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DECEMBER 2023 I DESIGN007 MAGAZINE 61 Terminal areas do not normally require rein- forcement since the laminated construction divides stress among all terminations. If addi- tional strain relief is required, additional rigid areas or mounting holes for mechanical fasten- ers can be provided. FR-4 and G10 stiffeners can be bonded to the circuit. Terminal areas also can be potted for strain relief and envi- ronmental protection. Pinheads can be potted or covered with a laminated Kapton strip, and pin-headers can be furnished to help position fingers for easy assembly. If necessary, secure the circuit within its package to prevent flopping and provide vibra- tion resistance. Mounting holes can be built into the circuit for internal or external mount- ing (Figure 10). For dynamic flexing 1-ounce copper is the best metal thickness. Use uniform dielectric layers, top and bottom. Provide largest pos- sible flex radius, at least 0.25" for extended life. Flex life in excess of 1 million cycles is attainable. Shielding and EMI Since flexible circuit conductors are loca- tion-fixed, designing sensitive conductors away from radiating lines can avoid the need for shielding. Grounded guard conductors can also isolate sensitive circuits and elimi- nate crosstalk. in copper foil, copper mesh foil, or silver epoxy compound are the most used materials for shielding. Limit shield use to minimize cost and maintain flexibility. Cop- per shields are recommended for reliability. A crosshatch pattern in artwork will increase flexibility. Silver epoxy, applied by spraying or silk screening is used for cost reduction and flex- ibility. It can terminate many points simultane- ously and has excellent isolation capability. Its poor bond strength, however, limits it to outer surfaces only. Another method of shielding is copper foil with a pressure sensitive electri- cally conductive adhesive. Designate any areas requiring flexing. Adding inner layer shields to multilayer circuits will increase the bend radius and reduce flexibility (Figure 11). In cases of high RF energy, video signals and digital pulses, a full 360-degree shield may be required. is can be achieved by incorporating guard con- ductors on the edge of the circuit. e guard conductors are exposed in the cover-layer and base dielectric at specified increments. Silver epoxy is then applied to the circuit's top and bottom, making contact at the exposed areas. e same result can be achieved with copper shields using plated through-holes to make connections to guard conductors. DESIGN007 Chris Keirstead is a sales manager with PFC Flexible Circuits. Figure 10: Mounting provisions. Figure 11: Sheilding techniques.