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86 DESIGN007 MAGAZINE I JANUARY 2020 • The width of the traces carrying the signal • The distance of the signal traces from the reference plane layer(s) • The thickness of the traces carrying the signals • The distance between signal traces in differential impedance applications The most common impedance requirements range from 50–75 ohms (single-ended) or 100– 110 ohms differential. Achieving these imped- ance values in flex circuitry requires the use of thicker dielectric materials than are normally used, resulting in an overall thicker and stiffer circuit (Figure 5). Plane Layers and Shielding Reference plane layers and external shield- ing play a key role in both impedance control and signal integrity. Fabricators can add plane layers using: • Additional etched copper layers • Screened conductive epoxies or inks • Laminated conductive films Copper plane layers are the standard for in- ternal planes that require a connection through plated vias. Copper planes will cause a flex to hold a pre-form better, while screened epoxies and inks and laminated conductive films will produce a more flexible circuit. Stiffeners It is wise to rigidize SMT, connector, and other termination areas on your flex circuit with mechanical stiffeners. Your flex fabricator can add stiffeners of various thickness made from epoxy-glass laminate (FR-4) or polyimide film. In SMT applications, stiffeners should be applied to the side opposite the SMT compo- nents. On through-hole connectors and other through-hole applications, stiffeners should be applied to the same side as the connector or through-hole component. Stiffeners applied to connector areas will require holes that match the connector footprint. Holes in the stiffener should be sized at least 0.015" larger than the access hole in the circuit. Thermal Pads Thermal pads should be used on any solder pad that is surrounded by a large amount of copper. Large areas of copper will sink heat away from a non-thermal pad and make it very difficult to solder. Rigid-flex Design Guidelines Since rigid-flex circuits are a hybrid of rig- id and flexible PCBs, there are special guide- lines that apply to this type of construction (Figure 6). • On rigid-flex circuits, ensure that all plated through-holes are in a rigid area (no PTHs in flex areas) • Specify adhesiveless flex materials and "cut-back" or "bikini" cover construction for rigid-flex designs. Acrylic adhesive is the "Achilles heel" of a plated through- Figure 5: Illustration showing a two-layer construction with higher impedance requirements. The added thickness of the controlled impedance section makes circuits less flexible.