We present technological results on the embedding of ultra-thin microcontroller ICs in flexible film substrates. The novel concept is based on the following technologies: face-up chip mounting in cavities on film laminates, photo-lithographic patterning of vias and interconnects embedding in polymer layer and compatibility with both sheet and roll-to-roll processing. The paper briefly reviews the benefit of embedding for ultra-thin dies in terms of mechanical robustness. For the technological demonstration, we used 25μm thin microcontroller IC and 50μm polyimide film substrates. Electrical interconnections were realized by sputtering of metal layers. Photolithography was performed on “wafer level” using aligner photomasks and a photo-sensitive polymer of 10μm thickness for embedding. The embedding process resulted in a mechanically flexible fan-out chip package of a thickness below 100μm. Perspectives and technological requirements for roll-to-roll manufacture as well as cost estimation for this kind of Thin Chip Foil Package are explained and discussed. Furthermore, we report our recent work on the development of an in-situ bending and electrical test equipment for flexible film modules. The new set-up was evaluated using ultra-thin test chips with daisy chain patterns that were ACA flip-chip bonded onto Polyimide films. It was found that reducing the chip thickness from 28μm to 12μm lead to a strong increase in mechanical strength of the chip-on-film (COF) assemblies tested under recurrent bending
The growing interest towards thinner and conformable electronic systems has attracted significant attention towards flexible hybrid electronics (FHE). Thin chip-foil packages fabricated by integrating ultra-thin monocrystalline silicon integrated circuits (ICs) on/in flexible foils have the potential to deliver high performance electrical functionalities at very low power requirements while being mechanically flexible. However, only very limited information is available regarding the fatigue or dynamic bending reliability of such chip-foil packages. This paper reports a series of experiments where the influence of the type of metal constituting the interconnects on the foil substrates on their dynamic bending reliability has been analyzed. The test results show that chip-foil packages with interconnects fabricated from a highly flexible metal like gold endure the repeated bending tests better than chip-foil packages with stiffer interconnects fabricated from copper or aluminum. We conclude that further analysis work in this field will lead to new technical concepts and designs for reliable foil based electronics.
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