Flexible Printed Circuits (FPC): Principle of Operation
Flexible Printed Circuits consist of one or more etched conductor layers (usually copper) on one or two sides of a dielectric film (usually Polyimide or PET). Line width and spaces of 80 um or less are possible but are dictated by copper thickness. A soldermask or adhesive coated dielectric coverlayer is often applied over the conductive pattern to insulate and protect the conductors and to dictate component placement areas. Additional layers may be laminated together to create flex or flex rigid multilayer FPC’s and plated through holes (PTH’s) are used to electrically connect between layers.
Shielding for EMC (electro-magnetic compliance) using silver ink or additional copper layers, and impedance control can be designed into FPC’s.
Selectively placed rigid or film dielectric material can be used to stiffen areas that need additional mechanical support.
Circuitry can be populated with SMT (surface mount technology) or through hole components using lead free solder assembly processes or conductive adhesive assembly processes (if lower temperature base dielectrics and coverlayers are used). 0402 (and up) sized SMT and through hole components can be placed on FPC’s and flip chip or wire bonding of bare semiconductor dies can also be done in certain circumstances.
FPC's can be designed as 'flex-to-install' (i.e. one time bend during installation) or to flex continually throughout their life, often in circumstances where millions of flex cycles are required (i.e. connecting to the moving head of a printer).