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Flex007-Jan2019

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JANUARY 2019 I FLEX007 MAGAZINE 83 ductor. The insertion loss (IL) of signal trans- port lines on the flex circuit can be quickly evaluated for various materials under high-fre- quency conditions. The dielectric layer thick- ness, dielectric material, and dielectric con- stant all affect IL. To compare ILs of various materials, we have specified a standard IL test- ing circuit layout and measurement method. Experimental Insertion loss is proportional to the dielectric constant (Dk), dielectric loss (Df) and signal frequency. For any material, a higher signal frequency leads to a higher IL. IL is defined as follows: where Pint is the input power and Pout the output power. In addition to its dependence on signal fre- quency, IL also varies with the dielectric char- acteristics of the material. The configuration of a double-layer FPC containing a flexible cop- per-clad laminate (FCCL) and coverlay (CVL) is shown in Figure 1. The FCCL consists of a dielectric layer sandwiched between two cop- per foils. The copper foils can be classified either as rolled-annealed (RA) or electrodepos- ited (ED) copper with a normal thickness of 12, 18, or 36 mm. The dielectric layer can be poly- imide (PI), modified polyimide, or LCP. The modified polyimide often contains fluorine or PTFE to enhance its properties. The dielectric layer has a normal thickness of 12.5, 25, or 50 mm. The CVL consists of polyimide and epoxy with a normal polyimide thickness of 12.5 or 25 mm, and epoxy thickness of 15 or 25 mm. During the testing of various FCCLs (includ- ing various dielectric materials, thickness, and copper thickness) and various CVLs under high-frequency data transport, we have speci- fied a standard fabrication process and an impedance measurement method to exclude other parameters unrelated to the material. The FPC for IL measurement is fabricated as shown in Figure 2. Figure 1: Configuration of a double-layer FPC. Figure 2: Fabrication of FPC for IL measurement.

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