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60 The PCB Magazine • November 2016 Impedance Considerations, Calculations Adding to the complexity of this design is the requirement for multiple impedances, both stripline and microstrip. Critical signals were kept off the microstrip layer (layer 2). Stripline Modeling Stripline modeling was straight forward. With our signal layers lying completely between the return planes there is no impact on imped- ance values due to proximity of adjacent layers. Calculations are straight forward and the result- ing values are consistent between unbonded layers in the flex. Loss per inch @ 3.2 Ghz are recorded for each line width. Part size, Panelization and Costs The rectangular shape of the product and 78 mm x 45 mm overall size allowed for good pan- elizations even with panelization considerations for the bookbinder manufacturing processes. Initial production was done 28 up on a 457 mm x 610 (18" x 24") panel. Production was moved to 60 up on a 610 mm x 914 mm (24" x 36") panel to reduce costs. A second low-cost geog- raphy manufacturing site was qualified using a 229 mm x 305 mm (9" x 12") panel size. General Development and Qualification Activities During product development, backdrill was added to the product requirements to improve signal integrity margin. The backdrill require- ment was added to vias with the three longest stub lengths covering 260 vias. The vias with the shortest stubs did not require backdrill. Fig- UNIQUE IMPLEMENTATION OF A RIGID-FLEX CIRCUIT Figure 8: Microsection of flex/rigid transition zone. Figure 9: Microsection of vias. Figure 10: Impedance calculation parameters.