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34 The PCB Design Magazine • June 2014 by Tom Woznicki Flex CirCuiT DeSign CoMPanY FLExDUDE Ninja Flex Circuits feature column One thing that I've always liked about flex circuits is that they look cool. They're shiny, orange, and clear—you can see those beautiful curved copper traces unlike their green rigid- board counterparts. But if you've looked inside a smartphone or read an iPhone teardown article, you don't see beautiful shiny flex circuits; you see flex cir- cuits that look like they were spray painted with black primer. Do not be deceived by their drab appear- ance—these are Ninja flex circuits! They have special cover films that give them pow- ers to suppress EMI, eliminate glare, control impedance, and reduce cost. They also dis- guise and protect intellectual property; it takes a great deal of effort to reverse-engineer them, and you literally destroy the circuit in the process. Figure 1 shows an iPhone 5s teardown by iFixit. Figure 2 is a close-up view of the camera module. You can see many different flex circuits all covered by either black cover film or shield- ing film. Black cover films are exactly as the name im- plies: Cover film made with black polyimide. It is widely used on flex circuits for LEDs and cam- eras. Just punch openings and laminate it to the base laminate as you would any normal cover film. DuPont offers Pyralux LF-B and halogen- free Pyralux HXC. Korean companies Innox and Doosan also make black cover film. Other companies, such as CEN in China, offer black polyimide which can be used with free-film ad- hesive to create cover film. A subset of black cover films are used to pro- tect components from static; the polyimide is electrically conductive, but with a very high re- sistance. These materials are not readily avail- able off-the-shelf as cover film. They are applied using free-film adhesive or by having a custom materials company make cover film from the polyimide sheets. DuPont has Kapton XC and CEN's antistatic polyimide is called BY. Figure 1: an iPhone teardown by iFixit. used with permission. Figure 2: Close-up of iPhone camera module. used with permission.

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