The question does not specify the expected number of flexing cycles across the device lifetime. For applications where the FPC will be flexed just once at installation, such as connections between LCD panel and attached backpack controller board, pretty much anything goes. At worst, infantile death of the FPC if it happens, will be detected in a test run. Assuming multiple flexing cycles: • The location of the via inherently becomes a weak point. Given that flexing already creates stress on the copper layers, this is a conduction breakage waiting to happen. You will notice that if there ever are vias on FPC, they are found on the parts unlikely to bend. • The copper used must also be the thinnest possible, to minimize risk of fracture. This means 1 ounce copper, or thinner even, if the application can stand it. • (Ref: mention of minimizing trace resistance because of high current, from OP's comments) In general, one tries to avoid high current over thin traces on FPC: Thermal conduct…

Single-Layer Flex Circuit IPC 6013 - Type 1 Description: One conductive copper layer bonded between two insulating polyimide layers Example construction: coverlay/copper/flex core Configurations: Single-sided access, dual-sided access, unsupported fingers Flex Core Materials: Standard thicknesses: 1/2 mil to 3 mil in either adhesive or adhesiveless constructions Standard copper thicknesses: 1/3 oz to 2 oz in rolled annealed or electro deposited formats Coverlays: Standard thickness: 1/2 mil to 2 mil polyimide, with 1/2 mil to 2 mil epoxy or acrylic adhesive Stiffeners: Component support or ZIF connector thickness requirments Materials: polyimide, FR4, stainless steel, aluminum PSA (pressure sensitive adhesives) available for attachment purposes EMI/RF shielding films available Applications: Suitable for either bend to fit or dynamic flex applications Double-Sided Flex Circuit IPC 6013 - Type 2 Description: Two conductive copper layers with an insulating polyimide …