting adhesives and isotropic conducting adhesives were investigated. An experimental comparison is made between the different techniques based on temperature/humidity reliability and flexural stability of the bonded LEDs. Additionally, finite element (FE) thermal modeling results of adhesively bonded LED-on-foil configurations are presented. The role of the different materials and the effect of their geometries on the temperature distribution in the simulated devices are discussed. The results are compared to experimentally observed temperature distributions using infrared thermal imaging in LED on PET foil reference devices. Finally a demonstrator device of 64 LEDs on flexible copper–PET substrate is presented.