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84 DESIGN007 MAGAZINE I JULY 2020 Next-Generation Flex Circuits: Transparent DuPont, one of the largest chemical com- panies in the world, developed a super engi- neering plastic film during the 1960s and soon after introduced Kapton®, a polyimide film that remains stable across a wide range of tempera- tures. DuPont also developed basic constructions of flex circuits as the major application of Kapton films. Nowadays, the flex circuit market reels in $20 billion a year. It is the primary wiring material for smartphones and smartwatches. If you remove the back cover of a mobile device, you will discover films that are orange in color. The industry term for this is "Kapton color." The color and low transparency of Kapton was never an issue for suppliers and contract man- ufacturers, so circuit designers did not require any particular color for heat-resistant films. It was Apple that decided it didn't like the Kap- ton color and required flex circuit manufactur- ers to paint the polyimide films black. Circuit designers never questioned the color of Kapton either (fortunately for polyimide film manufacturers). Clear films were required for some flexible circuits in display devices or photovoltaic cells; however, polyethylene tere- phthalate (PET) films were used instead. Low- temperature packaging processes were devel- oped for non-heat-resistant materials. The ever-changing electronics market requires upgrades to components, films, and circuits. New photo modules and medical equipment EPTE Newsletter by Dominique K. Numakura, DKN Research LLC Figure 1: An example of a transparent flex circuit. Several LEDs are mounted by soldering.

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