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44 FLEX007 MAGAZINE I JANUARY 2019 Transparent polyimide films were devel- oped by several chemical companies over the last few years. Some have equivalent physical properties as traditional polyimide films, but they can still absorb small amounts of light. Copper-clad laminates of films without a glue layer are available through electroless plating or sputtering. Typical flexible-circuit manufac- turers can generate transparent circuits using a standard photolithography and chemical etch- ing process. The new flexible circuit looks sim- ilar to a PET-based circuit due to its transpar- ency, but it has enough heat resistance when soldering and wire bonding (Figure 1). This could serve as the flexible substrate for optical modules, but more trials are needed to achieve transparent conductors. Material choices have included ITO film, silver nanowire, and micro- metal mesh; unfortunately, none provide a perfect solution. Customers have to choose one from limited candidates. Another example of modifying existing mate- rials involves elastic circuits with rubber-based substrates. Meandering lines are created on a thin urethane sheet through a photolithography and chemical etching process or screen print- ing of conductive ink (Figure 2). They require a copper-clad laminate with rubber-sheet via thermal lamination or electroless plating. Cop- per foil provides high conductivity, but elastic glue should be used as the bonding layer dur- ing the thermal lamination process. Electroless plating does not require a glue layer for reli- able bond strength (this is an advantage), but it requires a special surface treatment before the electroless plating to secure a dependable bonding strength with conductors. Silver paste screen-printed on an elastic sub- strate is the best solution as long as the appli- cations do not call for large current. Since the matrix material of the thick-film conductors is a soft plastic (such as epoxy and urethane), the printed conductors have a higher flexing endurance compared to metallic copper foil conductors. The new elastic circuits could be considered a valuable alternative for medical and healthcare modules attached to the skin. The demands for materials on wearable elec- Figure 1: LED lighting module built on transparent circuits.