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JULY 2018 I FLEX007 MAGAZINE 21 wider signal lines. The wider circuit trace can be more easily fabricated to meet the tight toler- ances needed for controlled impedance circuits. To achieve greatest flexibility, the ground plane can be hatched. Improving flexibility by cross-hatching the plane layers in the flex por- tion instead of increasing thickness is helpful. Remember that hatching will have an effect on the impedance value. Most fabricators have the modeling tools necessary to determine if cross-hatching (the preferred design) is feasi- ble and will allow for the needed impedance callout (Figure 3). In terms of impedance structures, there are several different constructions possible using flexible circuits. The first and simplest is the co-planar strip- line construction. The circuit is produced with just one metal layer, alternating single and ground features. These constructions are well-suited to higher characteristic impedance designs. A drawback of these designs is that they are susceptible to EMI noise. The second type is the microstrip circuit, a two-layer flex construction with one metal layer devoted to ground. These circuits, nomi- Figure 2: Stack-up and controlled impedance model.