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86 FLEX007 MAGAZINE I JANUARY 2019 each wire on the working panel, only IL data with corresponding impedance falling in the range of 100±10 ohm are viewed as valid. An Agilent Vector Network Analyzer, with a measurement bandwidth of 25 GHz was used to measure the ILs of dielectric material in var- ious FCCLs. All three dielectric materials have the same dielectric thickness of 50 mm, copper thickness of 12 mm, and CVL thickness of 50 mm with a total thickness of 174 mm (Figure 6). For FCCL measured under 10 GHz, LCP has a Dk value of 3 and Df value of 0.0016, modi- fied PI has a Dk value of 3.2 and a Df value of 0.003, and traditional PI has a Dk value of 3.5 and a Df value of 0.005 (Figure 7). Compared to the traditional polyimide dielectric material, modified PI and liquid crystal polymer are bet- ter suited as the dielectric material for making FCCL. The effect of dielectric thickness on IL has been studied for each dielectric material. The test was conducted under a constant copper thickness of 12 mm and CVL thickness of 50 mm for three dielectric thicknesses of 25, 38, and 50 mm, resulting in a total FPC thickness of 149, 162, and 174 mm, respectively (Figure 8). The data in Figure 9 indicates that a thicker dielectric leads to a lower insertion loss. Our trace layout and measurement method were also used to compare RA and ED cop- per foils based on their effect on IL of FCCL. Because the skin effect at the surface is more pronounced at a higher frequency, surface roughness will significantly affect IL. There- fore, the surface roughness of RA and ED cop- per foils are measured before the FCCL fabri- cation. Using the same dielectric material with equal dielectric thickness, copper thickness, Figure 5: Finished flexible circuit containing the test lines. Figure 7: IL for various dielectric materials. Figure 6: Configuration of test FCCL containing various dielectric materials. Figure 8: FPC with various dielectric thicknesses.