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92 DESIGN007 MAGAZINE I DECEMBER 2019 When DuPont developed the heat-resistant polyimide film Kapton in the 1960s, there was no large-volume application. Now, DuPont has developed a new circuit concept for flex- ible circuits, but the company did not have a business plan for the products. The compa- ny developed and commercialized the copper laminate Pyralux® as the raw material of re- liable flexible circuits and rigid-flex for aero- space applications. DuPont also developed ba- sic constructions of the circuit with process conditions. When DuPont promoted Pyralux, the company provided the technical informa- tion as the design guide and processing manu- al. The business strategy was successful, and Du- Pont enjoyed good prof- its. The basic construc- tion of the flexible cir- cuits was almost fixed. The industry has grown significantly in the 1980s and 1990s, especially in consumer electronics, such as personal com- puters and cameras. The flexible circuit segment made another expan- sion in the 21 st century with the growth of cell- phones/smartphones. The industry expanded and became larger dur- ing the last four decades. However, the basic con- struction of the circuits has not changed. Kapton® is still a major material as the base film of flex- ible circuits. The circuit density rose, and via hole size shrank. Most of the flexible circuits re- tain the Kapton color. Few mobile device manu- facturers seem to dislike the Kapton color, and they asked flexible circuit manufacturers to paint the circuits black, but customers don't re- quest changes to the base polyimide films. On the other hand, new demands for the flexible circuits have appeared in wearable de- vices and medical electronics, which require abnormal properties that cannot be satis- fied by traditional polyimide base flexible cir- cuits; one of these requirements is transpar- ency. Previously, it was not very critical for the applications; both cus- tomers and manufactur- ers of the flexible circuits did not pay attention to performance. Howev- er, the transparency of the circuits is critical for optical sensors and lighting devices. There- fore, the requirement about transparent circuit has become very keen (Figure 1). Almost five years ago, a material supplier in Japan commercialized a clear polyimide film as the base material of heat-resistant flexible cir- cuits. However, the etch- ing companies did not Transparent Flexible Circuits EPTE Newsletter by Dominique K. Numakura, DKN Research LLC Figure 1: Transparent/flexible circuit example.

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