Issue link: https://iconnect007.uberflip.com/i/271756
94 The PCB Magazine • March 2014 Europe. Figure 4 summarizes the relative per- mittivity versus frequency for Kapton®, Tef- lon® and the common adhesive materials used in flex circuits. Notice that Advanced Kapton used for high-frequency copper clad flexible laminates has lower dispersion compared to standard Kapton used for coverlay. Loss tangent values for these same materi- als are also shown in Figure 5. When analyzed in concert with Figure 4, one will notice that the materials with the least amount of disper- sion have the lowest loss tangent. Low loss and low dispersion are two of the primary reasons why Advanced Kapton and Teflon are used to maximize signal integrity for high-speed flex- ible circuits. Lower Loss Copper Not just the dielectric has a great impact on signal integrity. As thickness of layers reduces, the copper has an exponentially increasing im- pact on signal loss compared to the dielectric. If you suppose the air had dielectric loss at vary- ing levels, then Figure 6 shows the signal loss of a stripline for a thick structure, a nominal struc- ture, and a thin structure. Note that the loss is inherently larger due to the fundamental resis- tivity of copper for thin structures. For thick USING FLEX IN HIGH-SPEED APPLICATIONS continues Figure 4: relative permittivity versus frequency of flex dielectrics and adhesives. Figure 5: loss tangent versus frequency of flex dielectrics and adhesives.