Novel processing scheme for embedding and interconnection of ultra-thin IC devices in flexible chip foil packages and recurrent bending reliability analysis

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.