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March 2014 • The PCB Magazine 95 structures, decreasing the loss of the dielectric has a huge impact [2] . For thin structures the con- ductor accounts for most of the loss so lowering the dielectric loss only has a significant impact at high frequencies. Signal Integrity Analysis Tools One source of frustration for flex designers having to meet controlled impedance require- ments is that using the known permittivity values of flex materials in field solvers to calcu- late line width can be off. The reason for this is that most of these field solvers assume the use of glass reinforced copper clad laminates like FR-4. The software is optimized and verified by use of relatively thick layers and wide con- ductor lines compared to flex. In general, the designer or fabricator must bias the dielectric constant lower than the actual values reported on data sheets to get consistent line width pre- dictions. Overall Impact on Loss Performance It is possible to utilize Teflon and Advanced Kapton together to construct flexible circuit ma- terials [3] . The low dispersion and low loss tan- gent of these materials shown in Figures 4 and 5 combine to make the optimum platform for high-speed signals in a flex circuit. Figure 7 sum- marizes measurements taken from microstrip transmission lines made at the same time. Com- mon materials used in high-speed transmission (Meg4 and Meg6) are directly compared to a Teflon-Kapton composite (TK). As frequencies increase, the impact of the smoother RA copper becomes quite significant. The copper weight and type are denoted on the plot legend. Summary Takeaways • Flex circuits offer great advantages to de- signers, but there are big differences between flex materials and rigid materials. These include design, fabrication, and materials. Understand- USING FLEX IN HIGH-SPEED APPLICATIONS continues Figure 6: calculated values from the effect of thickness and dielectric loss.